Sick AW4

I apologize for the long post - wanted to pass along the best info.

I've got a 96 4.0L part time 4X4. I overheated the auto tranny in a towing operation in Sept 2010. I've serviced the tranny since then and the fluid is new and red. I haven't been on any long trips lately - it's doing three things that are troublesome. 1) it will drop out of TC lockup, sooner or later, just about all the time when running above 45 mph 2) it has rare slipping episodes at low speeds and 3) occasionally, it gets a little confused as to which gear it wants down below 30 mph. This latter problem is just for a split second and then it figures it out and goes on. The slipping is dealt with by just stopping and shutting it off and then immediately restarting and going on. The TC unlocking is the biggest problem - it happens more often and it is not convenient to stop and shut the vehicle off. When it happens, I can press down on the accelerator and it will normally lock back up for a few seconds until it is no longer accelerating and then it unlocks again. The cruise control works as designed so the break light switch would appear to be just fine.

Questions: 1) is it likely that the TPS was damaged when the engine and tranny overheated?
2) if lockup is normally achieved and then it drops out of lockup, a switchable hard wiring (going around the TCU) of the lockup would probably not improve the lockup situation?
3) if I unplug the TCU and shift manually to test the electrical, would it lock up if it was healthy
4) What's the best guess? solenoids? mechanical/hydrallic? some other electrical?

TPS can't be hurt by overheating tranny.

larue2000 said:

Questions: 1) is it likely that the TPS was damaged when the engine and tranny overheated?
2) if lockup is normally achieved and then it drops out of lockup, a switchable hard wiring (going around the TCU) of the lockup would probably not improve the lockup situation?
3) if I unplug the TCU and shift manually to test the electrical, would it lock up if it was healthy
4) What's the best guess? solenoids? mechanical/hydrallic? some other electrical?

Click to expand...

1. I doubt it, but your symptoms could be caused by a bad TPS. Giving it more throttle to get it to lockup again and the odd shifting certainly points in that direction.

2. Aside from having to manually turn it on/off, you'd get a check engine light, which might be an inspection issue.

3. Lockup does not occur with the TCU unplugged. It's only triggered by voltage applied to the t/c lockup solenoid.

4. Grab a multimeter and start doing some tests. http://www.transonline.com/transdigest/magazines/1997-10/Shift%20Pointers/index.html In particular check the resistance of the solenoids. I'm leaning towards a bad TPS myself.

Okay, I'm going to get off my butt and do some testing with a meter.

BTW, all these problems seem to occur when the tranny is all warmed up. When cold it seems to perform without any glitches.

I agree with Lawson's answers on all 4 of your questions.

If this were my vehicle, I would start by testing the throttle position sensor. Then solenoids.

If the TPS is failing, it isn't because of your overheat of the transmission. Rather, it is coincidental

This is not likely a major transmission failure, rather something electrical, so you want to resolve it. Testing hopefully will allow you to isolate it.

Consult the factory service manual for a flowchart if you will of other things you can try............

Good luck and let us know what you find!

Testing TPS: okay, with a digital multimeter, I measured 5.14V on source wire "C" (wire closest to left side of vehicle). Testing wire "B" (middle wire), it measured .77V with throttle closed. At WOT the voltage output was 3.77. Can't say there weren't any flat spots but it appeared to be a reasonably smooth progression. So far it's looking like the TPS is fairly normal? Should I test the resistance (ohms)? If so how do I go about that.

Thanks to all for the help.

Did a little more searching and reading about TPS output voltage. Is 3.77V enough - maybe not? If not, is it likely that it can be adjusted since the idle reading is withing specs?

MORE INFO: I took the TPS connector off and jumped B and C with the meter and got a flow of 4.31V which is pretty close to the magical 83% (of 5.14V) potential at WOT. So maybe a new sensor would approach that number?

Tested solenoids and got resistance as follows:
#1 = 11.0
#2 = 13.3
#3 = 18.8

No 3 is obviously out of range (11 - 15 ohms) but is it far enough? Would a weak TPS output be bad for a solenoid with high resistance. Maybe that is a dumb question but I don't know that much about electricity.

If it were me, I would just replace them all since you have to drop the pan anyway and you are in there anyway. I found a place that had all 3 solenoids for just under $200. Replaced all of them, with new fluid and filter and haven't had any issues with it since. Just my opinion and what I would do.

Looks like a bad set of solenoids - and to 1armguy's point should just change them all.

For the record, I buttoned it up after the first resistance test and went for a casual drive. Then I tested them again (the rig cooled for 4 or 5 minutes) and got 13.8, 14.7 and #3 was wandering all over the place (29 to 42). Then I went for another 10 mile drive (unplugged) to a friend's house at the top of a long hill. The tranny performed just fine with the manual shifting and that was a relief. I checked the resistance immediately upon shutting off the ignition. Got 15.0, 18.8 and 63.0! I think that solenoid 2 being out of range is actually good news - it goes a ways to explaining why there was confusion at lower speeds. The TPS may, in fact, be bad but we'll start with the solenoids and that may do the trick.

