Powerglide vacuum modulator question
Henry van Cleef
First of all, what year is your car, and do you have an aluminum-case or
iron-case transmission?
The can-type modulators generally die with a hole in the diaphram---lots
of smoke, loss of transmission fluid. Quick check is to take of the
vacuum host at the modulator and see that it isn't full of transmission
fluid. Function of the modulator in Powerglides, as I recall, was only
to modulate working oil pressure to the direct clutch---been years since
I looked at one, but I have the GM overhaul guide with oil charts, and
will check. Aluminum-case units ('62 partial, all '63 and later) are
entirely different from iron case units. Iron case units are different
for different years. You will need to specify year and engine to
identify the transmission.
--
Hank van Cleef---The Union Institute---History of Science and Technology
Unix, X11, networked systems vanc...@netcom.com, vanc...@tmn.com
Rick Inzero
It otherwise shifts incredibly smooth, both up and down, and
at the specified speeds (as spec'd in the service manual). It
just won't kick down into passing gear when you stomp on it...
I've played with the carb-to-trans linkage adjustment, and all that
seems to do is adjust the shift point, and not control this passing gear
downshift. From some reading, and these symptoms, it seems like it may be
a bad vacuum modulator (under the car, attached to the lower, rear,
side of the transmission). Unfortunately, the only details I can find
are (roughly) "replace the modulator". My questions are:
1) does a bad vacuum modulator indeed sound like the source of the problem?
2) how can you tell if its bad? (namely, can you "bench test" it?)
3) what should I expect when I take the bad one off? (What's behind/
under the modulator... air or several quarts of transmission fluid?)
Thanks in advance for your old-car wisdom!
(Note follow-ups directed to alt.autos.antique.)
---
Rick Inzero, President of the "Put The High Beam Switch Back On The Damn
Floor, Where It Belongs Society"
rochester!cci632!rdi
Computer Consoles Inc. (CCI) uunet!ccicpg!cci632!rdi
Rochester, NY r...@cci.com
"He who has the most mass wins." -Theodore Chen
Frank Knight
Well, it certainly does need to work, but it controls the shift
points based on vacuum. The problem you describe (flooring it
below 50 or so and get no downshift sounds more like the mechanical
downshift linkage adjustment (or the Tranny if it can't be adjusted
to me especially if your normal upshift points seem correct. The
vacuum mod. will produce downshifts, but not the one that occurs
based on accellerator position rather than intake vacuum. BTW,
its no problem to replace the modulator, just remember the O ring
and the spring where applicable.
Frank
Joseph C. Brandt
|> In article <1992Jun2.1...@cci632.cci.com> r...@cci632.cci.com (Rick Inzero) writes:
|>
|> My old Chevy Powerglide won't downshift to provide a "passing gear".
|> internal problems or (3) Learn to enjoy driving without passing
|> gear..... It makes for a leisurely and relaxing journey.
|>
|> -val-
|> --
Let's hear it for nbr. 3! I've been driving my '65 Impala that way
for years. WHen I get feel crazzzy, I slap 'er down into L.
/Joe
[7m
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[0m
Val Breault
My old Chevy Powerglide won't downshift to provide a "passing gear".
It otherwise shifts incredibly smooth, both up and down, and
at the specified speeds (as spec'd in the service manual). It
just won't kick down into passing gear when you stomp on it...
I've played with the carb-to-trans linkage adjustment, and all that
seems to do is adjust the shift point, and not control this passing gear
downshift. From some reading, and these symptoms, it seems like it may be
a bad vacuum modulator (under the car, attached to the lower, rear,
side of the transmission). Unfortunately, the only details I can find
are (roughly) "replace the modulator". My questions are:
1) does a bad vacuum modulator indeed sound like the source of the problem?
2) how can you tell if its bad? (namely, can you "bench test" it?)
3) what should I expect when I take the bad one off? (What's behind/
under the modulator... air or several quarts of transmission fluid?)
Thanks in advance for your old-car wisdom!
(Note follow-ups directed to alt.autos.antique.)
Yes Rick, it could very well be the modulator and yes, you can "bench"
test them very easily.
The modulator alters the shift point via a vacuum signal from the manifold.
Low vacuum (high MAP - Manifold Absolute Pressure) usually indicates
the engine is under a heavy load.
The modulator is little more than a spring loaded diaphragm with a
mechanical attachment (usually a pushrod) into the transmission. After
a long time, the diaphragm may become cracked. Three common symptoms
are: (1) Inappropriate shift points (2) Loss of transmission fluid
(3) Transmission fluid in the pipe that carries the vacuum signal to
the modulator.
To test, jack up the car and disconnect the vacuum line from the
modulator. If transmission fluid drips out then you've got a bad
modulator. No drips? Attach a vacuum tester and pump it down.
(You know - One of those hand held vacuum pump/gauge combinations)
If it leaks the modulator is bad. Neither of the above?? (1) You
may have a broken or misplaced pushrod, (2) The transmission may have
internal problems or (3) Learn to enjoy driving without passing
gear..... It makes for a leisurely and relaxing journey.
-val-
--
Val Breault - vbre...@gmr.com - N8OEF
The opinions expressed by the author do not necessarily reflect
the position of the General Motors Corporation.
