Transmission lock up problems

[Dixieboy30]

Hi everyone. I have a 1987 Trans Am GTA with the 350 TPI and 700r4 transmission. The car has 164k miles on it and is original to my knowledge. Ever since I bought it last year the torque converter will not lock up. It shifts through all of the gears just fine, but no lock up.

So far I have replaced the tcc solenoid, brake/cruise switch, and the tcc connector at the tranny. Supposedly you can force lock up by jumping the A and F terminals at the ALDL port and I have tried that too. If I turn the key on and jumper those terminals I get voltage at all three pins at the tranny. If I remove the jumper I lose voltage on the tan wire though. The only thing I can think now is that the converter itself is bad. Any ideas?

Oh yeah, there is a vibration right around 1600-1700 RPM, but I don't know where it comes from, it does it in park too.

[curtis73]

The vibe could be anything. That 350 is based on the same architecture of the first SBC in 1954. Mine rocks and rolls too.

I would strongly suspect the TC. The clutches wear and there is too much tolerance to achieve lockup. I would verify that the solenoid is getting voltage. Maybe a little light wired in so you can visually see that the car is trying to lock it up? If you get the light but no lockup, time for a new TC.

[Dixieboy30]

Originally Posted by curtis73

The vibe could be anything. That 350 is based on the same architecture of the first SBC in 1954. Mine rocks and rolls too.

I would strongly suspect the TC. The clutches wear and there is too much tolerance to achieve lockup. I would verify that the solenoid is getting voltage. Maybe a little light wired in so you can visually see that the car is trying to lock it up? If you get the light but no lockup, time for a new TC.

Thanks for the response. I'm fairly confident that the solenoid is getting voltage, so it probably is the converter being the car has this many miles on it. Any idea about what it would cost to have the converter replaced, along with a new rear main seal?

I don't drive this car very much, I've put less than 100 miles on it in the last 12 months.

[MagicRat]

Originally Posted by Dixieboy30

Thanks for the response. I'm fairly confident that the solenoid is getting voltage, so it probably is the converter being the car has this many miles on it. Any idea about what it would cost to have the converter replaced, along with a new rear main seal?

I would say $600-700 if using a stock-type rebuilt converter.

Originally Posted by Dixieboy30

I don't drive this car very much, I've put less than 100 miles on it in the last 12 months.

If that's all the mileage you are doing, don't bother with the repair.

First of all, it might not be the converter, because there are other related components in the transmission itself that may be at fault.

The transmission uses pressurized hydraulic fluid (as directed by the lock-up solenoid) to lock up the converter. There may be something wrong in this circuit, either a sticking valve, blocked passage or some other transmission (not converter) related function.

The lock-up function exists strictly to improve fuel mileage. In a transmission which already has overdrive, this lends a slight improvement at best.

In theory, it also reduces engine revs, and thus interior noise slightly, and a slightly lower trans operating temperature, But again, these differences are very slight at best. You can drive around just fine without any harm without the lock-up.
I have been driving a 700R4-equipped Chevy Van for some years with no working lock-up, and it's been just fine.

IMO unless you are doing extended periods of high-speed (80+ mph) cruising, the benefits are simply not worth the cost of the repair. Just install an auxiliary transmission cooler.... which you should have even if the lock-up did work, if you are doing long distance high speed driving.

If you are obsessive about having it working (and I can understand that) get a transmission shop to diagnose the problem, before throwing more money at unsuccessful repairs.

[Dixieboy30]

Well, one reason that I haven't been driving it is because of this problem, if it were fixed I would probably drive it more often. I keep hearing that driving it like this will cause too much heat to build up in the transmission and eventually cause it to start slipping. It does bug me somewhat though, because I know the rpm is higher than it should be. At 55mph it runs about 2k rpm when it should be around 1700-1800.

[MagicRat]

Originally Posted by Dixieboy30

I keep hearing that driving it like this will cause too much heat to build up in the transmission and eventually cause it to start slipping. .

Lock-up converters did not enter widespread use until the early 1980's. From the 1940's through the 1970's, almost all automatic transmissions did not have lock-up converters and they worked just fine.

FWIW, NONE of the transmissions from GM, Ford, Chrysler or AMC had lock-up converters during this time. This included many cars and trucks with more power and/or more weight than your GTA. They had the same cooling provisions as your car and functioned just fine.

Your 700R4 design fundamentally the same as all these non lock-up transmissions. Heat build-up simply is not an issue for normal driving. Yes, there will be slightly more heat generated, but not nearly enough to cause a failure unless you are doing some heavy towing or seriously-illegal high-speed driving.

Imho you should drive and enjoy your car. Do not let the lock-up issue worry you.
I think you should get an auxiliary transmission cooler, but I would suggest this even if your lock-up feature worked.

I do understand the annoyance factor, though. As I suggested earlier, since you are discussing having a professional mechanic swap converters, simply have them diagnose the problem first, before swapping converters.

They could give you an opinion on the engine vibration, too. Your engine and transmission combination are entirely supported by two rubber engine mounts and one rubber transmission mount. When worn or old, they do not isolate engine vibration very well and are quite easily replaced. Imho inspect them first.

[KiwiBacon]

The place I find lockups necessary is driving in hilly terrain. If it's not locking up you can often get out of the car and smell how hot it is.

For this reason I wired a TC lockup over-ride switch into my shopping trolley.

[Dixieboy30]

If mine won't lockup by jumpering the A and F terminals I doubt it would by wiring a switch in either. I think the problem is inside the transmission itself.


I do have another question though. If I decided to drive it like it is for awhile, would it be beneficial to switch to a 160* thermostat? The reason I ask is because the tranny fluid runs through the radiator. It has a 180* in it now.

[KiwiBacon]

Originally Posted by Dixieboy30

If mine won't lockup by jumpering the A and F terminals I doubt it would by wiring a switch in either. I think the problem is inside the transmission itself.


I do have another question though. If I decided to drive it like it is for awhile, would it be beneficial to switch to a 160* thermostat? The reason I ask is because the tranny fluid runs through the radiator. It has a 180* in it now.

A switch certainly won't help you right now, I was just pointing out that you don't need to be doing high speeds or towing to generate a lot of heat.

I wouldn't change the thermostat. While your cooling system isn't intended to cope with an unlocked tc at full power, a thermostat change will make little if any difference. You're better off fixing the real problem, which is the torque converter clutch or it's lockup circuits.

[MagicRat]

Originally Posted by Dixieboy30

I do have another question though. If I decided to drive it like it is for awhile, would it be beneficial to switch to a 160* thermostat? The reason I ask is because the tranny fluid runs through the radiator. It has a 180* in it now.

These engines, like most modern domestic cars, came with a 195 degree stat from the factory.

You should dump the 180 stat and go with a stock 195 degree stat.

Your fuel injection computer (ECM) relies (in part) on a coolant temp sensor in order to calculate the correct fuel/air mixture.

This ECM is designed for use with a 195 degree thermostat.

Your 180 deg thermostat is not allowing the engine to warm up fully. Therefore, the ECM is likely making the mixture richer than normal in a futile attempt to warm up the engine. This is decreasing your fuel mileage.

Also, tests have shown that cooler-running engines wear out faster. Maximum engine life is attained with a 195 degree stat. Obviously a 160 deg stat would be a move in the wrong direction.