1977 K10 Scottsdale Transmission wont shift

[GONZO]

I just got my Brother Michael's 1977 K10 Scottsdale. Total Rebuild. Engine Starts, Runs and Drives Forward and reverse. High and Low transfer works fine.
Transmission does not shift gears.
It was working before the rebuild and checking for a Diagnostics solution if a connection could be the issue. Any thoughts or suggestions to help get this beauty back on the road. Thanks

 

[PrairieDrifter]

What transmission..

 

[NickTransmissions]

I just got my Brother Michael's 1977 K10 Scottsdale. Total Rebuild. Engine Starts, Runs and Drives Forward and reverse. High and Low transfer works fine.
Transmission does not shift gears.
It was working before the rebuild and checking for a Diagnostics solution if a connection could be the issue. Any thoughts or suggestions to help get this beauty back on the road. Thanks

Was the transmission just rebuilt or the engine or the entire truck? Not sure if you're referring specifically to an individual component or the entire vehicle. If the trans was just overhauled, take it back to the shop under warranty

Regardless of whether it's a TH350 or 400, diagnostics are largely the same. First check vacuum at the modulator then check the governor. Governor gear stripped or valve not moving when you actuate the weights will cause a no upshift condition (as will an extremely worn governor bore) while very low vacuum, seized modulator valve or broken/failed modulator will also cause no or very late upshifts. If the governor, modulator are fine, your next suspect is the valve body.

If this trans was rebuilt, modulator should be new, governor new or overhauld with new governor gear and valve bore polished. Valve body should have been taken completely apart and overhauled.

 

[RanchWelder]

?

 

[CountKrunk]

My c20 has a (suspected) th475 (heavy duty) and when it wouldnt shift, the vacuum hose was broken. That was the problem.

 

[NickTransmissions]

My c20 has a (suspected) th475 (heavy duty) and when it wouldnt shift, the vacuum hose was broken. That was the problem.

Post the two-letter code on your ID tag if you know it and I'll let you know if it's a TH475 or HD TH400. The 475s went primarily into motor homes, school buses, box trucks and some diesel equipped C and P series trucks, etc...