95 Transsport won't shift into 4th gear after trans. hot!?!

My wife's transsport has a peculiar problem, which is only apparent on a long highway trip. When we start out, everything is fine, and the transmission behaves perfectly.

After about 30 miles of highway driving, we might come upon a small hill and the tranny downshifts to maintain speed, but it never goes back into 4th gear again! Even if we are on a flat stretch or whatever - we're talking speeds of 55 mph and above. I can tell we're in 3rd because of the higher rpms, and putting the prndl into 3rd makes no change in RPM.

If we come to a stop sign and then accellerate again up to highway speeds, the tranny doesn't go into 4th again.

Here's the weird thing: If I come to a stop sign, turn off the motor, then turn it on immediately, then accellerate again, the tranny WILL go back into 4th, until we hit another hill down the line and it will get stuck in 3rd once again.

This would seem to me to be a PCM thing - it must be looking at several variable to know when to shift into 4th - but what are these? Is high temp something that will prevent shifting into 4th?

Any help would be appreciated - thanks!

Too bad it is an OBD2...you need a scanner to pull the codes. I think some '95 were OBD1 if the diagnostic plug is exactly like this:
www.avigex.ca/xport/plug.jpg in which case just come back and tell.
Some error codes will prevent shift to 4...except I"m not aware this is "reversible"..that is, once the code is stored, 4 AND TCC AND cruise control will not come on until the code is cleared out.....
www.avigex.ca/xport/overdrive.jpg
In fact, can you tell if the TCC comes on? Normally, TCC will apply on 3rd gear too. If TCC does come on, I would then think the problem with 4 is down in the gearbox, leaky seal, sticky valve, worn o-ring...., but for the time being, this is only speculation. IN the GM book, the only transaxle condition that would apply to your description is "3-4 shift valve stuck". Other conditions listed would not explain that you do have 4th gear at times.

Yes, I can get the codes - are you certain that there will be some? A friend of mine has a scan tool that I have used before. Anything else I should do while I have it?

OBD2 provides a lot of codes with more detail than OBD1, so when available, one should not neglect the possibility that something interesting comes from that source. With the symptoms you describe, however, I'd think of it like being transmission hardware related..p.s. DOes the condition happens with cruise control engaged or with "man" control too?

OBD2 provides a lot of codes with more detail than OBD1, so when available, one should not neglect the possibility that something interesting comes from that source. With the symptoms you describe, however, I'd think of it like being transmission hardware related..p.s. DOes the condition happens with cruise control engaged or with "man" control too?

It makes no difference.

But I think the really key thing here is that when it starts happening, turning the ignition off and then back on will eliminate the problem. Almost like a PCM reset - so the PCM is for some reason not thinking it can go into 4th, unless it gets 'rebooted'. Any thoughts on that one?

BTW, I should be able to get some codes today (hopefully).

The only signal from the PCM that makes a difference between 3 and 4 is the A shift command.
www.avigex.ca/xport/transaxle.jpg
www.avigex.ca/xport/transmissionranges.jpg
www.avigex.ca/xport/transaxleconnector.jpg
THis is applied through pin F of the TCC connector. I'd find a way to pinch a small wire there with keeping plug connected to measure voltage on this pin in action. 12V is OFF condition, 0V is ON condition (PCM provides a ground to complete the circuit..

Okay, here's what I found:

I borrowed my friend's EZlink by AutoXray, nice tool. I had to drive 75 miles with the vehicle, and this tool gives live data as you're driving, as well as codes.

The vehicle drove just fine, and the scan tool was displaying everything from throttle position, to EGR to current gear etc. I was monitoring several key variables associated with the transmission, such as transmission temperature, current gear, TCC slippage, TCC engaged, requested gear etc.

The outside temp that day was ~60F

The trouble soon began when the transmission temperature reached approximately 210F (which took about 30 min). When that happened, the TCC would no longer engage (something I didn't notice before). It would still be in 4th gear though. Soon after, I came to a stop and then accellerated again up to 60. The transmission no longer shifted into 4th. The transmission temp was 215F at this point. The peak temperature reached by the transmission was 235F, which was much hotter than the reported coolant temp (205F).

The returned code from the scan tool was P0740 Torque Converter Clutch Circuit malfunction. No other codes.

