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AW4 does not shift - help me finish troubleshooting

bigalpha

bigalpha

Moderator
Location
Tucson, AZ
1995 XJ, AW4
- When in 'D', the tranny defaults to 4th gear.
- I can drive it and manually change the gears from 1-2, 3, 4.
- 4th will downshift to 3rd if I goose the throttle.
- I can drive the Jeep with the computer unplugged.
- Replaced TPS with new - tested with meter; meets voltage guidelines.
- Jeep runs pretty smoothly, and does not stumble. Idles low every once in a while, I think it's vacuum related.
- Jeep in 'Park', ignition 'ON', TCM unplugged - Tested TCM connector - 'Power In' is satisfactory.
- Jeep in 'Park', ignition 'ON', TCM plugged in - C14-C16 registerd 0 ohms.
- I replaced the computer with a junkyard computer from a 95 XJ Sport; no change. No guarantee the computer was working.
- I checked all fuses and all are good (unless fuses go bad w/o actually blowing the fuse?)
- Wiring harness at the TCM is in good shape with no cuts or breaks in the wiring
- The ground at the TCM is grounded properly.
- Gauges seem to be working properly.
- No CEL.
- Before I replaced TPS, I pulled codes and it gave me "24: check TPS and wiring". After replacing the TPS, the code is gone.

At some point, there was some alarm system installed on the Jeep. I pulled the siren out the other day because it's not even hooked up anymore. By the fusebox, there is a tan/light brown wire with a black strip that has been cut. I have no idea what it goes to. It disappears into a bundle of wires taped together under the dash.
Fusebox:
2893897416_3e088b06ae.jpg


Stray wire:
2893056331_3b68b8497c.jpg


I believe that about sums up what I've done so far. I would LOVE to get this problem fixed; especially if it's a (relatively) cheap problem I can fix at home. I'm open to questions and comments. I can get better/new pictures of anything to help diagnose this problem.

Now for my questions:

1. If the solenoids were bad, the tranny would not shift, right?
2. What can cause C14-C16 inputs to not receive any input from the solenoids?
3. What are other troubleshooting steps I can do to narrow down my problem?
4. How can I diagnose a bad TCM?
5. If the TCM is dead and I replace it (and it fixes the problem) will the tranny start shifting properly immediately w/o disco the battery?
6. Assuming a sensor is at fault, and I replace the sensor, will the tranny start shifting properly immediately w/o disco the battery?
7. Can a fuse go bad w/o actually blowing?
 
With the TCU unplugged, measure the resistance between C14 on the wiring harness connector (not the TCU itself) to a chassis ground. Repeat for C15 and C16. This should test the resistance of each of the three solenoids in the tranny. They should be between 11-15 ohms (not volts).

With the Jeep in Park, ignition on, and the TCM plugged in you should see voltage on pin C16.

You've already done the best diagnosis for a bad TCM be swapping it out with another. I would be very surprised if both your original and a junkyard unit were both bad.

If the tranny solenoids are not getting powered at all by the TCM to control the shifting, then you get the symptoms you describe.
 
The Tan/Black wire might be a headlight wire. The FSM shows tan/bk wires between the dimmer switch, headlight switch, and the headlamp delay module. Maybe the previous owner snipped the wire to the delay module so the light would go off right away? Or maybe the alarm guy wired in so the alarm could flash the headlights?
 
lawsoncl said:
With the TCU unplugged, measure the resistance between C14 on the wiring harness connector (not the TCU itself) to a chassis ground. Repeat for C15 and C16. This should test the resistance of each of the three solenoids in the tranny. They should be between 11-15 ohms (not volts).

With the Jeep in Park, ignition on, and the TCM plugged in you should see voltage on pin C16.

You've already done the best diagnosis for a bad TCM be swapping it out with another. I would be very surprised if both your original and a junkyard unit were both bad.

If the tranny solenoids are not getting powered at all by the TCM to control the shifting, then you get the symptoms you describe.
Click to expand...

Lets say the computer is bad. If I replace it, will the tranny start to shift immediately; or is there a delay while the rest of teh components realize the TCM is working again?

I'll test teh ohms with the connector unplugged.
With the computer plugged in, I should see voltage at C16 ... like, actually V, instead of ohms?
 
bigalpha said:
1995 XJ, AW4
- I checked all fuses and all are good (unless fuses go bad w/o actually blowing the fuse?)
Click to expand...

Indeed they can, that's why there are the little testers that come with big fuse kits. The one for your TCU is under the hood, little 10A mini fuse...red, IIRC.
 
ECKSJAY said:
Indeed they can, that's why there are the little testers that come with big fuse kits. The one for your TCU is under the hood, little 10A mini fuse...red, IIRC.
Click to expand...

Alright. Well, maybe I'll just swing by and get a 10A fuse on my way home tomorrow. That would be pretty damned crazy to have a tiny, itty bitty fuse cause all these problems. Actually, I would be stoked!

lawsoncl said:
With the TCU unplugged, measure the resistance between C14 on the wiring harness connector (not the TCU itself) to a chassis ground. Repeat for C15 and C16. This should test the resistance of each of the three solenoids in the tranny. They should be between 11-15 ohms (not volts).

