68 c4 reverse problem when hot

jerthemost said:

I recently bought a 68 mustang coupe. it has auto (i assume c4) ps, pb, 289 2v. the tranny was supposedly rebuilt just before purchasing. when i start the car after it sitting overnight it goes into reverse just fine but after i drive a few miles it will only go in reverse if i rev the engine several times. it will then suddenly jump into reverse and then back up ok. so it only does it when it gets warmed up. all of the other gears work fine all of the time. any ideas would be appreciated.

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does your shift "firmness" also change when it gets warmed up?

it almost sounds like a pressure issue to me, once your fluid gets warmed up, the pump can't provide enough pressure (hence having to rev it some and build up pressure for it to kick in). I'm trying to remember exactly the valve layout on the C4's......its been a while.


also i wouldn't be surprised if its something in the Vb.....trust me.... about 2% of the ppl that work on ANY of the VB's get it right the FIRST time....then about 40% the second time....If your working on an AOD have fun

I'm guessing you're either low pressure (like mentioned above) or there's a sticky valve somewhere, although this wouldn't explain the hot/cold issue.

jerthemost said:

thanks for your response. the tranny is not overdrive. what are you refering to by "vb"?

would you think that the pump would be part of a rebuild? or is there any way to test it you know of?

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i know a C4 has no OD in it...i mentioned the AOD because the're notoriously more painful to work on its Valve Body (or "vb") than a C4 is. they are however almost the same transmission, essentially the C4 is just missing the front OD clutches and such...some other stuff too...anyway....

The pump should have been replaced when they did the rebuild. Also have you ever put a pressure gauge on it and measured what kind of PSI your seeing at idle?

Although after i made my first post...I started to second guess my original assumption of the pump being bad or i (think) you would see more issues with the tranny. After thinking about it, I'm wondering if the valve for your reverse band isn't a little sticky yet, how recently was it rebuilt? miles....etc. since the rebuild. That would attribute to why it would just suddenly "jump" into reverse. I didn't put the tranny together and I'm assuming you didn't either??? if so, you'd know how tight that valve was. could have a bad seat on the seals or something thats causing it to hang.

there's lots of stuff that can go south on a tranny when you start digging into them. I've watched an older gentleman turn a non-working C4 into a functional one once more simply by moving a ball in the VB...lol...First thing i would do is try to get ahold of a known good pre-assembled valve body and swap it in, if you see the same issue....at least its not the complicated part in the tranny thats bad!!! otherwise...well...then your problem is fixt!