Category Archives: Motoring

Gas Tank Skid Plate Reconstruction

A common issue with Grand Wagoneers is rusting gas tank skids. Gas tanks on Wagoneers are plastic. To support the weight, the gas tanks are fastened to a metal skid. Then the skid is bolted to the frame. This is a great setup as the gas tank can not rust out and clog fuel filters but Jeep made the skid plates out of thin metal with poor rust prevention so they do not last.

Pulled gas tank out of the wagon. Dirty!
Pulled gas tank out of the wagon. Dirty!

 

I disconnected the fuel lines and fill lines then dropped the tank. Skid is full of dirt and rust. The straps that hold the tank to the skid where rusted through so removing the tank from the skid was easy.

Frame inspection and cleanup.
Frame inspection and cleanup.

I then inspected the frame where the gas tank lives. This is also a very common area for rust to happen. This looks bad but the frame was 100% intact. There was ton of dirt build up on the frame that at first looked like rust. The rusted holes in the left side of the picture is left over parts of the skid plate that was still attached to the frame. I cleaned up the frame and painted it with some chassis saver to prevent any further rust.

Skid plate cleaned up after removal of gas tank.
Skid plate cleaned up after removal of gas tank.

Then I cleaned up what was left of the skid plate. I was hoping to repair the skid by patching it but the skid was to far gone at this point.

Skid plate triangle brace. Not much left of it!
Skid plate triangle brace. Not much left of it!

Then pulled the skid plate triangle brace and cleaned it up. Swiss cheese steel. I decided to rebuild the triangle brace first since it will be easier.

Brackets for the triangle brace.
Brackets for the triangle brace.

Built the new bracket parts for the triangle brace. I added nutserts to make install easier later.

Tracing the old brace into cardboard.
Tracing the old brace into cardboard.
Brace traced.
Brace traced.
Cardboard cutout.
Cardboard cutout.
Testing fitting the brackets and cardboard cutout.
Testing fitting the brackets and cardboard cutout.

To make the rest of the triangle brace, I traced it onto some cardboard. Then I cut out the cardboard and mocked it up with the braces I cut out steel.

Cardboard template transferred into steel.
Cardboard template transferred into steel.
Steel cleaned of rust and scale.
Steel cleaned of rust and scale.

Then I copied the cardboard into some 14AWG steel. The steel was bought LTP (Less Then Prime) which means it is rusted but it was inexpensive. Cut the steel out and then cleaned up with a flappy disc on the grinder.

Brackets tacked to the steel triangle.
Brackets tacked to the steel triangle.
Comparing the original rusted bracket to the new one.
Comparing the original rusted bracket to the new one.

I then tacked the brackets onto the steel and compared it to the original bracket. Looks good to me!

Outside welded and grinded smooth.
Outside welded and grinded smooth.
Inside welded. New bracket complete!
Inside welded. New bracket complete!

Triangle brace is complete. Time to start fabricating the skid plate for the gas tank.

Skid plate pieced back together for measurements.
Skid plate pieced back together for measurements.

I started measuring the skid plate and started cutting some steel.

Skid plate construction. Checking fitment.
Skid plate construction. Checking fitment.
This is my construction technique. Measure, make cardboard cut out, test fit, cut out of steel, and tack up.
This is my construction technique. Measure, make cardboard cut out, test fit, cut out of steel, and tack up.
More test fitment after making the large side panel.
More test fitment after making the large side panel.
Other side of the large panel.
Other side of the large panel.
Rest of the side panels cut out and tacked up.
Rest of the side panels cut out and tacked up.
The gas tank has a lower bulge for the gas pickup. Made a drop down section and fitted up some steel to cap it.
The gas tank has a lower bulge for the gas pickup. Made a drop down section and fitted up some steel to cap it.
Skid plate seam welded and grinded. Lots of metal dust. My respirator was completely black after this. Wear your PPE people.
Skid plate seam welded and grinded. Lots of metal dust. My respirator was completely black after this. Wear your PPE people.
Test fitting the tank with the gas tank straps.
Test fitting the tank with the gas tank straps.
Painted the triangle bracket, skid, and associated brackets in chassis saver to prevent rust.
Painted the triangle bracket, skid, and associated brackets in chassis saver to prevent rust.
Bottom side painted. I drilled holes in the skid for water drainage as you can see here.
Bottom side painted. I drilled holes in the skid for water drainage as you can see here.
Gas tank fitted, vent lines plumbed, and fuel sender installed.
Gas tank fitted, vent lines plumbed, and fuel sender installed.

