adjusting the vacuum modulator

[Austin86250]

1986 6.9 c6
my 1st to 2nd gear was very rough
then my truck started to run through all the gears by 18 mph 1st to second 10 mph then 2nd to 3rd 18 mph
which way would I turn the screw in the modulator to try and correct the problem?
or should I just get a new modulator?
I believe my vacuum pump works
any input would be appreciated

 

[Cubey]

Turn the screw clockwise for a later upshift, counterclockwise for earlier.

Rough shifts may be something bad internally.

You would have no power brakes if the vacuum pump was bad.

 

[Brian VT]

...and lack of vacuum would result in late shifting, not earlier, I believe.
You can try turning the VM adjuster screw in. If that's not the trick then you'll probably need to mess with the Vacuum Regulator Valve. If that's not the trick then the tranny may have it's own issues.

 

[Austin86250]

i thought that i heard somewhere that a out of spec vacuum modulator causes hard shifts

 

[Brian VT]

Could be.
As mentioned, the VM isn't the only thing that controls it. But it is the easiest/cheapest thing to check.

 

[Austin86250]

Could be.
As mentioned, the VM isn't the only thing that controls it. But it is the easiest/cheapest thing to check.

what are some of the other things what control it?

 

[Brian VT]

Vacuum pump, Vacuum regulator valve, kickdown rod?, transmission guts.

 

[Ky85]

Here is a link to replacements

New ‘IDI-Online’ C6 VRV (Vacuum Regulator Valve) - IDI Online

We have new or already-rebuilt replacement C6 IDI VRV's (Vacuum Regulator Valves) for sale. Also, we can rebuild your existing VRV if it is not working or broken. Since the IDI C6 VRV (Vacuum Regulator Valve) was discontinued years ago, they are becoming scarcer and scarcer. We have manufactured...

www.nickpisca.com

 

[gandalf]

In case you need it, I have a pdf (C6-auto.pdf) which includes a good section on the VRV (Vacuum Regulator Valve) and adjustment. It's too large to paste in here, but I'd be happy to email it if you think it might help you. PM me with your email address if you'd like a copy.

 

[Brian VT]

The link above also has a good section on testing/adjusting the VRV.

 

[franklin2]

what are some of the other things what control it?

Road speed also affects the shifts. The governor in the transmission spins with road speed, and along with the vacuum modulator, controls the shifts. The modulator and the vacuum thing on the throttle can be adjusted too far one way and cause early lazy shifts, or the other extreme which will cause late hard shifts.

Ford did some Rube Goldberg stuff now and again, and the VRV on the throttle to adapt a gas transmission onto a diesel engine is one of them.

 

[Big Bart]

Austin,

The VRV on the IP is what Ford used to determine the shift points of the C6. The original modulator valves were not adjustable. However the new aftermarket ones are. The VRV acts like a carb on a gasser and sends more or less vacuum to the modulator to control shift points and smooth shifting.

If the VRV is either not working or not working at the right time you can have early, late, or very late and hard shifting.

It would be easier to try to adjust the modulator, but I am not sure which way to turn. I would turn it one turn in and see if you are going in the right direction. See if the VRV is set close enough the modulator can over compensate.

Ford said the VRV valves were in spec or bad.(Most bad ones were tossed out.) But you can adjust them. You can take the plastic cap/cover off the top by the vacuum ports. Now you can see a plastic nut on the neck of the VRV. You can twist the nut and it will adjust the shift point. I have always turned a half turn and drove, half turn and drove, till it was were I wanted it.

 

[Isaac Ristow]

You first need to check your vacuum at the vacuum manifold then check all the vacuum lines. Next I only adjust the VRV on the IP the modulator shouldn't need adjusting they either work or they don't set from factory. Check the condition of the nipples on the VRV also and be very careful around them as they tend to snap off and a VRV is hard to source and expensive. The way I adjust a VRV is to have the tools in the cab to adjust it mine is torx but most are bolts drive it and pull over and move it forwards or backwards a very slight amount drive again see if its better or worse just keep messing with it until it feels right. There is a correct way to set it with a mightyvac and a vacuum gauge but I've found it easier to just set it to how it drives. I like to set them just past where it wont drop to 2nd at 25~ mph

 

[Big Bart]

I use a air conditioner vacuum pump to create constant vacuum when doing the Ford vacuum check. Much easier than a hand pump.

But I find it is easier to drive and adjust drive and adjust. The Ford way does not always get you where you want to be.