TH400 shifting problem

[turbos10]

Hey all,

My truck is an 87 with a manual pump conversion 6.5T mated up to a TH400. The vacuum pump is plumbed to the passenger side of the IP and then on to the modulator valve. I am not sure what the device on the IP does, so I would love if someone could explain. The issue I have is that when the truck first warms up everything shifts just as it should. However, after running a while it is as if the shift points move up. After completely up to operating temp you have to be at 3/4 to full throttle to get it to shift. The transmission shift hard and does not show any signs of slippage.

The other side of this is that if you shift it manually it will shift up. However, it will drop down a gear if you dont acclerate up to the point engine RPM is ~2500(dont have tac, but educated guessing). If you dont maintain that it will drop back from high to second or second to first. Then you are cruising at 3k RPM with no acceleration at all.

From a dead stop at 1/2 throttle it will actually stall at a speed and not shift unless you push the throttle on down.

I realize the TH400 shifts partially based off of vaccum via the modulator valve and that pressures and RPM also have some effect, but I am no turbohydro expert. I have to think it has something to do with the vacuum pump operation or the IP device(unknown to me) killing vacuum. The most baffling thing to me is that when it is cold and first warms up it is fine.....

Any help is appreciated.

[njdevi11]

The vacuum input on the th400 is a modulator for a gas engine, it works like this:

When a gas engine is at idle the valve body is closed and there is a lot of vacuum created by the engine sucking air in, the transmission recognizes this as a low strain situation (like your on flat land driving at a constant rate) so then it upshifts.

When there is is a WOT situation there is very little vacuum on a gas engine since the valve body is all the way open. The transmission recognizes this and downshifts for more power.

The vacuum on a diesel does not vary since there is no butterfly valve. (or since its running off a pump) Therefore apparently you have some kind of vacuum regulator on your IP that is linked to your throttle. It sounds like this could be a fault. I know there are cable actuators you can replace the vacuum one with that would link directly to the accelerator to control the trans. However these are over $100. This is part of the reason why I'm switching to a 4 speed.

[turbos10]

Thanks for the reply. I went and talked to a transmission shop today and learned a little bit more. You are right about the vacuum regulator on the IP. After talking with the guy at the shop he said he can modify the governor in the transmission and get me back to steady consitant shifts and make it stay in gear when warm. He said he has setup several TH400's like this and can do it so you dont need the vacuum at all. He said the TH400's really need alot of vacuum to work well and the diesel setups with the pump and regulator just dont work very well.

I am assuming the regulator reduces vacuum at higher throttle positions to mimic a gasoline engines vacuum curve which is closely related to throttle position.

I am going to let him tinker with it and hopefully he will get it lined out. He said he can do it for less than $100.

[midniteplowboyy]

If you can find one, get a mechanical modulator cable for a H1 hummer with a TH-400.

[turbos10]

If you can find one, get a mechanical modulator cable for a H1 hummer with a TH-400.

Hehe...next time I pass through Ft. Hood I will have to make a...hrrrmm....detour. Wonder if they lock those things up at night;)

Seriously, though...who would carry parts for hummers?

[njdevi11]

I just did a google search for "th400 cable modulator" and found a site. I'll send you a PM with the link, Its $113 OUCH!

ONCE AGAIN: THAT was a huge factor in me going to the 4-speed.
Other factors:
I like a standard transmission more.
I think either my t-case or trans crapped out
The np205 is much stronger and I can twin stick it.

[turbos10]

I just did a google search for "th400 cable modulator" and found a site. I'll send you a PM with the link, Its $113 OUCH!

ONCE AGAIN: THAT was a huge factor in me going to the 4-speed.
Other factors:
I like a standard transmission more.
I think either my t-case or trans crapped out
The np205 is much stronger and I can twin stick it.

Much appreciated!

Luckily my truck already has the NP205 and the transmission is rebuilt so I would like to get the good out of it. I think that it was built for a gas engine rather than a diesel. From talking with the trans shop it takes some tweeking to work well with a diesel.