How long should a Modulator HOLD Vacuum?

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- bigbossfan80
  
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posted
20 years ago

Thu, Jan 29, 2004 2:56 AM

I just bought a MityVac hand-held Vacuum Pump and went out to my car, an 89 Cutlass Supreme, to test it out.

One thing I tested, the transmissions Vacuum Modulator, has me questioning whether or not it's faulty, or leaking etc.

I've had hard upshifts as well as shifts into Reverse for a while, and haven't figured out the culprit yet.

Anyways, I pumped the Modulator with 16" hg to see if it held, and it pretty much started to fall right away. It fell to 0" hg in about 20 seconds.

I applied the vacuum at the manifold end of the long metal vacuum line that connects to the modulator. Before I go out of my way to remove the airbox and reach down to the Modulator itself and apply vacuum, in order to determine whether it's the line itself that's leaking, or the Modulator diaphragm, I figured I would first make sure that 20 seconds is too quick for the vacuum to leak-off.

So, does anyone happen to know?? I've done some searches online and find answers anywhere from 1 minute to 5 minutes. I'm not sure if it depends on the Modulator, or if it's a pretty universal figure.

THANKS!:)

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- HLS
  
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posted
20 years ago

Fri, Jan 30, 2004 12:12 AM

In theory, they should hold vacuum almost indefinitely. Theory aint reality sometimes, and in the real world microleaks will cause pressure decreases.

I dont think 20 seconds is nearly long enough in any case.

I would double check the hoses, etc, and try a new vacuum modulator if you dont find anything obvious.

Usually vacuum modulators are not so expensive unless you go to a dealership to get one.

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- HLS
  
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posted
20 years ago

Fri, Jan 30, 2004 12:15 AM

What engine/transmission does your Cutless Supreme have?If it is a 440T4, that transmission had both a modulator and a TV cable I believe.

GM wanted over $100 for some of the modulators, but you could pick up aftermarket mods for a lot less...Redline was recommended to me once, but it turned out that this turdo transmission was just fried.