libengan

Howdy - new user here and was hoping someone can help me with an issue I'm having with my '88 Cherokee Laredo XJ 4.0L i6 Automatic 4x4

The issue seems to be with the automatic transmission shifting (AW4 4-speed).

When accelerating from a dead stop in "D", there is almost no power, like it's stuck in 3rd gear, and I have to floor it to get going.

But if I accelerate from a dead stop while in "1-2", it takes off no problem, and I can then shift up to "3", and finally up to "D" as I get going.

I having limited knowledge of transmissions, but it seems like it is getting stuck in 3rd, and not shifting back down to 1st or 2nd?

I have tried adjusting the throttle valve cable using my FSM proceedure, but it had no effect.

Any thoughts? Thanks!

SuperRA

That sounds to me like your TCM (Transmission Control Module) is not functioning. This could be due to one or more issues.
*Fuse is blown
*TCM is bad
*Wiring is bad

The fuse is located under the dashboard on the passenger side. (its a single fuse holder attached on the harness) The TCU is also located in the vicinity and if you follow the harness, it should lead you to it mounted on the inside of the lower dashboard.

There is also a fusible link feeding the TCM as well, but it also feeds the engine controls. So if your engine is running fine, I wouldn't suspect it.

libengan

That sounds like a good place to start. I'll see if I can check the TCM tomorrow afternoon, and let you know what I find. Thanks!

BlueRidgeMark

What's this? A new thread from a newbie with a sensible, informative title. No groveling, either. ("Oh, PLEASE HELP!")

Not only that, but it was followed by a clear, readable, lucid explanation of the problem, along with information on the troubleshooting you've done.

Really, really weird!

cruiser54

Gotta love it!!

Now I'll muddy the waters with a photo showing said fuse and TCU.

libengan: Might also perform Tip 10 in my Pink Link below.

libengan

So I finally had the time today to get the lower dash off and access the TCU.

At least by appearances, the TCU actually appears to be the newest part on my Jeep (I've only had the Cherokee for about a month now). The wires and connector all look good and clean, and the circuit inside the TCU is clean enough to see my reflection in. I also checked the fuse, and it's also intact. In fact I checked all fuses, and they're all good.

I haven't done the trans plug refreshing like cruiser54 suggested, but I'll be getting some electrical contact cleaner this week and hit that too.

All that to say, I took it all out and put it back together, and still having the shifting issues.

Any other thoughts? One other thing that I'm noticing is getting worse is the engine goes into a high idle speed every time I throw it into park (but not necessarily when I start it up). I'm wondering if this is a throttle issue, and if it isn't also affecting the shifting?

cruiser54

Do my Tip #1. You'll see why these symptoms may have one thing in common. The engine dipstick grounds.

libengan

Ok so I completed tip #1 (including adding the new grounds), and tip #10 cleaning out the electrical connectors.

Still having the same shifting issues. Any other thoughts?

Thanks.

cruiser54

Your TPS may be shot.....

Couple of ways to go about this.

Clean both it's plugs and do Tip 5 at this point. Needs to be done anyway.

libengan

Ok so I cleaned the TPS plugs, and did the sensor ground test, and was getting readings like 1-4 Kohm. That is Kilo ohms by the way. And it would change every time I wiggled the C101 connector. So I did the C101 connector elimination in tip #27.

After the C101 elimination, I am now getting resistance more in the range I shoud (I think) Sensor Ground Test now shows 1.1ohm to 1.5ohm resistance

I also did the TPS test just for kicks and found the following:
Flat 3-wire connector:
.57V(OUTPUT) / 4.83V(REFERENCE) = 11.8%
Square connector:
4V(OUTPUT) / 4.53V(REFERENCE) = 88.3%

So in conclusion, the sensor ground test showed that the resistance is a little high.
The TPS values are a bit off too, but I need to clean my throttle body before trying to adjust it.

Would these things be enough to make the TCU flat out not function?

Thanks!

cruiser54

You're doing a great job.

Clean the throttle body and adjust the TPS on the Engine side. Flat 3 wire connector.

Then, check the Trans side connector and report back.

libengan

I happened to be at a salvage yard this weekend, and found a transmission control module on a '92 cherokee. Just for kicks, I swapped it out with mine, and now it's shifting in automatic again!

I think the shifting could be smoother, so I do intend to clean the throttle body and fix the sensor issues, but at least it's shifting again!

Thanks again for your help!

cruiser54

There's a difference between the 88 and 92 TCUs.....

libengan

Sorry, I meant I pulled it off a '90. Any compatibility issue there?

cruiser54

Nope. You're good to go.