The Stovebolt Site Forums The Shop Area Driveline Hydra-Matic Transmission, Dramatic rpm drop when putting in drive

Hello,
my girlfriend owns an 1954 GMC 3100 with an hydra-matic transmission attached to a 235 chevy engine(truck had a 248 original). We got the 235 swapped from a friend as it was good and running and the 248 was completely junk.
I have a question regarding a drasitc rpm drop of the motor when I put it gear 1-4, 1-3, R.
When I put it in 1-2 it stalls the motor. The idle speed of the motor in neutral is about 370 rpm.
Does anyone have an idea what might cause this (break bands not set correctly etc...)?
The truck is a frame off restoration and is road ready since about 6 weeks.
I would appreciate any hint that might help me to get the problem solved.
Thank you, Mario

Mario,

If you do not have a GMC Shop Manual, I'll look up the idle speed setting. For a 1954 Chevrolet truck with Hydra-Matic transmission, the idle speed is 400-450 RPM, with shift lever in neutral.

I do not have extensive experience with my Hydra-Matic but I found the shifting pattern (up-shift and down-shift) to be very sensitive to the idle setting and to the adjustment described in the Maintenance & Adjustments Manual.

After you reset the idle speed, if it still stalls in 1-2 consider the torque converter. I believe they had a locking torque converter that engaged above a certain speed in 3rd and higher gears. never engaged in 1st. maybe it is being activated by mistake or stuck on.

There is no converter in the old dual range hydramatic.
It is called a fluid coupling, they do not amplify the torque, like a torque converter, they are missing the stator that a torque converter has. There is no lockup clutch in the fluid coupling. Direct is accomplished by a separate splined shaft.
First like mentioned the idle speed is way too slow, I would think it should be 400 to 500 in gear. The clutch for direct could be binding too ? I'd have to study things more.

Sorry, I was thinking of the big truck 5 speed GMC that was like the 6 speed powermatic Chevy had.

All the 1954 GMC Hydra-Matic trucks with transmissions had the same idle speed: 400-450 RPM. These were the H-Ms used in up to 3.5 ton trucks.

They all had 4 forward speeds (and, possibly a reduction unit, giving 8 forward speeds).

Idle speed is too low. There are also some throttle pressure adjustments that you can make. Tim has pointed you the right direction with the service info link.

The engine will not idle in the 1-2 range. It is normal for that gear to drag it down. That gear is strictly for dropping into while trying to "get out of trouble". The service manual will pretty much say the same thing. It should idle in 1-4,1-3,N, and R though.

Thank you for your tips so far. So basically you would not consider the break band set is wrong? rather the idle speed or throttle linkage might cause the problem?

usually dying when put in gear is result of vacuum leak.

1-2 doesn't have the slippage built into it that the other positions do. it will NOT idle in the 1-2 position.

I am not saying that your transmission is perfectly adjusted. I am just trying to say that if you are just going by the dying in 1-2 position, then you are being over critical.

What Greg is saying is exactly correct. This is a 55 manual the advice is applicable.

http://chevy.oldcarmanualproject.com/shop/1955truck2/55ctsm07150.htm
Look at the section titled "Engine Idle" #2

If increasing the idle speed does not work I would ensure you engine is tuned properly. When you have this working then you can dive into potential transmission problems.