C6 Hard Shifting & Vacuum Modulator
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Cross-Country
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C6 Hard Shifting & Vacuum Modulator
I've got an 86 f150 w/ a c6 trans. It shifts into second and third pretty hard, and seems to be worse when the engine warms up. A quick google search tells me this tends to be a failing vacuum modulator. I crawled under my truck today to take a look, and everything seemed to be okay. I pulled off the vacuum hose and no trans fluid came out, which I've heard is the tell tale sign of a failed modulator. Is it possible that it's still bad even without signs of fluid in the vacuum hose? Is there any other way to determine if it's bad?
Cargo Master
Joined: Jan 2019
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From: Caraway, AR
Yes. Get a hand vacuum pump, $20 at a harbor freight or on amazon. Half that much again at the parts houses. You can test all the diaphragms with it, modulator, egr, vacuum advance. Just give it a couple of pumps, it should hold pressure. If the needle goes straight back to zero, diaphragm is bad.
Moderator
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From: Virginia
If you have a vacuum modulator problem, along with the hard shifts the shifts will also be delayed and the rpms of the engine will be high before it shifts. Do you also have delayed shifting? If you do, you might also want to check for vacuum leaks in the modulator line and where it hooks up in the engine compartment. A lousy vacuum signal to the modulator will cause hard delayed shifting.
Mountain Pass
Joined: Feb 2016
Posts: 112
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If you have a vacuum modulator problem, along with the hard shifts the shifts will also be delayed and the rpms of the engine will be high before it shifts. Do you also have delayed shifting? If you do, you might also want to check for vacuum leaks in the modulator line and where it hooks up in the engine compartment. A lousy vacuum signal to the modulator will cause hard delayed shifting.
Thread Starter
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Cross-Country
Joined: Nov 2020
Posts: 50
Likes: 5
If you have a vacuum modulator problem, along with the hard shifts the shifts will also be delayed and the rpms of the engine will be high before it shifts. Do you also have delayed shifting? If you do, you might also want to check for vacuum leaks in the modulator line and where it hooks up in the engine compartment. A lousy vacuum signal to the modulator will cause hard delayed shifting.
Cargo Master
Joined: Jan 2019
Posts: 3,369
Likes: 16
From: Caraway, AR
I believe there is also an adjustment you can tinker with on the vacuum modulator itself by removing the vacuum hose from the modulator's nipple. Just beyond the nipple is a flat head screw inside the modulator. A small screw driver is all you need to adjust it which can initiate the shift quicker in low rpm or hold the gear longer with more rpm. The later would produce the harder shift. If the modulator doesn't hold vacuum at least it is an easy and cheap item to replace. If you have good vacuum from the manifold, the modulator holds good vacuum and adjustment screw doesn't make a difference I wonder if the truck could have an aggressive shift kit installed? My C6 was shifting hard going into second and fixed the problem by pulling the valve body and installed a shift kit (Transgo) upon disassembly I found a small spring had broken binding the valve operation.
Moderator
Joined: Jan 2001
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From: Virginia
If you just bought it it indeed may have a shift kit in it. You would have noticed if the rpms were too high because of a modulator problem, it's very noticeable. I am thinking your tranny has been rebuilt or modified and that is the reason for the harsh shifting. What does the fluid look like on the dipstick? If it's very new, it will be a very light pink color on the stick. A normal used color will be red. Fluid with a lot of miles on it but still normal will be a dark red. Fluid that is very old and starting to get burnt will have a brown look to it, and may have a burnt smell to it.
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