Encountering a Bizarre transmission shifting problem
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Junior Member
Joined: Jul 1999
Posts: 47
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From: Washington, D.C. metropolitian area
Car: 1982 Trans Am
Engine: LU5 - Cross-Fire
Transmission: TH-200C
Axle/Gears: 2.93
Encountering a Bizarre transmission shifting problem
My 82 Trans Am which has a 200C 3-speed automatic transmission is having a strange shifting problem. Car runs fine for the first 30 minutes and then it has a problem down shifting to 2nd gear. For instance when I speed up to 35 mph and then coast down to 10 mph it will stay in 3rd gear and will not down shift to 2nd unless I press the accelerator pedal down and rev the engine and then it will drop into second. Also if I coast down from 3rd gear to a complete stop it will not drop out of 3rd until it goes to a almost a complete stop and it then downshifts to 1st gear.
Now here is what is strange the problem started two years ago, well seven months ago my speedometer stopped working due to a cruise control transducer problem. When the speedo stopped working the transmission started shifting like it should and the problem I listed above did not occur. So for seven months the car ran fine with no shifting problems. Three weeks ago I fixed the speedo problem by installing a new transducer and speedo cable and then the transmission problem is now back. If I disconnect the speedo cable, I can run my car 10 hours straight and it will shift perfectly. When I reconnect the speedo cable the problem returns. I just replaced the VSS (vehicle speed sensor) and this did not fix the problem. It seems when the speedo works, the speedo via the VSS sends info to the ECM and then to the transmission. Somewhere between the VSS to the tranny something electronically is messed up and sending the transmission the wrong information. Could it be a bad prom chip? Faulty wiring? Something electronically wrong with tranny? Mechanically the transmission works fine, it seems to be something electronic. Thanks in advance for any help.
Now here is what is strange the problem started two years ago, well seven months ago my speedometer stopped working due to a cruise control transducer problem. When the speedo stopped working the transmission started shifting like it should and the problem I listed above did not occur. So for seven months the car ran fine with no shifting problems. Three weeks ago I fixed the speedo problem by installing a new transducer and speedo cable and then the transmission problem is now back. If I disconnect the speedo cable, I can run my car 10 hours straight and it will shift perfectly. When I reconnect the speedo cable the problem returns. I just replaced the VSS (vehicle speed sensor) and this did not fix the problem. It seems when the speedo works, the speedo via the VSS sends info to the ECM and then to the transmission. Somewhere between the VSS to the tranny something electronically is messed up and sending the transmission the wrong information. Could it be a bad prom chip? Faulty wiring? Something electronically wrong with tranny? Mechanically the transmission works fine, it seems to be something electronic. Thanks in advance for any help.
Supreme Member
Joined: Jul 1999
Posts: 18,457
Likes: 16
From: Loveland, OH, US
Car: 4
Engine: 6
Transmission: 5
The ECM does not shift the transmission. The speedo is not connected to the transmission's shift logic in any way, nor is the VSS (little yellow box behind the cluster). I doubt those 2 things actually have anything to do with each other, it's probably more of a coincidence.
IIRC the 200 uses a vacuum modulator, but I could be wrong. If so, try a new one of those. If not, and it uses a TV cable, check its adjustment.
IIRC the 200 uses a vacuum modulator, but I could be wrong. If so, try a new one of those. If not, and it uses a TV cable, check its adjustment.
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