Hello mrbrian,

Send me a PM and I can forward you a document with lots of pictures that I wrote about adjusting the vacuum and transmission pressure. Bear in mind that it is based on a 1981 300SD with an adjustable VCV. If you end up with another non-adjustable VCV at least you'll be able to adjust all the other related components. If you find an adjustable one, you'll be able to adjust everything.

The document is too large to attach (1.8 Mb) otherwise I would.

Hope it helps.

So I finally got around to getting the VCV back on the car so I could test it while it was hooked up to the car's vacuum pump.

The gauge read 15 at idle and as I actuated the linkage lever the reading quickly dropped to 0. Now I'm thinking the VCV is probably ok.

It got dark before I was able to get the linkage attached and take the car for a drive. I'll try and test drive it tomorrow and monitor the readings.

I gave the main vacuum line a good inspection and can't see any potential leak sources. I'm wondering if maybe the brake booster is leaking and this could explain both the intermittent failure of the power brakes I have experienced in the past as well as the shifting harshness. Anybody know how to test the brake booster for leaks?

I don't think you should be able to get the VCV to drop vacuum to zero by just moving the linkage while the car is running, stationary. It should only drop the vac to zero when the transmission should be shifting. Linkage movement with the car sitting still doesn't mimic driving conditions.

Really? When you press the accelerator, the linkage moves the arm in the same way - why would it be different? On a gas engine the vacuum would be close to zero at full throttle so the VCV should mimic the same kind of situation. Anyway, I'll do a test drive and see what the car is shifting at. Could be the linkage arm needs adjusting. Anyone know if the VC is supposed to be at 0 when the car shifts?

The entire purpose of the VCV is to bleed off vacuum, so as to have the vacuum at 0 at the point when the car should shift. That is why it is there. If it is out of adjustment you will never get the transmission to shift as it should.

What you wrote suggested you had the car running, stationary, when you tested the VCV. If so, whatever movement you made of the linkage was nothing like driving conditions. An unloaded engine sitting still with the car in P or N would severely over-rev if you moved the linkage as far as it would go under driving conditions.

Guys, here are some manuals you'll find useful.

126 FSM, a couple of transmission manuals, trailer hitch plans, 01 DodgeRam FSM

I'll leave the link active until tomorrow then deactivate the sharing.

link down. send me a pm if you can't find the manuals online.

To those dealing with trans vacuum issues, there is a section in the manual that lists modulator pressure. MY 84SD has a 722.303 trans and green modulator for example. The trans ID is above the pan on the passenger side. The exact measured pressure suggested in the manuals varies by trans and modulator color but all trans with the same color modulator use similar pressure readings.

Set the modulator then make a gauge by drilling a 1/8" hole in a nickle (US). Follow the FSM and set the VCV to 0 with the gauge installed. Then adjust the VCV up or down until you have the firm shifts you want without flare. All of the needed info is in the manuals.

Also, 15" vacuum is a little low. Try plugging all of the vacuum systems and find out what the pump is putting out. IIRC, both of my SDs are above 18".

Like I said, I'll deactivate the link tomorrow. Professor Google turned all of this up so it isn't secret. Happy reading.

Harsh shifts are a symptom of too little vacuum given the wear and settings on your particular trans. Details are in the trans manuals above. Just go through the process. Make sure the vac pump is putting out, then adjust the modulator & VCV.

Go back and fix any ancillary vacuum leaks that you have to plug in order to get the shift right. Usually the climate vac pods and possibly the door vacuum locks need attention - and can affect the trans.

So today I drove around and watched a vacuum gauge as it was plumbed into the line going to the modulator. Looks like the vacuum reading at idle was around 9-10. As I accelerated, the reading quickly dropped to around 2-3, then stabilized around 5-6. Most shifts were happening around 4-6 or so. Still hard shifting of course. The only thing I can figure is that I must have a vacuum leak somewhere else that I haven't found yet. Does this seem like a likely cause to you folks given my readings?

Also, thanks to Mahone for you most excellent document. I intend to read through it thoroughly, thanks very much.

Junkman, I wasn't able to use the links... I'll PM you. Thanks.

The FSM calls for the VCV linkage to be disconnected for the test. There is a 10mm gauge that measures where the vac is supposed to drop. I made a gauge by drilling a 1/8 hole approximately in the middle of a nickle. Disconnect the linkage, open the VCV 10mm based on your fancy custom gauge which approximates what the pedal would do without reving the engine and adjust the VCV. Leave the cap off the VCV, hook up the linkage and go drive. Harsh shifts and you need to increase vacuum (or plug some leaks which cause low vacuum). Soft shifts and you decrease vacuum.

The modulator pressure needs to be correct and vacuum leaks need to be under control or at least consistent before you mess with the VCV.

Zach - the SD is designed so that vac drops immediately to zero as soon as you hit the accelerator pedal; car does not need to be in motion. I assume his CD is just like the SD. The car should upshift quite smoothly with no vac and you can test it by driving around with the vac line unhooked. The only thing you should notice is clunky downshifts.

You only need vac for a downshift. If you unhook the vac line from the vcv and tee in a Mityvac to the tranny, you can drive with zero vac and test upshifts and you can pump up the Mityvac (ie provide vac to the tranny) before coming to a stop to test for smooth downshifts.

In short:

upshift - smooth with zero vac
downshift - clunky with zero vac but smooth with vac

Of course I am assuming the SD rules apply to the CD.