quick backstory:

we got our 1978 F100 back in February and drove it here and there (short trips) every now and again with no issues from the drive train whatsoever...then about 2 weeks ago me and my wife took the truck for a short cruise and the tranny starting slipping in and out of gear in a parking lot...came home and checked the trans fluid and it was WAY low so we parked it....while parked over the next week it started leaking trans fluid big time from around the pan gasket and a little bit from the bottom bolt where the gear selector attaches(hadn't leaked a drop previously).

So this Saturday we went ahead and dropped the trans pan, redid the gasket,filter, etc....buttoned it all back up, filled with fluid...no more leaking so far, but the slipping in and out of gear is still happening....while your driving down the road in gear at decent rpm's it is fine, but when the rpm's drop or when coming to a stop it tends to come out of gear and then slip back in.

Before I give in and just say the trans/torque converter is hosed and needs rebuilt, is there anything else I can try? Any chance it could be something vacuum related? I'm searching here. LOL....even with gas pedal to the floor it doesn't slip, but at idle it slips a good amount.

 

Typically if its vacuum related it wont shift out of first gear. Slipping is usually due to low fluid. Not to insult you, but did you check the tranny oil when fully warmed up? makes a big difference on the C6 trannys. I recommend at least a 15 min warm up min.

 

I had a similar issue with my C6 a while back. It ended up that the vacuum modulator was bad and replacing it has seemed to fix the issue. I also find that over filling the transmission just a little bit tends to help. When the transmission is warm, the fluid should be at least at the top of the operating range. I try to keep mine a tad bit over the top and have not had the slipping in and out of gear problems since then.

Ultimately, I think that a rebuild is the only way you'll fix it permanently. I'm fully expecting to have to do the same at some point. I can't speak for yours, but my transmission is 45 years old and has never been opened up. There is just a limit to how long a person should expect these things to work right. The good news is that a transmission rebuild kit is not very expensive and these are not very complicated transmissions either. If you do it yourself, you can rebuild the thing for $100-$150. They are also not terribly expensive to have repaired for the same reason. Around here (Seattle) there are guys rebuilding these transmissions for $500-$1000 depending on where you take it provided you carry it in and drop it off vs. them having to remove and install it.

 

Yeah I checked it when fully warm...the insulting part( to the poor truck lol) is that I never checked the tranny fluid after we bought it and got it home...had I checked that and seen how low it was originally these issues might have been avoided....when we drained the pan it only had about 2 quarts in it if that.

 

I don't know if this one has ever been worked on or not...the inside of it looked pretty nice and clean though....no nasty bits or pieces of anything in the pan and the fluid was still reddish.

Was the vacuum modulator difficult to replace? I figure I'll try that first, only due to the fact that the tranny is only slipping in and out at low rpms and idle when the vacuum might not be as strong.

 

Vacuum Modulator is about a 5 minute job. You just need a 1/2" wrench if I remember right, similar to the clamp that holds a distributor on. There is a small rod in the end of the modulator, which you need to put into the new one, then you just stick it in the transmission and reinstall the clamp. It's a good idea to have some new vacuum line while you're down there and replace it as well. Takes about 6 inches or so. Also a good idea to replace it up at the engine and double check that there are no vacuum leaks.

 

Hey, where did you buy your modulator from? Looking online the part #'s and description seem confusing for getting the right one for the C6

 

I got mine at NAPA for my 77..... black stripe.