Torque Converter Lockup

[MikeVA]

I have spent all morning looking at the transmission threads to find a solution for my current problem without much success.
I have a 94 XLT with 175,000 miles, in the last couple of months the front end has been rebuilt, Vacuum shift modulator replaced, bands adjusted, trans fluid changed, new filter and trans drain plug installed. When I was chasing down a couple of vacuum leaks, I installed a vacuum gauge and after repairing all the leaks it holds 20 psi at warm idle and 12 to 15 under heavy acceleration, cruise control at 60 on level hwy it will hold around 15 to 17 psi.

Now the torque converter is locking up way to soon and slow to unlock with the dreaded shuddering when you come to a complete stop. I have seen a number of threads describing the failure to unlock, but not locking up to soon at the same time. Is the a way to test the signal to the solenoid to see when the PCM is sending a lock or unlock command (voltage or ground) without dropping the pan? If I have to drop the pan and dump out 4 or 5 quarts of fluid to test it, I might as well change it unless you have to remove the valve body to replace it.

Any help would be greatly appreciated.

Mike

 

[MrShorty]

When I've tested the PCM to TCC solenoid signal, I've carefully backprobed the 60 pin connector at the PCM. You could also access the circuit at the pan connector just above the pan. On my '92, there's a third connector on the driver's side wheel well under the hood where you can access the TCC solenoid circuit. Any of those should allow you to do some basic electrical testing without dropping the pan.

For this circuit, the PCM acts as a simple ground side switch (no pulse width modulation to confuse the issue), so you simply need to watch for a change in state (usually back and forth from 0 to 12 V depending on how you wire the meter in) to see what the PCM is doing. You can even use the output state test (see my notes on pulling EEC-IV codes thread) to see if it is working without driving the vehicle.

 

[MikeVA]

I ran a voltage check on the alternator, it ran a steady 14.63v, and ran a scan test. KOEO first returned a 566- according to my book -3 to 4 shift circuit failure. Disconnected the battery cable, cleaned and reseated the electrical connectors on the transmission and the PCM connectors and took it for a test drive to see if the scan would return anything different. Ran KOER test and it came back clean. Now the real question, what do I need to check or replace next?

 

[MrShorty]

Did cleaning the connectors resolve the KOEO 566? Your original post didn't mention any problems with the 3-4 shift. Are there issues there as well? Did cleaning the connectors do anything to resolve the TCC issue? Do the two circuit (3-4 shift and TCC solenoid) look ok (intact, no crosswiring)? Do these circuits appear to respond correctly to the output state test?

 

[MikeVA]

I cleaning the connectors cleared the 566 code at least while still in the driveway, I removed the battery cable to clear the PCM memory and ran the KOEO again and it came back clean with a 111 code then ran a KOER test an it came up clean.

Took it for a quick run around the block and the converter was still locking up to soon, its possible that the 3 to 4 shift is occurring to soon instead and I just didn't realize it could be the 3 to 4 shift instead of the TCC lockup.

When I put it in drive for the trip back to the house everything seemed to work fine. I mostly drive about 6 miles to work in stop and go traffic during the week so it would rarely go into 4th or lockup the converter during normal use.

Now it seems like it will drop into 4th and lockup the converter at 30-35 mph on level road and shudder moderately when coming to a complete stop, like the brake circuit is not sending the unlock signal to the converter although the BOO test during the KOER test worked fine.

All the wiring at the transmission looks good and remarkably clean, still has the factory spiral wrap intact and tight. The harness connector looked good, but I couldn't see any way to reach it last night to test it. I'm going to run the KOEO again this morning before I go to work to see if the 566 code is back and then do the output state test this afternoon.

 

[Maniak]

If the Key on Engine Off is still throwing a TCC or 3/4 solenoid code then its really only two things.. Wires going to the solenoid, or the solenoid itself.

If you disconnect the connector from the transmission and test the impedence of the solenoid you should get 26 to 40 ohms.. If you do then more likely you have a melted or broken wire. At one time we had melted wires to our a4ld. The harness that went to the 3/4 and tcc came out of their harness and was sitting against the exhaust.

~Mark

 

[MikeVA]

I drove to work this morning in drive and everything seemed normal.
On the way home, I put it in OD and the same lockup symptoms returned.

I ran the KOEO and KOER test again this afternoon, both came back clean. Output state test seemed normal.

It will go all the way into 4th and the TCC lockup at 35mph on level road, tach reads 1000 rpm. I did notice when I came to a complete stop, the shudder would happen and the dash voltage meter would drop from the l in normal to the n for just a second until the converter unlocked, I'll try to run it on a digital voltmeter Thursday.

