AW4 Transmission Computer Testing

[reson46]

I am experiencing erratic shifting in my '88 AW4 that I have encountered in the past when the TPS has gone bad.

 

I started my diagnosis by checking the input voltage to the TPS, terminal "D" of the sensor connector. With the key on this should be around 5 volts. Mine was nowhere close.

 

Next I break out the FSM and start looking through the wiring diagrams. It shows the source of the TPS input as pin "D1" from the transmission computer. I check the voltage of this wire coming straight out of the transmission computer wiring harness. It is also very low.

 

Now I start checking the inputs to the transmission computer thinking that it might not be getting enough voltage. I check the voltage to pin "D14", which comes from the battery, and it has 12 volts. I also check the voltage for pin "D16", which comes from the ignition, and it is getting power with the key on.

 

I now know that the computer is getting good input power, but for some reason is not outputting the correct voltage for the TPS. This leads me to think that the computer may be bad. Luckily I have a spare, so I plugged it in. It gives me the exact same results.

 

I could possibly have two transmission computers that are bad, but I tend to think that this is unlikely.

 

Is there a way to test the transmission computer to know definitively if it is bad? If possible I'd like to be able to test both of these computers before I go grab another one out of the junkyard.

 

Is there anything else that could be causing these symptoms?

 

Thanks,

Willy

[HOrnbrod]

I think it's unlikely both TCUs are bad and showing the same symptoms. Recommend testing the AW4 solenoids first. Between the AW4 and TCU there is a harness with a 7-pin connector right on the rear firewall, light gray or white in color. This harness supplies the signals from the TCU to the three solenoids in the tranny that control shifting and TC lockup. The coils on these solenoids can go bad and cause erratic shifting and should be tested with an ohmmeter. Open up the connector and check pins E, F, and G to ground, pin D on the tranny side of the connector. Each should read 13-15 ohms. If not, they have to be replaced which involves pulling the tranny pan. But ohm them out first to make sure they are okay.

 

[reson46]

How would the solenoids impact the input voltage to the TPS? Do the solenoids provide some type of input to the TCU? I thought they just did what the TCU told them to - shift and lock up the torque converter - without providing any input to the TCU.

 

Thanks,

Willy

[HOrnbrod]

How would the solenoids impact the input voltage to the TPS? Do the solenoids provide some type of input to the TCU? I thought they just did what the TCU told them to - shift and lock up the torque converter - without providing any input to the TCU. Thanks, Willy

 

The solenoids could impact the voltage; I don't know how the TCU is wired internally. It only takes a minute to check the solenoids, so it can't hurt. Upon further checking:

 

From the 1988 AW4 Manual:

 

THROTTLE POSITION SENSOR (TPS) TEST

NOTE: Digital voltmeter must be used to check TPS.

1) Note location of TPS electrical connector. See Fig. 5.

Turn ignition on. Using digital voltmeter, check output voltage at

center terminal with throttle plate closed (idle position) and wide

open (full throttle).

2) With throttle plate closed (idle position), output voltage

should be greater than 200 millivolts. With throttle plate wide open

(full throttle), output voltage should be less than 4.8 volts.

3) Ensure output voltage gradually increases as throttle

plate is moved from closed to wide open throttle. If no voltage

exists, check for defective wiring circuits or connections. Replace

TPS if defective.

 

Also, the manual shows TCU pin D2 as the voltage feed for the TPS; D3 as the TPS ground. TCU pin D1 is marked unused.

 

Willy, shoot me a PM with an email address and I can sent you the 88 AW4 Trans Diagnosis manual if you want. Good info.

[reson46]

Also, my manual shows TCU pin D2 as the voltage feed for the TPS; D3 as the TPS ground.

 

D2 is the voltage feed from the TPS to the TCU. D1 is the voltage feed from the TCU to the TPS. The TCU steps down the voltage from 12 volts to 5 volts as the input to the TPS. The power goes from TCU->out TCU D1->TPS->in TCU D2. Whatever is returned to the TCU at D2 is the position of the throttle.

 

I've got the procedure for checking the output voltage of the TPS. The problem is if the input voltage to the TPS from the TCU is off from the start, the output voltage of the TPS will never be correct.

