AW4 Transmission Control Problem!

Hey guys, i'm new to the forum and have been searching for weeks for an answer to my problem. I've got an 88 cherokee 4.0 with an AW4 tranny and it won't shift out of first gear. I did the manual shift test with the TCU unplugged and it shifts fine. I checked the TPS as per the service manual and its fine, and even put in a new one for good measure.
I did all the electrical diagnostics on this site: http://www.transonline.com/transdigest/magazines/1997-10/Shift Pointers/index.html.

Everything checks out except the brake switch shows 0.4v instead of 0 with the brakes off, the TCC solenoid has 21 ohms of resistance, and the TPS ground has 0.8v instead of 0.1.
But i figure none of those things should stop it from going into second, so i had a buddy probe the c15 terminal on the TCU harness (shift 2 solenoid) while i was driving, and the TCU wouldn't send voltage to it. It only sent voltage to shift solenoid 1. So i replaced the TCU with another and still the same story! I'm at my wits end tryin to figure this thing out, so I'm hoping one of you might have some answers.

Any help would be greatly appreciated!

Try the neutral start switch (nss). Could need cleaning or replaced.

Had trans problems with a past jeep of mine... feel your pain, good luck!

The NSS voltages checked out fine when i did the volt testing at the TCU harness, is it possible it's still screwing up even tho the voltages look fine?

If it shifts fine with the TCU out of the loop, then this is likely not a mechanical issue, but an electrical one.

I didn't take the time to read that article that you linked to and it may very well be accurate, but I think I would be sure that I followed the troubleshooting flowchart provided in the Factory Service Manual instead.

There are a few AW4 gurus on this forum, who I'm sure will chime in after treating their New Years Day headache!

I don't think the NSS would cause this.

It's sound you you have narrowed it down to maybe a VSS, vehicle speed sensor. If the TCM doesn't think you are moving it won't shift, the other would be rpm feed to the TCM. Just guessing, I'm sure there are more guys out there that have had AW4 issues that can chime in.

Wouldn't be the throttle valve cable would it? It'd be easy to check....
http://www.naxja.org/forum/showthread.php?t=1015963

The answer to your question and problem is in the data you posted!
The .4V in part, and especially the .8V on the TPS ground is your problem, both should be under 0.1 V, .005 is even better. The TCU does not get correct data from the TPS if the ground is poor, dirty, loose corroded. The 87-90 jeeps are notorious for all sorts of ground problems with sensors and the controllers. Read up on the Renix ground posts here, my thread story is the "RenX files" in the OEM forum on how I chased the same transmission problem for 2 years, thinking it might be the engine, then the transmission, chasing my tail, before replacing both theTPS sensor ground and the TPS fixed my poor power - transmission problem.

QUOTE=t.loretto;245183883]Hey guys, i'm new to the forum and have been searching for weeks for an answer to my problem. I've got an 88 cherokee 4.0 with an AW4 tranny and it won't shift out of first gear. I did the manual shift test with the TCU unplugged and it shifts fine. I checked the TPS as per the service manual and its fine, and even put in a new one for good measure.
I did all the electrical diagnostics on this site: http://www.transonline.com/transdigest/magazines/1997-10/Shift Pointers/index.html.

Everything checks out except the brake switch shows 0.4v instead of 0 with the brakes off, the TCC solenoid has 21 ohms of resistance, and the TPS ground has 0.8v instead of 0.1.
But i figure none of those things should stop it from going into second, so i had a buddy probe the c15 terminal on the TCU harness (shift 2 solenoid) while i was driving, and the TCU wouldn't send voltage to it. It only sent voltage to shift solenoid 1. So i replaced the TCU with another and still the same story! I'm at my wits end tryin to figure this thing out, so I'm hoping one of you might have some answers.

Any help would be greatly appreciated![/QUOTE]

Ok so ran a new ground wire directly from the tps to the tcu harness, and I'm still getting a voltage reading. Weird thing is, when i unplug the tcu theres no voltage in the wire, so there can't be a short at the tps side of things. When i plug in the tcu's i get .86v with one, and .69v with the tcu i got from the wrecker. So i checked the tcu ground at the d7 wire and get .01v, no problem there. Is it possible both tcu's have a bad tps ground lead?

t.loretto said:

Ok so ran a new ground wire directly from the tps to the tcu harness, and I'm still getting a voltage reading. Weird thing is, when i unplug the tcu theres no voltage in the wire, so there can't be a short at the tps side of things. When i plug in the tcu's i get .86v with one, and .69v with the tcu i got from the wrecker. So i checked the tcu ground at the d7 wire and get .01v, no problem there. Is it possible both tcu's have a bad tps ground lead?

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Not sure why you did that? The new ground(s) need(s) to be run directly to the negative post of the battery (the only true ground), to the negative battery cable post clamp, from any sensor and ECU/TCU ground connection that has a poor ground. You can try cleaning up the engine to body frame to battery grounds, but I did that and still found bad sensor ground wires (like 8-16 ohms or more) that were causing me headaches, so I ran extra direct sensor to battery grounds on mine right from near the sensor to the battery. The voltage see is a back voltage caused by a poor ground connection somewhere.

I've spliced the ground wire from tps to tcu and ran a ground directly to the battery, now i'm reading .02v on the tps ground, but still no shifting. I'm not sure if thats what you mean by running a new ground, sorry but wiring and electronics are not exactly my forte :dunno:. I'm also now getting a semi-decent ohm reading on the tcc solenoid(16.5), and the brake switch reading has gone down to .15v with brake off. Where Do i go from here?

