Aw4 replacement

The Aw4 in my 91 XJ has all but completely failed, I've been reading up and seeing a lot of people saying that a replacement from the boneyard is probably preferrable to a rebuild.

So my question is: What years do I have to choose from to get a swappable trans, and what all will I need to pull with it?

Thanks in advance for any helpful info.

The issue is that out spline count changed. I _think_ the change was in 91 but I'm not sure. The 4th gear ratio changed abit at the same time. Do a search to double check. Also you want to stay pre 1997 as the sensor on the AW4 changed and it won't work with your 91 trans computer.

whats wrong

Yeah, I looked it up and 91 is when these changes took place.

At first I was heating up on the way back to the Vegas region from LA. I pulled over and waited about 6 hours until after dark. When I got back onto I-15 there was a slight rumble as I accelerated but it went away as soon as it came and I assumed it was the rumble strip.

I cruised along just fine until I had to slow down and get around a slow moving vehicle, as soon as I got into the next lane and try to pass there was a dullish clunking sound and vibration as the rpm went up a bit and speed dropped and held at about 55. Not wanting to be stuck in the far left lane I raised and lowered the r's a bit and the jeep lurched and took off at speed again.

All was fine until the stop and go traffic near home as I had to adjust the r's just to get moving again after a stoplight, I brought it home and parked it and started reading about what I should do. It feels like the trans isn't engaging and needs a kick in the teeth to get it moving, after all that it's obvious it wasn't doing much good for the rig so I'm assuming I've completely destroyed something.

Start by checking the fluid level. :}

The AW4 is a fairly robust tranny and the vast majority of shifting related problems are not mechanical. Try pulling the fuse to the TCU. I think on a 91, it's still in the wiring harness right at the TCU. The tranny should shift normally using the shifter - ie (1-2) gives you first, (3) third, and (D) 4th with no converter lockup. If it does, the problem is electrical and not mechanical. The usual electrical problems are a bad TPS (likely if it's the original), the switch on the brake pedal (affects lockup), and bad solenoids (you can test their resistance at the TCU connector).

In your case, it sounds like either low fluid or a bad solenoid, but that's a semi-wild guess.

The fluid level is still in the optimal range.

Before I go tearing into things, where would the TCU be located?

I rebuilt the engine myself with an extra set of hands, but I've never really touched a tranny before.

I found a fuse that was labeled as transmission and removed it. I drove around the block and it shifted into 1-2 and 3 just fine, but once I put it in D it wouldn't engage.

The TCU is under the passenger side of the dash, bolted to the bottom kick panel.

Ok, we have removed the lower panel of the dash, and the plastic of the passenger side. We can see all the wires and the TCU, but are unable to locate the fuse.

I'm not 100% sure the fuse is still in the wiring harness on a 91. I know it's there on the Renix years. You could just unplug the TCU instead and make sure it shifts manually with the shifter.

Might want to add the NSS (Neutral safety switch) to that list of possible electrical gremilins that can spook the AW4. It tells the TCU where the shifter is located.

I would also suspect the TPS. Also part of your description sorta sounded like a loose flywheel bolt(s), so check them.

TPS and the tranny solenoids are easy to test with an ohm meter.

What is the fluid history of this tranny. Been changed or flushed recently?

The test data and test methods for the 3 tranny solenoids and TPS is in several threads in this forum.

Monster said:

The fluid level is still in the optimal range.

Before I go tearing into things, where would the TCU be located?

I rebuilt the engine myself with an extra set of hands, but I've never really touched a tranny before.

I found a fuse that was labeled as transmission and removed it. I drove around the block and it shifted into 1-2 and 3 just fine, but once I put it in D it wouldn't engage.

Click to expand...

I just tested the solenoids, they click and ohms read 14.5 at each.

The TPS was giving me a hard time when I bought the vehicle about a year and a half ago. Can't remember if I replaced it then or what.

The fluid hasn't been messed with since I bought it, just monitored. I dropped the pan to find dark red fluid and minimal sludge in the pan considering the 162k miles on it.

I didn't think to check the bolts again, I'll go check that out when I get home.

And thanks for the NSS lead, I'll look into that as well.