Thanks to all for reading and contributing. I'll update if there are any unexpected developments.

The 11 to 15 ohms may be a factory spec for room temperature solenoids. They would read a little higher when hot, as the wire, coil, resistance will increase as the wire temp increases. Number 2 may be OK. #3 is obviously having a thermal failure! Swap 1 and 2 and see if the problem moves, unless you like buying parts and passing out cash, or are in a hurry.

Here are a few threads that might help you out sourcing and replacing the solenoids.

http://www.naxja.org/forum/showthread.php?t=987463
http://www.naxja.org/forum/showthread.php?t=996259

The second thread has a link to http://www.transmissionpartsusa.com/AW4_A340_transmission_solenoids_kit_Toyota_p/940-00094393a.htm which has the set of 3 solenoids for $139

MAJOR UPDATE: before I spent the money to change out the solenoids, I wired (with a toggle switch) around the TCU (D16 to C14). I was amazed to see that solenoid 3 functioned perfectly - has not missed a beat. When it drops out of lockup, I can flip the switch and the TC will lock up and stay that way. So, solenoid 3 appears to be good or good enough. This is not a perm solution but does provided valuable info. Question: is the strength of the signal coming to C14 from the TCU weaker (less voltage or amperage) than the battery voltage available at D16?

Further, I ran into a post on fixing what must have been an intermitent ground problem. I did the same thing and it totally fixed my low speed shifting problem. And it has improved the lockup problem. See: http://www.naxja.org/forum/showthread.php?t=1054498

This beast is not throwing any codes ('96) but does the TCU remember things that happen to it? I've read that a disco of the battery will make certain problems go away for a while.

Thanks to all for the help. I'll update the lockup problem when more info is available.

Interesting. The solenoids also ground through the transmission body and engine. It seems odd that 2 of the solenoids measured okay, but the third was measuring all over the place. I would have though a bad ground would affect the measurements for all three. It is highly possible if the ground was that bad that it was affecting the TCU itself.

The loss of ground was rare (once or twice on a 5 mile errand run around town), extremely brief and very intermitent. Primarily affecting the low speed shifting confusion, I would notice it most frequently when accelerating in the second half of a turn. Must be the grounding in the tranny losing contact.

The brief loss of ground would therefore affect the solenoids, not directly, but via the corruption or loss of the signal(s) from either the TPS or OSS or both. So, in that sense, the TCU is involved.

Lastly, I am very confused about the testing of solenoid 3. I may retest everything at the connector next to the dipstick. As indicated, the new ground only "improved" the lockup problem but the toggled wire around the TCU continues to work. The final solution there will probably be a new TPS or OSS.

Finally got around to changing out the TPS. No change in lockup problem.

larue2000 said:

The TC unlocking is the biggest problem - it happens more often and it is not convenient to stop and shut the vehicle off. When it happens, I can press down on the accelerator and it will normally lock back up for a few seconds until it is no longer accelerating and then it unlocks again.

Click to expand...

I do not believe that is possible, as it is entirely backwards to normal opperation, unless the acceleration movement is jarring the brake petal assy enough to affect the TC lock up switch that tells the TCU when the brake is, and is not being applied. The TC only locks up when you stop accelerating and you do not have the brake applied, and unlocks if you accelerate enough to down shift.

larue2000 said:

Did a little more searching and reading about TPS output voltage. Is 3.77V enough - maybe not? If not, is it likely that it can be adjusted since the idle reading is withing specs?

MORE INFO: I took the TPS connector off and jumped B and C with the meter and got a flow of 4.31V which is pretty close to the magical 83% (of 5.14V) potential at WOT. So maybe a new sensor would approach that number?

Click to expand...

Can you be more specific as to what you did here? If you disconnected the TPS, it would not show any voltage, so your statement is confusing. IF you tested B to C on the harness only, with no TPS attached, then you should have read 5.14 V if C is a good ground?

EDIT: OK I see that B and C are not grounds, so not sure what your tested, or what it means?

For the record, I buttoned it up after the first resistance test and went for a casual drive. Then I tested them again (the rig cooled for 4 or 5 minutes) and got 13.8, 14.7 and #3 was wandering all over the place (29 to 42). Then I went for another 10 mile drive (unplugged) to a friend's house at the top of a long hill. The tranny performed just fine with the manual shifting and that was a relief. I checked the resistance immediately upon shutting off the ignition. Got 15.0, 18.8 and 63.0! I think that solenoid 2 being out of range is actually good news - it goes a ways to explaining why there was confusion at lower speeds. The TPS may, in fact, be bad but we'll start with the solenoids and that may do the trick.

Click to expand...

Where did you take the readings on Sol #3? At the solenoid, or at the TCU? The high resistance might have been the wiring/connectors and between the TCU and the solenoid!

larue2000 said:

Question: is the strength of the signal coming to C14 from the TCU weaker (less voltage or amperage) than the battery voltage available at D16?

Click to expand...

Only if the connectors are dirty, or the wire is broken, or making poor contact at the connectors!