With the Jeep in Park, ignition on, and the TCM plugged in you should see voltage on pin C16.
Click to expand...
Jeep 'ON', in 'Park', TCM unplugged:
C14 had nothing.
C15 had nothing 90% of the time. Then it would jump around all the way to like 34 then to 0.
C16 had nothing.

Jeep 'ON', in 'Park', TCM plugged in:
C16 read 12.24V.
 
bigalpha said:
Alright. I tested the connector unplugged from the TCM. C14 had nothing. C15 had nothing 90% of the time. Then it would jump around all the way to like 34 then to 0. C16 had nothing.

When I plugged in the TCM, C16 read 12.24V.
Click to expand...

If you have voltage at C16, then the computer and power to it is fine.

The resistance measurements on the solenoids are troubling. A really high resistance or no connection on the solenoids means you've either got a wiring problem or bad solenoids (I'm assuming you had it set to read ohms). It would be unusual to have three bad solenoids, so I would start checking the wiring to the transmission.

I don't have a 95 in the driveway or my FSM handy so this is from memory. The wiring goes from the TCM, through the firewall on the passenger side. In the passenger side of the engine bay up against the firewall there should be two squarish plugs (8-pin?), grey and black, that connect to the wiring that goes down to the tranny. One is for the neutral safety switch that is on the passenger side of the tranny. The other has the wires for the solenoids and the output shaft speed sensor, and heads over towards the drivers side. The wiring for the solenoids passes into the tranny on the drivers side through a rubber grommet near the bottom edge (it's not a plug, the wires just go through the grommet).

If all else fails, the next step is to pull the pan and filter off the transmission and measure the resistance of the solenoids directly. It requires draining the tranny, but that would eliminate the wiring between the solenoids and TCM.
 
Alright, I'll dig around my FSM some and see if I can find the wiring diagram you are talking about. I assume that the voltage at these 8 pin connectors should be the same as they are at the TCM? If so, can I test these connectors directly?

I assume since power is coming to the TCU, that the fuse/power input wouldn't be causing this issue.

Is there something else easy that I could test the resistance of; to make sure I'm testing the ohms properly? I know that sounds kind of dumb, but I just want to make sure I'm doing everything properly.

I was kind of hoping you wouldn't say drop the pan and test the solenoids directly. If I do this, and the solenoids are bad, are they available at Advance/Napa/etc? I've been using the Jeep as a DD since I'm trying to sell my Subaru, so I'd hate to have it out of commission for a long period of time.

Thanks guys for all your help!!!
 
Yes, the voltage/resistance measurements at those connectors should be the same.

If you have a speaker handy, it should measure somewhere around 4-8 ohms. Touching the leads together should be zero, or pretty close to it.
 
lawsoncl said:
The Tan/Black wire might be a headlight wire. The FSM shows tan/bk wires between the dimmer switch, headlight switch, and the headlamp delay module. Maybe the previous owner snipped the wire to the delay module so the light would go off right away? Or maybe the alarm guy wired in so the alarm could flash the headlights?
Click to expand...

What part of the FSM did you find that? I'd like to have my headlights stay on for a bit after I got out of the Jeep. Maybe I could rewire it back.
 
lawsoncl said:
Yes, the voltage/resistance measurements at those connectors should be the same.

If you have a speaker handy, it should measure somewhere around 4-8 ohms. Touching the leads together should be zero, or pretty close to it.
Click to expand...


I'll dig around after work and see if I can find the solenoid wires at the connection and test it there.

I thought that if the solenoids were bad, the transmission wouldn't shift?
 
bigalpha said:
What part of the FSM did you find that? I'd like to have my headlights stay on for a bit after I got out of the Jeep. Maybe I could rewire it back.
Click to expand...

Section 8W of the FSM.

bigalpha said:
I thought that if the solenoids were bad, the transmission wouldn't shift?
Click to expand...

The solenoids are how the TCM tells the transmission to shift. If they are not working, then the tranny will either shift into the wrong gear or not shift at all. If solenoids 1 and 2 are not working, you will end up with 4th gear in (D), 3rd in (3), and 1st in (1-2). Solenoid 3 controls the t/c lockup.
 
If I can manage to find those two squarish plugs, do I need to disco them to test the solenoids?

So, it'd be: ignition 'ON', 'Park', and unplugged?
 
The problem with that is mine is a 95, so the wiring is in different places. Should the voltages still come out the same?
 
bigalpha said:
The problem with that is mine is a 95, so the wiring is in different places.
Click to expand...
Here's what is written at the top of page 2.......
Jeep AW4 Transmission Application
1987-1990 – Jeep 2.5L & 4.0L Cherokee & Comanche
1991-1993 – Jeep 4.0L Cherokee & Comanche
1994-1997 – Jeep 4.0L Cherokee
bigalpha said:
Should the voltages still come out the same?
Click to expand...
So, I would guess so. But I'm not really sure.
 
They changed the computer in 97, so the connector from my Jeep doesn't fit the 97 (I know, I tried).

Shift pointers shows wires at certain places and certain colors in the connector. Mine are not all the same. That's why I ask.
 
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