On the return trip, the transmission was cool when starting out, and then - sure enough, at around 210F no TCC, and then at 215F, no 4th gear. At this point I pulled over, turned the car off, then back on, and the temperature was still hot, and upon accelleration things were fine - for the first few minutes, but then the same symptoms came back, so I'm thinking the PCM has a timeout function or something - to assure the sensors are reading correctly.

I talked with the guy who I got the scan tool from, and he said that the transmission is running too hot, and that it shouldn't be getting above 200F. Is this true?

In retrospect, if this really is the issue, what could be causing it? A plugged radiator? Wrong fluid?

Also, what is the purpose of the temp sensor in the tranny? There isn't an idot light for it, yet it must be used for something - does the PCM use it to determine shift points? It must have a purpose internally - since it reports the temp back to the PCM (thus allowing me to monitor it).

Anyway, lots of fun getting this data, now to try to make some sense out of it!

Any help would be appreciated. Thanks.

Additional info:

The code 740 (TCC error), according to the manual, the PCM will disable 4th gear if this condition exists for more than 10 seconds. It actually compares vehicle speed to the RPM to determine whether the TCC is actually locked.

So the PCM is doing what it should, but the bigger question is why the tranny is running hot. Plugged radiator? Wrong fluid?

The manual also said that the normal range for the tranny temp is up to 90C.

HTH. Any ideas would be appreciated.. Thanks.

Additional info:

The code 740 (TCC error), according to the manual, the PCM will disable 4th gear if this condition exists for more than 10 seconds. It actually compares vehicle speed to the RPM to determine whether the TCC is actually locked.

So the PCM is doing what it should, but the bigger question is why the tranny is running hot. Plugged radiator? Wrong fluid?

The manual also said that the normal range for the tranny temp is up to 90C.

HTH. Any ideas would be appreciated.. Thanks.


About 2 years ago I was experiencing the same problem with the trans not staying in 4th (OD). After having the trans rebuilt because it was returning a code stating that the torque converter was bad I was driving along and it dropped out of 4th again (1 week after getting it back). After some choice words for the trans shop I discovered what the problem had been all along, the brake light switch was going bad and tripping the trans out of overdrive. What led us to the brake light switch was the fact that if you were on cruise it would also disengage the cruise control.

About 2 years ago I was experiencing the same problem with the trans not staying in 4th (OD). After having the trans rebuilt because it was returning a code stating that the torque converter was bad I was driving along and it dropped out of 4th again (1 week after getting it back). After some choice words for the trans shop I discovered what the problem had been all along, the brake light switch was going bad and tripping the trans out of overdrive. What led us to the brake light switch was the fact that if you were on cruise it would also disengage the cruise control.

excellent point, thanks for sharing!

:)

excellent point, thanks for sharing!

:)

I have been following your problem. I have the same problem with my 94 van. I also replaced the tranny which didn't solve the problem. Any solutions yet?

I have been following your problem. I have the same problem with my 94 van. I also replaced the tranny which didn't solve the problem. Any solutions yet?


Well..........

I'm assuming you have the 3.8 with the 4sp tranny.

Turns out my van's TCC was slipping and failing slowly. The heat build-up in the tranny is due to the tranny slippage, and the PCM will not allow the tranny to go into 4th gear after a certain temperature is reached. This is a safety feature, since the transmission is under the greatest load in 4th gear.

I ended up having the transmission rebuilt, and the shop put in heat treated components, which I guess is an 'upgrade' that GM should have done from the factory.

I myself installed a transmission cooler in series with the van's. My readings show that the cooler drops the transmission temp by at least 20C.

All problems have been solved at this point. Hope that helped.

I am curious as to how much your tranny rebuild was. My parents have a 95 Trans Sport SE. They have recently sunk alot of money into it repairing parts to keep the engine running. Coil packs and such. Now the transmission has gone. The only gear left is first. I don't know if it is worth it to get it rebuilt or cut their losses and junk the van. Thanks.

I am curious as to how much your tranny rebuild was. My parents have a 95 Trans Sport SE. They have recently sunk alot of money into it repairing parts to keep the engine running. Coil packs and such. Now the transmission has gone. The only gear left is first. I don't know if it is worth it to get it rebuilt or cut their losses and junk the van. Thanks.

We shopped around, and got it completely rebuilt for $1200, which includes a 2 year 36,000 mile warranty. It is from a shop that is a family run business with a great reputation. I'm really happy with them. They also installed high strength heat treated components.

Thanks. I was expecting up to $1,400. So that makes sense. Thanks again.