Vacuum Modulator Unadjusting Itself?
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5th Wheeling
Joined: Aug 2020
Posts: 44
Likes: 0
From: Louisville, Kentucky
Vacuum Modulator Unadjusting Itself?
Hey everyone,
Quick question, pretty much as the title says. I have a '79 F100 with a '68 C4 in it, and lately I've been having an issue with the vacuum modulator on my transmission. Every fifth or so time I go to drive my truck, once I put it in drive it takes about three seconds to engage, and it hardly accelerates, I just get a lot of revs. When this happens, I have to get under the truck, take the vacuum line off the transmission, and screw the adjustment screw back in on the modulator, then it will finally drive like normal. Sometimes the adjustment screw is so far out I can wiggle it by hand, and almost take it out without unscrewing it. Is there an internal problem with my transmission, a faulty modulator, or do I have a vacuum leak somewhere I'm not seeing?
Quick question, pretty much as the title says. I have a '79 F100 with a '68 C4 in it, and lately I've been having an issue with the vacuum modulator on my transmission. Every fifth or so time I go to drive my truck, once I put it in drive it takes about three seconds to engage, and it hardly accelerates, I just get a lot of revs. When this happens, I have to get under the truck, take the vacuum line off the transmission, and screw the adjustment screw back in on the modulator, then it will finally drive like normal. Sometimes the adjustment screw is so far out I can wiggle it by hand, and almost take it out without unscrewing it. Is there an internal problem with my transmission, a faulty modulator, or do I have a vacuum leak somewhere I'm not seeing?
Tuned
Joined: Jan 2019
Posts: 364
Likes: 91
I would say that the mod has more effect on shift timing than what your problem is, Once the trans engages does it work OK the rest of the drive and the no engagement thing is only on startup? If so,the converter is draining back and needs to fill before engagement.That said,stick a new mod on it and see what happens.
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5th Wheeling
Joined: Aug 2020
Posts: 44
Likes: 0
From: Louisville, Kentucky
I would say that the mod has more effect on shift timing than what your problem is, Once the trans engages does it work OK the rest of the drive and the no engagement thing is only on startup? If so,the converter is draining back and needs to fill before engagement.That said,stick a new mod on it and see what happens.
Laughing Gas
Joined: Nov 2013
Posts: 1,207
Likes: 60
From: S.E. Louisiana;so far USA
Once I screw the adjustment back in, it works great, sometimes for the rest of the day, sometimes for the next three days, it's hard to tell when it's gonna unadjust itself. Sometimes when I start it then put it in drive, I hardly have any drive at all, it just coasts forward and I get a lot of revving. I do have reverse, and manual 1 and 2 when the screw is out too. I have a new modulator, I'll put that in and see if that helps
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