With the skid plate and triangle bracket fabricated and painted I installed the gas tank. Before that I cleaned out the gas tank with a pressure washer and dried it out. Then the gas tank received new gas tank straps, vents, vent hoses, and sending unit.

Gas tank with new skid installed under the wagon!
Gas tank with new skid installed under the wagon!

I then installed the skid under the wagon. Fit great and now I do not have to worry about the gas tank falling out.

Coolant System Repair on the Wagoneer

Time to start fixing things on the Wagoneer. It drives around town fine but when the engine is turned off after driving it tends to puke radiator fluid everywhere. Looking at the radiator it is clear this happens around the filler neck. Wiggling the filler neck it looks like there is a crack at the base. Since this is a brass/copper constructed radiator I should be able to braze/solder this back together.

Radiator filler neck with lots of radiator fluid (glycol) crusted around it.
Radiator filler neck with lots of radiator fluid (glycol) crusted around it.

While I am in here I am going to also replace the T-Stat, water pump, and clean up the heater box. Basically a cooling system rebuild.

Radiator peacock. I zip tied the transmission lines out of the way so the coolant can flow without hitting anything.
Radiator peacock. I zip tied the transmission lines out of the way so the coolant can flow without hitting anything.

Draining all the coolant. On the Wagoneer the drain peacock is at the bottom which is nice. On the TJ Jeep it is horizontal and tends to shoot coolant all over the steering suspension. Only thing in the way was the two transmission fluid lines which I was able to zip tie out of the way.

Fluid drained.
Fluid drained.

With the fluid drained I removed all the hoses in the system. There are three that attach to the radiator; Upper radiator hose, Lower radiator hose, and the hose that goes to the overflow bottle. I inspected these as I removed them and they looked to be a in shape and I will be reusing them.

Pulled T-stat housing.
Pulled T-stat housing.

I pulled the T-stat housing and it looks like it belongs on the Titanic. I will clean this up with a brass wire wheel on the drill before reinstalling.

T-stat is installed upwards. Lots of gunk on the T-stat as well.
T-stat is installed upwards. Lots of gunk on the T-stat as well.

The T-stat itself sits in a grove on the block and the gasket is put over it. The previous mechanic just gooped RTV all over it and was a pain to remove.

Close up of the T-stat. Nasty.
Close up of the T-stat. Nasty.

Not much left of the original gasket. Lots of rusty sludge build up on it as well. Makes sense that the Wagoneer sat for years before I got it.

Removing the fan and fan clutch.
Removing the fan and fan clutch.

At this point it makes sense to make more room to work by removing the fan clutch. Since the belts are still installed it makes it easy to remove the 4 bolts on that hold the fan clutch to the water pump.

Bolt that adjusts the tension on the power steering pump belt.
Nut/Bolt that adjusts the tension on the power steering pump belt.

 

Bolt that adjusts the tension on the alternator belts.
Nut/Bolt that adjusts the tension on the alternator belts.

Next I removed the belts. There is a nut on the bottom of the alternator that will need to be loosened. Doing so allows the alternator to swing inwards loosening the belt. A nut on the back of the power steering pump does the same thing.

Power steering pump belt part number.
Power steering pump belt part number.
Alternator and A/C compressor belt part number.
Alternator and A/C compressor belt part number.

 

I wrote down the part numbers in case I need more belts. The belts look fairly new so I am going to reuse them.  With the belts removed I pulled off the water pump pulley to get a good look at the pump.

Rusty water pump.
Rusty water pump.

 

Oh boi, previous mechanic RTV the pump on with Black RTV. Gonna be fun to remove. There are lots of bolts on this thing and brackets that attach to other accessories. I removed all the bolts and laid them out in order of how they came out. The alternator bracket and power steering bracket need to be loosened so the pump can full come off. Removing the pump was not actually that bad as the RTV did not have a good seal on the surface.

