shift issues m1008

[ohfisherman]

just picked up a low mileage m1008. it was not shifting right when i got it. replaced all vacuum lines and the modulator and it still is not right. checked vacuum at the pump and needle bounces wildly from 0-15. put in a new pump and now pulls a strong 15-17 with no needle bounce. still does not shift unless i tach it out. any ideas?

 

[Westech]

stuck governor or bad vacuum modulator on the side of the trans. Replace the modulator first (about 10 bucks) and go from there.

 

[CUCV850]

check, how much vacuum at the modulator?

 

[Crash_AF]

Sounds like your Vacuum Regulator Valve is not adjusted properly or is not working. It's on the passenger's side of the injection pump.

To test, put a vacuum source on the lower port of the VRV and a gauge on the upper. With the throttle open to nearly WOT you should have 8 in/hg at the upper port.

If you don't, loosen the two screws holding the VRV to the pump and rotate the valve until you get 8 in/hg on the gauge then tighten it up. If you can't, the VRV is bad and should be replaced. It's a pretty expensive part (almost $100 IIRC).

Later,
Joe

 

[burbn10]

Sounds like your Vacuum Regulator Valve is not adjusted properly or is not working. It's on the passenger's side of the injection pump.

To test, put a vacuum source on the lower port of the VRV and a gauge on the upper. With the throttle open to nearly WOT you should have 8 in/hg at the upper port.

If you don't, loosen the two screws holding the VRV to the pump and rotate the valve until you get 8 in/hg on the gauge then tighten it up. If you can't, the VRV is bad and should be replaced. It's a pretty expensive part (almost $100 IIRC).

Later,
Joe

I just fixed the identical issue. It was the VRV. I found it on Amazon.com for $85 with free shipping. AC Delco part #14057219. I'd check to see if you have vacuum at the modulator first though. High RPM hard shifts are usually caused by a lack of vacuum to the modulator, causing the governor to control all the upshifts at near max RPM. I agree with Joe 100% on this one, but doesn't hurt to check the cheap stuff first. Could be as simple as a hole in the vacuum lines going down to the trans or a $10 modulator... Also check your VRV for position. If it is rotated all the way towards the radiator, it will do this same thing too. The Army had mine adjusted all the way forward to compensate for the part not working right.

 

[Crash_AF]

I agree with Joe 100% on this one, but doesn't hurt to check the cheap stuff first. Could be as simple as a hole in the vacuum lines going down to the trans or a $10 modulator...

He said he replaced the modulator, vacuum lines and the pump in his OP but I agree, check those things first before dumping ~$100 on a valve.

Also, forgot to mention that you should check to make sure the lines are hooked up properly at the VRV, the lower line is the one that goes to the pump, the upper line goes to the modulator valve on the tranny.

Later,
Joe

 

[ohfisherman]

thanks for all the help. the Vacuum Regulator Valve was only held in with one bolt and was the problem. it was bad so i think somebody put it on after they took the other one off. i am guessing this because i had no brake lights and checked everything before noticing the switch on the brake pedal was missing. i think my truck was a donor at one time. thanks again.

 

[pbrstreetgang]

TM for these parts and this fix

Can anyone provide the TM page for this fix? I have found the regulator valve, but I don't know where to look for the modulator. I got underneath my CUCV last night but could not find what I was looking for.

What size hose is needed to replace the hoses, as I understand, this is an easy and cheap fix. If these don't go, then I will then replace the valve.

 

[Crash_AF]

TM 9-2320-289-20 Section 3-21.1 Page 3-64 (Page 197 in the PDF) covers the VRV replacement and adjustment.

TM 9-2320-289-34 Section 5-3 Page 5-6 (Page 366 in the PDF) covers the modulator valve replacement.

Later,
Joe

 

[OD Heavy Hauler]

How much vacumn should be at the modulator? I checked mine one day and it only had 7"hg. I thought it to be a little low and this truck still shifts a little hard I think.

 

[burbn10]

How much vacumn should be at the modulator? I checked mine one day and it only had 7"hg. I thought it to be a little low and this truck still shifts a little hard I think.

I believe it's supposed to be around 8" of vacuum, so it's not too far off. Just adjust the VRV back towards the air cleaner a bit to increase vacuum and that should soften up and lower the shift. TH400 transmissions were known to have a quick shift to them anyways.

 

[pbrstreetgang]

Where can you get the modulator and what is the part number?

 

[OD Heavy Hauler]

Thanks burbn10. I may try a little adjustment once I get past my starter issue when its running again.

 

[burbn10]

Where can you get the modulator and what is the part number?

I don't know what the part number is, but I just got mine from Advance Auto Parts. Told them it was for a 6.2L K30 with a TH400 trans. It was in stock. $10. Napa has them too.
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[Crash_AF]

According to the -34P the GM number is 359917

You should be able to get them from GM or any parts house as Burbn said, it's a very common part.

Later,
Joe

 

[deepseeman]

So I need to jump in on this thread, instead of making a new one. just replaced tranny and she was running great. all new vacuum lines/modulator were installed too. its been 3 months now and just this past friday she started doing hard shifts. re checked lines-all good. have plenty of vacuum from pump, but when i go to feel for vacuum at the modulator, i have none while at idle. i cant adjust the VCV as i installed a 6.5 turbo and the valve is already against the intake. do you think its the CVC?