99 alero tranny question.

[rnj]

My 99 alero, 3.4 just developed a weired slipping or tc lock issue. The car shifts fine through all gears, but cruising at 65-70mph, and let off the gas to deccelerate, then give it gas again, I can hear the engine rev, the a few seconds later feel the trans engage. I took the car to have trans svc and they said fluid looked like 100,000 mile fluid. No shavings or debris in pan. What else could cause this, I believe it has a computer controlled tranny, is there a modual or something that may need to be replaced????

[Blue Bowtie]

The transmission shifting is controlled by the Powertrain Control Module (PCM) or "computer" on your vehicle. The torque converter clutch (TCC) application is performed by the PCM operating a solenoid valve inside the transmission, which in turn applies transmission oil line pressure to the TCC.

The PCM on your model uses a "PWM" scheme (Pulse Width Modulation) to rapidly open and close this solenoid valve so that oil pressure is gradually applied to the TCC over about a half-second time span. The PCM then operates the solenoid valve full-time to maintain pressure once the TCC is engaged. This prevents hard application or "slamming" of the TCC, which not only makes the application smoother and ride less jerky, but also extends the TCC life.

This all works just wonderfully - Or at least until the PWM solenoid valve starts to get sticky from contamination or "junk" in the transmission. When it's sticky, it doesn't comply with the PCM signals as quickly as it is supposed to, but usually "slams" into TCC lockup once the PCM commands it on.

The TCC solenoid can be tested (electrically) from outside the transmission. If the solenoid coil resistance is within specification and is not grounding, the solenoid itself is probably O.K. (electrically). If the solenoid is out of tolerance, that may be the cause of the problem.

A technician with a bi-directional scanner can command the TCC solenoid on and off for several cycles with the engine running, vehicle raised, and transmission in gear. This procedure will exercise the solenoid valve assembly, thereby potentially allowing the valve to flush out any contamination which might be causing sticking. The same procedure should be possible without a scanner by grounding and ungrounding the appropriate transmission harness wire under the same circumstances. Forcing the valve open and shut may solve the problem. The fresh fluid in the transmission is certainly going to help.

The problem may eventually solve itself with just the clean fluid in the transmission, but the TCC needs to be exercised through normal use before that is going to happen. One way to do that is to drive the vehicle normally at that speed (without cruise control), making sure the TCC is engaged. Tap the brake pedal lightly with your spare foot while maintaining speed. The PCM should "see" the brake pedal signal and release the TCC. Once the brake pedal is released, the PCM will attempt to apply the TCC again. Multiple cycles of this with a hot transmission can help clean out any sludge in the TC solenoid valve.

This kind of symptom is a prime reason to change the transmission oil and filter more frequently than the recommended interval. If you have a drain plug installed in the transmission, the task is no more difficult than changing the engine oil.

[rnj]

WOW!!! Thanks for that run through.

[rnj]

Thanks for Bnaylor on the function of the electronic tranny. I had the trans serviced, fluid filter, and it is running very smooth, been about a month and a half since service. The car had 114000 miles on it without ever have sercied the trans. soo yous people out there with around that many mile, go get it serviced before you need a new trans.

[mechtech1]

Are you sure it is really slipping after lifting and then reapplying the accelerator? The reason I ask is, that at least on my 99 V6 Alero, the convertor unlocks anytime the throttle is closed, then when throttle is reapplyed it takes a few seconds for it to lock up again. But at no time is the trans actually slipping, since it would accelerate fine with more throttle opening.

Lee Richardson
Mech-Tech

[rnj]

Well, the rpms would fluctuate up and down on the highway, 60-70mph but the car would not accelerate the speedo would move, but no pull from the car. Also in the mornings, it would sometimes take a few seconds to engage from park. Since after about a week of having the trans serviced, its been fine ever since.

[mechtech1]

In that case it does sound like something is wrong with it.

Lee

[meem023]

Hi, I have a 99 Alero with 105k. just recently the trans appears to be slipping although it just happened out of nowhere. I bought the car used a couple months ago, since then i have replaced alternator, belts, battery, spark plugs and wires and now this issue. The car tends to "slip" after it has been driving for a short while and it does it in all gears. Im hoping there may be something other then the trans causing this. please any suggestions?!?

[BNaylor]

Originally Posted by meem023

Hi, I have a 99 Alero with 105k. just recently the trans appears to be slipping although it just happened out of nowhere. I bought the car used a couple months ago, since then i have replaced alternator, belts, battery, spark plugs and wires and now this issue. The car tends to "slip" after it has been driving for a short while and it does it in all gears. Im hoping there may be something other then the trans causing this. please any suggestions?!?

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