Cleaning up the filler neck of the radiator.
Cleaning up the filler neck of the radiator. Crack is visible now.

Before I tried brazing the filler neck of the radiator I first removed the fan shroud as It is made of plastic and I did not want to melt it.

I cleaned up the filler neck area with a brass wire brush on the drill.

Tools used to braze the filler neck on the radiator.
Tools used to braze the filler neck on the radiator.

I coated the filler neck joint with lots of acid based flux and then heated it up with the propane torch till I could pop it off with some pliers.

Removed filler neck for cleaning and rebrazing.
Removed filler neck for cleaning and rebrazing.

After the filler neck came off I cleaned up the mating surfaces with the brass wire brush on the drill. Then I coated both surfaces with more flux and pressed the filler neck back into the radiator. Then heated up the joint again and brazed it with the solder.

Filler neck after brazing.
Filler neck after brazing.
Close up showing the crack is gone.
Close up showing the crack is gone.

Looks like I did a decent job. Will know at the end if it holds pressure :)

Cleaned up T-stat gasket surface.
Cleaned up T-stat gasket surface.

Since the T-stat housing area and water pump mounting surface was so corroded I hit the entire area with the brass wire brush on the drill. Cleaned them right up.

Broken bolt on the water pump mounting area :X
Broken bolt on the water pump mounting area :X

During the clean up I found that the previous owner just left this broken bolt for me to find. Woooooooo….

Looks like someone already tried to extract it as there is a hole drilled into it.

I finished the job by drilling a 1/8″ pilot hole into it and then used a #7 drill bit to clean up the hole. I then put in a 1/4-20 tap to chase the threads. If I ever sell this engine to someone I would go back and drill it out and helicoil it but this fix will probably work for what I need.T-stat installed!

T-stat installed!T-stat installation was next. On my water related gaskets I like to use Permatex High Tack Gasket Sealant on both sides of the gasket. I know some say to never use a sealant or use only blue RTV but I have never had to go back and redo a gasket with this stuff. Unlike RTV the High Tack Gasket Sealant is easy to clean up if you need to replace the part again in the future. If also sticks to the housings better which is great when you do not have mounting/alignment dowels.

After the T-stat housing I installed the water pump and put the bolts back in the same way they came out. I looked up the torque numbers in the Factory Service Manual and torqued the T-stat housing and water pump to spec.

Removing the studs from old water pump.
Removing the studs from old water pump.

I then removed the studs from the old water pump using the stacked nut method. These studs need to be reused on the new pump.

Fan clutch studs installed.
Fan clutch studs installed.

Then I put the studs on the water pump. Then I put the water pump pulley on and reinstalled the belts but kept them loose till the end as I wanted to make sure I did not have to remove them later.

Using the belts to get friction while tightening the fan clutch stud nuts.
Using the belts to get friction while tightening the fan clutch stud nuts.

I then installed the fan clutch/fan assembly. To prevent the pulley from rotating as I was tightening them I used one hand to press down the belts which prevent it from rotating.

Installing the fan shroud was the last part of finishing up the front of the engine for this project.

Removing the heater box. Two bolts and the blend door cable need to be removed from the engine side.
Removing the heater box. Two bolts and the blend door cable need to be removed from the engine side.

Now I wanted to clean out the heater core box since I had the coolant out of the heater core.

First I started by removing the two external bolts and the blend door cable on the top of the heater box.

Dropping the A/C module under the dash.
Dropping the A/C module under the dash.

On the inside there are two more bolts that hold on the heater box. To get to these bolts the A/C module needs to be dropped from the dash. The A/C does not need to be discharged for this. There are around 6-8 bolts that hold it up under the dash. After removing the box kinda just falls down.

Wobble extensions to remove the behind dash bolts.
Wobble extensions to remove the behind dash bolts.
Where to snake the socket.
Where to snake the socket.

The easiest way I found to do this is to use lots of wobble extensions on a deep socket and snake this contraption in between the A/C module and the dash. Makes it fairly painless.

Lots of leaves built up in the heater box.
Lots of leaves built up in the heater box.

Then pull the heater box from the engine compartment. Mine had quite a bit of leaves that prevented the blend door from moving all the way.

