C6 Issues
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New User
Joined: May 2006
Posts: 11
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C6 Issues
I have a 77 F150 4X4 with a 400 and a C6 that was seating for the last month while I was out on a training Op. Before I parked her, she was running out fine but when I got into it yesterday and drove home she was holding each gear longer and was really firm/hard on the shifts. This morning warmed her up and checked the fluid level, just fine. I jump in her to run some errands she was still holding 1st and 2nd too long with really hard shifts and wouldn't go to 3rd unless I dropped it 2 then back to D. I was wondering if anyone has had anything like this happen to them and if any of y'all had any solutions that I could take care of at home? I am thinking of dropping the pan and changing the filter then filling her with new fluid along with a quart of Lucas trans. treatment. This is something I have been planning since I haven't had that long and am not sure the last time that the fluid was changed. Any help y'all have is greatly appreciated.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Check the vacuum line that runs to the vacuum modulator on the transmission. It is usually a 1/4" hose that runs from the vacuum tree on the back of the intake manifold. Usually it starts out as a rubber hose, then is connected to a steel line that runs down past the exhaust, which then connects back to a small length of 1/4" hose that finally attaches to the mod. itself (passenger side, just behind the pan). It is very possible the hose dry-rotted while the truck sat. The diaphragm in the mod. could have ruptured as well.
Elder User
Joined: Dec 2007
Posts: 739
Likes: 1
C6 modulator is very easy to replace, and not that much $$. Most likely? Rubber hose or the steel line has a few pinholes leaking vacuum and causing the valve body in the C6 to think you're always on heavy accel.
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Elder User
Joined: Dec 2007
Posts: 739
Likes: 1
Pretty much the same thing. If it has the modulator valve? Any break in it's design would create hard/late shifting.
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i ain't rite
Joined: Mar 2005
Posts: 65,556
Likes: 5,590
From: Marlboro Mental Hospital.
your diesel sounds like a bad vacuum pump, or an improperly adjusted V.R.V.
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i ain't rite
Joined: Mar 2005
Posts: 65,556
Likes: 5,590
From: Marlboro Mental Hospital.
the V.R.V. is the vacuum regulator valve. it is on the driver side of the injector pump, and controls the vacuum signal to the modulator valve. it very rarely goes bad though. check for vacuum at the booster, or fender canister. if you have low vacuum, recheck it directly at the pump to rule out a leak. you will need at least 17-19 inches of vacuum for the trans to work properly. you can have good brakes with as little as 12 inches of vacuum. this is the reason most people don't realize the pump is going, cause there are way more stick trucks than there are automatic trucks.
when i did the stick to C6 swap in my 88, i went nuts for 4 weeks trying to get the trans to shift properly. running up to 2500 rpm, and then slamming into the next gear, or not shifting at all till i let off the throttle. i finally figured vacuum pump and tested it. it was only making 13 inches of vacuum. i put a new pump on, and all the trans problems went away with the now 22 inches of vacuum.
when i did the stick to C6 swap in my 88, i went nuts for 4 weeks trying to get the trans to shift properly. running up to 2500 rpm, and then slamming into the next gear, or not shifting at all till i let off the throttle. i finally figured vacuum pump and tested it. it was only making 13 inches of vacuum. i put a new pump on, and all the trans problems went away with the now 22 inches of vacuum.