th400 shift question
 

My gmc has a th400 in it behind the 427. The previous owner stated the tranny had been built for towing. the fluid and filter are getting changed this week. My question is the tranny will stay in first until 65 mph or roughly 6000 rpms. That isn't even close to normal compared to other th400 trucks I've drove. Seems like it is sticking in first. Is there anything else I can do to make it shift lower?
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Re: th400 shift question
 

The vacuum modulator and /or vacuum to the modulator would be the first thing to check out.

Jim

Re: th400 shift question
 

also check the down shift switch...not sure where your is but mine is on the side of the carp...theres a plug on the DS near the speedo cable try pulling it off and running it...if that fixs it then its the switch

Re: th400 shift question
 

Is this with a "normal" throttle opening when starting off, or are you talking about the pedal being floored? If floored, does it upshift when you let up on the pedal?

Ray

Re: th400 shift question
 

raycow, yes this is under full throttle. then when you let up it shifts. if you floor again though(under 55 mph) it downshifts right back into first.

Re: th400 shift question
 

Well then, I don't know if you could call it a malfunction. I am thinking the transmission may have been deliberately modified for racing. It could certainly be returned to stock configuration by any competent shop. If you want to try it yourself, I would recommend looking at the governor first.

Ray