06_LT1

Yesterday morning, I was planning to drive the '96 C4 to work. However, when I shifted the car into reverse to pull out of the garage, it slammed VERY hard into gear - hard enough to chirp the tires, even though I was on the brakes.

The car was not idling high - everything seemed normal.

I shifted into park, and then into drive, and then back into reverse, with the same effect each time - the car would slam into gear, like the torque converter wasn't working right. Once in gear it didn't stall or act strange, but it was very much not working right.

I came back home after work, fired it up, and the car worked as expected.

The only difference I know of was the ambient temperature was in the 50's when I noticed the problem, and I had left the man door to the garage open so it got colder than usual. When I tried it again later the same day, the temp was in the 70's.

Any idea guesses about what might be going on? This is a '96 LT-1 auto with a little under 30K miles on it. The u-joints are in good shape - there is no extra slop when shifting. This is the ONLY time the car has ever acted this way when going into gear.

Thanks for any suggestions...

kg4fku

check your ATF fluid per the manual for proper level. Also check color (should be a pinkish color with no metal flakes) and should not smell burnt. I haven't worked on an auto in a while but it shounds like a high pressure issue. Might be time to change fluid and filter.

arbee

Please explain your thoughts on how changing the oil is going to correct a high pressure situation.

kg4fku

Might not do anything at all. However, it will show if there are heavy particulates in the fluid and if the fluid is really burnt (sign of overheating and self-destruction) and give you an indication of overall health of the transmission inside. Also as the car is almost 19 years old and has 30K miles, it isn't going to hurt anything to change the fluid.

Unless the OP has access to high pressure line gauges or scanners and knows how to use them, there really is nothing simple that I know of that the OP can really do to diagnose an automatic transmission other than check/change the fluid.

We really don't have enough information to properly diagnose the issue.

What we don't know is how the car acts driving down the road and physical conditions.
-Are the shifts smooth and complete or are they hard?
-Does the trans have a shift kit installed?
-Is the fluid level low or high?
-Does the trans leak and if so does it leak from the front(torque converter), rear (seal) or middle (pan)?
-Does the torque converter lock up early, late or not at all?
-Does the torque converter not unlock?
-Are gear changes early or late?
-Does the trans delay going into any gear (drive or reverse)?
-Do gears slip when changing (over revving of engine)?

Additionally we don't know the service history of the car and storage practices of the OP for the car. Is the car driven regularly and when driven is it allowed to warm up completely to allow sufficient internal lubrication?

Personally, I believe that there may be an issue with the line pressure being too high. If that is the case then there are multiple possible reasons for this but keep in mind that the OP said the problem went away later (not able to be replicated).

This also leads me to believe this could be an electrical issue vice a completely mechanical issue.

Probable smoking guns:
- Loose/corroded PCM connections.
- Connectivity through the pass through connector at the transmission connector.
- All grounding concerning the PCM, Transmission, Battery, Terminals at the battery, Battery to chassis ground, Engine block to chassis ground. All grounds.
- Positive terminal not serviced.

Keep in mind that I am just a shady tree mechanic and I don't know everything and it has been many (many) years since I helped tear apart an automatic trans.

Just trying to help

kg4fku

OP -

Do you start your car and let it idle without moving the car? If you do, how often would you say you do this before you actually move the car (put the trans into drive or reverse)?

When was the last time the car was moved (actually put into drive or reverse under its own power) prior to you noticing this issue?

kg4fku

So I did some digging and found some troubleshooting information.

1.
Problem: Shifts into gear harshly, car feels sluggish off the line, No 1st, 4th or Torque Converter Control (TCC) lockup available, Manual 2nd, 3rd and Reverse are only available gears, CEL is on

Symptom:Transmission is either in limp-home mode or has lost electrical power. If there are lots of error codes in the PCM, check the under hood fuse that powers the Transmission, and if it pops again, look for a short in that circuit like an O2 sensor harness touching exhaust. Otherwise, check PCM codes for a particular fault in the Transmission causing the PCM to put it in limp-home mode.

Possible solution: check the under hood fuse that powers the Transmission

2.
Problem: Shifting suddenly becomes very hard. When going from park to either reverse or drive it slams into gear. 1-2 and 2-3 shifts are also harsh. All gears seems to work. May be intermittent.

Symptom: CheckThrottlePosition Sensor (TPS).for smooth electrical response.

Possible solution: If the response is jumpy or erratic at all replace Throttle Position Sensor (TPS).


What is "limp mode" ? This is a fault mode in which the transmission either loses electrical power or the PCM deactivates all of the electronics. The Transmission will default to max line pressure which will cause harsh shifting. The driver will have Reverse and 3rd, with manual 2nd available by putting the gearshift in 2 or 1 (doesn't matter). 1st, 4th, and Torque Converter Control (TCC) lockup will not be available. It will not hurt the car to drive it like this for short periods, but use manual 2nd to get the car moving, and be aware that the Transmission will generate more heat than normal while operating in limp-home mode due to increased torque converter slippage during 2nd gear starts and no lockup in 3rd gear.

84wuzmy1st

Shifting from P to D or R when the idle is really high does this I thought... wait for the car to warm up first and the RPM's to drop?

arbee

Here is a cut from the OP's first post.




The car was not idling high - everything seemed normal.