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vacuum modulator C4

7821 Views 11 Replies 6 Participants Last post by  camachinist
Hi Guys,

I have a question about the vacuum modulator used in a C4. I wanted to adjust it a little but when removing the hose i saw fluid(ATF) in the hose and the modulator. Is this allright or is this trouble..

It was rebuilt about a year ago...

Thanks in advance
"The Netherlands"
1 - 12 of 12 Posts
The hose should be dry. Replace the modulator if there is ATF in the hose.
This means your modulator is shot. ATF is sucked into the hose by the manifoldvacuum because the membrane of the modulator is broken.
Vacuum modulators are cheap (even here in the Netherlands) and easy to replace. Good luck....
Is sounds like a leaky diaphram. I would change it, they are under $20 for a new modulator. A leaky diaphram will not work well and suck trans fluid into manifold.
The modulator diaphragm is probably bad....if you've had some ATF consumption recently or untraceable smoking, this might be it...

When removing, be careful to note and retain the throttle valve actuator rod which inserts into the modulator; the throttle valve won't work without it...

Make sure the replacement modulator is adjustable, if you like tweeking shift points and quality...
Last time my father drove behind me and he said when i pulled fast there was sometimes some kind of white smoke. I am going to buy a new (adjustable) modulator. Strange they didn't replace it at the rebuilt shop..
Before the rebuilt i tried to remove it also but couldn't get it unscrewed.. Any tips??

"the netherlands"
I think you need a real thin wrench to get in there to remove it. Go to a bicycle shop to buy the tool.
It can get pretty tight from years of use...also, depends on whether it is threaded in or held in by clamp (depends on year of trans)...

Examine and try other suggestions and get back to us if you can't budge it...if screwed, it loosens by turning CCW as any normal fastener...
it is a screw in type.
I will make some tool to get it removed. It must be a little thin though because of little space..

I think i have to flush the oil too don't i??

"The Netherlands"
If you noted debris in the oil or a strange color, flushing would be helpful.

As was mentioned by another poster, a thin wrench (I just ground down an old one) is used to remove the modulator...

Remember to be careful of the thottle rod (little rod which inserts into modulator and actuates throttle valve) when removing and replacing...sometimes a bit of grease is helpful in holding it in place..

Let us know if you have any problems...
What i actually meant with flushing is: is it needed to put the oil out of the trans to remove the modulator? So i don't get my garage flooded with ATF?

Thanks in advance
"The Netherlands"
The throttle valve has tight clearances and you should only have a minor seep...just put a drain pan under the area to protect your floor. If it does leak more, the drain pan will catch the fluid while you're doing your work. Better than pulling the pan, eh? *G*

After replacement, run engine to warm fluids and check trans level per normal, filling as required...
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