Shifting, help me understand....

[tgtaylor64]

I have a 85 M1009. Shifting is quick and smooth. I am in third by 20 to 25 mph; resulting in a very slow acceleration. The kick-down switch on the accelerator is working fine. If I almost floor it, it will stay in first or kick-down. I have replaced the vacuum lines and modulator valve. I have also rotated the vacuum adjustment on the IP all the way towards the front. It seems that the adjustment has no effect on the shift points. My question is, how does the manifold vacuum and the accelerator position interact? Should staying in first longer (starting from a stop) using moderate pedal be controlled only by the manifold vacuum? Alas, just one more thing....

 

[Warthog]

There are multiple parts to the shifting pattern of the TH400.

Unless it has been modified, all TH400s need a correctly working vacuum source to funtion properly. The diesels do not produce a vaccum so a pump has been installed.

So it starts with the vacuum pump. The pump should produce ~20in of vacuum.

20in of vacuum is way to much for the transmission so it needs to be reduced. This is where the Throttle Vacuum Valve comes into play. It senses the position of the throttle and corrects the vaccum to ~8in at partial throttle.

This 8" of vaccum is supplied to the modulator on the side of the transmission. The modulator then tells the tranny when to upshift.

If the tubing has a air leak or the pump, modulator or throttle valve are not working, then it won't work right.

When you are doing a full throttle acceleration, the tranny needs to know when to downshift. This is the job of the kickdown switch on the gas pedal. It closes and sends an electrical signal to the transmission to energize the downshift relay.

These are two totally different systems.

So in short, for the transmision to work properly ALL the pieces have to be in place and working correctly.

The TMs has a troubleshooting section for checking out the shifting pattern.

Also check out this thread.

http://www.steelsoldiers.com/cucv/50568-cucv-throttle-vacuum-valve-adjustment.html