TH400 shift timing issues

[burbn10]

Okay. I'm normally pretty good with transmissions, but this one is confusing me... I got my M1008 and the Army had the vacuum pump hooked up directly to the transmission modulator. This caused it to practically skip 2nd and shift into 3rd at 18mph no matter what throttle position was. I understand that part. I hooked up the vacuum lines correctly to the vacuum regulator valve on the injection pump and replaced the modulator. Truck shifted great for a few days. I loved it. Here's the issue.... I normally leave work at 11pm or so when the air gets cold and damp. My transmission suddenly reverts back to it's old ways like when it was hooked directly to the vacuum pump when it gets cold and wet out.... Any ideas? I hate to adjust the VRV or the modulator because during the day when it is warm and fairly dry, the truck shifts great.

 

[Warthog]

Have you checked/replaced all of the rubber vacuum lines? On top of the engine and the small piece on the transmission?

You can set the VRV easily.

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

 

[Wolf.Dose]

The modulator on the injection pump is of poor design. We experianced it very often that it does not work propperly. When there were more CUCV truck arround in our area, a friend of mine, who lives on selling parts for US vehicles, usually had a greater number of these in stock for it was a known problem. Even longer storage make them last shorter.
So try to replace it to a really newly made one, if you get one.
Wolf

 

[scottodog]

Just had the same problem with one of my trucks it was the govenor bad the teflon gears were still good but the springs must have been bad. Easy to change and fixed my quick shift from 1st to 3rd.

 

[Crash_AF]

I agree with Wolf, it sounds like the VRV is probably leaking a little while cold and as the parts warm up it closes up the leaks and it functions properly.

To test, get a hair dryer and heat the valve up while the engine is cold. If it shifts fine, that's your problem.

Later,
Joe

 

[burbn10]

Have you checked/replaced all of the rubber vacuum lines? On top of the engine and the small piece on the transmission?

You can set the VRV easily.

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

Yeah. All the lines are good. It shifts great when it is warm and dry out, just not when cool and damp. The vacuum pump produces good vacuum and the entire system holds vacuum great. Just is doing something weird in damp weather... Not sure yet if it is a temperature or dampness issue yet.

 

[burbn10]

I agree with Wolf, it sounds like the VRV is probably leaking a little while cold and as the parts warm up it closes up the leaks and it functions properly.

To test, get a hair dryer and heat the valve up while the engine is cold. If it shifts fine, that's your problem.

Later,
Joe

I have it parked inside the building tonight, so I'll see if it shifts better when I head home tonight. If it does, I'm going to use a spray bottle and dampen the components to see if it makes any difference.

 

[burbn10]

Does anybody know the correct GM part number for the VRV? The guy at the local GM parts department gave me a number, but the part shows a wiring pigtail and mine doesn't have one.... I don't think it's the right one and they want $150 for it...

 

[Warthog]

From the -20p manual, Figure 10, Vacumm Valve and Hoses, Item #10

14057219 VALVE, VACUUM REGULA

www.gmpartsdirect.com has them for $89.21,list price is $150.49




http://www.gmpartsdirect.com

 

[Wolf.Dose]

I just wanted to take out my manual to fid the number, however, Warthog did it already.
Wolf

 

[jimm1009]

Here are the valves associated with the tranny control.
See PDF attachments. Right-click and "open in new window"
jimm1009

 

[burbn10]

From the -20p manual, Figure 10, Vacumm Valve and Hoses, Item #10

14057219 VALVE, VACUUM REGULA

www.gmpartsdirect.com has them for $89.21,list price is $150.49

Thanks a lot. I looked right over that.

 

[scottodog]

If you had the vacuum pump hooked up directly to the transmission modulator and it still shifted soft try taking vacuum away and see if it shifts hard if it doesnt change it would most likely be either the modulator valve or govenor on the transmission if all the lines are good and you have good vacuum. New modulator is around 20.00 and a new govenor is 29.95 here Turbo 400 Governors & Governor Components - TSR Racing

 

[cjtroutt]

Here is the part number AC DELCO #14057219 Transmission Vacuum Regulator Valve. Bought one last month it's works very well.
Also it's importian to have vaccum lines the one on the end is to trans 2 one is to vac pump.
Here is an photo of mine setup on cummins 6BTA

 

[cjtroutt]

If you had the vacuum pump hooked up directly to the transmission modulator and it still shifted soft try taking vacuum away and see if it shifts hard if it doesnt change it would most likely be either the modulator valve or govenor on the transmission if all the lines are good and you have good vacuum. New modulator is around 20.00 and a new govenor is 29.95 here Turbo 400 Governors & Governor Components - TSR Racing

Scottodog is right on 100%

 

[burbn10]

If you had the vacuum pump hooked up directly to the transmission modulator and it still shifted soft try taking vacuum away and see if it shifts hard if it doesnt change it would most likely be either the modulator valve or govenor on the transmission if all the lines are good and you have good vacuum. New modulator is around 20.00 and a new govenor is 29.95 here Turbo 400 Governors & Governor Components - TSR Racing

I understand how the system works. I am confused as to why this only happens in damp weather. Truck shifts and operates like a champ when it is warm and dry. Sounds like an electrical issue to me, but there is only the downshift wire going to this trans, the rest is vacuum/hydraulic control. I unhooked the vacuum line to the modulator and the governor did fine. It shifted very high and hard, but that is what it is supposed to do without the modulator having vacuum. But point is that it is working. Modulator is new and working and I'm getting 20 inches of vacuum at the modulator at idle from the VRV.

I messed with the rotten thing last night and I noticed the VRV is adjusted all the way forward towards the radiator and I am still getting full vacuum to the modulator. So I am pretty sure the VRV is bad. Why moisture is angering it is still confusing, but it is.... Maybe the rubber in there is just so messed up that it won't seal when the weather is cool. I tried to heat it with a heat gun and it helped a little, so I'm going to get a new VRV this week and give that a try. Even if it isn't bad, it can't hurt to put a new one on... I'm going to go from there.

 

[burbn10]

Success! Installed new VRV and truck shifts better than ever! It's good weather today, so the real test is going to be when it rains, but I think this is it. Thanks for all the help everyone!