The heater box opens up by removing all the bolts on the outside. There are five pieces; two external fiberglass shells, inner metal plate, blend door, and the heater core.

I took my heater box all apart and cleaned it up. Then I reassembled it.

External shell ready for reassembly.
External shell ready for reassembly.
Reassembly of the bottom of the blend door.
Reassembly of the bottom of the blend door.
Reassembly of the top of the blend door.
Reassembly of the top of the blend door.
Metal inner plate installed.
Metal inner plate installed.
Heater core installed. View 1.
Heater core installed. View 1.
Heater core installed. View 2.
Heater core installed. View 2.
Heater core installed. View 3.
Heater core installed. View 3.

 

First put the blend door back into the shell, next put the metal inner plate in. Next install the heater core and then the top of the shell. When putting the bolts back in do not tighten them up all the way till all the bolts and the heater core tabs at the top and bottom are all lined up.

Reinstalled heater box blend door cable.
Reinstalled heater box blend door cable.

 

With the box cleaned up I reinstalled the heater box and put the dash back together. Then reattached the blend door cable.I hooked all the hoses back up on the radiator and heater core and refilled the engine with new coolant and put a new radiator cap on.

New radiator cap and the brazing seems to be holding pressure.
New radiator cap and the brazing seems to be holding pressure!

A trip around the block and to the grocery store shows that the Wagoneer has stopped puking radiator fluid! Success so far!

New Vehicle Acquired. Jeep +1

This past Friday I picked up a 1990 Jeep Grand Wagoneer. It was in Dallas, Tx so I took the Friday off and drove up with my Dad and picked it up. Drove back to Houston with it (~300miles) was a pretty fun road trip!

Wagoneer in the driveway after the long day.
Wagoneer in the driveway after the long day.

It is a fully loaded Grand Wagoneer with all the trimmings. Everything functions except the rear window wiper and sprayer. The wiper looks to just be a bad wiring and the sprayer is missing from the back. If you know where to get a sprayer let me know….they seem to be unobtainium.

5.9L of raw Murica fury.
5.9L of raw Murica fury.

The 5.9L AMC 360 engine is mostly intact and runs ok. The valve seals are blown so the engine smokes a bit on start up.

When I got the Wagoneer the oil pressure gauge wasn’t working so I just had to hope it had oil pressure to make it back to Houston.

The first thing I did was an engine oil flush and replaced the oil pressure sender and wire. Engine idles at 40psi cold and ~10psi hot. The hot 10psi is not ideal but it does have 188K miles on it. I am debating rebuilding the oil pump or just leaving it.

Stuck in Dallas traffic after picking up the Wagoneer.
Stuck in Dallas traffic after picking up the Wagoneer.

My long term plan with the Wagoneer is to not 100% restore it but just make it nice. Fix all the problems, modernize the electronics, and swap in a LQ4 V8 engine and 4/5 speed automatic transmission. To get there however will take time.

My short term goal is to turn it into a decent daily driver and rock the AMC 360 engine while I fix the major problems and figure out how much I want to restore.

Here is the “short term” list.

  • Cooling System
    • Radiator has a cracked filler neck. Fix the copper/brass radiator by soldering/brazing the crack
    • Replace T-stat
    • Replace Water Pump
    • Flush System
    • Replace temperature sender
  • Engine
    • Another engine oil flush
    • Open valve covers to check rockers and push rods
    • New spark plugs
  • Transmission
    • Oil flush
    • New filter
    • New fluid
  • Fuel System
    • Repair gas tank skid
    • Clean tank
    • Replace rubber hoses and steel if needed
    • New fuel sending unit
    • Tank vent and hoses replacement
    • New fuel filter
  • A/C (very important!)
    • Recharge R12
    • Fix electrical
  • Steering
    • Replace leaking steering gear box
    • New hydraulic hoses
    • New fluid for power steering
    • Ball joints
    • Tie rod ends
  • Suspension
    • Replace sagging leaf springs with new suspension
  • Drivetrain
    • Flush and replace oil in transfer case
    • U-Joints
    • Rebuild Axles
King enjoying the Wagoneer.
King enjoying the Wagoneer.

