C3 Vacuum Modulator Assembly

[Bill K.]

My 1972 corvette has an Turbo Hydra-Matic 400 transmission that shifts to early and will not down shift to pass on the Highway. It has been determined that the Vacuum Modulator Assembly is either the wrong one or is bad. Can anyone tell me which Vacuum Modulator Assembly (color of the strip) I need?

[Roy B.]

Re: C3 Vacuum Modulator Assembly

If it shifts to early it's the Modulator or you are leaking vacumm or none going to it. Any Car or Corvette unit of that year 400 will work.

[Joe L.]

Re: C3 Vacuum Modulator Assembly

Bill----

The vacuum modulator assembly which you require was last available under GM #8629941. Unfortunately, this piece is GM-discontinued. It might still be around in dealer inventory, though. However, although it would be 100% functional for your application, it is configured completely different than the original modulator used for your car which was GM #3963812.

I don't know what, if any, color code was used for these valves. You should be able to use the above-referenced part numbers to obtain a correct SERVICE valve from an auto parts store. I feel pretty confident that functional replacements for these modulators are available in the aftermarket from manufacturers like Standard Automotive Products or Borg Warner.

[Joe L.]

Addendum

Bill-----

The NAPA part number for the modulator for your application is ATP-17499. This is an adjustable-type modulator and has a RED stripe.

[Bill K.]

Re: Addendum

Thanks Joe