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Transmission overheating question

M

MachineMan

NAXJA Forum User
Location
San Jose, CA
My transmission over heats and gets up past 260F on a mild day under moderate loads. I have a inline temp guage.. I've installed and external cooler and flushed out the system with non-synthetic DexronIII and still it overheats. It seems to shift OK, the engine temps are fine, and the cooling lines aren't smashed. If it stays above 260F for 15min or more it starts to leak fluid from inside the bell housing somewhere.

What should I do next? Is there anything in the tranny pan that I could replace for this problem? Could it be an electrical TCU sensor problem?

1990 XJ with AW4.
 
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Is the Torque Converter locking up?

A typical problem is the TC unlock switch on the brake pedal pulls loose and the trans hunts in and out of the TC lock, or never locks.

The other possible problem is a deeper one, more prone to the earlier 87-90 XJ's. The TC rides on the AW4 pump, and the pump bushing/bearing wears allowing the TC to walk around at the TC to trans seal. The later model AW4s changed the bushing/bearing design to eliminate this wear spot (if 200K miles is considered an unfair time for a wear problem to expose itself). Make sure the TC/flexplate bolts and the trans housing to engine bolts are tight (sometimes tightening it up will center the alignment and stop the leak, for a while).
 
Ed, I just checked the connector you mentioned. On the brake pedal lever theres 2 brown flat connectors and both are hooked up. One of them runs into a harness with a jumper plug and fuse which ultimately connects to that yellow box under the steering column. The other connector goes into the main system harness somewhere.

The leak only occurs after the tranny overheats so probably only a consiquence of something else wrong.

What next?
 
I just checked out the wiring on the tranny. On the drivers side theres two 2 wire connectors, one white and one black. The white one is hooked up but the black one is just dangling and not plugged into anything. Can't see anyplace it should go and not sure if its important.
 
MachineMan said:
Ed, I just checked the connector you mentioned. On the brake pedal lever theres 2 brown flat connectors and both are hooked up. One of them runs into a harness with a jumper plug and fuse which ultimately connects to that yellow box under the steering column. The other connector goes into the main system harness somewhere.

The leak only occurs after the tranny overheats so probably only a consiquence of something else wrong.

What next?
Click to expand...


The yellow box is the cruise control box, the other wire IIRC ends up at the TCU under the passenger dash. The nylon switch itself has knurled plastic that presses into a bracket hole, and it can back out, making the TCU (& cruise) think you are hitting the brakes (and want the TC to unlock). It needs to be pushed in and seated well.

The trans will leak if overfull (CRS, thanks jpx3, the obvious), through the breather on top and/or the TC to trans seal (with wear). Check the dip stick level with the trans hot (a very common problem is overfilling).
 
The fluid level is good, Its just above the add mark when warmed up and in park.

If I was to measure the continuity of this brown brake switch should it be open when the brake is not pressed? There's also there's white connector higher up which I belive is for the brake lights.

Is there an easy way I can test if my torque coverter fully locks up?
 
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What tires and gears are you running?
 
I have 35" tires with 4.56 gears.

There more I think about this problem, Ed is probably on to something with the torque converter slipping. I can drive 20miles on flat highway and it will be OK. But once I start going up hills it overheats..

So how do I check the torque converter engagment and what things can effect its locking mehcanism?
 
MachineMan said:
The fluid level is good, Its just above the add mark when warmed up and in park.

If I was to measure the continuity of this brown brake switch should it be open when the brake is not pressed? There's also there's white connector higher up which I belive is for the brake lights.

Is there an easy way I can test if my torque coverter fully locks up?
Click to expand...


Someone had the FSM on line, and having the wiring diagrams will help (a lot, compared to working off memory).

The brown brake pedal switch should be the brake lights. It will close on pressing the pedal.

The upper white switch should be the cruise/tran switch. I cannot remember the normal status, but a search for the trans controller threads (or Compushift) will result in a diagram of the circuit (maybe MadXJ.com?).

You should feel four distinct shifts and the TC lockup (that feels like a soft shift). Testing by making your own trans controller override can help with a diagnosis.