Btw thanks for all the help Ecomike, you've kept me from going crazy! (so far)

So i just probed the solenoid wires while my friend drove, and at around 10 or 15 mph the tcu cuts the signal to solenoid one but still won't send signal to solenoid 2

t.loretto said:

I've spliced the ground wire from tps to tcu and ran a ground directly to the battery, now i'm reading .02v on the tps ground, but still no shifting. I'm not sure if thats what you mean by running a new ground, sorry but wiring and electronics are not exactly my forte :dunno:. I'm also now getting a semi-decent ohm reading on the tcc solenoid(16.5), and the brake switch reading has gone down to .15v with brake off. Where Do i go from here?

Btw thanks for all the help Ecomike, you've kept me from going crazy! (so far)

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If you are already on a jeep forum, chatting with me trying to fix a jeep, it is too late on the "crazy" part for both of us!

I recall their being some kind of hidden resistor between the brake switch and TCU and ground that I ran across once before and that the brake switch ground voltage never goes to zero for that reason. I posted some notes here on it somewhere about 3 years ago.

Sounds to me like you may have the ground problem and back voltage fixed, and normal now. So has this ever worked right with the switch Mods?

Is solenoid #2 wire defective somewhere? Where are you looking for the #2 signal, near the TCU, near the AW4, in the custom switch network?

Not sure I am going to be much help past this point.

I don't have switch mods, the tranny and system are stock. I was just driving one day and it felt like the jeep slipped out of gear, i slowed to a stop and tried going again and ever since it hasn't shifted out of 1st. Goes through the gears fine with the tcu unplugged tho.

I'm checking the voltages at the tcu harness. As far as i understand, when it's supposed to shift the tcu removes voltage from shift solenoid 1 and puts it on solenoid 2, but it doesn't. I figure the wiring to the solenoids is intact cause the resistance is in range, 14 ohms. The only thing i can think of is the speed sensor not pulsing properly. With the key on engine off the speed sensor shows 5v, but i suppose that doesn't necessarily mean it's actually working. When i get a chance this weekend i guess i'll throw in another one to see what happens

Solenoid1 is on for 1st gear; 2nd gear has both on; third gear has just sol2 being activated; then 4th has neither powered. Only one solenoid changes per gear shift.

Have you verified that the rear speed sensor is working? If you probe the sensor with a meter while driving, you should see voltage jumping around. Pin C3 to ground, a tan wire I believe. On an 89, the sensor is just a magentic relay that closes once per revolution so the voltage is jumping between 0 and 12 volts.

I finally gave in and took it to a shop today, turns out it was the speed sensor. Thanks for all the suggestions you guys, too bad i didn't have the patients to figure it out! Hope this helps someone one day with the same symptoms

lawsoncl said:

Solenoid1 is on for 1st gear; 2nd gear has both on; third gear has just sol2 being activated; then 4th has neither powered. Only one solenoid changes per gear shift.

Have you verified that the rear speed sensor is working? If you probe the sensor with a meter while driving, you should see voltage jumping around. Pin C3 to ground, a tan wire I believe. On an 89, the sensor is just a magentic relay that closes once per revolution so the voltage is jumping between 0 and 12 volts.

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t.loretto said:

I finally gave in and took it to a shop today, turns out it was the speed sensor. Thanks for all the suggestions you guys, too bad i didn't have the patients to figure it out! Hope this helps someone one day with the same symptoms

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As Maxwell Smart used to say: "Missed it by that much." :doh:

Glad you got if fixed.

I hope the shop didn't ding you too much for the labor to replace it. On a 4wd, it hides behind the plate that hold the xfer case linkage which means pull all that off to get to it. I've been noticing a fair number of tranny speed sensor failures on Renix era jeeps lately. I guess that makes sense as the sensor is a reed relay and eventually has a mechanical failure.

yep, most manufacturers of reed switches (admittedly, for non automotive markets - automotive parts usually have much higher ratings) seem to rate them at 10 to 100 million cycles... figuring 3.55 gears and 28" tires, each revolution is about 25 inches of travel. 100 million cycles only gets you around 40k miles, so I'm not all that surprised to see failures after the ~200k miles I have seen on most RENIX jeeps around here (approx 500 million cycles of the output speed sensor.)

For the non-geeks amongst us, this is what a typical reed switch looks like. There is one of these molded into the plastic snout of the sensor that sticks into the tranny tail cone. It gets cycled on-off once per revolution by a magnet on a rotor on the output shaft. At 60 mph that's around 40 times a second. Hi reliability ones are rated up to 1 billion cycles which by Kasteins estimate would be about 400k miles.

More tech info at http://www.hamlin.com/technical-detail-reed-switch.cfm

The sensor changed with the move to the HO setup, and again in 1997.

lawsoncl said:

For the non-geeks amongst us, this is what a typical reed switch looks like. There is one of these molded into the plastic snout of the sensor that sticks into the tranny tail cone. It gets cycled on-off once per revolution by a magnet on a rotor on the output shaft. At 60 mph that's around 40 times a second. Hi reliability ones are rated up to 1 billion cycles which by Kasteins estimate would be about 400k miles.

More tech info at http://www.hamlin.com/technical-detail-reed-switch.cfm

The sensor changed with the move to the HO setup, and again in 1997.

Click to expand...

Wow, didn't realize they made high reliability ones that were that good.

The sensor changed from 90 to 91 and in 97? I thought it only changed from 97 to 98, when they went from the reed setup to the reluctance / coil pickup type of sensor and changed the number of pulses per rev.

Actually, just checked...

87-90 OSS part number: 8350 3722
91-93 OSS part number: 8350 3722
94-96 OSS part number: 8350 3722
97 OSS part number: 8350 3722
98-99 OSS part number: 4897 734AA (also used for the ISS)

So the sensor is exactly the same 87-97 and then changes to a different part for 98-01 (I did not check 00-01 but they should be the same part.)