Shifting problems.

[opuckett00]

I have a 1978 Chevy K10 with 350 and THM 350 trans. So here is what happens: I start the pickup take it for a drive and it shifts fine, no problems going through the gears (1st to 3rd), then once the pickup is good and warmed up (not overheating) it will not stay in 3rd gear. If I let off the gas it will go into third, but as soon I begin to apply the gas, it will downshift to 2nd. I have checked my vacuum line, it's good. Replace vacuum modulator, its new. Just not sure what's next?

[roundhouse]

Tee a vac gauge into the line and watch the gauge as you drive

See if there is any change from when it's cold vs hot
And tee in the line at the trans end
And then tee it in at the carb end
if it's different that would indicate a problem in your existing line
You could have some issue wih a vac leak at the intake manifold that only happens when the engine is warm

As your internet mechanic That would be my first guess
Also temporarily use a new rubber line all the way from the carb to the trans
I'm guessing your truck has a section of line that's metal
  Id totally Bypass that with a new rubber  line to see what happens



Could be some issue right behind the mod valve

Isn't there some ball and spring right behind the valve inside the trans ?
(or is that just fords)
I also don't remember if it's just fords wih the kickdown linkage ?

Also i think there are several different mod valves
with a different colored stripe around them ?

And some are also adjustable
I've seen some with a screw up inside where the vac lines goes on and you can adjust the kickdown sensitivity   that way


Otherwise it sounds like a fluid issue
May e check the cooling lines and make sure they are not kinked or bent causing a restriction


Sent from my iPhone using Tapatalk

[78BIG-TEN]

Are all shifts normal or are they delayed,binding detent cable could cause it

[opuckett00]

What about the governor? Could that have anything to do with this issue. I have had the governor out before all these problems started, could I have messed something up?

[78BIG-TEN]

yes the governor could be sticking causing delayed or no upshift.and the gear could be stripped\broken as well

[jaredts]

Is your vacuum line mostly steel, or all rubber?  It should be mostly a hard line because the rubber can collapse.  Have you tried unhooking the kickdown cable and driving it?

[opuckett00]

Its all steel except where is hooks to the modulator.  And i haven't tried driving the pickup with the kick down cable disconnected. 

[Engineer]

You do have your vacuum line connected directly to the manifold, correct?

There are thermostatic vacuum switches that control vacuum to things like emission controls, and the vacuum advance. You do not want to be connected to any of these.

Do like roundhouse suggested and verify that your vacuum signal is functioning correctly. If it is then you know to look for an internal issue in your transmission.