Transmission slips in overdrive
Thread Starter
Intermediate
Joined: Sep 2020
Posts: 36
Likes: 15
From: Montreal
Transmission slips in overdrive
Hi Guys,
The auto trans on my 1987 Vette is acting weird. It slips at low speed when it's in overdrive (D with a square around). It shifts perfectly when in drive (D without overdrive).
I used an additive shortly after buying the car this summer to stop a leak from the trans main seal (ATP AT-205) and I think that it started doing that right after. When the leak stopped, I changed the trans oil. It seems to have improved but it's still doing it.
To make to car runs right all the time, I drive in (D) in the city and I put in (D w/ overdrive) on the highway.
Any ideas ?
Thanks.
The auto trans on my 1987 Vette is acting weird. It slips at low speed when it's in overdrive (D with a square around). It shifts perfectly when in drive (D without overdrive).
I used an additive shortly after buying the car this summer to stop a leak from the trans main seal (ATP AT-205) and I think that it started doing that right after. When the leak stopped, I changed the trans oil. It seems to have improved but it's still doing it.
To make to car runs right all the time, I drive in (D) in the city and I put in (D w/ overdrive) on the highway.
Any ideas ?
Thanks.
Thread Starter
Intermediate
Joined: Sep 2020
Posts: 36
Likes: 15
From: Montreal
It's when I lauch the car. Should be first and maybe second speed. I am wondering how that slippage at low speed could be linked with the fact that my trans is in D with overdrive ?!? Why the first gears are not acting the same if I chose D with or without overdrive...
Safety Car
Joined: Sep 2010
Posts: 3,752
Likes: 190
From: Indiana PA
It's when I lauch the car. Should be first and maybe second speed. I am wondering how that slippage at low speed could be linked with the fact that my trans is in D with overdrive ?!? Why the first gears are not acting the same if I chose D with or without overdrive...
Your using the overrunning clutch in Drive but not in Overdrive. Your using the forward clutch and roller in Drive and Overdrive. If your positive its in 1st and 2nd it probably is a worn forward clutch slipping. You notice it more because of the higher torque multiplication at low speeds.
Does the fluid smell burnt?
Any deposits in the pan when you had it off?
A basic line pressure check would be a good idea.
A quality old school transmission shop would be a good place for them to have a diagnostic test done.
other than the TCC lock up these transmissions are old school mechanical devices.
Drifting
Joined: Mar 2013
Posts: 1,952
Likes: 239
From: Venice Cali.
I just had mine rebuilt, sometimes in OD it slips a little after braking. I adjusted the TV cable and it stopped but was too harsh of a shift. I adjusted it back a little and it kind of happens a tiny bit but it is very smooth. I think the cause is TV pressure. Good Luck
Thread Starter
Intermediate
Joined: Sep 2020
Posts: 36
Likes: 15
From: Montreal
Thanks 93Rubie
The fluid doesn't smell burnt. There was some shaving/sparkle in the pan but nothing to worry about. There is an old school trans garage not so far. I will stop there in the coming days...
The fluid doesn't smell burnt. There was some shaving/sparkle in the pan but nothing to worry about. There is an old school trans garage not so far. I will stop there in the coming days...
Race Director
Joined: Jan 2012
Posts: 17,062
Likes: 166
From: United States
FYI, my 93 drives the same way. I drive in D until I hit 50mph, or highway..
But, I also have a 2800 rpm stall, matched to 3.5 gears so I get up on rpm quicker.
I think you are fine, as long as fluid isnt burnt. Dont adjust the TV cable unless you follow service manual. You can easily burn a 700R4 up
But, I also have a 2800 rpm stall, matched to 3.5 gears so I get up on rpm quicker.
I think you are fine, as long as fluid isnt burnt. Dont adjust the TV cable unless you follow service manual. You can easily burn a 700R4 up
Melting Slicks
Joined: Feb 2003
Posts: 2,677
Likes: 812
From: Saskatoon Sask.
The symptoms you describe point to a failing sprag. The forward clutches are active in both D and D over. The over run clutches just lock up the sprag hub to provide engine braking.(this would also bypass a slipping sprag when accelerating) If the forward clutch were to slip in D, it will also slip in D over. Clutches that are slipping will slip less in the lower gear because less torque is required to get the car moving. You can narrow this down a bit by trying the following. Start out in D. Before it shifts to 2nd, move selector to D over and see if it slips. Start out again and do the same thing but don't move the selector until it has shifted into 2nd. See what it does. You say the fluid doesn't look burnt so that would point to clutches not slipping. However, the comment that there is "sparkle" in the pan would suggest metal particles. The sprag is made out of metal.
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Thread Starter
Intermediate
Joined: Sep 2020
Posts: 36
Likes: 15
From: Montreal
Well, it has been a while. I don't even have the car anymore. But I finally knew what was the issue. The guy I sold the car to had some issue with the engine. While doing the inspection, they found out that it had a shift kit installed. I don't really know what this is but it was the issue since the car acted normally after removing it. I posted this in hope that it might help someone with the same problem that I had.
