C6 Reverse Problem
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Tuned
Joined: May 2006
Posts: 268
Likes: 12
C6 Reverse Problem
I have an 88 F250 7.3L diesel with a C6 transmission. About 4 or 5 years ago it would refuse to back up when I put it in reverse. I took out the valve body, cleaned it and installed a shift kit. It worked fine up until a few weeks ago. Now its doing the same thing, won't back up. The forward gears all work. I pulled the valve body and cleaned it. I also changed the vacuum modulator to a green stripe (couldn't find a stripe on the old one) and I noticed the modulator pin has a white stripe in case that matters. Put it back together and now I can feel it slightly engage to reverse (a little pull) but that is all. Still won't backup. Any ideas or is it just time for a rebuild? The old fluid was pretty dark.
Thanks.
Thanks.
Cargo Master
Joined: May 2001
Posts: 3,208
Likes: 13
Check the fluid pressures. Port is on the left side by the shift linkage.
Nuetral---engine idling--brakes applied---normal oil pressure 50-80 psi
Drive, 2 or 1---engine idling---brakes applied---normal oil pressure 50-95 psi
Reverse---engine idling---brakes applied---normal oil pressure 60-150 psi
The c6 is notorious for low / reverse piston seal to wear and leak. causing slip / loss of reverse due to low oil pressure. so check your oil pressure.
Nuetral---engine idling--brakes applied---normal oil pressure 50-80 psi
Drive, 2 or 1---engine idling---brakes applied---normal oil pressure 50-95 psi
Reverse---engine idling---brakes applied---normal oil pressure 60-150 psi
The c6 is notorious for low / reverse piston seal to wear and leak. causing slip / loss of reverse due to low oil pressure. so check your oil pressure.
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Tuned
Joined: May 2006
Posts: 268
Likes: 12
Thanks, I didn't know about the pressure port. However, I did read about the low pressure problem so when I put in the shift kit a while back, I put in the Sonnax power booster valve that is supposed to fix that. When I pulled it apart this weekend, I cleaned it all inside and out including the boost valve. I might have overfilled the transmission this weekend since I have the truck up on car ramps but wouldn't think that would cause the problem. Thinking I may just go ahead and replace the valve body and do a rebuild on the transmission myself since it doesn't sound like anything easy. I pull an older (heavy) 30ft 5th wheel trailer with the truck.
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