Home » Public Forums » GMCnet » [GMCnet] Shifting and Vac Modulator
| [GMCnet] Shifting and Vac Modulator [message #99010] |
Wed, 08 September 2010 14:46  |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
|
Senior Member |
|
|
Good Afternoon Guys
Okay, I'm not sure I have this all 100% right, but I'm trying to figure out
what's going on.
As I understand the way the vacuum modulator works, it monitors engine vacuum.
Engine vacuum reacts very accurately to engine load with high vacuum produced
when the engine is under light load and diminishing down to zero vacuum when
the engine is under a heavy load. When an engine is under a light load or no
load, high vacuum acts on the modulator which moves the throttle valve in one
direction to allow the transmission to shift early and soft. As the engine
load increases, vacuum is diminished which moves the valve in the other
direction causing the transmission to shift later and more firmly.
My Problem
My transmission shifts extremely early, to the point that if I take off from a
stop sign, before I get through the intersection I'm already in 2nd gear, if I
cross the intersection and head up a hill, I have to manually downshift to get
up the hill at a decent pace.
In analyzing the situation, could my problem be that my auxillary brake booster
vac pump is not connected right and is not segregated from the vacuum being used
on the vacuum modulator??
I started investigating things thinking I could have a bad vacuum modulator,
then this came to mind.
Thoughts?? Thanks for the help.
Ray
Ray & Lisa Erspamer
78 Royale Center Kitchen
TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale@att.net
414-745-3188
Web Site: http://ray-lisa.page.tl/
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
|
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99011 is a reply to message #99010] |
Wed, 08 September 2010 15:04  |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
|
Senior Member |
|
|
Ray,
As for thoughts, could you disconnect the aux vacuum pump and test?
As for the modulator -- as I understand it -- the diaphram can go bad. What about testing with a Mitey vac and see if it holds vacuum without leaking down.
Dennis Sexton
73 GMC
Germantown, TN
USA
-----Original Message-----
From: Ray Erspamer <78gmc-royale@att.net>
To: gmclist@temp.gmcnet.org
Sent: Wed, Sep 8, 2010 2:46 pm
Subject: [GMCnet] Shifting and Vac Modulator
Good Afternoon Guys
Okay, I'm not sure I have this all 100% right, but I'm trying to figure out
hat's going on.
As I understand the way the vacuum modulator works, it monitors engine vacuum.
ngine vacuum reacts very accurately to engine load with high vacuum produced
hen the engine is under light load and diminishing down to zero vacuum when
he engine is under a heavy load. When an engine is under a light load or no
oad, high vacuum acts on the modulator which moves the throttle valve in one
irection to allow the transmission to shift early and soft. As the engine
oad increases, vacuum is diminished which moves the valve in the other
irection causing the transmission to shift later and more firmly.
My Problem
y transmission shifts extremely early, to the point that if I take off from a
top sign, before I get through the intersection I'm already in 2nd gear, if I
ross the intersection and head up a hill, I have to manually downshift to get
p the hill at a decent pace.
In analyzing the situation, could my problem be that my auxillary brake booster
ac pump is not connected right and is not segregated from the vacuum being used
on the vacuum modulator??
started investigating things thinking I could have a bad vacuum modulator,
hen this came to mind.
Thoughts?? Thanks for the help.
Ray
ay & Lisa Erspamer
8 Royale Center Kitchen
ZE368V101144
auwatosa, Wisconsin 53226
mail: 78GMC-Royale@att.net
14-745-3188
eb Site: http://ray-lisa.page.tl/
______________________________________________
MCnet mailing list
ist Information and Subscription Options:
ttp://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Dennis S
73 Painted Desert 230
Memphis TN Metro
|
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99012 is a reply to message #99011] |
Wed, 08 September 2010 15:24 |
Charles
Messages: 455
Registered: January 2004
Karma: 0
|
Senior Member |
|
|
Dennis,
Are you saying that is one pulls a vacuum on a
modular valve, with a Mitey vac or some other source,
the modulator valve shouild hold that vacuum?
Charles
----- Original Message -----
From: "Dennis S" <dennisfsexton@aol.com>
To: <gmclist@temp.gmcnet.org>
Sent: Wednesday, September 08, 2010 3:04 PM
Subject: Re: [GMCnet] Shifting and Vac Modulator
>
> Ray,
> As for thoughts, could you disconnect the aux vacuum pump and test?
