Turbo 400 Vac Modulator
Thread Starter
Race Director
Joined: Jan 2007
Posts: 10,682
Likes: 696
From: Danville Illinois
2025 Corvette of the Year Finalist - Modified
2024 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
2019 C3 of Year Finalist (appearance mods)
Turbo 400 Vac Modulator
One of the things on my list for this year is to replace the vacuum modulator on my turbo 400 trans.
Not having any problems, have the original one on there, car is a 75 with upper end mods with t/b 400 and an B/M shift kit for the last 45 years.
At 74 i will not be doing this, looking for info & tips ?
Not having any problems, have the original one on there, car is a 75 with upper end mods with t/b 400 and an B/M shift kit for the last 45 years.
At 74 i will not be doing this, looking for info & tips ?
Last edited by Mod75; Jul 16, 2021 at 11:41 PM. Reason: I cant spell
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Ya I'd leave it be. I get wanting to preempt a failure. Problem I have with folks doing work on my car is trusting them to do it right and not break anything else in the process. Unless you know someone specifically, competence is low these days for old vehicles in the auto repair business. No OBDII port to plug into
You could buy one and have it on hand in the event that it does fail. That at least would get you half way there if it should fail.
You could buy one and have it on hand in the event that it does fail. That at least would get you half way there if it should fail.
Last edited by REELAV8R; Jul 17, 2021 at 12:06 PM.
Race Director
Joined: Mar 2016
Posts: 14,396
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From: Napa Valley California
I would replace it with a new OEM part.
I had an 89 Chevy 3500 1 ton dually and drove it for many years.
I was driving home from work one night and noticed when I came to a stop there was a ton of blueish white smoke coming out the exhaust.
I thought it was a head gasket and drove it straight to Chevy who diagnosed it as a head gasket as well.
The next day and $900.00 later the same thing happened.
When I backed into my driveway the transmission was slipping so badly it would not back into my garage.
I did some research and found it was possibly the transmission vacuum modulator.
When they go bad the engine vacuum will suck all the fluid from the transmission into the intake and burn it in the cylinders.
From the time I noticed it smoking I only drove it about 50 miles and that was enough to destroy the TH 400.
At the time the modulator was about $3.95 and I changed it in about 30 minutes.
But the $900.00 head gasket swap and $2,000.00 transmission swap was costly.
I had an 89 Chevy 3500 1 ton dually and drove it for many years.
I was driving home from work one night and noticed when I came to a stop there was a ton of blueish white smoke coming out the exhaust.
I thought it was a head gasket and drove it straight to Chevy who diagnosed it as a head gasket as well.
The next day and $900.00 later the same thing happened.
When I backed into my driveway the transmission was slipping so badly it would not back into my garage.
I did some research and found it was possibly the transmission vacuum modulator.
When they go bad the engine vacuum will suck all the fluid from the transmission into the intake and burn it in the cylinders.
From the time I noticed it smoking I only drove it about 50 miles and that was enough to destroy the TH 400.
At the time the modulator was about $3.95 and I changed it in about 30 minutes.
But the $900.00 head gasket swap and $2,000.00 transmission swap was costly.
Racer
Joined: Apr 2009
Posts: 331
Likes: 44
From: Cincinnati Ohio
It is a simple replacement. One bolt to release the clamp, remove vacuum hose and slide out old one and slide in new one. The old ones after a long time start to leak oil. If the diaphragm goes out it can pull transmission fluid into intake and you will wonder where it all went. The last one I bought was only around $15. Don't overcomplicate a simple task.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Leave it alone. It is an original part and one that was made WELL (the reason it still lives). Today's aftermarket stuff is cr@p compared to original OEM parts. Wait until you have problems before you toss out the original modulator.
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Le Mans Master
Joined: Aug 2006
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
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It is a simple replacement. One bolt to release the clamp, remove vacuum hose and slide out old one and slide in new one. The old ones after a long time start to leak oil. If the diaphragm goes out it can pull transmission fluid into intake and you will wonder where it all went. The last one I bought was only around $15. Don't overcomplicate a simple task.
Are you sure that you need a new one?
Thread Starter
Race Director
Joined: Jan 2007
Posts: 10,682
Likes: 696
From: Danville Illinois
2025 Corvette of the Year Finalist - Modified
2024 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
2019 C3 of Year Finalist (appearance mods)
So the consensus is to leave it alone, no problems with it so far, one owner 75 with 45k on it.
Having just turned 74 i don't need to go looking for something to do, so its leave it alone.
Having just turned 74 i don't need to go looking for something to do, so its leave it alone.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Another potential "gotcha" is that sometimes the modulator valve sticks in the vacuum modulator as you remove it, gets dropped and then is installed either dirty or backwards, or both. I've seen it happen. I have to agree with the "If it ain't broke, don't fix it"
Instructor
Joined: May 2021
Posts: 127
Likes: 28
Are you able to pop off the modulator without draining the oil? There is a decent leak at my modulator and I drained the oil before pulling it out. It is hard to tell where the leak is coming from, but since I see oil at the seam, I am worried the modulator may be bad and not just the O-ring. I was considering replacing the modulator, but I do not want to mess with adjusting a new one. My vette shifts nice. Is there a way to adjust it to match the old unit before installing it?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Static fluid level is above the modulator center line, so oil will leak out if you don't drain any oil from the pan first. A leak at the modulator is most likely due to the O-ring seal getting old, hard, and shrunk from original size over years of service.