C4 – Engine stalling in reverse or drive

[Greg]

I recently bought C4 transmission to replace the one i have on my 302w motor. My old one has a broken low reverse band. Otherwise car was running fine and was shifting okay with the old box. Will try to get the old one fixed up later. The replacement has come with its own torque converter. The replacement transmission is identical to my old box.

I have installed the transmission to together with its own torque converter to the existing flywheel. Car starts up and idles beautifully.

The issue now is when shifting into reverse or drive or manual gears it takes the shift into gear but within a second the engine dies. Shifting to reverse makes a much louder sound just before the engine dies. If I keep the revs up high and lay of the brake then it slips into gear with the hard clunk sound and stalling the engine.

I have checked for vac leaks and looked at the vacuum modulator. All seems fine. Also had my mechanic check the idle and timing. Engine is running and sounding beautiful. Any suggestions on what too look for next in order to resolve the issue?

Betweem the two boxes I would say they are from the mid into late 60’s or 70’s. Its has a 26/24 spline input shaft. The identifying stamps on the box are as follows.

RF-C4AP-7A105A 4K0A3
C6AP-70027-C
DA0P70006AA

Here is something i checked today Unfortunately i dont have anything to measure the pressure so checked it manually.

Car is idling fine whether idle is set lower medium or high on the adjustment screw. When revving on either idle with the pipe out of the vacuum modulator there is increase of vacuum.

With the pipe out and not blocked off by my thumb the idle is rough but when blocked off goes back to normal idle.

Only things that cannot be checked is the pin on the modulator. It is a threaded modulator so as it stands mounted onto the vehicle it is hard to get a spanner to loosen out the modualtor. The pan and the transmission mounting is on the way. I am assuming it is fine though cause the box as new seals and looks like its has been serviced recently.

Also checked that the oil is being sent through the cooler and back to the box.

Here is the experience when shifting and stalling :

1) In Park the car idles fine.
2) When brake is applied engine does not stall.
3) When shifting into reverse while brake is applied it takes the gear but instantly a loud lockup at the diff side is heard and engine is stalled all happens at once intstantly.
4) Then with engine in neutral I start the car. Car idles fine.
5) Brake is applied while in neutral car does not stall.
6) Shifting into reverse from neutral it does the same as shifting from park into reverse.
7) Starting in neutral. Holding brake trying to shift into drive. Does the same as you would shift into reverse. Engine stalls soon as you shift into gear. Does take the gear but locks up instantly.

I’ve watched Eric’s video on how the torque converter works. He mentioned when the clutch inside locks up leads to the symtoms i am experiencing. I just want input from other peeps here and confirmation that it is before i head off to get another one.

Topic

[Mike]

Try putting your original torque converter in the replacement transmission.

[Greg]

Lol Yeeeesssss Buddy. Guess they dont call you evil for nothing. Before I go ahead after listening to one person’s advice Lol I would love to hear all the other experienced members on this groups comments. Which seems rare here so far. Is the torque or not! That was the confirmation i am looking for. Not ouch my finger just got hurt putting it there. And some comes around and says well put that other finger in there. Well there must be something wrong with the “there” that the finger got put in. If so what is before I insert another finger in there Lol Oh well Nevermind

[Caleb Stovall]

Lol Yeeeesssss Buddy. Guess they dont call you evil for nothing. Before I go ahead after listening to one person’s advice Lol I would love to hear all the other experienced members on this groups comments. Which seems rare here so far. Is the torque or not! That was the confirmation i am looking for. Not ouch my finger just got hurt putting it there. And some comes around and says well put that other finger in there. Well there must be something wrong with the “there” that the finger got put in. If so what is before I insert another finger in there Lol Oh well Nevermind

What?

[Luke]

Keeping in mind that I’m not a automatic transmission guru. To me It sounds like a torque converter, TCC or TCC solenoid issue. Best of luck!

[Author]

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