Eric Youngblood
|> My old Chevy Powerglide won't downshift to provide a "passing gear".
|> It otherwise shifts incredibly smooth, both up and down, and
|> at the specified speeds (as spec'd in the service manual). It
|> just won't kick down into passing gear when you stomp on it...
|> I've played with the carb-to-trans linkage adjustment, and all that
This is what controls the downshift for "passing". This is how the transmission
(inconjuction with the line pressure governer) decides whether or not to down- shift to a lower gear.
|> seems to do is adjust the shift point, and not control this passing gear
|> downshift. From some reading, and these symptoms, it seems like it may be
|> a bad vacuum modulator (under the car, attached to the lower, rear,
|> side of the transmission). Unfortunately, the only details I can find
|> are (roughly) "replace the modulator". My questions are:
|> 1) does a bad vacuum modulator indeed sound like the source of the problem?
From everything I've read in my service manuals and maintenance books the
vacumn modulator is only responsible for part-throttle & light acceleration shifts
I have spent time tuning my TH350 to maximize the power characteristics of the
Pontiac 400 in my TA. The modulator will only affect shifts up to a certain point.
When you are using WOT the transmission is using line pressure to determine shift
points, and its the governer that regulates this pressure.
|> 2) how can you tell if its bad? (namely, can you "bench test" it?)
Since you have good smooth shifts during normal acceration and at proper speeds
it doesnt appear that it is malfunctioning, if it were you would expect :
a) late part throttle upshifts (probably harsh)
- or -
b) upshifts tooo early.
|> 3) what should I expect when I take the bad one off? (What's behind/
|> under the modulator... air or several quarts of transmission fluid?)
Yes, there is transmission fluid behind it, but you wont lose much (use a pan
to catch it to avoid a small mess) This is also how a bad modulator diaphram
can cause your car to 'burn' transmission fluid as it gets sucked into the
engine through the vacumn line.
Some of my personal experience with modulators (adjustable type)
1) Delay shift point & produce more positive shifts (part throttle)
2) Cause sooner shifts & much smoother (part throttle)
3) Adjustments had no effect on WOT shifts or downshift points.
Personal Experience in Governer tuning (lighter weights & springs) :
1) Lighter weights produce later WOT shifts
2) Lighter springs produce farther spaced shift points (WOT & part throttle)
3) Cause sooner & more *aggressive* "passing" downshifts. Depending on vehicle
speed the downshift may go from 3rd-->1st or 3rd-->2nd.
Hope this helps,
$0.02
Ericy
Gerald Ryder
|> It otherwise shifts incredibly smooth, both up and down, and
|> at the specified speeds (as spec'd in the service manual). It
|> just won't kick down into passing gear when you stomp on it...
|> I've played with the carb-to-trans linkage adjustment, and all that
|> seems to do is adjust the shift point, and not control this passing gear
|> downshift. From some reading, and these symptoms, it seems like it may be
|> a bad vacuum modulator (under the car, attached to the lower, rear,
|> side of the transmission). Unfortunately, the only details I can find
|> are (roughly) "replace the modulator". My questions are:
|> 1) does a bad vacuum modulator indeed sound like the source of the problem?
|> 2) how can you tell if its bad? (namely, can you "bench test" it?)
|> 3) what should I expect when I take the bad one off? (What's behind/
|> under the modulator... air or several quarts of transmission fluid?)
|>
|> Thanks in advance for your old-car wisdom!
|> (Note follow-ups directed to alt.autos.antique.)
|>
I own a 1955 Chevy with a 2sp, cast iron power glide in it. About 15 years ago I
had the same problem as you describe. It did turn out to be a bad vacuum modulator
valve. I had it fixed at a transmission shop for about $75.00 in (1977) and it has
worked fine since.
--
gerald
Scott D. Berry
>It otherwise shifts incredibly smooth, both up and down, and
>at the specified speeds (as spec'd in the service manual). It
>just won't kick down into passing gear when you stomp on it...
>I've played with the carb-to-trans linkage adjustment, and all that
>seems to do is adjust the shift point, and not control this passing gear
>downshift. From some reading, and these symptoms, it seems like it may be
>a bad vacuum modulator (under the car, attached to the lower, rear,
>side of the transmission). Unfortunately, the only details I can find
>are (roughly) "replace the modulator". My questions are:
>1) does a bad vacuum modulator indeed sound like the source of the problem?
a direct cable connection for downshifting (connects to the throttle)
>2) how can you tell if its bad? (namely, can you "bench test" it?)
into the intake manifold, accompanied by a large amount of white/blue/gray
smoke out the rear pipe. Make sure the vacuum line is not plugged or leaking
>3) what should I expect when I take the bad one off? (What's behind/
> under the modulator... air or several quarts of transmission fluid?)
dirt gets in. THIS IS CRITICAL TO PRESERVING THE TRANSMISSION.
Behind the modulator is a valve unit which also could be
stuck due to deposits, etc. Not too much fluid should leak out, as usually
the modulator is above the fluid level. Also be careful to keep track of
any springs which go with the valve, keeping it in the correct order.
>
>Thanks in advance for your old-car wisdom!
>(Note follow-ups directed to alt.autos.antique.)
>
>---
>Rick Inzero, President of the "Put The High Beam Switch Back On The Damn
> Floor, Where It Belongs Society"
> rochester!cci632!rdi
>Computer Consoles Inc. (CCI) uunet!ccicpg!cci632!rdi
>Rochester, NY r...@cci.com
>
>"He who has the most mass wins." -Theodore Chen
Scott D. Berry Assistant Professor of Physics Florida State University
315 Keen Bldg, B-159, Tallahassee, FL 32306-3016 (904) 644-1218
INTERNET: be...@redbeard.physics.fsu.edu BITNET: berry@fsu