C6 shifts hard.

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Jul 7, 2014 | 09:10 PM

Ordnance

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C6 shifts hard.

Started driving my 77 F250 after work and it doesn't want to shift for a damn. It rides first gear until 35, then kicks down hard into second. Dont think i've felt a shift to third, and I had it up to 75.

Obviously something isn't right, I've read that a vacuum line that controls the shifting is prone to fail. however I don't know where said line is physically on both the trans and the engine. All the haynes manual says is to check fluid and have the transmission serviced by professionals. Can someone point me in the right direction?

Jul 7, 2014 | 09:19 PM

redroad

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From: Pa.

Vacuum modulator is on passenger side of trans
and the vacuum line will run from the same vacuum tree your power brake booster is connected usually behind the carb .. The vacuuum modulator can go bad ..

Jul 7, 2014 | 09:23 PM

mikeo0o0o0

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From: Stanley, VA

X2 on the vacuum modulator. If it happened suddenly, check the vacuum line and make sure it hasn't become disconnected or damaged.

Jul 7, 2014 | 09:29 PM

mikeo0o0o0

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From: Stanley, VA

Here's how the line is routed on a '73 w/ 390.
Also, IMHO Hayne's ( and Chilton) are pretty much worthless.

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Jul 7, 2014 | 09:36 PM

lbcarguy

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From: Long Beach, CA

Check kick down lever

Also, check the kick down lever connected to the carb at one end and on the trans next to the shift linkage. Make sure it is not stuck in the down position (most downward position at the transmission). Make sure it is fully UP.

Mike

Jul 8, 2014 | 08:38 PM

raven3

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From: Bakersfield,CA

Had a bad modulator on 73 C6, because when the vacumm line was removed, oil seeped out of the modulator vacuum connection.

Jul 9, 2014 | 10:16 AM

Ordnance

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had to wait all day for the pavement to cool off enough to actually lay on it. (Arizona) got underneath the truck and jiggled the kickdown lever a few times, and checked my vaccuum line. everything looked good, so my guess is the modulator. I'll replace it this weekend. Other than getting one with the matching color stripe, is this part plug and play, or do i have to make adjustments to it prior to install?

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Jul 9, 2014 | 11:49 AM

mikeo0o0o0

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From: Stanley, VA

Quote:

Originally Posted by Ordnance

I'll replace it this weekend. Other than getting one with the matching color stripe, is this part plug and play, or do i have to make adjustments to it prior to install?

Pretty much plug 'n play. It's held in by a clamp that is retained by a single bolt. Unbolt the clamp and you should be able to pull it out. It uses an o-ring for sealing. There is a short pin that may or may not come out with the modulator, you reuse it.
There is some adjustment that you can do but usually it's not necessary. If you do adjust it, the adjustment screw is in the end of the nipple you connect the vacuum line to and you use a very small screw driver to adjust.