Sounds like a lot but afterwards it should be a nice daily. The Wagoneer already rides like your sofa going 70mph down the freeway so it can only get better!

ABCC or Automotive Butt Climate Control

Climate controlled seats in your car are probably one of the greatest inventions since sliced bread if you live down in humid hot climates (like Houston!). Since trying them out in my Dad’s Tahoe years ago I have always wanted them in my Jeep but no one really makes after market seats that have them and there are no “kits” like adding heating elements to seats.

This changes now!

2015 Ford F-150 seat climate control module.
2015 Ford F-150 seat climate control module.

I managed to snag a 2015 Ford F-150 seat climate control module unit on ebay for $40. I picked this unit cause it looked compact and looking at the wiring from pictures I found on the internet it looked like I could reasonably make this work.

Ford wiring diagram for climate controlled seats. Looks simple enough!
Ford wiring diagram for climate controlled seats. Looks simple enough!

So how these work is via the peltier effect. The peltier unit has two copper heatsinks attached, one for each side. When voltage is applied to the peltier unit one side gets hot and the other cold. This is transferred to the copper heatsinks where the fan blows creating a stream of lava hot air and arctic frost air.

Exhaust port of the unit. This is typically vented back into the cabin under the seat or goes into the climate control system of the vehicle.
Exhaust port of the unit. This is typically vented back into the cabin under the seat or goes into the climate control system of the vehicle.
This port goes into the tubing that goes into the seat cushion. Depending on the polarity of the peltier it can be hot or cold.
This port goes into the tubing that goes into the seat cushion. Depending on the polarity of the peltier it can be hot or cold.

This is the peltier unit removed from the housing. The blue and yellow wires ended up being the peltier unit and after some testing the green and gray wires seem to be a NTC 200K thermistor. With 12V on the peltier in heating mode the resistance is 54K ohms. With 12V on the peltier in cooling mode it is 240K ohms. Room temperature resistance is around 170K ohms.

Peltier unit top.
Peltier unit top.
Peltier unit bottom.
Peltier unit bottom.

The fan is interesting. There are three wires. Red, black, and white. I figured red is power (12V), black is ground, and white is a pulse that I can measure and find the RPM for feedback control similar to the thermistor on the peltier.

Fan part of the Ford module.
Fan part of the Ford module.

Wired up the fan with 12V on red and ground on black. No go. Attached the white wire to 12V and it spun up! Pulling 1.4 Amps. Cool. Then the red wire fell off the wire clip and it was still spinning. 12V on the white wire (still at 1.4A!) and nothing on the red…. Weird.. Why is that? Maybe the red wire was feedback? Looking at the signal on the scope proved that was not the case. Time to open it up!

Interesting construction of the fan.
Interesting construction of the fan.

I took the fan out of the plastic housing. Looks like the PCB is directly on the aluminum or steel backing plate? The blades/cage of the fan is attached with a blind c clip in a bronze bushing on the back of the fan. This was pretty difficult to remove but a sharp pick and screwdriver I managed to remove it.

The PCB exposed. TELL ME YOUR SECRETS!
The PCB exposed. TELL ME YOUR SECRETS!

Yup! The PCB is on an aluminum or steel substrate (need to check which one) instead of the typical FR4 base. All single sided routed.

Red wire is labeled VM, Black is GND, and the White wire is VSP. I manged to get a part number from the large IC labeled IC1 on the upper right. The LB11988 is a brushless motor controller from On Semiconductor. T1 is labeled 065886A and is a DPAK package. I dunno what this part is at this point. Looking at DPAK mosfet parts the pinout makes sense. It also could be a large BJT. I then drew up a schematic.

Fan power front end schematic.
Fan power front end schematic.

The VCC pin on the LB11988 powered the internal control circuitry for the IC and VS is the motor power from I can gather. The datasheet is a bit vague here.

If the Red wire is disconnected the White wire provides all the power to the motor. That means that the T1 part either has a blown gate if its a mosfet (mosfets are not supposed to pull significant or any power through the fate) or T1 is a BJT with crazy base junction current.

I removed the T1 part and tested so see if it was a BJT and sure enough I was able to measure the diode effect from base to collector and base to emitter. Looking at DPAK BJTs that are automotive grade shows that some of them can have up to 2A of base current….Checks out I suppose!