Anyone have a link to the FSM or trans controller handy?
 
i know people that have run 33's and 3.55 against advice from, well everyone, and they ruined AW4s left and right. sounds like you are running correct gear/tire.

codes? i would almost be inclined to take it to a tranny shop and see if they diag anything. i know on 97+ they have a code for the TCS (transmission control sensor) and that relates to the tranny not shifting when its supposed to. we pulled a friends and his fluid was freakin black and smelled like it had been on fire. but it sounds like you have been into yours and maintain it. dunno.

-jpx3
 
Not sure if they can read tranny codes from 1990??

If I pull the fuse on the TCU can I still drive it and manually shift through all the gears for testing? If I start off in 4th maybe I could tell if its slipping ??
 
Update:

Under the dash at the top of the brake pedal lever theres a large white button switch that also has some kind of vacumm hose hooked up to it. If I disconnect the 2 wire plug and measure the switch the reisstance its infinite with the pedal pressed and at rest. So pressed or not the switch seems to be doing nothing electrically..

If I manaully force the switch button farther in(as if the pedal was farther out towards the driver) then I get 10ohms... Is this switch related to the transmission or some other braking operation?
 
MachineMan said:
Update:

Under the dash at the top of the brake pedal lever theres a large white button switch that also has some kind of vacumm hose hooked up to it. If I disconnect the 2 wire plug and measure the switch the reisstance its infinite with the pedal pressed and at rest. So pressed or not the switch seems to be doing nothing electrically..

If I manaully force the switch button farther in(as if the pedal was farther out towards the driver) then I get 10ohms... Is this switch related to the transmission or some other braking operation?
Click to expand...


That is the cruise/trans controller input switch, for the TC lockup override when you hit the brake. It unlocks the TC when you hit the brake to freewheel the trans and save you some gas mileage, and tries to prevent the trans from overdriving the brakes.

It's the switch to make sure it's knurled fitting is seated well.
 
jpx3 said:
i know people that have run 33's and 3.55 against advice from, well everyone, and they ruined AW4s left and right. sounds like you are running correct gear/tire.

codes? i would almost be inclined to take it to a tranny shop and see if they diag anything. i know on 97+ they have a code for the TCS (transmission control sensor) and that relates to the tranny not shifting when its supposed to. we pulled a friends and his fluid was freakin black and smelled like it had been on fire. but it sounds like you have been into yours and maintain it. dunno.

-jpx3
Click to expand...
I dont agree at all,4.56's min w/ 33's.With 35's,4.88's gotta be marginal,most overheating issues Ive seen are a result from cooling capacity VS gearing.Changing the gearing has "always" reduced the temps.
 
The switch is threaded plastic cylinder that goes into a sheet metal holder. Looks like I could thread it in more to make the switch have contact when the brake pedal is up. There's no knurled fitting on it though, just about 1.5" of plastic threads.

So is this it and possibly my problem?
 
I had one of those switches expire on my 89 due to an ATF leak/bath... after one too many upside-down trips under the dash, I just followed the instructions of the BOR tech article for manual 1-2 and manual TC lockup switches (used a pair of 'push-pull' switches mounted on either side of the handbrake on the console - approx where the power mirror adjuster would go)

Back when I used to DD this Jeep, I'd leave the TC unlocked until I got on a stretch of road where I could lock in 3rd or OD. I also found that the 1-2 switch also works for a 3-4 switch when the shifter is in the OD position. (to 'force' an instant downshift & hold 3rd)

It was at times pesky pulling & pushing the knobs (about like splitting a 2spd truck axle) but I generally drive with my right arm on the console lid anyway, so it became almost "normal."

If you are considering the mods (they're easy & cheap) look at www.bigoffroad.com tech pages... I think the article is titled "First gear, please."
 
MachineMan said:
The switch is threaded plastic cylinder that goes into a sheet metal holder. Looks like I could thread it in more to make the switch have contact when the brake pedal is up. There's no knurled fitting on it though, just about 1.5" of plastic threads.

So is this it and possibly my problem?
Click to expand...


That is it.

Threading may be impossible, but a little gentle application of leverage can slip the threads past the sheet metal holder.
 
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