C-4 Shift issues
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4th Gear
Joined: Nov 2022
Posts: 4
Likes: 1
C-4 Shift issues
I purchased a used “rebuilt” C4 1972 vintage 2W drive transmission, it was missing the vacuum diagram, pin and valve modulator. I purchase a used valve modulator, made a 1.6 inch pin, and purchased 2 vacuum modulators. The transmission goes into reverse no issues going backwards. In drive I believe the transmission is starting in second gear, at 35 mph I am at 3,000 rpm, at 50 MPh I am at 4,500 rpm. I have tried to turn the vacuum modulator screw with no luck. I noticed the valve modulator had a adjustment screw not sure how to adjust it. I have check fluid level which is Ok.
Any advice would be great.
Any advice would be great.
Logistics Pro
Joined: May 2015
Posts: 4,469
Likes: 965
From: Tri Cities, TN
I'm not sure "used" and "rebuilt" can be in the same sentence. The modulator valve controls the line pressure based on load, "vacuum signal", adjusting on it or putting the wrong parts in it without having a gauge on the line pressure port can lead to blown seals. Should be 50-90 psi idling in neutral. If that trans still has an ID tag on it use that to be sure you have the right parts in the modulator.
If you are certain your manual and kickdown linkage are properly adjusted and you pulled the pan and filter to made sure they were clean before you installed it. My guess would be stuck governor or sticking valves in the valve body.
If you are certain your manual and kickdown linkage are properly adjusted and you pulled the pan and filter to made sure they were clean before you installed it. My guess would be stuck governor or sticking valves in the valve body.
Logistics Pro
Joined: May 2015
Posts: 4,469
Likes: 965
From: Tri Cities, TN
One thing I forgot to ask. Will it shift manually, and can you force a kickdown by going wide open throttle?
If it stays in second gear no matter what you do it most likely has a valve body problem.
If it stays in second gear no matter what you do it most likely has a valve body problem.
Thread Starter
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4th Gear
Joined: Nov 2022
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I do not know which way the adjustable pin on the left goes into the transmission. The bottom photo is how I installed based on diagram, non-shifting transmission. The 1.6 inch pin does go into the adjustment screw top photo. Can you provide guidance.
Logistics Pro
Joined: May 2015
Posts: 4,469
Likes: 965
From: Tri Cities, TN
Bottom photo is correct. You know there are two different diameters of the modulator valves, .434 OD, 64-69 and .372 OD. 70 and up. If your trans happens to be older than you think and you stick the small diameter one in it, it would be the same as not having one in it at all.
Hotshot
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From: Phoenix, Az.
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Mountain Pass
Joined: Nov 2021
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From: Ringgold Ga
Know it’s been a minute
but can you remove the plunger part while the trans ISS till in the car, like with a magnet, I want to pull and verify mine is in the correct direction as I have no third. And trans was apart, has new pin and modulator as well as shift kit. After having valve body out and checking everything this is where I’m at, that it may have been put in backwards as I guess that’s pretty common.
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Thread Starter
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4th Gear
Joined: Nov 2022
Posts: 4
Likes: 1
No First Gear
It has been awhile. I removed the governor and valve body. Cleaned both, added a shift kit to the valve body and reinstalled both. The transmission now starts in second gear and on the first first of the day will shift to third at 4K RPM. After the first shift, the transmission will shift at 2,800 rpm into third every time; however it starts in 2 gear. If I move the shifter into the 1-2 gear it starts in second and stays in second. I do not have a throttle kid down linkage. In drive I have a 2 speed transmission with a street strip 2,800 RPM stall converter. Any thoughts?
Logistics Pro
Joined: May 2015
Posts: 4,469
Likes: 965
From: Tri Cities, TN
There are several things that can cause your problem. I think your main problem is you purchased a "used" supposedly rebuilt trans. Aside from that the number 1 reason for starting in 2nd is governor related. Now that could be the governor itself or you could have a broken seal ring on the governor distributor body or leakage in the distributor sleeve. Replacing a governor is easy the rest of that stuff takes a complete tear down to even get eyes on it.
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