Vacuum Modulator
Thread Starter
Pro
Joined: Mar 2004
Posts: 686
Likes: 0
From: Southeast
Vacuum Modulator
I did a search on this and could not find it. I have a 71 with the TH-400. Awhile back, my vacuum line going to the modulator broke off and made the tranny shift goofy. I had that fixed while it was at the shop getting something else done to it. Therefore, I know nothing about where it is and where the vacuum hose runs to. The tranny is shifting weird again and I'm thinking it's the same problem once again as the symptoms are the same. Where does the vacuum hose go from the modulator? Where exactly IS the modulator? Sorry for the ignorance, but thanks for the help.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
You'll have to go under the car to see it. The modulator is a goldish-colored device -a stepped cylinder about an inch and a half in diameter and about two inches long- on the lower passenger side of the transmission. It will have a rubber hose going into the end facing the rear or, if the hose came off, a tube where the hose attaches.
The vacuum supply hose (it's a metal tube, actually with rubber hose on each end) runs from the modulator, up the firewall, and into a vacuum fitting at the rear of the intake manifold just behind the carburetor. You'll also see another fat vacuum line running to this same manifold fitting.
The modulator is easy to test with one of those hand-held vacuum pumps. You just connect it up to the modulator, pump it down and see if it holds a vacuum. If it holds, you're good to go. If it doesn't, then the diaphragm is ruptured. Takes about two minutes to replace the modulator with simple hand tools.
Hope that helps.
The vacuum supply hose (it's a metal tube, actually with rubber hose on each end) runs from the modulator, up the firewall, and into a vacuum fitting at the rear of the intake manifold just behind the carburetor. You'll also see another fat vacuum line running to this same manifold fitting.
The modulator is easy to test with one of those hand-held vacuum pumps. You just connect it up to the modulator, pump it down and see if it holds a vacuum. If it holds, you're good to go. If it doesn't, then the diaphragm is ruptured. Takes about two minutes to replace the modulator with simple hand tools.
Hope that helps.
Melting Slicks
Joined: May 2002
Posts: 2,431
Likes: 52
From: Florida
The vacuum source is on the rear of the intake manifold on a branch fitting screwed into the intake. There is sometimes a restrictor pinch on the line at the intake fitting. This can be cut off to improve vacuum signal. Usually rubber hose deteriorates on both ends of metal line. If you remove modulator and fluid drips from hose end, it's bad. The test mentioned above though is a more reliable test.
Good luck,
Jim
[IMG]
[/IMG]
Good luck,
Jim
[IMG]
[/IMG]
Last edited by jimvette999; Oct 20, 2005 at 07:48 AM.
