73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Stewart G Griffin on August 26, 2006, 12:58:00 pm
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Please discuss, in great detail, the process of replacing the vacuum modulator.
A) What's the deal with "red", "Brown", "yellow", "green", "blue" stripes?
B) Is there a difference between a large canister and a small canister?
C) Does the modulator valve come with the cansiter when you buy it at pepboys, advance, autozone etc.?
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The actual modulator valve is inside the transmission in the valve body. The vacuum diaphragm stripe is supposed to indicate it's sensitivity based on the color code of the oem unit. Most are adjustable with a pocket screw driver so it will delay the upshift according to your preference.
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Yeah i would greatly appreciate some more info on changing this because i happen to need to change mine...
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Oh, we're bringing this up again........
Do you have any actual, specific questions because replacement is pretty straightfoward. That is if you can actually diagnose that the modulator needs replacement unlike in my case which wasted 3 weeks of wages. So about 3K!
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They are simple to replace....just take the 7/16" bolt (or is it 3/8--been a while) loose, take the vacuum line off and pull it out. Reverse to install. Ensure you have the rubber o ring in place on the new one.
You can adjust these things to change the shift characteristic by turning the little tiny screw inside the modulator.
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Just curious, how did you arrive at the conclusion that you need a new modulator?
i'll tell you my tragic tale later, if you want to hear it.
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Usually they begin to leak atf through the diaphragm. It's connected to intake manifold vacuum so if it's leaking enough you will start to consume atf and will see white smoke from the tail pipe.
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Well, my truck will shift up through the gears manually so i figured the tranny must be fine, and a few of my friends have had these older trucks and they said that half the time that's what is the problem. Plus it was a ten dollar part so i am starting cheap, but if it is really going to be a pain, like Stewart said then im just going to drop the trans and put in one of the other 2 i have laying around... What's the story though Stew i would like to here it, might give me something to look out for!
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I think he is referring to you bringing up a post from 3+ years ago in 2006 and "bringing this up again".
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Well, since it's been brought up from the dead already :D does replacing the modulator do anything for DOWNshifting? I'd really like my truck to downshift at a higher RPM beyond 2000.
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One bolt holds the vacuum modulator in.
The modulator responds to engine load based on vacuum signal. High vac = low load low vac = high load. So when you stomp down on the throttle manifold vacuum drops. This will cause modulator pressure to rise either forcing a downshift or delaying an upshift.
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did you try adjusting the detent cable to get it to downshift after 2000 rpms?
(http://www.delbridge.net/detent3.jpg)
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Thanks guys. Good pic Andy. No I haven't tried that as I'm not 100% sure what it looks like on the side of my Eddy carb but I'll have a look now that you posted that.
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Alright i'm snowed in as in massively snowed in, so i figure now is the time to tell the story:
No, if you're just replacing the modulator it's not a big deal because as you said it's $12 and takes about 5 mins to change.
What i was concerned about is your diagnosis; What happened in my case was i had puffy white smoke thru the right side exhaust pipe(on true duals). So i thought that it was a head gasket. Out came the crate engine and in went a spare 305---which is actually the engine in there now; Another tragic tale for another thread or see my other posts.
The "new" 305 "did" the same thing---smoke out the right tailpipe. i'm starting to think SBC's have problems with head gaskets? The swap takes a whole day, because i have little experience with switching engines.
At this point, i become determined to "beat this problem" no matter what. So, i tell my bosses that i can't work until further notice--personal stuff; That's the way the economy was back then---that i could take off like that. Of course i was not paid.
i change the right side head gasket on the crate engine(i notice a little water and oil in cylinders 4+6 but at this point i sort of ignore it thinking it was there because the head gasket was bad. This will end up haunting me about 2 yrs later as you can see in my other posts)
i then fire the crate engine up on the test stand. The fact that it is on the test stand is significant because the vacuum modulator isn't hooked up on a test stand. There was absolutely no smoke so i think i got the problem solved.
Out comes the "bad" 305 and in with the crate (this right here takes at least 8 hours).
Smoke still comes out of the right side tailpipe!!! i'm thinking that the gasket re-set itself after being run on the test stand or something?
Dejected, i go for a ride. During this "ride of sadness" the truck starts not being able to shift out of 2nd. But, at the same time, the smoke stopped! i'm not sure what is going on at this point, but i'm happy that the smoke stopped.
i then put 2+2 together and realize that the tube going to the intake manifold has fallen off at the vacuum modulator. So, this is why the trans won't shift out of second and.........
So, after some reading, i find that yes, vacuum modulators can cause cloudy white smoke out of the tailpipe. So, $12 later and about 5 mins problem solved.
But three weeks of not working =approx 3grand gross, when the problem could have been solved for $12 and 5 mins.
i would say that the most difficult aspects of car repair are, in order:
1) proper diagnosis
2) having the right tools (a good facility to work on it---lifts, compressed air and special jacks are good)
3) getting everything torqued down properly. Here's is where service manuals are good.
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Stuart, you should have asked here first. I'm sure it would have been suggested within the first few replies you got.
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Well, maybe. But by august 26, i had found out that it was the modulator and that's why i was asking about the stripes. BTW, i think red is stock if i remember right?