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AW4 torque lockup not disengaging when changing from 3rd to 4th--normal?

ehall

NAXJA Member
NAXJA Member
I have noticed that the '94 AW4 I just swapped into my '91 has a shift flare from 3rd to 4th, where the engine rev up and the tranny shifts kind of hard. I hooked up my mechanics scanner to the TCU diagnostics port and found some interesting things. Specifically, when the '91 TCU "Comfort" mode is selected, the shift pattern is as follows:

1st
2nd
3rd
3rd w/ torque lockup
4th w/ torque lockup

The lockup solenoid does not disengage before shifting to 4th, which produces a hard studder since the tranny is getting all of the crank power

Conversely, when the "Power" mode is selected, the shift pattern is as follows:

1st
2nd
3rd
3rd w/ torque lockup
4th

The TCU does not command the lockup solenoid to engage in overdrive when Power mode is active

This all seems a bit weird to me. Does anybody know if this correct behavior?
 
I don't think it's quite normal, because it should lock up in third and fourth in both modes. Later models using the same transmission and control module are wired to run only in power mode, and they get the lockup.

In "D" position, lockup should occur in both third and fourth gear. My recollection from when I had an 87 was that under certain conditions it would stay locked during the 3-4 shift, but it was somewhat hard to quantify, especially since mine had no tachometer. I think it would stay locked if accelerating very gently and steadily without full throttle, but when it did that it was not much more abrupt than a normal shift. It would only behave that way if I drove so that it stayed in third for a while, and then kind of edged up to the fourth shift without much throttle. Imagine, for example, climbing a long rise with a heavy load, and then cresting it without changing throttle. It would just sort of drop into 4 without any intermediate fuss. Under very hard acceleration it would not take the time to lock in third at all, and in between, it would lock in third, unlock and then relock in fourth. So depending on circumstances, one could count 5, 6 or a different 5 steps from a standstill. I had always meant to set it up with some little LED's or something on the solenoid lines to try to confirm what was happening, but I never got around to it, and got a 5-speed instead. It's a lot easier to figure out the sequences on that! :jester:
 
After thinking about it overnight, I have concluded that my TCU is probably borked

For one thing, the original '91 tranny used to have a quick shift out of third. I didn't pay attention if this was 3rd to lockup, 3rd w/ lockup to OD, or what, but ...

The scanner shows what the TCU commands, so if the tranny is shifting from 3rd w/ lockup to OD w/ lockup in Comfort and not engaging OD lockup at in Power, then that is a TCU problem and the tranny has nothing to do with it. The TCU dictates the shifts not the tranny.

Most likely this was also happening with the old '91 as well, but the wear on the tranny parts after 340k miles were hiding it--instead of jerking with a flare, the OD w/ lockup was just slipping a lot making it feel like a quick shift even though it was still going from 3rd w/ lockup to OD w/ lockup

I found a '91 TCU on eBay for $40 shipped so I will try that
 
I have an indicator light ( LED) mounted on my dash close to the TCU that indicates when the TC is getting the power to the solenoid to lock the TC.
Mine locks frequently in second gear, except when I am pushing the acceleration hard. Once it locks up in second or third it stays locked until I touch the brake pad or drop below about 20 mph and 1100 rpm.

Sounds like you have bad clutch pack in the tranny or a sticking solenoid (or loose wire? to the solenoid) or trash in the valve body somewhere.

Does not sound like the TCU to me.
 
You might want to test the solenoids and grounds while your swapping it out!
 
I found a '91 TCU on eBay for $40 shipped so I will try that
Brown Santa brought this today and I drove around for a while with it installed. It did not correct the studdering shift from 3rd/lockup to 4th/lockup in Comfort. However it did seem to give me 4th/lockup in Power (I'll have to confirm with a scanner), and the shifts in Power mode are firmer than Comfort again.

The solenoids work fine based on my drive testing.

I have also noticed that the jeep jerks into gear pretty hard when it is put into Reverse and Drive after warm-up. There are a couple of isolating dampeners in the tranny that are supposed to alleviate this, so it's possible that those are failing.

I'll have to look more. I still need to know if the lockup is supposed to disengage when the tranny shifts from 3rd to 4th. I need this info to figure out if I can bypass a branch on the diagnostic tree in my head.
 
