Hey guys. To give a long story short I bought a 1989 f250 with a 7.3L idi and a c6 transmission. It's been going great for the couple days I had it. But today after I did a fuel filter change this morning it's been shifting terrible. It will hold onto 1st gear up until 25 mph and 3500 rpm. I tried adjusting the vrv and I turned the modulator counter clock wise 1/2. In both scenarios I would adjust, then test over and over but the results felt the same no matter what. Yes I checked vacuum lines. Is this a sign of the vrv going bad or the modulator? Or is the transmission near the end of it's life. I need to pick some brains here before i go crazy. Thanks 😊

 

put a vacuum gauge on the pump and see what you get. you need AT LEAST 19 inches vacuum for trans to shift properly. when you step on the brakes it should recover almost immediately. if it does not, the vacuum pump is shot, or you have a vacuum leak.

 

I just had my VRV rebuilt by one of our members, @genscripter He did a great job. Here's his website, with instructions on testing a VRV.

https://www.nickpisca.com/diesel/

Be careful test driving your truck to set the vacuum modulator and VRV. If you can't easily dial it in you can torch the transmission.

 

You don't need 19inhg, but that really helps. The bigger problem is the driver's elevation. You could have a great vac pump at sea level putting out 20inhg, but at colorado altitude putting out 13inhg.

Ultimately, you need at least 13inhg to be the BARE minimum before you'll notice shifting issues thru the VRV.