With the mysterious of the unit solved it is time to develop a controller for it. I have read that the peltiers used in these units are powered via 16V but I can not confirm. Maybe a 12V -> 16V step up is needed that is rated for 8A. Lots of power need :)

Honda ATC110 engine stand complete and running

I was able to get the engine stand completed and engine running again.

Cheapo moped muffler and beer!
Cheapo moped muffler and beer!

The old exhaust system was pretty much cracked and fell apart when I tried to remove it from the three wheeler so I had to make a “new” setup. I bought the cheapest muffler I could find on eBay ($15 moped muffler) and some spare steel tubing I had and I welded up a temporary exhaust. I reused the old exhaust flange that mounts to the engine but I would like to replace all of it with new material when I finally transplant this engine in the go cart.

Straight exhaust setup reusing the original header flange and some spare tubing. Ugly but it works.

I then mounted the fuel tank I bought on ebay for $6. Fairly tiny with only 1L of capacity but the tank was pretty cheap and doesn’t leak… can’t complain about that.

1L fuel tank, hoses, and fuel filter from ebay.
1L fuel tank, hoses, and fuel filter from ebay.

I made the fuel tank mount out of some old aluminum sign material I had forever. I think I have been holding onto that material for over a decade now! Knew it would be useful some day :)

Fuel tank mounted on the engine stand.
Fuel tank mounted on the engine stand.

The fuel tank mount is designed to be unscrewed giving me more access to the engine when I start disassembly.

And it fired right up after filling the tank up. Glad I got the wiring right!

 

What is next is to disassembly and clean the entire engine. Once it heats up it smokes pretty good so I will need to at the minimal replace the piston rings and lap the valves. Probably end up honing the cylinder walls for the new rings.

Pulling a Honda ATC110 Engine

Those that listen to my podcast know that I enjoy working turning wrenches on vehicles and engines. Before my friend Stephen moved to Colorado he dropped off his old Honda ATC110 3 wheeler. It was in pretty bad shape. The drive train was all twisted up from when the chain fell off and jammed up. Fenders where cracked and the front fork was tweaked a bit. Only good thing was the 110cc engine fired up right away.

Honda ATC110

This gave me an idea. When I was younger I always wanted to build a go cart but never had the funds, tools, and my parents where very wary of the idea. Now I have a perfectly good 110cc engine, a welder, and no longer under the supervision of my parents :) .

Honda 110cc engine yanked out of the 3 wheeler.

After a couple 11mm and 14mm bolts the engine was freed from the old 3 wheeler chassis. Time to get rid of the old chassis and parts I am not going to use.

Chopped up 3 wheeler

I looked online and these chassis’ don’t really sell and would be a pain to get rid of otherwise. A sawzall and cut off wheels made quick work of the pressed steel chassis and tubes.

Honda ATC110 engine stand.

With some scrap material I had left over from other projects, I fabricated an engine stand for the 110cc engine.

Honda ATC110 engine wiring.

I then wired up the various electrical bits of the engine. These engines have very little electronics to drive them. An alternator generates the power for the sparkplug which is controlled by the CDI (Capacitor Discharge Ignition unit). I found the wiring diagram for this engine here.

Honda ATC110 engine wiring.

On the other side of the engine stand I mounted the throttle and ignition coil.

What is left on the engine stand to build is an exhaust system and adding a fuel tank. For the exhaust I found a $15 mini bike/moped muffler and some 1″ 16AWG steel tubing. The fuel tank is a $6 plastic mini bike tank. Next update on this project I hope to have those installed and the engine running on the stand. This engine needs a complete tear down as it leaks from every seal and I would rather not get my new go cart all oily!

Jeep Bluetooth Radio Hack for a TJ Jeep

I have been working on my old 1999 TJ Jeep and I always wanted to have Bluetooth connectivity in for the stereo. I could have bought an after market head unit but I never really liked the look of them and they tend to be easily stolen out of the Jeep (softtop!). The stock head unit matches the dash and is less likely to be taken. Thus the solution was to hack in a Bluetooth module into the radio!