>
> As for the modulator -- as I understand it -- the diaphram can go bad.
> What about testing with a Mitey vac and see if it holds vacuum without
> leaking down.
>
>
>
>
> Dennis Sexton
> 73 GMC
> Germantown, TN
> USA
>
>
>
> -----Original Message-----
> From: Ray Erspamer <78gmc-royale@att.net>
> To: gmclist@temp.gmcnet.org
> Sent: Wed, Sep 8, 2010 2:46 pm
> Subject: [GMCnet] Shifting and Vac Modulator
>
>
> Good Afternoon Guys
> Okay, I'm not sure I have this all 100% right, but I'm trying to figure
> out
> hat's going on.
> As I understand the way the vacuum modulator works, it monitors engine
> vacuum.
> ngine vacuum reacts very accurately to engine load with high vacuum
> produced
> hen the engine is under light load and diminishing down to zero vacuum
> when
> he engine is under a heavy load. When an engine is under a light load or
> no
> oad, high vacuum acts on the modulator which moves the throttle valve in
> one
> irection to allow the transmission to shift early and soft. As the
> engine
> oad increases, vacuum is diminished which moves the valve in the other
> irection causing the transmission to shift later and more firmly.
> My Problem
> y transmission shifts extremely early, to the point that if I take off
> from a
> top sign, before I get through the intersection I'm already in 2nd gear,
> if I
> ross the intersection and head up a hill, I have to manually downshift to
> get
> p the hill at a decent pace.
> In analyzing the situation, could my problem be that my auxillary brake
> booster
> ac pump is not connected right and is not segregated from the vacuum being
> used
> on the vacuum modulator??
>
> started investigating things thinking I could have a bad vacuum modulator,
> hen this came to mind.
> Thoughts?? Thanks for the help.
> Ray
>
> ay & Lisa Erspamer
> 8 Royale Center Kitchen
> ZE368V101144
> auwatosa, Wisconsin 53226
> mail: 78GMC-Royale@att.net
> 14-745-3188
> eb Site: http://ray-lisa.page.tl/
> ______________________________________________
> MCnet mailing list
> ist Information and Subscription Options:
> ttp://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Charles Wersal
Duncanville, Texas
26 foot 1975 Glenbrook
Pandora's Box
|
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99013 is a reply to message #99012] |
Wed, 08 September 2010 15:48 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
|
Senior Member |
|
|
Charles,
Perhaps just asking if or suggesting it might be a good test.
The Maintenance manual, section 7-23, gives some test procedures (with a caution about using over 6 psi for a soap test). The swab down the vacuum line for transmission fluid should be the first test.
It just seems to me that a vacuum leak down test would also apply to testing the modulator while still installed.
Dennis
| Charles wrote on Wed, 08 September 2010 15:24 |
Dennis,
Are you saying that is one pulls a vacuum on a
modular valve, with a Mitey vac or some other source,
the modulator valve shouild hold that vacuum?
Charles
----- Original Message -----
From: "Dennis S" <dennisfsexton@aol.com>
To: <gmclist@temp.gmcnet.org>
Sent: Wednesday, September 08, 2010 3:04 PM
Subject: Re: [GMCnet] Shifting and Vac Modulator
>
> Ray,
> As for thoughts, could you disconnect the aux vacuum pump and test?
>
> As for the modulator -- as I understand it -- the diaphram can go bad.
> What about testing with a Mitey vac and see if it holds vacuum without
> leaking down.
>
>
>
>
> Dennis Sexton
> 73 GMC
> Germantown, TN
> USA
>
>
>
|
Dennis S
73 Painted Desert 230
Memphis TN Metro
[Updated on: Wed, 08 September 2010 15:48] Report message to a moderator |
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99014 is a reply to message #99013] |
Wed, 08 September 2010 15:50 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
|
Senior Member |
|
|
Spelling correction...
That should read "swab down the vacuum line"
Dennis
-----Original Message-----
From: Dennis Sexton <dennisfsexton@aol.com>
To: gmclist@temp.gmcnet.org
Sent: Wed, Sep 8, 2010 3:48 pm
Subject: Re: [GMCnet] Shifting and Vac Modulator
Charles,
erhaps just asking if or suggesting it might be a good test.
