Transmission downshift question
Thread Starter
Burning Brakes
Joined: Feb 2000
Posts: 851
Likes: 112
From: Torrance California
Transmission downshift question
I'm trying to help a friend of mine diagnose a transmission problem on a '90' L98, automatic. It actually use to be mine which I sold to him ten years ago. The problem seems to be on coast down, foot off the gas with slight braking, as the transmission shifts down from 2nd to 1st, a audible clunk is heard or felt. On acceleration there is no problem and the transmission functions well in all other modes. The fluid and filter has been changed but the problem is still there. Is there anything externally we can look at? Internally? Any idea's would be appreciated. Thanks. 
Thread Starter
Burning Brakes
Joined: Feb 2000
Posts: 851
Likes: 112
From: Torrance California
He checked all the U-joints in and out of the rear end and axles, everything seemed to be tight. The car only has 60K miles. I was kind of wondering but I can't remember if this year would require any soft ware updates in reference to the tranny like on the newer cars. We were hoping it was something we could address externally and not have to drop and open up the transmission which would have to be left to a professional.
Pro
Joined: Mar 2013
Posts: 628
Likes: 13
From: Indiana
Did you look at the C channels mounting holes. They are known to wear out oval shape and can cause clunking. This is between the trans and rear. If loose someone has a kit or plate that helps secure these bolts.
Race Director
Joined: Apr 2003
Posts: 17,699
Likes: 1,667
From: moraga ca
If it is a clunk, most likely it is an issue outside the transmission. Something causing play in the drivetrain or the tranny or rearend moving around. Check again the ujoints and cbeam mounts and rear end mounts.
Thread Starter
Burning Brakes
Joined: Feb 2000
Posts: 851
Likes: 112
From: Torrance California
We will re-examine the u-joints and look at the 'C' beam mounting. Do you think the TV cable my be out of adjustment? I've combed through the archives in an effort to duplicate the same problem. One forum member found that a "brake pedal switch" can influence strange transmission problems like causing the TC to kick in and out. Not exactly the problem we have though. We welcome any more idea's. Thanks.
Race Director
Joined: Apr 2002
Posts: 10,036
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From: Anaheim CA
These transmissions are strictly mechanical -- no software to upgrade. That didn't come until the 4L60E.
I don't think the TCC lockup speed changed over the years. It locks at 40 MPH. I don't remember what the unlock speed is. I think it's 37 MPH. That shouldn't affect the 2 to 1 downshift. The TCC does unlock if you put on the brake.
It wouldn't hurt to adjust the TV cable. It only takes a few seconds. It mainly affects upshifts.
I don't think the TCC lockup speed changed over the years. It locks at 40 MPH. I don't remember what the unlock speed is. I think it's 37 MPH. That shouldn't affect the 2 to 1 downshift. The TCC does unlock if you put on the brake.
It wouldn't hurt to adjust the TV cable. It only takes a few seconds. It mainly affects upshifts.
Last edited by Cliff Harris; Nov 15, 2013 at 01:51 AM.
