Intermittent Torque Converter Lock Up -1998 Jeep Cherokee XJ - AW4

[Brynjaminjones]

I've been struggling for a few years with an intermittent torque converter lock up in my 1998 Jeep Cherokee XJ, which has a 4.0 and an AW4 transmission (which I believe is from the Toyota A340 family).

Background:
The torque converter often doesn't lock up when it's supposed to, particularly when hot. I've installed a light that shows me when 12v is being supplied to the lock up solenoid, and I've found that the torque converter often fails to lock even though the light is on.
It works pretty much perfectly when cold, but often won't lock at all if the transmission is hot. This unfortunately means that I can't use my XJ for towing at all, and have stopped taking it on long trips.

I don't believe the torque converter lock up clutch slips - if I can get it to lock, it works perfectly. It's a very binary problem - it either works or it doesn't.

I've recently noticed that even when it is working properly, it often takes several seconds after the light comes on for the lock up to actually take place.

The lock up solenoid was replaced about 9 months ago. The issue seemed better for a while, but soon came back.

The fluid in there was *very* old and brown, and smells burned.

Other than this issue, the transmission operates 100% perfectly. It has never slipped, missed a shift, or had a delayed engagement.

My thoughts so far:
I've ruled out electrical issues - the car is commanding the lock up when it should. The solenoid is new and passes any resistance checks, even when hot.

I've been slowly doing pan drain/refills to replace with fresh fluid. I'm using a multi-vehicle ATF which says its compatible with Dexron III and Dexron VI.
I do not think the lock up clutches slip. The slow lock up when it works, plus fact that it gets worse with heat, suggests to me that it might be struggling with fluid pressure in the lock up circuit specifically.

Because the transmission otherwise operates perfectly, I do not think it's suffering from systemic low pressure.
All the transmission shops around here are only interested in doing a full rebuild instead of diagnosing the issue, but I do not feel like my transmission is failing.

Questions:
1. The fluid I've been using has a hot viscosity rating of 6.4 cSt and is Dex VI compatible. Other Dex III fluids seem to have viscosity ratings closer to 7.7 cSt. Should I try a thicker fluid?
2. I've found a valve body repair kit from Sonnax, which apparently fixes an issue where pressure gets bled off at the lockup relay valve. Is this worth exploring? See here.
3. Any other thoughts? I've been struggling with this issue for years, and I've found it very difficult to find advice!

 

[BC1]

"It works pretty much perfectly when cold, but often won't lock at all if the transmission is hot".
This seems to point to the low viscosity fluid. Try using a fluid that meets the spec and/or add another cooler.

 

[Brynjaminjones]

"It works pretty much perfectly when cold, but often won't lock at all if the transmission is hot".
This seems to point to the low viscosity fluid. Try using a fluid that meets the spec and/or add another cooler.

Yeah, that's where my head is going, although this problem has been going on long enough that it surely can't just be down to the fluid.

Does Dex III thin or thicken with age? The fluid that was in there before was in there for about 10 years, and was disgusting.

 

[SwampSurvivor]

My high mileage transmission has no issues with Dex VI and lockup.

But if yours was full of garbage ... who knows.

I would suggest Supertech gallon jug of specific DexIII but you're not in the USA ...

 

[BC1]

The Dex/Merc will thin out much faster than the VI, so you need to keep it refreshed. Change all the disgusting fluid now through a cooler line. Then a drain/fill every so often.

 

[Lyman]

I would try going with an actual dex/merc fluid. Not a substitute.

 

[Brynjaminjones]

Thank you all. I need to do more changes anyway to continue to clean up the fluid, so I'll find some genuine Dex III with a higher viscosity rating and see how I get on.

I still don't feel like this problem should be happening though - surely it shouldn't be this sensitive to viscosity. I'll be happy if I can minimise the issue with the fluid, but ideally I'd find a 100% fix.

Does anybody have any thoughts on the Sonnax lockup relay valve shared in the OP?

 

[OilUzer]

How are you determining if the TC is not locked when the light is on?

By the RPM drop or checking TC temperature against the pan temp?

 

[zrxkawboy]

I still don't feel like this problem should be happening though - surely it shouldn't be this sensitive to viscosity.

I agree...I don't think your problem is fluid-related. Dex III starts at a higher viscosity than Dex VI, but shears to a lower viscosity in service. The Dex VI stays much closer to its starting viscosity.

 

[Brynjaminjones]

How are you determining if the TC is not locked when the light is on?

By the RPM drop or checking TC temperature against the pan temp?

Mostly by RPM drop, but also by general behaviour. I've been driving this Jeep for the last 13 years so I'm generally very used to its characteristics and what the TC lockup feels like.
When the TC locks up, there is a noticeable drop in RPM and change in tone. Once it's locked, I can easily test whether it's locked by gently squeezing the throttle. If the engine loads up without increasing RPM, then I know it's locked because there is no slippage. If the RPMs flare with the gas pedal, I know it's not locked.
Also, I can get the RPMs to rise by braking when the TC is locked, because braking unlocks it.

When the TC fails to lock, there is no sign (other than the light!) of any attempt to lock whatsoever - no RPM fluctuation or anything like that to suggest that the lockup clutch itself is slipping.

I agree...I don't think your problem is fluid-related. Dex III starts at a higher viscosity than Dex VI, but shears to a lower viscosity in service. The Dex VI stays much closer to its starting viscosity.

That's what I'm thinking here too.
With that said, I would be very happy if a thicker fluid was able to improve the symptoms. Back when it ran on Dex III, it did seem to get better for a couple of months after a fluid change.
The Dex VI equivalent has only been in there a week or so.

 

[Thermo1223]

If it's a pressure problem then it is rebuild time. Seals are leaking, check valves aren't holding pressure, and the pump may be as well if it only does it hot.