Brown Santa brought this today and I drove around for a while with it installed. It did not correct the studdering shift from 3rd/lockup to 4th/lockup in Comfort. However it did seem to give me 4th/lockup in Power (I'll have to confirm with a scanner), and the shifts in Power mode are firmer than Comfort again.

The solenoids work fine based on my drive testing.

I have also noticed that the jeep jerks into gear pretty hard when it is put into Reverse and Drive after warm-up. There are a couple of isolating dampeners in the tranny that are supposed to alleviate this, so it's possible that those are failing.

I'll have to look more. I still need to know if the lockup is supposed to disengage when the tranny shifts from 3rd to 4th. I need this info to figure out if I can bypass a branch on the diagnostic tree in my head.

Like I said before, once it is locked up it stays locked while shift from 2 to 3 and 3 to 4, unless you floor it (IIRC), once locked even at WOT, although mine seems to stay locked even when floored if it is already at or over 50 mph,
 
Can someone explain torque converter lockup? I've never owned an auto XJ until recently, and have never really experienced it on other rigs.

I know I'm experiencing it when I'm cruising at highway speeds in drive: when I push the skinny pedal (even slightly) the RPM's jump for a few seconds and then kick back down.

:flame: on.
 
neither of mine have routinely locked up in 2nd so obviously yours and mine are different

If you are cruising, not accelerating, in second gear ( D ) at about 25 to 30 mph, all AW4s will lock up the TC.
 
One possibility--since '92 and up does not have the Power/Comfort TCU option, maybe the '92 and up valve body changed too. IE, it's possible that the donor '94 tranny is not intended/designed for the lockup to be engaged by my '91 TCU

Anybody know if they have the same or different part numbers?
 
Eric, i can check the part numbers for ya. Compare my 94's to the 88 TCU i have "somewhere" in garage. About to head to work, i'll take a look tomorrow if someone hasnt already given you the info.

FWIW, i'm running an 88 trans in my 94 without any issue (~35K miles now). Often wondered what it would take to get a power/comfort switch operational in my 94.
 
The AW4's are the same, but the 91 TCU is different. The 90 TCU has a direct connection to the Dual TPS, The 91 is not a dual, but a single sided TPS. The 91 TCU gets its TPS input from the ECU. The 91 and newer can be set up with the comfort/power switch. Search here and you will the find the write ups on it, possibly in MOD forum.
 
FWIW, i'm running an 88 trans in my 94 without any issue (~35K miles now). Often wondered what it would take to get a power/comfort switch operational in my 94.
With your '94 TCU, you're running it in "Power" mode all the time, which works fine for me as well.

Adding a switch should be pretty easy. http://www.naxja.org/forum/showthread.php?t=944484 has the diagrams which are also in the FSM.
 
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One possibility--since '92 and up does not have the Power/Comfort TCU option, maybe the '92 and up valve body changed too. IE, it's possible that the donor '94 tranny is not intended/designed for the lockup to be engaged by my '91 TCU

Anybody know if they have the same or different part numbers?

All the AW4's have a solenoid for the t/c lockup, so it's a safe bet that the TCU would still be able to lock the t/c. I have a 99 AW4 running with an 89 TCU and it locks up just fine. Only issue I had was the difference in the rear output shaft speed sensor.
 
The AW4's are the same, but the 91 TCU is different. The 90 TCU has a direct connection to the Dual TPS, The 91 is not a dual, but a single sided TPS. The 91 TCU gets its TPS input from the ECU. The 91 and newer can be set up with the comfort/power switch. Search here and you will the find the write ups on it, possibly in MOD forum.

The two main changes to the TCU in 91 were changing the diagnostic lines to OBDI vice serial, and the TPS input voltages changed. Instead of running opposite of what the ECU received, they flipped it around to that it accepted the same voltage (0-5 volts versus 5-0 volts) and eliminated the need for a dual output from the TPS.

It changed again in 96 with the move to OBDII. The next change was in 98 (I'm about 90% sure) when the AW4 had some sensor changes. The connector on the 98+ TCU looked very different too. All the pre 98 TCU units had the same connector, although it was color coded and had keying tabs to differentiate the major changes. Basically if the color of the connector matched, it would swap with your TCU.
 
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