First I bought a used radio on ebay. Part number for the radio was P56038933AB and I was able to pick one up for $25 as I wanted to keep my radio functional in the Jeep till I got the Bluetooth working. Then I picked this module on amazon. I chose this module as it had a wide input voltage, every single connection and signal was brought out to a header, and it had good reviews.

Next I opened the radio. It had T15 Torx (jeep thing :| ) machine screws that held it all together. Took it apart all the way to the bare radio.

Next to figure out how to get the Bluetooth signal injected into the radio! Most people that do these kind of hacks just blindly poke around the radio till they find and audio signal and then inject there. This is fine but I wanted to make sure I was getting the best audio quality out of my cheap Bluetooth module and early 90’s designed, base model head unit. I also wanted to keep the volume knob on the radio functional. This meant injecting the Bluetooth signal in the circuit before the power amplifiers and before it was gained for volume.

To figure out where to inject the audio. I first wrote down all the IC Manufactures, Part numbers, and Package size. Clicking the links will show an image of the IC.

Part Number Manufacture Package
SC433377CB 04827914AB Motorola DIP-42
4652138 022AZ9836 ST Microelectronics DIP-42
04744263 Motorola DIP-28
04231192AA Philips TO-220-13

Next I started searching for stuff like “ST Audio DIP42” into google. After finding promising datasheets, I then verified their pinouts buy tracing the power and ground sources in the radio. Here is the cross reference of what I was able to find. I also found this interesting PDF which is a early 90’s listing for ST Micro parts. I was able to find the TDA7340S IC with it.

Part Number I/C Cross Reference
SC433377CB 04827914AB Probably a 6800 series MCU
4652138 022AZ9836 TDA7340S
04744263 MC13022A
04231192AA TDA8947J

The IC that does the sound muxing, volume, and tone control is the TDA7340S and I figured this would be the best place to inject the Bluetooth audio signal.

Above is the block diagram. After the audio mux the signal travels to an external effects loop which consists of in series 1uF electrolytic capacitors. Then the audio signal travels into the volume and tone control parts. Perfect! If I inject the audio right before the capacitors I will retain the volume and tone control of the radio.

I thought about using the unused PHONE IN and then using a MCU to sniff the I2C buss and inject the right commands to switch it to that input but figured that would be more work then just injecting in the effects loop.

The next part is to make sure the signal from the Bluetooth adapter is compatible with what the TDA7340S is expecting. At max volume on my phone the Bluetooth adapter outputs a ~50mV DC offset signal with a ~1.7V to ~1.9V Pk-Pk. The signal in the effects loop was 4.65V DC offset signal with a 800mV Pk-Pk.

Bluetooth Module output at max volume.
Output of the TDA7340S effects loop. This would need to be matched to ensure compatibility.

Since these signals are different I needed to adjust the signal of the Bluetooth adapter. I would need to apply a DC offset and then negative gain the signal from the Bluetooth signal. The opamp circuit below should do the trick. The feedback circuit is set to half gain and by applying half of VCC (via a voltage divider) to the other input of the opamp we can DC offset the signal.

I tested it by having my phone play a tone via a tone generator app called “Frequency Sound Generator”. Signal below.

The input is in red and the output is in yellow. Signal now matches closely to what the TDA7340S expects their to be in the effects loop!

Then to switch between the TDA7340S signal and the Bluetooth signal I found an analog switch IC made by Maxim, MAX4544CSA+. The opamp I decided to use was a AZ4558C. I decided to use it because it is a decent audio amp and fairly inexpensive. Lastly, since its stereo we need to double everything. Below is the schematic and layout I did in Eagle.

The files for the board can be found on my github. I uploaded the files to MacroFab and ordered the board.

I then soldered the board into the radio. I desoldered one leg of the the effects loop 1uF caps which are designators C112 and C113, then soldered hook up wire on the legs of the capacitors and into the hole in the PCB. The power for the Bluetooth module is pulled from the bottom of the radio. To switch the audio from the TDA7340S to the Bluetooth the input of the MAX4544CSA+ is pulled up. A switch on the dash will be used for this. See image below for how everything is hooked up.

Power for the Bluetooth module.

Then I tested the radio and made a video.

And then installed it into the Jeep!