| [GMCnet] Vacuum Modulator Adjustment [message #61040] |
Wed, 21 October 2009 19:58  |
kenneth hugelier
Messages: 89
Registered: August 2008
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There seems to be a lot of transmission knowledge on line now so I thought I might tap in to some of it. I have an adjustable vacuum modulator and I would like to know which way to turn the screw to make the trans. stay in first and second a little longer. I know, I could just turn it and see what happens, but I thought I might save a little time (and maybe a little trouble). Thanks.
ken hugelier
77 PB
Det. Mich.
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| Re: [GMCnet] Vacuum Modulator Adjustment [message #61189 is a reply to message #61040] |
Thu, 22 October 2009 10:13  |
Gary Casey
Messages: 448
Registered: September 2009
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Turning it in will increase the modulator pressure and keep the transmission in lower gears longer. It will not, however, change the full throttle shift points. It will also increase the line pressure at every manifold pressure, which will be most noticeable during light throttle shifts, and they may get too harsh before you get much increase in shift points. If you are just trying to increase the shift points and not make the shifts more harsh, the best way is to modify the governor. I'm going to work out a method on mine, but other priorities, like a dissassembled engine, are higher on the list.
Gary
________________________________
From: ken hugelier <kahugelier@sbcglobal.net>
To: gmclist@temp.gmcnet.org
Sent: Wed, October 21, 2009 5:58:55 PM
Subject: [GMCnet] Vacuum Modulator Adjustment
There seems to be a lot of transmission knowledge on line now so I thought I might tap in to some of it. I have an adjustable vacuum modulator and I would like to know which way to turn the screw to make the trans. stay in first and second a little longer. I know, I could just turn it and see what happens, but I thought I might save a little time (and maybe a little trouble). Thanks.
ken hugelier
77 PB
Det. Mich.
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| Re: [GMCnet] Vacuum Modulator Adjustment [message #61198 is a reply to message #61189] |
Thu, 22 October 2009 18:09 |
GMCWiperMan
Messages: 1248
Registered: December 2007
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Several years ago Bill Bramlett, a name perhaps not familiar to most of
you but one of the premier GMC mechanics, devised a modification for the
TH425 governor. That information may save someone a lot of trial and
error tuning:
1. Carefully grind off the rim of the dome to separate it from the
mostly-flat base. Grind off only enough to allow the pieces to separate
so there's still a lip on the dome. Clean up the rough edges with a
fine file.
2. Clean the whole thing up to remove as much debris-catching oil as
possible.
3. Using tape, plastic, rags, whatever method you can devise, protect
the lower, valve, portion of the assembly from drilling debris.
3. In each of the two weights (the two 1" square pieces of steel) drill
5 each 1/4" holes. That will lighten the weights, slowing the opening
of the valve in the bottom of the governor, and increasing the shift speeds.
4. THOROUGHLY clean the entire assembly to ensure no debris is left to
interfere with operation of the governor and its attached valve.
5. Just before installing the governor, apply a small bead of of RTV
around the mating surfaces of the dome and its base.
6. Install a new seal around the bottom of the base.
7. Being very careful to properly seat the seal without cutting it,
install the governor and latch the bail over the dome. The bail with
keep the dome on the base.
Having offered all that, I must admit that I've never drilled the fly
weights on either the governor I run nor my spare because I'm content
with the original shift speeds. However the spare does have the dome
cut off and I wish the installed one did. Why? Because I'm told that
it's possible to R&R the governor even with headers installed if the
dome can be removed. Again, that's just what I've been told. But with
the installed one unmodified, I'm only half way to finding out for sure
-- hope I never need to.
Ken H.
Gary Casey wrote:
> ...If you are just trying to increase the shift points and not make the shifts more harsh, the best way is to modify the governor. I'm going to work out a method on mine, but other priorities, like a dissassembled engine, are higher on the list.
> Gary
>
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