 

[Brynjaminjones]

If it's a pressure problem then it is rebuild time. Seals are leaking, check valves aren't holding pressure, and the pump may be as well if it only does it hot.

I would be inclined to agree, if it weren't for the fact that this transmission otherwise behaves 100% correctly. Surely a systemic low fluid pressure issue would have some other symptom?

This is why I'm trying to dig deeper into this "lockup relay valve". It seems like this is one of the few hydraulic components involved in the operation of the torque converter lockup, and the description of the repair part on Sonnax's website suggests that it can be responsible for pressure loss in that circuit.
I don't want to throw parts at this without proper diagnosis though!

 

[Brynjaminjones]

Okay, I've maybe gotten somewhere with this.

This transmission has a "throttle valve" cable that links the transmission to the throttle, which control line pressure. I've been adjusting this cable for years, but I think I've been adjusting it slightly too tight.

A couple of days ago I loosened it up slightly, and weirdly my torque converter is now locking up much quicker and more consistently.
It's not fixed though - there are still some times where it tries and fails to lockup when I'm quite far into the throttle, and after this it then won't lock up until it's cooled down again.

 

[meep]

I might put a small ammeter inline with the lockup solenoid. 1) your light says it’s getting power, but 2) an ammeter would tell you if power is getting through it. At this age I’ve seen metal fatigue in wires even though the exterior looks good. By small, that solenoid probably pulls under 2 amps.

AW4 is a fabulous transmission. Overheating was a problem in the Jeep application.

 

[thescreensavers]

Okay, I've maybe gotten somewhere with this.

This transmission has a "throttle valve" cable that links the transmission to the throttle, which control line pressure. I've been adjusting this cable for years, but I think I've been adjusting it slightly too tight.

A couple of days ago I loosened it up slightly, and weirdly my torque converter is now locking up much quicker and more consistently.
It's not fixed though - there are still some times where it tries and fails to lockup when I'm quite far into the throttle, and after this it then won't lock up until it's cooled down again.

How are you adjusting the cable it's pretty easy to adjust, just push it the adjuster all the way in the mash the throttle and it should be set.

I did a full fluid exchange with Valvoline DEXRON VI/MERCON LV ATF it has over 200k miles and works perfectly

 

[Brynjaminjones]

I might put a small ammeter inline with the lockup solenoid. 1) your light says it’s getting power, but 2) an ammeter would tell you if power is getting through it. At this age I’ve seen metal fatigue in wires even though the exterior looks good. By small, that solenoid probably pulls under 2 amps.

AW4 is a fabulous transmission. Overheating was a problem in the Jeep application.

Funny you should mention this - I did think to myself "it's getting 12v, but what if it's not enough current to work the solenoid when hot?". If this continues to be a struggle, I'll see if I can get something hooked up.

How are you adjusting the cable it's pretty easy to adjust, just push it the adjuster all the way in the mash the throttle and it should be set.

I did a full fluid exchange with Valvoline DEXRON VI/MERCON LV ATF it has over 200k miles and works perfectly

Yeah, that's pretty much what I've always done. I used to press the button then open the throttle body by hand. I realised that it would sometimes overshoot, so I started holding the cable to add resistance as I opened the throttle, making sure it wouldn't go too far.

When I checked my adjustment the other day, I realized it was tight enough that the TV cable bracket would flex slightly every time I went to WOT, because I was hitting max cable travel. I've re-adjusted so the cable doesn't hit max travel before WOT, which means it's about 1-2 clicks looser than it was before.
I hadn't expected that to make any difference, but my TC is behaving slightly better.

 

[thescreensavers]

Yeah, that's pretty much what I've always done. I used to press the button then open the throttle body by hand. I realised that it would sometimes overshoot, so I started holding the cable to add resistance as I opened the throttle, making sure it wouldn't go too far.

When I checked my adjustment the other day, I realized it was tight enough that the TV cable bracket would flex slightly every time I went to WOT, because I was hitting max cable travel. I've re-adjusted so the cable doesn't hit max travel before WOT, which means it's about 1-2 clicks looser than it was before.
I hadn't expected that to make any difference, but my TC is behaving slightly better.

Personally I'd use the skinny pedal that's your input, you don't drive by using your hand on the throttle body. I've never adjusted those by Opening the TB manually.

 

[Dragfluid]

You say no shops around you talk about wanting to do any diag?
Here is a shop that belongs to ATRA. How far are they?
Devon Gearbox Centre [12528070]
Huxley Close
Plymouth, PL7 4JN
Phone: 11-441752 343368

 

[Brynjaminjones]

You say no shops around you talk about wanting to do any diag?
Here is a shop that belongs to ATRA. How far are they?
Devon Gearbox Centre [12528070]
Huxley Close
Plymouth, PL7 4JN
Phone: 11-441752 343368

Sorry for the slow reply - I've been away and forgotten about this for a little while.

I did speak to them recently - they're 5 miles away and directly opposite the body shop I use! Unfortunately they're not taking on work on automatics at the moment.
I spoke to another local transmission specialist who was very helpful, but said that due to the age of the transmission they wouldn't be interested in doing anything other than a full rebuild.

In the meantime, I've found a nice, high quality synthetic Dexron III fluid that I think I'll try. It's got a much higher viscosity at 100C - 8.2 cSt instead of the 6.4 cSt of the fluid I've been slowly refilling with until now.
It's Mobil ATF 320 - see HERE.

 

[ltslimjim]

I added a bottle of Lubegard Red ATF additive to an older 99 Dodge Stratus with TC lockup engagement issues.

It helped until I sold it. The transmission had been severely neglected and likely on ATF+3. I converted it to ATF+4 with the LG additive. Got somewhat better but the damage had been done. Likely needed a rebuild later in its life.