The Maintenance manual, section 7-23, gives some test procedures (with a caution
bout using over 6 psi for a soap test). The swap down the vacuum line for
ransmission fluid should be the first test.
It just seems to me that a vacuum leak down test would also apply to testing the
odulator while still installed.
Dennis
Charles wrote on Wed, 08 September 2010 15:24
Dennis,
Are you saying that is one pulls a vacuum on a
modular valve, with a Mitey vac or some other source,
the modulator valve shouild hold that vacuum?
Charles
----- Original Message -----
From: "Dennis S" <dennisfsexton@aol.com>
To: <gmclist@temp.gmcnet.org>
Sent: Wednesday, September 08, 2010 3:04 PM
Subject: Re: [GMCnet] Shifting and Vac Modulator
>
> Ray,
> As for thoughts, could you disconnect the aux vacuum pump and test?
>
> As for the modulator -- as I understand it -- the diaphram can go bad.
> What about testing with a Mitey vac and see if it holds vacuum without
> leaking down.
>
>
>
>
> Dennis Sexton
> 73 GMC
> Germantown, TN
> USA
>
>
>
-
ennis S
3 Painted Desert 230
ermantown, TN
______________________________________________
MCnet mailing list
ist Information and Subscription Options:
ttp://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Dennis S
73 Painted Desert 230
Memphis TN Metro
|
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99024 is a reply to message #99010] |
Wed, 08 September 2010 17:58 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
|
Senior Member |
|
|
Ray,
I did a quick search on the forum and found this post by Manny from May, '10
Manny wrote...
Early shifts are mostly because of great engine vacuum. The spring in
the modulator can be adjusted. turning the screw clockwise increases
the tension (higher speed shifting). Turning the screw
counter-clockwise decreases the spring tension (sooner shifts).
Turning the the screw wither clockwise or counter-clockwise doesn't
give you a liner affect. Sometimes the spring, springs back.
I suggest turning in 1/2 turn or 3/4 turn increments until you get the
desired shifting speed you want.
--
Manny Trovao
mannystrans@hughes.net
Manny's Trans / Power Drive
San Jose, California
408-937-1583
End quote..
So you might want to do a search and read the other posts to see all comments in context.
Also, there are a number of different modulator styles -- from the OEM, big black round style to a brass colored replacement.
Dennis
Dennis S
73 Painted Desert 230
Memphis TN Metro
|
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99030 is a reply to message #99010] |
Wed, 08 September 2010 19:07 |
Gary Casey
Messages: 448
Registered: September 2009
Karma: 0
|
Senior Member |
|
|
Ray,
You have it exactly right. The vacuum modulator pushes on a valve that
regulates a pressure, called modulator pressure, of all things. This pressure
controls the shift points and the line pressure. A spring in the modulator
pushes on the valve and manifold vacuum pulls against the spring. Originally,
these transmissions used an absolute pressure modulator, but I suspect over the
years most have been changed out for a "vacuum modulator." The original was
designed to compensate for altitude - it compensated the shift feel correctly,
but the shift points were not good at altitude. The vacuum modulator does the
opposite - the shift points are well compensated at altitude, but the shift feel
will get firm.
You said normal part throttle upshifts are early, but what about higher throttle
settings, short of WOT? Do they occur at the same speed and are they
accompanied by a really soft shift? If so, the vacuum modulator or the
modulator valve is suspect. The vacuum modulator vacuum hose was generally
tapped into its own port, but I don't think that is very important. I doubt if
the problem is related to the vacuum pump or any manifold connections. The
shift points should extend out at higher throttle openings and you should be
able to get a part throttle 3-2 downshift without going to full throttle,
although not at a very high speed. If you can, the vacuum modulator system is
probably working okay.
At light throttle a standard THM425 will have quite early shift points - at
least compared to what a normal person might think it should have. Something to
live with unless you are ready for a project :-)
Gary Casey
'73 23 with a "heavy-duty" rebuild and firm shifts
As I understand the way the vacuum modulator works, it monitors engine vacuum.
Engine vacuum reacts very accurately to engine load with high vacuum produced
when the engine is under light load and diminishing down to zero vacuum when
the engine is under a heavy load. When an engine is under a light load or no
load, high vacuum acts on the modulator which moves the throttle valve in one
direction to allow the transmission to shift early and soft. As the engine
load increases, vacuum is diminished which moves the valve in the other
direction causing the transmission to shift later and more firmly.
