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Old 11-22-2019, 02:17 PM   #1
poolplayer1
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Default 1955 station wagon with C4 tranny

I was just wondering if anyone here has any experience with the C4 tranny.I seem to have a problem with mine. As soon as I take off and within a few minutes,the tranny will kind of kick and than shift,like it holds back and than will shit. It does not have a smooth shift. I had it completely rebuilt about 25 yrs ago and i only have about 1000 miles on it since it was parked most of the time. Since i retired in 2015,I have been driving it on and off and it was always shifting nice and smooth. When I rebuild the 302 engine earlier this year,I also removed the tranny and had a transmission shop here in town reseal the tranny cause the tailshaft seal was leaking. The guy there said that the clutches were kind of worn out and needed to be replaced but I told him that the tranny had everything replaced (other than the body itself). What do you guys think may be the problem? Could it the modulator causing this problem? Thanks for your help.
I know that if I take it back to the same guy,he is going to say that it needs rebuilding. Its hard to find honest people anymore.
PS, Its full of fluid.
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Old 11-22-2019, 02:25 PM   #2
40cpe
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Default Re: 1955 station wagon with C4 tranny

It is possible that it is a vacuum or modulator problem. The shift points are adjustable by adjusting the modulator, but that shouldn't be a factor if it hasn't failed or been tampered with. Check your vacuum hoses and connections first. Of course, since the vacuum comes from the engine manifold, the engine vacuum should be healthy.
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Old 11-22-2019, 03:55 PM   #3
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Default Re: 1955 station wagon with C4 tranny

Thanks for your reply. Yes, the vacuum line is connected to the intake manifold. I will check to make sure that I have good vacuum there. I remember the tranny guy said that he had removed the tail shaft so he probably removed the modulator too but I don't see how by doing that,it could of messed something up. Will check some things and than post again.
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Old 11-22-2019, 07:55 PM   #4
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Default Re: 1955 station wagon with C4 tranny

I have been rebuilding these units since 1971 so I have some knowledge of them. First, yes the modulators are adjustable but there are some limitations. If the modulator has an adjustment screw, it can only be safely turned in or out about two turns from flush with the end of it's tube. Any more and it can break free from the diaphragm and can act erratic. The adjustment is really there to make small pressure adjustment in line pressure when checked at 10 inches of vacuum. We all have used it to change shift points but that was not it's purpose. Second, is it a screw in modulator or push in with a clip on one of the extension bolts? Next, what length pin is installed between the modulator and the throttle valve? Is the kickdown linkage hooked up and adjusted properly? Is the manual selector linkage adjusted so that the manual valve is correctly positioned? Is the fluid between add and full when the unit is fully warmed up (10 miles of driving)? Any signs of air bubbles in the fluid? "T" a vacuum gauge into the modulator line down at the trans and drive the unit an observe the vacuum at idle and cruise. Any fluid coming out of the modulator or hose, water or ATF? These are all things to check out that you should be able to do. I can get more specific after you check these things out.
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Old 11-23-2019, 02:07 PM   #5
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Default Re: 1955 station wagon with C4 tranny

Thanks flatheadron,I will check all this things that you mention here.I should know more on Monday when i get under the car and check things out.The modulator is a push-in type and not the screw in type. I did check the fluid after I drove it for about 15 miles and it checked just a little under the full mark,so that should be ok. The kickdown linkage is not connected at all. The modulator does have a small screw that can be adjusted but I have never done any adjustments and the tranny was shifting really good.
Question: If I have to replace the modulator,when I remove it from the tranny,will I loose alot of fluid or not enough to worry about it?
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Old 11-24-2019, 10:01 AM   #6
flatheadron
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Default Re: 1955 station wagon with C4 tranny

You will lose some oil so place a pan under the trans to catch and reuse if clean. Also, if you have a mic or caliper, measure the length of the pin that's installed. There were five lengths available. Check to see that if you pull downward on the kickdown lever, that it springs back up to ensure it's not stuck.
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Old 11-24-2019, 12:16 PM   #7
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Default Re: 1955 station wagon with C4 tranny

Ok, thanks again.I will do all that on Monday and will let u know.
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Old 11-27-2019, 04:13 PM   #8
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Default Re: 1955 station wagon with C4 tranny

After checking all possible things affecting the vacuum modulator and not finding any problems, for example, vacuum line from intake to modulator is new,i have 15psi of vacuum to the mod.,I decide to replace the modulator. On the first test run,it would change gear real soon. Drove it back home and adjusted the little screw on the mod.1.5 turns clockwise,Took it for a test run again and this time,it changes gears just like it was before,nice and smooth and at the right time. Thanks to you guys that gave me advice and guided me in the right direction.Apreciate your help very much.
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Old 11-27-2019, 04:17 PM   #9
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Default Re: 1955 station wagon with C4 tranny

that's good news. thanks for letting us know the outcome.
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Old 11-29-2019, 09:41 AM   #10
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Default Re: 1955 station wagon with C4 tranny

It's the little things that get you sometime. Glad that things turned out ok and it was something simple.
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