 

Thanks,

Willy

[cruiser54]

Here's what you wanna do and I think Hornbrod is steering you in that direction.

 

Cruiser’s Trans Plug Refreshing

 

 

Over near the trans dipstick tube are 2 rather large connectors. One is black and the other gray. These 2 connectors carry all the info between TPS, TCU, NSS, speed sensor, and trans solenoids.

Unplug each one, visually inspect for corrosion or bent pins, spray them out with electrical contact cleaner and plug them back in.

 

Revised 3-28-2012

[reson46]

Don,

 

Thanks for the manual. I found another test to try when I get a chance - 7A.

 

I find it interesting that figures 6 and 125 both indicate that pin D1 is not used but my FSM manual shows it as the TPS voltage supply - which I am positive is correct. There is obviously a wire that comes out of D1, and it has voltage. Why does this pretend it doesn't exist? :hmm:

 

Thanks,

Willy

[HOrnbrod]

Don,

 

Thanks for the manual. I found another test to try when I get a chance - 7A.

 

I find it interesting that figures 6 and 125 both indicate that pin D1 is not used but my FSM manual shows it as the TPS voltage supply - which I am positive is correct. There is obviously a wire that comes out of D1, and it has voltage. Why does this pretend it doesn't exist? :hmm:

 

Thanks, Willy

Willy, I just looked in my 91 FSM to verify. TCU pin D1 is not used, and D2 is listed as the TPS input pin and goes to TPS pin 3. It's also paralled as a feed to pin 22 of the ECU. So my FSM agrees with the AW4 manual as far as the TPS input circuit. :dunno:

[reson46]

Don,

 

Thanks for the manual. I found another test to try when I get a chance - 7A.

 

I find it interesting that figures 6 and 125 both indicate that pin D1 is not used but my FSM manual shows it as the TPS voltage supply - which I am positive is correct. There is obviously a wire that comes out of D1, and it has voltage. Why does this pretend it doesn't exist? :hmm:

 

Thanks, Willy

Willy, I just looked in my 91 FSM to verify. TCU pin D1 is not used, and D2 is listed as the TPS input pin and goes to TPS pin 3. It's also paralled as a feed to pin 22 of the ECU. So my FSM agrees with the AW4 manual as far as the TPS input circuit. :dunno:

 

Differences between HO and Renix? I know the throttle bodies are different. Are the TPSs and computers different?

 

Willy

[cruiser54]

Willy, in my 87 manual D1 is TPS supply and is red, D2 is TPS input and is grey with tracer, and D3 is TPS ground tan w/ orange.

 

All 3 of those wires run through the dreaded C101 connector on the firewall above the brake booster in an 88. I'd be doing this first:

 

Renix Jeep C101 Connector Refreshing

 

 

The C101 connector on 1987 and 1988 Renix Jeeps was a source of electrical resistance when the vehicles were new. So much so that the factory eliminated this connector in the 1989 and 1990 models. The factory recommended cleaning this connector to insure the proper voltage and ground signals between the ECU and the fuel injection sensors. We can only imagine how this connector has become a larger source of voltage loss and increased resistance over a period of almost 25 years. The C101 connector needs to be cleaned at least once in the lifetime of your vehicle. Chances are it’s never been done before.

 

Almost every critical signal between the engine sensors, injectors, and the ECU travel the path through the C101.

 

The C101 is located on the driver’s side firewall above and behind the brake booster. It is held together with a single bolt in it’s center. To get the connectors apart, simply remove the bolt and pull the halves apart. You will find the connector is packed with a black tar like substance which has hardened over time.

Take a pocket screwdriver or the like and scrape out all the tar crap you can. Follow up by spraying out both connector halves with brake cleaner and then swabbing out the remainder of the tar. Repeat this procedure until the tar is totally removed. This may require 3 or more repetitions. Wipe out the connectors after spraying with a soft cloth.

 

If you have a small pick or dental tool tweak the female connectors on the one side so they grab the pins on the opposite side a bit tighter. Apply a true dielectric grease, not the stuff that came with your brake pads, to the connection and bolt it back together.

