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Discussion Starter · #1 ·
Truck is a 77 C10 468bbc about 450 wheel hp. Trans is a th400 that’s been built, red clutches and steel, all ati parts, b&m shift kit.

While out testing it yesterday it had been running great. After about an hr or so I went to go home and noticed that it would not shift into 3rd gear. Just before the issue I had did a small 2ng gear pull but let out before shifting into 3rd. I shift the trans manually with an aftermarket floor shifter. I stopped to check the fluid and make a visual check on everything on the side of the road. After a small 15 minute cool down it went back to shifting into 3rd again. I made it about 5 miles down the highway and it automatically shifted down to 2nd on its own. I limped it the short distance home and parked it.

Can y’all give me advice on what to start checking out. Obviously the fluid, but what about the vacuum modulator? If it’s bad will that cause the issue I’m having? Or possibly the governor? Please tell me this is nothing internal!! I have a event next weekend on the 9th that I must attend so I’m in a crunch. The trans has been flawless other then a small pan leak and I’m waiting for the new pan and gasket to show up.
 

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Items you list can affect the trans staying in 3rd gear.

Low vacuum at the modulator. Bad modulator. Leak in the vacuum line. If the vacuum line is all rubber hose: sometimes the hose will collapse when it is hot & soft. this will cause a bad vacuum feed to the modulator

Low fluid level ... waaay too full is bad too.

Over heating the transmission

Governor problems. Check the plastic gear for wear. Inner valve for sticking
 

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Discussion Starter · #3 ·
Items you list can affect the trans staying in 3rd gear.

Low vacuum at the modulator. Bad modulator. Leak in the vacuum line. If the vacuum line is all rubber hose: sometimes the hose will collapse when it is hot & soft. this will cause a bad vacuum feed to the modulator

Low fluid level ... waaay too full is bad too.

Over heating the transmission

Governor problems. Check the plastic gear for wear. Inner valve for sticking
I’ll get to it this evening. Thanks for the help.
 

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If everything external checks out ok and you still have the problem, pull the pan while waiting for the new one and pull the valve body to be sure nothing got into it. I'm not a tranny expert but had mine apart for various reasons and I can't think of anything internal further than the vb. There are some tight clearances in there that could cause a heat related shifting problem
 

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T400 problem

I would check the modulator, modulator valve and the governor. Were the governor weights changed? Shifter adjustment correct? My bet would be a worn governor.
 
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