C4 shifting too fast
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Postmaster
Joined: Apr 2004
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From: Santa Clarita
C4 shifting too fast
I have a C4 transmission in my truck. When I cruise in it, the transmission shifts into second gear really quick. Is there anyway of delaying the shift from first to second, like the modulator valve? or maybe a problem with the vacuum? It's really annoying. 
Postmaster
Joined: Jun 2004
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From: Riverside, So Cal
Ed -
I had the same complaint about my FMX. My tranny guy showed me where and how to adjust the shift point at the modulator valve. When that didn't work, I traced the problem to a leak (break) in the vacuum line from the manifold to the modulator. Sounds like a vacuum may be your problem, if it ever shifted right.
What he showed me was to pull the vacuum line off the modulator and turn the screw (deep) inside with a small flat blade screwdriver. (Push the vacuum line back on). Once my vacuum was restored, I dialed it in perfectly using the time honored trial and error method. Screw it in = shifts later, out = shifts earlier (I think). I am assuning that a C4 works/looks the same as my FMX.
Big help, huh?
I had the same complaint about my FMX. My tranny guy showed me where and how to adjust the shift point at the modulator valve. When that didn't work, I traced the problem to a leak (break) in the vacuum line from the manifold to the modulator. Sounds like a vacuum may be your problem, if it ever shifted right.
What he showed me was to pull the vacuum line off the modulator and turn the screw (deep) inside with a small flat blade screwdriver. (Push the vacuum line back on). Once my vacuum was restored, I dialed it in perfectly using the time honored trial and error method. Screw it in = shifts later, out = shifts earlier (I think). I am assuning that a C4 works/looks the same as my FMX.
Big help, huh?
Last edited by Randy Jack; Mar 20, 2005 at 10:32 PM.
Postmaster
Joined: Dec 1999
Posts: 2,813
Likes: 6
From: Connecticut
Usually if a vac line is leaking or cracked, they don't shift. Check it anyway as they dry out over time. Make sure it is ok at the modulator end. THat's where they go bad the most. That elbow at the end.
Barry
50 F-1
Barry
50 F-1
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Postmaster
Joined: Apr 2004
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From: Santa Clarita
Thanks guys. I think I might just replace the whole the vacuum line anyways, just to make sure. So Randy, how many times did you have to jack the truck up, turn the screw, test drive, come back jack the truck up, adjust, you get the picture, till it was right? LOL I was wondering if I can just screw it in a little and leave it? Would that work? I hate crawling under the truck if I don't have to. I'm not as slim and trim as I use to be. LOL
Postmaster
Joined: Jun 2004
Posts: 4,190
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From: Riverside, So Cal
Ed - I'm not particularly slim either. I had to go under my (low) truck probably 6-8 times. I'm a slow learner. However, I didn't have to jack it up to get inder it. My Kamikaze driveway comes down a steep hill to the garage. Right in front of the garage, the driveway goes back up as an apron to the garage. I park the truck with the front wheels on the drive and the back on the garage apron and scootch under the running board to adjust it. It's the only time I am glad I have that cockamaymie driveway. LOL.
Cargo Master
Joined: Jan 2005
Posts: 2,497
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From: Tucson AZ
you didn't mention if this was a rebuild or not, but make sure the modulator you have has a white stripe there are others that fit but the range of adjustment is different. Randy is dead on about the high tech fine tuning procedure but I think the baseline is bottom it out and return 1 full turn
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