Idle speed must be too high.
THM-400 trouble?
Thread Starter
Racer
Joined: Nov 2006
Posts: 379
Likes: 7
THM-400 trouble?
Hi
I got a '68 with a thm-400 transmission. The transmission shifts very smooth. But when it is warm and I put gearshifter in reverse or forward, it makes a loud 'klonk' and starts pulling backwards/forwards without me activating the speeder. I do not think the idle is set too high. It does not happen when the engine/transmission is cold. The car has around 60.000 miles on it, but has not been driven for a lot of years.
I have changed the oil and filter.
A guy I know, said it could be a wornout converter?
Any ideas?
I got a '68 with a thm-400 transmission. The transmission shifts very smooth. But when it is warm and I put gearshifter in reverse or forward, it makes a loud 'klonk' and starts pulling backwards/forwards without me activating the speeder. I do not think the idle is set too high. It does not happen when the engine/transmission is cold. The car has around 60.000 miles on it, but has not been driven for a lot of years.
I have changed the oil and filter.
A guy I know, said it could be a wornout converter?
Any ideas?
Last edited by Ronaldo; Jun 1, 2008 at 05:14 AM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Most likely, it is a worn out rubber "snubber" at the differential. It is supposed to absorb the jolt the drivetrain receives when reversing engine torque on it (fwd to rev; rev to fwd). It is an easy and inexpensive change. It could be U-joints, but if you had one that bad you would feel it at other times, also.
Thread Starter
Racer
Joined: Nov 2006
Posts: 379
Likes: 7
Most likely, it is a worn out rubber "snubber" at the differential. It is supposed to absorb the jolt the drivetrain receives when reversing engine torque on it (fwd to rev; rev to fwd). It is an easy and inexpensive change. It could be U-joints, but if you had one that bad you would feel it at other times, also.
It does not do this when the transmission is cold.
Thread Starter
Racer
Joined: Nov 2006
Posts: 379
Likes: 7
Thanks, the idle sounds like it is just above where the engine cuts out but I will try to adjust the idle a tad down and see if it helps. The tach is unfortunately not working... This is my first car with an auto tranny.
Thanks both of you
Thanks both of you
Pro
Joined: May 2007
Posts: 593
Likes: 0
From: Warrenville IL
OK, I've got a similar problem but mine is worst in reverse. Plus I've just replaced the rear differential snubber bushing and all the u-joints less than 500 miles ago.
At first start-up if the car isn't warm, reverse can even cause the car to stall.
Other than that the trans is smooth as silk. Under reasonable rpm's shifts smoother than my 2003 Civic.
Any ideas?
At first start-up if the car isn't warm, reverse can even cause the car to stall.
Other than that the trans is smooth as silk. Under reasonable rpm's shifts smoother than my 2003 Civic.
Any ideas?
Thread Starter
Racer
Joined: Nov 2006
Posts: 379
Likes: 7
It was the idle that was too high. I set the idle the lowest it could and then it went in gear fine without noise and pulling. But now the car can't idle when the engine is cold
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Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
There is a "fast idle" adjustment that is performed on the right side of the Q-jet...as well as the normal (warm) idle speed adjustment on the left side. You will have to find how yours is to be adjusted. Basically, you set the normal idle (as you already have), then you manually lock the choke into "fast idle" position [you'll have to read the adjustment instructions in GM manual, etc.] and then turn the "fast idle" screw to get it up to around 1200 rpm or so. Then, when your car is cold and you tap the throttle to initialize the choke, it will be set to 1200 rpm idle until the engine warms up and drops the choke. If you have converted your regular choke to an electric version, you will need to adjust (or repair) your choke to get fast idle on start up.
Thread Starter
Racer
Joined: Nov 2006
Posts: 379
Likes: 7