I recently bought this truck, has a brand new 351w crate motor and a c6 transmission. I drove it back and forth to work a couple days, ~60 miles round trip and noticed it was losing a lot of transmission fluid, it was leaking where the transmission and transfer case mate. So I topped it off one last time and drove it home. I should note that I was topping it off with Type F fluid, because I was unaware that after 1977 apparently the c6 needs dex III/Mercon but I've read that Type F will not hurt it.

Anyways, I pulled the transfer case off, and replaced the input shaft seals on the transfer case. I think the main cause of my leak was that whoever messed with it before didn't have a gasket at all between the transfer case and tranny. So I made one with black RTV. I let it cure for a day just to be safe, topped off the transfer case, and the transmission because I'd lost fluid on the last drive home. This time I used Dex III/Mercon. After I got everything topped off I was excited to go for a test run, seemed fine until I got up to about 20 mph and the transmission wasn't engaging third and the RPMs just rose like it was in neutral. I dropped it into 2nd and it was fine, back to drive, nothing, third doesn't engage. What the hell? It was perfectly fine before and I didn't touch anything on the transmission other than to fill it. I've checked the fluid level a thousand times an it's right on the money when warm and in park. I've read that if it's the clutch pack, then reverse would be out and it's not.

The vacuum modulator on the transmission isn't connected to anything, but it's been like that the whole time and was fine. Would the vacuum modulator not having a vacuum source cause this problem? If so why would it suddenly show up?

I'm about to call a transmission shop tomorrow, but I'm really dreading it. The symptoms of a bad modulator don't seem to be what I'm having, but I might go get some vacuum hose and hook it up to the carb just to see. Whoever put this crate motor in has the vacuum port on the carb plugged off where the modulator would go, but like I said, it was shifting fine before. Would some Type F mixed with the Dex III/Mercon cause this? I'm going to flush it all out soon but I just don't see why they would immediately cause this problem.


Not sure if it's relevant, but I adjusted my idle on the carb as it was a bit high for my liking, A/F mix should be fine, thing is with my vacuum gauge hooked up I could never get the vacuum higher than 15, I think the ignition timing is off a bit. I had already called a guy to get him to adjust the timing tomorrow, but that was right before I realized the tranny won't shift to third and I'm not driving all the way in 2nd gear.

 

When the vacuum modulator is not connected the transmission should shift at the WOT shift points. So to shift 2-3 you'll need to get the engine speed above about 4500 RPM in second gear. You're probably not going fast enough for the 2-3 shift. Connect the vacuum modulator to engine vacuum.

 

Thank you for the response, I plan on getting a new modulator just in case and some vacuum hose tomorrow and checking it before I call a shop. But like I said, it's just strange it would shift fine before I fixed the leak when it was disconnected then too. I haven't tried getting the engine speed all the way up to 4500. When I had it in drive it was like after second it would slip into neutral or something and then engage second again if I gave it enough gas. I'll reply again when I hook up the modulator.

 

Ok so the issue seems to be resolved. This morning I took it for a test run and floored it till I got to like 5k rpm and it FINALLY shifted into third. I went and got some vacuum hose and a new modulator just in case after pulling the old one. I ended up finding the vacuum line that would have gone to the modulator tucked up on the frame out of sight. I pushed the pin in on the old modulator and fluid came out of the other side so that's why I think it was bad. I'm guessing maybe the previous owner started sucking up tranny fluid and just pulled off the line? Because I seriously don't remember doing that when I dropped the transfer case. It's still strange that it was shifting fine before doing the transfer case seals.

Anyways, thanks a lot. New hose+modulator fixed it and it's shifting fine now. It's also running smoother due to the vacuum leak caused by that hose being disconnected