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Ford f-100 transmission problem

Home > Discussion Forum > Mechanic's Corner - Transmissions

Email Author email sambo  Reply to Message reply to message  Post New Message post new message      search forums
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Ford f-100 transmission problem sambo 11-18-03  
I have a '79 Ford F-100 Custom. Problem is, upon cold-start, it won't leave low speed. If I pull over, or let it warm up for about 10 minutes, it usually will shift fine. I just changed trans. fluid and filter (I did not drain torque conv. or cooler as fluid looked ok in pan). When I dropped pan, I noticed a screw that had held some kind of shifting arm that came down out of trans. had stripped out and was rolling around pan. I replaced it with a self-tapping screw a little bigger. Fluid looked OK and filter not too bad, no burnt smell in fluid. Any suggestions??? I like to tinker with this truck and I hate to change trans. if it is not necessary. Oh yeah, it's a 302 engine with a C4 trans. I think, I'm not 100% sure on transmission, but I think its a C4...Thanks
Re: ford f-100 transmission problem Phil Feinstein 11-19-03  
The C4 has a vacuum modulator on the back corner that was notorious for its diaphragm ripping. Pull the vacuum hose off the back of it and see if the hose has trans fluid in it. If it does, replace the modulator. The modulator opens and closes the accumulator circuit and with the diaphragm broken you have less line pressure than what you need to shift.

Hope that helps!
Re: ford f-100 transmission problem sambo 11-19-03  
Thanks for advice, I checked and the modulator is there right above the filter and pan. When I pulled the hose off, it was dry as a bone. Should I still suspect this. It seems strange that the truck runs fine sometimes and sometimes it don't. I even thought about sensors, but I don't know if these old trannys have sensors or not or even where to look.
Re: ford f-100 transmission problem Phil Feinstein 11-20-03  
If the hose is dry, this might not be the problem. The thing to do to clear this is put a vacuum pump on the end of the diaphragm and see if it holds vacuum. If it does, it's probably OK. You should also check for vacuum while the engine is running at the hose that connects to the modulator. No Vacuum could cause your problem.

The C4 is a purely mechanical trans, just like the C6. I don't think there's much of anything in the way of electronic controls under the hood of your truck. The fact that it starts working normally after warming up says to me something is sticking and the trans fluid loosens it. The piston that the modulator moves could be sticking in the valve body and comes loose after warming up. If the modulator and the vacuum hose to it are both good, this is the next thing I'd check.

Hope that helps!
Tranny in 77 ford john kilpatrick 11-28-03  
I have a 77 ford f100 explorer edition and it originally had a 6 cylinder and a heavy duty tranny and i put a 302 and a c4 tranny in it and cant get the transmission linkage to match up and i really dont want to put in a floor shift. can anybody help?
Re: ford f-100 transmission problem rob 9-16-05  
Location of vacuum modulator on transmission f100 1980 truck
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