Topic: C4 wont change when cold

[Tiny]

Just had my C4 fully rebuilt but since that time I have had this problem.  I put it in Drive (Large Dot) stays in 1st for a couple of blocks even up to 30 mph.  Wont change.  To get it to change into 2nd I flick the lever up to the small dot before it finally shifts into 2nd but not change into 3rd.  Only after it has warmed up considerably will it then change into 3rd. After that I put it back into the Large dot and it then performs flawlessly all day. No problem at all with upshifts or downshifts.

It all occurs when the car is cold, but once it is warmed up everything is fine.  Not sure what it could be.  Someone has suggested it is the Modulator valve which is new.

Any ideas.

[OldGuy]

I think that the suggestion that the modulator valve is faulty has merit. Have you checked to see if you have any vacuum leaks at the modulator valve connection (curved hose) or any where else that is associated with the manifold-vacuum-line-to-modulator-valve plumbing? Often times the curved hose becomes "spongy" from the presence of oil and can cause a leak.

You can also check to see if the diaphragm in the modulator valve has a hole in it by connecting a hand-held vacuum pump to the modulator vacuum line at the intake manifold connection and pulling a vacuum with the pump. If you don't get a measurable vacuum reading while performing this function OR the vacuum bleeds off (back to atmospheric pressure) rapidly, then you have a leak some where in the circuit.

I hope that his info. helps.

Frank

[Tiny]

Great thanks Fran.  Just a question, if the vacuum hoses check out okay I might put my old modulator back on.  When removing the modulator do you tend to loose a large quantity of fluid?

[OldGuy]

You will loose some fluid while changing out the modulator. I've found that you can minimize the amount by jacking up the rear of the car so that the car slopes down in front. Higher is better! Be careful to retrieve/re use the actuator rod that is in the throat of the modulator. You will need a thin opened-end wrench for the operation.

I hope that changing out the modulator helps to cure the issue. It's certainly a place to start and involves minimal effort.

Frank

[Tiny]

Thanks Frank, will let you know how it goes in a few days

[67gtasanjose]

Be careful to retrieve/re use the actuator rod that is in the throat of the modulator.

Make sure there is one in there too!

[Tiny]

Not sure what you meant 67gtasanjose, 'make sure there is one in there too........

Do you mean actuator rod?

[MattDoscher]

Yes, the actuator rod.  They will sometimes either fall out onto the ground or remain stuck inside the tranny.  It could also be too short or too long.

Matt

[67gtasanjose]

Not sure what you meant 67gtasanjose, 'make sure there is one in there too........

Do you mean actuator rod?

YES!