 

 

 

Revised 11-29-2011

[HOrnbrod]

Yes, that makes good sense. But first I'd do a quick continuity check from D1-D2-D3 to the appropriate pins on the TCU.

[reson46]

Don,

 

Thanks for the manual. I found another test to try when I get a chance - 7A.

 

I find it interesting that figures 6 and 125 both indicate that pin D1 is not used but my FSM manual shows it as the TPS voltage supply - which I am positive is correct. There is obviously a wire that comes out of D1, and it has voltage. Why does this pretend it doesn't exist? :hmm:

 

Thanks, Willy

Willy, I just looked in my 91 FSM to verify. TCU pin D1 is not used, and D2 is listed as the TPS input pin and goes to TPS pin 3. It's also paralled as a feed to pin 22 of the ECU. So my FSM agrees with the AW4 manual as far as the TPS input circuit. :dunno:

 

Don,

 

I discovered why I was finding so many differences between that AW4 diagnostic document and my FSM wiring diagrams. That diagnostic document is definitively not for Renix MJs/XJs - even though it says 1988 Jeep Cherokee on the title page. That document only includes HO diagnostics.

 

All the diagrams and wire colors are for an HO. I ran through several of the tests as far as I could by matching up corresponding wires from the TCU. I pretty much hit a dead end when a step required me to test an ECU wire. :wall:

 

Willy

[reson46]

After testing and tracing as many wires as I could figure out how to, making sure every ground was good, and even grabbing a third TCU from the junk yard, I still wasn't able to determine the cause of my low TPS input voltage. With Trail Jamboree coming up next week I had to figure something out, so I decided to give the TCU the middle finger.

 

I had already wanted to implement some of these changes, and had even wired in a TCU power cut off years ago. Now this gives me full manual control of every gear and the torque converter lockup. I even got fancy with my new estate sale acquisition - a label maker. It will definitely take some practice before I remember every position.

 

Willy

[HOrnbrod]

Don,

 

I discovered why I was finding so many differences between that AW4 diagnostic document and my FSM wiring diagrams. That diagnostic document is definitively not for Renix MJs/XJs - even though it says 1988 Jeep Cherokee on the title page. That document only includes HO diagnostics.

 

All the diagrams and wire colors are for an HO. I ran through several of the tests as far as I could by matching up corresponding wires from the TCU. I pretty much hit a dead end when a step required me to test an ECU wire. :wall: Willy

 

Well crap. That's why everything I saw agreed with my 91 AW4 wiring even though the doc was labeled as an 88 XJ AW4. Well, that's good to know and I hope the mistakenly titled doc I sent didn't hold you up too much. I had no idea. For sure the solenoids are all fine as they are all working with your manual shifter. Sure would like to know the cause of your low TPS input voltage though. As I'm sure you do too. :dunno:

[reson46]

Don,

 

I discovered why I was finding so many differences between that AW4 diagnostic document and my FSM wiring diagrams. That diagnostic document is definitively not for Renix MJs/XJs - even though it says 1988 Jeep Cherokee on the title page. That document only includes HO diagnostics.

 

All the diagrams and wire colors are for an HO. I ran through several of the tests as far as I could by matching up corresponding wires from the TCU. I pretty much hit a dead end when a step required me to test an ECU wire. :wall: Willy

 

Well crap. That's why everything I saw agreed with my 91 AW4 wiring even though the doc was labeled as an 88 XJ AW4. Well, that's good to know and I hope the mistakenly titled doc I sent didn't hold you up too much. I had no idea. For sure the solenoids are all fine as they are all working with your manual shifter. Sure would like to know the cause of your low TPS input voltage though. As I'm sure you do too. :dunno:

 

No worries. I appreciate the help. It didn't hold me up much. It gave me a couple of ideas. It is good to know why there were so many inconsistencies.

 

After Trail Jamboree I'll probably look into it again when I get a chance. I left a switch position for TCU on. It would be nice to let the TCU do the thinking when running around town. The wife already complains that I've made the thing too complicated for her to drive with all the switches and gauges.

 

Thanks,

Willy