One strange thing happened during the scans though, when I started the KOEO test my scanner reported a 4 cyl engine instead of a 6, its a digital Innova scanner. I don't remember it doing this before and all of the test seemed to work normally. Any chance this could be a PCM or ground to PCM problem?

 

[MrShorty]

A ground problem does seem possible. Shifting from 2nd to 4th/locked would be consistent with a short to ground in the TCC solenoid and 3-4 shift solenoid circuits between the PCM and the transmission.

when I started the KOEO test my scanner reported a 4 cyl engine instead of a 6,

I think you are referring to the KOER test here (just to be sure).

Output state test seemed normal.

It will go all the way into 4th and the TCC lockup at 35mph on level road, tach reads 1000 rpm. I did notice when I came to a complete stop, the shudder would happen and the dash voltage meter would drop from the l in normal to the n for just a second until the converter unlocked, I'll try to run it on a digital voltmeter Thursday.

Were you able to watch what the computer was doing during the output state test and the test drive?

 

[MikeVA]

I think you are referring to the KOER test here (just to be sure).
Were you able to watch what the computer was doing during the output state test and the test drive?

You are correct, it was the KOER test, sorry it was getting late.

I can see the scanner during the output test, but not while driving.
It would be easy to make it a set of extension cables so the scanner could be inside where I could see it while driving, but I didn't know it would display anything unless the PCM was in test mode? Any chance there is a pinout for the bulkhead connector on the drivers side somewhere on the site? I think it would be fairly easy to tap in a voltmeter there and see what voltages are being sent to the 3-4 solenoid and TCC solenoid while driving.

 

[MrShorty]

Any chance there is a pinout for the bulkhead connector on the drivers side somewhere on the site?

doubtful. I haven't seen one here. I have used the wiring diagrams in Chilton's DIY manual for this circuit, and it has proven adequate for me. Autozone has changed their site recently, but I still think you can access an online version of Chiltons for free if you register.

 

[Maniak]

I never found one either. When I was having trans issues (no 3/4) I just tapped into the wires as they went down to the transmission and hooked my volt meter to them to make sure the computer was telling them to energize/de-energize when I thought they should be..

In my case it showed me that the solenoids were being told to kick correctly, so I changed my solenoids out. I replaced the solenoids and things were fine (fixed that problem.. still lost the trans a year later though).

~Mark

 

[MikeVA]

I think I found the correct wires at the PCM connector. There is harness with a pair of 8 pin connectors that comes up the drivers side of the engine by the valve cover that seems to have the correct wires in it. I didn't have enough time or help to run a continuity test on it yet, that may have to wait until Saturday. My meter reads 33.8 ohms between Red-Blue with Yellow stripe and 34.5 ohms between Red-Purple with Yellow stripe just by following the color code out of the transmission connectors.

Are the solenoids in the transmission normally open (what I suspect?) or normally closed? It appears from the diagrams that they are in the unlocked position during normal driving and the PCM sends either a voltage or ground to the solenoid to engage the 3-4 shift and TCC lockup and removing the signal would unlock the 3-4 shift and TCC lock. I think somewhere in one of the transmission threads that I read, there was an inhibit circuit that would not allow the 3-4 shift or TCC lock below 35mph. Is it possible that the 3-4 shift and TCC are locking as soon as the inhibit threshold is reached due to a PCM error or am I grasping for an easy answer?

 

[MrShorty]

I believe they are normally open, but I'm not certain.

...PCM sends either a voltage or ground to the solenoid...

ground

I think somewhere in one of the transmission threads that I read, there was an inhibit circuit that would not allow the 3-4 shift or TCC lock below 35mph

You are right. I have never seen any mention anywhere of this being an electrical circuit, so I'm pretty sure it is a hydraulic circuit in the valve that prevents 4th gear and lockup when the transmission is in 2 or 1.

Is it possible that the 3-4 shift and TCC are locking as soon as the inhibit threshold is reached due to a PCM error or am I grasping for an easy answer?

Certainly possible. As I noted above, the first circuit fault I would be looking for is a short to ground between the solenoid(s) and the PCM. A short to ground would explain it shifting into 4th/locked early, and waiting to downshift too long (specifically until the hydraulics decide to shift into 2). I would normally look for that short to ground in the wiring between the PCM and the transmission before I tried to determine if the short was inside the PCM.

 

[MikeVA]

Weird as it is after all the leg work of finding wires and checking resistance, apparently the problem has resolved itself. I drove it about 50 miles today and the transmission worked perfectly.

It must have been a loose connection or something.

I'm sure it will come back again at some point, but at least I know where to look next time.