This is a 1988 F250 7.3L diesel. The C6 rebuild is complete and reinstalled. I did the rebuild very, very carefully all the way through but purchased a rebuilt valve body. This was a full rebuild including a new torque converter, rebuilt valve body, new intermediate piston and new modulator (green stripe). On the test drive I find that it will not shift up unless I run the rev's up and then let my foot off the accelerator. It does shift all the way up by letting my foot off at each shift, and will shift automatically on the way down.


I checked the VRV today. First, I have rock solid engine vacuum at 23 inches on the supply line to the VRV. I drew a vacuum on the line running to the modulator and it held a constant 23 inches so no leaks there. I then removed the VRV from the injector pump and drew 23 inches on the intake to the VRV with a pressure gauge on the output. I get no more than 7 inches on the output without any rotation of the VRV (at essentially idle) and it drops to 0 when I open it all the way (at essentially wide open throttle). Should the VRV start at or near idle vacuum near 20 inches or more and then drop to 0 at WOT? Is there a calibration adjustment inside the VRV? If so, what range should the vacuum vary on the output over the full rotation of the VRV. Thanks...

 

I double checked again tonight and the line to the modulator holds vacuum fine. Took the VRV to the bench and hooked a vacuum pump to the supply side and a vacuum gauge to the transmission side. With 23 inches in I get only 5 inches out going to zero when I open the valve up. It also appears to leak down. So, anybody know what the transmission side range should be closed (idle) and open (WOT)? Should that be 23 inches at idle and 0 inches at WOT? Anybody know how to take the thing apart to work on fixing it?

 

I've never seen a VRV, much less touched one. But if the purpose of it was to simulate a gas engine's intake manifold vacuum so a vacuum-operated trans like the C6 could be used behind a diesel, then high teens/low twenties at idle, near zero for WOT.

Since a diesel has a more restricted RPM operating range than a gas engine, I wonder how they fiddled with the C6 to handle that issue with respect to shift points vs. engine load. For a given engine load and trans output shaft RPM, you can get an upshift earlier (at lower RPM) with more vacuum to the trans' vacuum modulator.

What do they do with the C6's kickdown rod? Do they attach it to the go-pedal linkage? That one might be easier to adapt going from a gas application to a diesel, by changing the effective crank diameter of whatever works the rod.

I have the 1994 manual set, never really looked at the diesel supplement. The C6 was still an available trans in 1994, at least for gas engines. If I get a chance tomorrow, I'll hunt it up and see if it mentions the VRV in the diesel area.

 

Not much info. The diesel supplement says what the VRV does, that's all.
In the C6 trans area, found a short bit on the VRV check/adjustment (remember this is from a 1994 7.3L):

Remove throttle cable from the throttle lever on the right side of the FIP.

Remove and relocate the throttle return spring to have one end on the throttle level ball stud, other end onto the throttle cable support bracket. this seems to hold the throttle lever open during testing

With VRV still attached to the FIP, engine OFF, disconnect the two vac hoses, attach a vac pump and gauge to the VRV upper (vac supply) port.

Attach a vac gauge to lower (to trans) port.

"It may be necessary to continually pump the vacuum as it bleeds off (this is normal)".

Apply minimum 20" of vacuum to the VRV and maintain. Cycle the throttle lever 5 times from idle to WOT with vac applied.

Insert a .515" gauge block between the FIP boss and the WOT stop. Hold the throttle stop against the gauge block, vac gauge on trans port should read 6 - 8" of vacuum.

If not within that limit, adjust the VRV to 7.5" +/- .5" of vac by loosening slightly the screws attaching the VRV to the FIP, and turning the VRV, slightly left/right to obtain required vacuum. Tighten screws to 75 - 90 inch pounds.

If proper vac reading cannot be obtained, replace the VRV.

They then go on to say that after the VRV has been adjusted, remove the gauge block, hook up the throttle cable and return spring again, and with the 20" min. vac applied, cycle throttle from idle to WOT 5 times again, then to curb idle (not fast idle) position. Vac output towards trans should be a minimum of 13". If not, replace VRV and readjust.

So other than some vac bleed off being normal, and some vac numbers vs. throttle position, not much info.

It seems to me that they are checking just two points on a throttle lever position (with VRV installed on the FIP) to vac output curve. Just a basic good/no good. They don't get into whether the change in vac vs. lever position is linear, or via some non-linear function. Maybe that is built into the design of the internal valving/porting of the VRV.
That would make sense, to do the vacuum transfer function needed to simulate a gas engine in a sealed gizmo, that a dealer service tech just does a quick sanity check on, if it fails that, replace with a new one.

Which does not help one with a decades-old truck, with a part that is probably long obsolete!

 

The vacuum modulator should be adjustable (a little screw where the vacuum tube hooks up). Also, don't forget the pin between the vacuum modulator and the transmission internals... this is a required part and can easily get lost (happened to me). It will also not shift right if you don't put the pin back in.

I used a black stripe vacuum modulator on mine... is the green one the right item for your application?

If you're trying to tune your vacuum regulator, why not hook it up to the vacuum modulator to see if/when it activates? Also, the vacuum regulator can be bought at a box store (quick search online).

 

Thanks for all the info. Only working on the VRV right now as the modulator is new. Yes, green stripe is what is listed for the 7.3L diesel.


I think I have had a break through on this. I found my old one (had replaced it two years ago). The one I have in there now and the old one both do the same thing, that is, 5 to 7 inches max going to zero. I had heard something about maybe being able to adjust them. As I was playing with my old one, I turned the little black ring with the 4 nubs on it (had to pop the nubs loose as they were glued) and it moved the vacuum up to about 18 inches going down to 8 inches to the transmission. So, it appears you can calibrate the range. I am guessing the diaphragm has stiffened a bit over the years causing the range to shift out of normal.


It appears in the VRV set up instructions that the two critical points are 13 inches and 7 inches. So, going to recalibrate one of mine so the range is say 15 inches dropping to 5 inches (this is on the bench and at the two extremes of the valve range). I won't get to this until Saturday but if this works, it means that people have likely been throwing away good ones all these years.

 

Success!!! I adjusted the VRV diaphragm (black ring right below the vacuum ports) to 18 inches closed (idle position) with 23 inches input from the vacuum supply. Opening it up (WOT position) it read 5 inches to the transmission. Note: This is on the bench set up. I put a drop of super glue on one of the nubs on the adjusting ring to lock it in place and mounted it back on the injector pump. I then set the VRV to 7 inches at WOT with the spacer in place as described in the adjusting instructions and tightened the mounting screws. Took it out for a test drive and it is now shifting normally. So, don't throw away your VRV if it doesn't adjust as described in the instructions. Instead, recalibrate it first on the bench. Good luck....