Let me start off as saying I was a Ford tech for around 4 years and certified in just about everything except transmission (never got to it). I still worked on them, just not much. As for my issue - I bought a 86 f250 with a 6.9l and a c6 yesterday for 700 bucks which is a complete steal around here. He was selling it cheap because "the transmission is going out" (he was unsure but it has issues). When driving in first, it runs great with no issues. The problem is 95% of the time it will not shift into second gear and thus will not get to third so it winds up the engine to max rpm and then just holds there. I revved it out because diesel 6.9 wont be hurt by it and to see if it would shift late. If I let off the throttle and coast, it will sometimes go into second but the second I give more than 1/16 throttle it goes back to first. It is also very hard to tell if it is shifting what so ever. I, don't believe it is second slipping because I can manually shift to first and most of the time it doesn't downshift making me think it is still in 1st. Some of the time (like if I was rolling down a hill so it went in second) I can shift into first and it will down shift. The truck came with a second transmission in the bed but I'm unsure if it is a diesel c6 or a gasser c6 so I don't want to plan on just swapping transmissions. From the little experience I have with c4/c6 transmissions, I plan on replacing the modulator tonight and checking the idle vacuum at the modulator and adjust the vcv if need be, but from the old plug it with your finger test, it seems to have decent vacuum and all the lines seem to not be rotten. I already disconnected the kickdown as that was the first thing I suspected. I was also planning on adjusting the second gear band. Is there any other simple things to try before rebuilding it? The governor being bad would cause early shift if I remember correctly and the servo being bad would cause it to slip second.

 

That variable vacuum thing mounted on the injection pump lever is known to go bad once in awhile. The problem is, I do not think they are available anymore and the aftermarket is not making them. Some people have toyed with the idea of going to a manual valve body. If you have a vacuum gauge, I think you can monitor the vacuum going to the modulator as you open and close the throttle. It needs to smoothly change up and down as you work the throttle. You can also check to make sure you have plenty of vacuum. I caught mine in time, but a lot of trucks have the main vacuum line from the pump circling around the pass side inner fender, where it rubs all the time from the engine shaking. Mine hadn't worn through but it would have if I hadn't moved it and tied it up. Run your hand along this hose and see if it has a flat spot or a hole in it.

 

I'll check vacuum tonight and see. I lost my vacuum gauge and pump so I'm waiting for amazon to deliver one. I will post back with results

 

I think I've posted this before with the same caveat about knowing nothing about this part but for what it's worth: New ?IDI-Online? C6 VRV (Vacuum Regulator Valve) Progress. ? IDI Online

 

I'm checking that tomorrow. As for today I replaced the modulator and found the old one and my "known good" one were junk. It worked better after, but all this did is point me to a new problem. I can more reliably get it to shift into second, BUT now second gear is pretty much neutral unless I rev it up to like 3k (tach not working). So now I'm thinking servo as the band didn't snap. If that servo is bad, do you think the band is toast?

 

What does the fluid look like? I have tried to kill a c6 several times, and just changing the fluid fixed it. I was towing and it turned a very dark red to almost brown. I turned the engine by hand till the drain for the torque converter came around to the bottom, and drained it and pulled the pan. It took about 13-14 quarts. Fixed it right up till later I did it again. Changed the fluid again and this time I added a aux cooler in front of the radiator, and it never did it again after the cooler install..

 

Fluid looks fine. A little darker than fresh fluid. Nothing near what I saw on just about every vehicle I worked on at ford. Tiny bit of burned smell but nothing I would worry about.

 

The governor can get sticky from lack of use, or even when in use sometimes...........& can affect both up & downshifts. Driving in reverse flushes the tailshaft housing (& governor) with fluid. I have needed to do this with two different C6's, & in both cases it fixed the problem.

I just used an empty piece of quiet road & drove quickly in reverse for about a mile. You could put the rear axle up on stands to do the same thing.
I don't know if this will fix your issue, but it will cost virtually nothing to try.

Also, the gas vs diesel version C6's have different bell housing bolt patterns.......you can probably compare by measurement to see what you have.

 

I have read about cleaning the governor but no one had mentioned driving in reverse. Everyone was talking about removing it and cleaning it. Maybe I'll try that but I'll have to use jack stands. Does idle work find or do you need to have some rpm behind it? I may end up just rebuilding this thing anyways. For 250 bucks and a day of labor I would have a brand new transmission with a shift kit and a nice servo. I also did find a new old stock VRV I may get just because, but it is 250 bucks.

EDIT: As for the bellhousings being different, I have read you can make a gasser c6 work if you enlarge the bolt holes a bit. Although idk if I want to try that, especially when it costs more for a used trans than for me to rebuild mine.

 

I don't know for sure.......but I reversed enthusiastically both times & it was successful

 

Ken ... What issue did the reverse drive fix ? Slipping or not shifting ? My C6 takes off in first fine but when it shifts to 2nd it feels like a downshift and putting it in 2nd to move feels like a brake is on (it rolls freely in neutral) ... should I try your reverse trick or do you suspect something else ?

 

So finally got my new vacuum tester in. Pulled the vrv off to bench test (I don't have a .528 spacer anyways) just to see how it's doing. It's completely shot. I can pump that thing by hand as fast as I physically can and I can only get up to 5 in of vacuum. Can't even get to 20 to test it as it won't hold vacuum for more than .2 of a second. Pulled it apart and found it to be leaking internally. I just ordered a brand new one (new old stock) to see what that does. In the mean time I'm going to adjust the band to spec and probably clean the governor and pull the Piston out to check it. If the Piston is shot I'll throw in one of the c6 r ones off Amazon. I'm thinking I have 2 issues. Vrv won't put it in second, and the band or piston is out of spec cause second to slip. If those don't fix it, then I can just leave the new parts in and throw a 100 dollar rebuild kit at it. Anything else you'd suggest before rebuilding? Local shop wanted 2 grand for a rebuild and I really don't feel like doing a rebuild myself but I'm out of options on that.

 

Governor would be a shifting issue from what I understand on these transmissions. Yours sounds like something in the second gear is bound up if it's bragging. Maybe the band broke loose and is cocked sideways? I'm not sure tbh

 

I didn't have slippage or drive problems at all......but before flushing the governor (reverse driving), the trans would not consistently downshift as the truck slowed to a stop, e.g. at lights or an intersection; then would start off in 2nd.
There was no problem with downshifting under load, or when passing etc.

The downshifts as you come to a stop aren't very noticeable unless you are watching for them, but it was pretty evident when starting off again if you were still in second gear.

I had two C6 trucks that did this on occasion, & the 'reverse drive' fixed both.

 

Well I just did around 10 25 mph reverse pulls about 150 feet long up my street. I'm waiting on my new vrv, so I'll see when that comes in. Definitely seems to have helped second thought when I manually out it in second, but without the vrv or a manual valve body I can't grab third gear