Hey folks,

Hoping someone here can point me in the right direction; 82 C2500 6.2/th400.

Basically truck wont shift out of first gear (done this since I bought it); replaced the vacuum pump (original problem), getting good vacuum now, but the transmission still wont shift. Getting a good 20" of vacuum at the IP, however the output to the TH400 confuses me - starts at about 5" at idle and drops as RPMs rise - I was under the impression it should raise as RPMs increase?

Can anyone give me some pointers as to which direction to take? Kinda need the truck running this weekend as my daily dropped a cylinder and blew a headgasket last week...

Thanks in advance,

Andy

Could be a pinched / partially plugged line between the IP and the vacuum modulator... that's what I found on my truck shortly after I bought it. But, I had delayed, high-RPM upshifts. If you're not getting any upshift at all and you're over 30-35 MPH, I don't think the vacuum modulator is the issue.

And, the vacuum amount at the modulator is supposed to drop with increased throttle (it mimics the way a gasser motor would behave under load.)

Well, 5" is way too low for an idle reading. You probably have a vacuum leak somewhere, or your vacuum regulator valve is way out of adjustment. In addition, the vacuum should vary with throttle position, not engine rpm.

The way it works... You get basically a constant vacuum from the vacuum pump. The vacuum regulator valve takes that vacuum reading and varies the vacuum output to the transmission to simulate the output of a gasser. Vacuum from a gas engine is high at idle and decreases as throttle position/load increase. A line runs from the VRV to the modulator on the transmission...

Edit...

So, in short, if you don't get enough vacuum at your transmission, it will act like it is at high throttle (or above - that would mean no upshift at all). If you get too much, it will act like it's idling...

Yeah, Bypassed the regulator ran the full pump vacuum straight to the line to the modulator; basically starts in second; shift to 3rd at 30km/h (whats that 15mi?). So transmission isn't screwed...

I was about to go see how that regulator/tps/doohickey is adjusted, but I have a strange feeling I ran the truck out of fuel (was reading real low, sloped driveway... :( ) and of course now it wont start - so it's time for andy to read that faq on bleeding the ip :)

If you run it with the vac reg bypassed long it will burn up the trans. The vac signal also controls line pressure. high vac= low pressure low or no vac = high pressure.

Right, the vacuum modulator works in conjuction with the governor to vary shift points and line pressure to accomodate changes in load and throttle position.

Ok, Finally got her purged (found a big leak at the secondary(?) fuel filter - had to take care of that too) and running again... Remind me never to push the boundary on a diesel again :D

With 20" off the pump, and 20" at the regulator/tps/doohickey - adjusted the regulator (leaned it as far upright as it would go) that's putting out 7-8" at idle. Not quite enough for the modulator - drive test shows pretty much will not shift unless you get to speed, coast at idle for a second then it'll shift.

So the question is - where do I go from here? Could almost drive it like this, but it's be much safer to have it working correctly. Got under the truck, unfortunately I've got a non-adjustable modulator in there; and I can't get a adjustable one for 1-2 days according to the local parts shops...

Is that regulator/tps/doohickey at all user serviceable? Wondering if blasting it out with some brakekleen or similar could help...

Might be repairable, I don't know. I never took one apart. My guess is you'll have to get another one though...

The Vacuum Regulator Valve is best described as a "controlled vacuum leak". If it won't hold 11" hg at idle, it will need to be replaced. There are more options for THM400 control.

With a good VRV in place, idle positon vacuum should read about 11" hg. At WOT, it drops to around 5" hg., give or take a couple of inches hg. I usually follow this procedure for a baseline on a THM400......

Set the VRV to full upright position, take a vacuum reading at idle, if around 11" hg, leave it and check for an adjustable modulator. Pull the vacuum line off the trans end at the modulator and look inside the vacuum fitting of the modulator. If you see a screwdriver slotted screw in the hose end, the modulator is adjustable, if not, I purchase an adjustable modulator for my local NAPA store. Install it and back the adjustment screw (screwdriver slotted screw in the end of the vacuum fitting) out, counter-clockwise about two turns, so the screw is just at the end of the hose bib (vacuum fitting). If the screw gets too far out and seems loose, apply a drop of 3M weatherstrip cement to the threads to hold it in place.

This, coupled with a known good VRV will usually make a THM400 shift, 'in the ballpark'. You may need to adjust the modulator in or out to tailor the shift points. I try to turn the slotted screw in to adjust the shift points slightly higher than normal to protect the trans from soft, slipping shifts.

The VRV is not a very good way to vary the vacuum to a trans but that's all they could come up with on the diesel, due to lack of manifold vacuum and no way to create it except with the vacuum pump. With the above procedure, I've had good success getting some longevity from the THM400 and keeping the shift points close to normal.

Hope this helps....

Don't suppose anyone has a part number for this VRV/TPS/regulator/(I still like doohickey)? For some strange reason I'm not seeing it on any of the online sites (though I did find a spring labeled as Vacuum regulator valve); it'd be good to have a reference for the parts stores...

GM #14057219 Gr.#4.205 Catalog date, 7/86 It's old info, may have superceded several times, but the group number and the old, original part number should lead the partsguy to the right place.

I know this is a little old but if it isn't shifting out of first at all then it is the governor and nothing to do with all the other stuff you are replacing. If the governor is functioning properly then it will still allow the trasnmission to shift reguardless of vacuum. It is determined by the RPM's the governor spins that will allow it to change gears, the modulator will determine WHEN to change for smoother shifts.
I had to replace my governor just recently, bought mine off Ebay for about $40. That has gotten me back to actually changing the gears but now I need to fine tune the modulator or check my vacuum.
This is the place I bought my governor from. http://cgi.ebay.com/ebaymotors/TH400-Governor-New-Sonnax-Turbo-400_W0QQitemZ8059293973QQcmdZViewItem?_trksid=p450 6.m20.l1116
Hope you get it shifting again.