78 C10 TH350 transmission

[tom78]

Hello folks, I'm a newbe here. I have question about my 78 C10 TH350 transmission. It shifts from 1st to 2nd almost immediately when taking off, 2nd to 3rd is normal and I  also have no passing gear. The detent cable was not hooked up so I replaced it with a new one, still no passing gear. Any suggestions why the quick shift from 1st to 2nd and no passing gear would be greatly appreciated.

[Irish_Alley]

did you adjust it?

pull up on this tab

then
open the throttle all the way up and push the tab back down then release the throttle

[tom78]

Had a mechanic install and adjust it. He told me it must be something internal. My main concern is the transmission not shifting properly, I can live without the kick down linkage.

[Irish_Alley]

where did you get the trans from? was it new or used?

[mcintyrederek]

Shifting quickly through the gears could also be a result of a bad vacuum modulator or governor.  Quick shifting means the transmission thinks there's a lot of vacuum and no heavy loading on the engine so if the governor thinks the wheels are up to speed, it shifts. 

Verify connections of vacuum line to the modulator and change it even if you think it looks nasty.

Another suspect could be the governor.  As wheel speed increases, so does the centrifugal force of the governor weights and it pulls on or out to let tranny fluid go where it's supposed to.  If the springs are weak or missing on the governor then it's definitely a possibility. 

As for the "passing gear" it's the mechanical linkage cable that has already been mentioned, and how to adjust it explained as well.  The TH400's don't have a cable, they have an electric solenoid that downshifts when you put the pedal close to the floor.

Good luck.

[mcintyrederek]

Pics..

Vacuum modulator location.

Governor location.

Governor removed and cap shown.

[jaredts]

The last advice was pretty good, just keep in mind as you troubleshoot that the cable has nothing to do with when it shifts on a th350.  It only forces a downshift when you stomp the gas.  The vacuum modulator sees high vacuum=quickly shifts through the gears at low rpm.  Low vacuum=higher rpm shifts.  Some vacuum modulators are adjustable with a tiny screw inside the vacuum nipple (remove the hose).  Might be worth checking before you check out the modulator itself and the governor--they are the two things that I would look into after that (as already stated).

[rich weyand]

Could be the spring is broken on one side of the governor.  The weights in the governor control how high both shifts are, the springs control the gap between the 1-2 and 2-3 shifts.  A very wide gap (early 1-2 shift) could be from a broken spring.  It takes just a couple minutes to pull it and check, depending on how hard it is to get the cap off.

[mcintyrederek]

Yep, the TH350 caps are more of a pain to pull off than the TH400 where the cover unscrews.

The governor should just slight right out, along with a few ounces of transmission fluid, so just be sure you have something to catch it if you're working on clean concrete.

jaredts is right, the cable that connects to the other side of the carb has nothing to do with the pattern of shifting upward or downward.  It just senses the throttle position and when the throttle is wide open, or close to it, it forces a downshift regardless of what the governor thinks.

BTW, governor weights are critical in how the shifting of the vehicle behaves.  There are numbers stamped on the weights and in my TH400 the springs were color coded.  The tension on the springs and the weights are critical and set up to behave correctly with the engine specifications, the rear axle ratio, and even the tire size.  Varying any of these without changing the weights or springs can cause too long of a delay in shifts or shifting too quickly.

Correct on the vacuum modulator, a small 1/8" slot screwdriver in the vacuum hole turned CW will give a longer duration between shifts, and unscrewing it (CCW) will give softer shifts, less time in between.

[tom78]

Thanks for the info guys. I will check them out. The trans is the original transmission.