Chevy TH350 question

[Batphreak]

Obviously this has nothing to do with a diesel. My toy truck is a 1972 GMC K1500 with a TH350 Auto and 350 engine. The shifting has been driving me nuts and I don't know much about chevy trannies. If I get after the throttle and let it shift on its own the 1-2 shift happens at about 20mph and 3400RPM, then the 2-3 is around 50mph at the same RPM. This is WAY too short for my tastes, first things I checked were the throttle valve cable (adjusted it properly) and checked the vacuum modulator (it isn't leaking, so I'm sure its working correctly). My next thought is the governor, but I really don't know how to diagnose that. Anyone know much about these old trannies? I'm too much of a Dodge guy.

 

[TBurow]

is this the stock trans(not rebuilt from another vehicle)? if not the governor is probably not calibrated for a truck that weighs the same as yours and had a different rear axle ratio.

the easiest thing would be to get a governor from a truck with the same gear ratio as your truck from the junkyard, or get the right springs and weights for your vehicle and put them in the governor

also you could put in an adjustable vaccuum modulator if yours isnt adjustable. adjustable ones have a small slot where the vac line hooks up to you can use a screwdriver to adjust the shift points but not very much. also set the tv(downshift) cable a little tighter and it will shift a little later also.

hope that helps

 

[MMeier]

just do away with the kickdown cable and adjust the govenor as stated above, need to downshift, grab the shifter!

 

[TowPro]

Are you sure the modulator is not leaking? Put a paper match into the hole (not the match end), if its wet at all, replace it.
Also check the vacuum hoses back to the modulator. Most likely the auditude compensating moduler (size of coke can) has already been replaced. The replacments use a "pre formed bent hose". I have seen many of those hoses kink or crack. Some of the modulators have a screw inside the vacuum hose, i think if you turn the screw tighter it will shift later.

 

[Batphreak]

It is the stock transmission, never been rebuilt. I'll check the modulator again, the one I have is adjustable, but I thought the adjustment was only good for 2-3mph difference. I need like 10-20 more before it shifts. I've shifted it manually and it can go a lot more before it really needs to shift. The truck does have much bigger than shtock tires, 33's, I've been thinking the tires are throwing off the governor, but they should make it turn slower (just like the speedo) which should give me higher shift points. The line to the modulator is still the old hard line, with rubber on each end, I'll pull it out to check it carefully.

 

[BJMarshall]

The modulator will have little or no effect on the full throttle shifts. If you had a vacuum leak, you would have a late 1-2 shift and no 2-3 shift. You can get a governor calibration kit from B&M. It's easy to do, but it takes a lot of time to get it exactly right. Also, look for wear in the governor bore, but this usually causes late shifts not early.

In order to get proper full detent shifts, the cable needs to be tight when the truck is at full throttle.

 

[TowPro]

I meant check the rubber lines on both ends of the metal line. They can crack (leak) or colapse (no vacuum passes through).

you can test the metal line in place with a hand vacuum pump. Plug one end, put the pump on the other. If it holds vacuum the metal line is OK.