C4 won't shift

[Puff Daddy]

So, today I was running around town. Stopped for about an hour, and when I got back on the road, it won't shift out of 1st gear. I can manually shift to 2nd, and the reverse works fine. First I thought it might be a vacuum problem, but I don't see any evidence of that. Any recommendations?

 

[jeffncs]

Change the vacuum solenoid at the rear of the trans. That’ll directly impact shifting of it failed.

Before ordering a replacement, look at the color ring on the old one where the vacuum line connects. Also check to see if it’s push in or screw in. Beware - it’ll leak of taken out. Push-in will have a little bracket holding it in place.

 

[Puff Daddy]

Change the vacuum solenoid at the rear of the trans. That’ll directly impact shifting of it failed.

Before ordering a replacement, look at the color ring on the old one where the vacuum line connects. Also check to see if it’s push in or screw in. Beware - it’ll leak of taken out. Push-in will have a little bracket holding it in place.

Thanks. I appreciate the response. Is that the same as the modulator valve?

 

[Puff Daddy]

Thanks. I appreciate the response. Is that the same as the modulator valve?

I found it. Blue ring, push in. I assume I need to drain the transmission before changing it?

 

[jeffncs]

No - don’t drain the fluid!

Buy the replacement part and have it ready to go.

Unplug the vacuum line. Take out the bolt holding bracket.

Be deliberate now.

There’s a pin in the center of the modulator on the trans (wet) side which needs to be moved into the new modulator.

Pull the old modulator, move the pin to the new mod. Push the new modulator into the trans.

If you do it quick enough, you’ll only lose a small amount of ATF so have a rag ready.

 

[Puff Daddy]

Also, can anyone tell me exactly each of these hoses are? I'm almost certain at least one is a vacuum hose for the transmission.

 

[Broncobowsher]

I see 2 hoses going to the vacuum modulator. There were 2-hose vacuum modulators that worked with the EGR system. Modified vacuum signals to work with the carb running EGR that doesn't have manifold vacuum at cruise. So both hoses go to the modulator.

Tip: if you pull off a vacuum hose and ATF comes out of it, the modulator is bad and leaking ATF into the engine. Also tends to swell the hoses and then they fall off.

 

[Puff Daddy]

Well this one definitely swelled and fell off. In fact, neither of them are connected. I think I know where the small hose goes, but no idea where the fat one goes.

 

[Broncobowsher]

If you have a single hose modulator (sounds like you do) it is the simplified version. That single vacuum line goes to straight manifold vacuum.
Converting to single vacuum line modulator is fairly common. Especially if you have emissions delete done to the engine.