My Problem
My transmission shifts extremely early, to the point that if I take off from a
stop sign, before I get through the intersection I'm already in 2nd gear, if I
cross the intersection and head up a hill, I have to manually downshift to get
up the hill at a decent pace.
In analyzing the situation, could my problem be that my auxillary brake booster
vac pump is not connected right and is not segregated from the vacuum being
used
on the vacuum modulator??
I started investigating things thinking I could have a bad vacuum modulator,
then this came to mind.
Thoughts?? Thanks for the help.
Ray
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99055 is a reply to message #99050] |
Wed, 08 September 2010 22:15 |
midlf
Messages: 2212
Registered: July 2007
Location: SE Wisc. (Palmyra)
Karma: 1
|
Senior Member |
|
|
| Larry wrote on Wed, 08 September 2010 21:44 |
If all else fails try the governor. JWID
|
Ray - if you don't have headers it is easy, I'm told here, to change the regulator. I have a spare you can try, or two if I'm counting the transmode.
However there have also been numerous comments here over the years on how the vac line can get plugged at the restriction in the steel line and cause problems. Others can elaborate on this as I don't remember the details.
Steve Southworth
1974 Glacier TZE064V100150 (for workin on)
1975 Transmode TZE365V100394 (parts & spares)
Palmyra WI
|
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99056 is a reply to message #99055] |
Wed, 08 September 2010 22:16 |
midlf
Messages: 2212
Registered: July 2007
Location: SE Wisc. (Palmyra)
Karma: 1
|
Senior Member |
|
|
| midlf wrote on Wed, 08 September 2010 22:15 |
| Larry wrote on Wed, 08 September 2010 21:44 |
If all else fails try the governor. JWID
|
Ray - if you don't have headers it is easy, I'm told here, to change the regulator. I have a spare you can try, or two if I'm counting the transmode.
However there have also been numerous comments here over the years on how the vac line can get plugged at the restriction in the steel line and cause problems. Others can elaborate on this as I don't remember the details.
|
Ray - check your cell phone. I have left the work number I will be at until 0600AM Thursday.
Steve Southworth
1974 Glacier TZE064V100150 (for workin on)
1975 Transmode TZE365V100394 (parts & spares)
Palmyra WI
|
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99095 is a reply to message #99056] |
Thu, 09 September 2010 09:27 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
|
Senior Member |
|
|
Thanks Steve, I will do some additional research and then decide what to do.
BTW, I still have your bearing seal retaining piece that you let me use for the
pattern for the part I needed for my Warner Bearing Tool set, need to get that
back to you.
Ray
Ray & Lisa Erspamer
78 Royale Center Kitchen
TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale@att.net
414-745-3188
Web Site: http://ray-lisa.page.tl/
________________________________
From: Steve Southworth <midlf@centurytel.net>
To: gmclist@temp.gmcnet.org
Sent: Wed, September 8, 2010 10:16:55 PM
Subject: Re: [GMCnet] Shifting and Vac Modulator
midlf wrote on Wed, 08 September 2010 22:15
> Larry wrote on Wed, 08 September 2010 21:44
> > If all else fails try the governor. JWID
>
>
> Ray - if you don't have headers it is easy, I'm told here, to change the
>regulator. I have a spare you can try, or two if I'm counting the transmode.
>
>
> However there have also been numerous comments here over the years on how the
>vac line can get plugged at the restriction in the steel line and cause
>problems. Others can elaborate on this as I don't remember the details.
Ray - check your cell phone. I have left the work number I will be at until
0600AM Thursday.
--
Steve Southworth
1974 Glacier TZE064V100150
Palmyra WI
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
|
|
|
|
| Re: [GMCnet] Shifting and Vac Modulator [message #99104 is a reply to message #99050] |
Thu, 09 September 2010 10:01 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
|
Senior Member |
|
|
Hi Larry
That's exactly what's happened to mine, by 15 mph I'm in high gear.
What's really made it noticable is that I just finished installing the new
Springfield Ignition system that I purchased from Dick Patterson. With the
coach shifting so quick it's hard to tell if the timing curve change is doing
any good or not, with the early shifting it seemed to perform WORSE with the new
ignition system.
Ray
Ray & Lisa Erspamer
78 Royale Center Kitchen
TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale@att.net
414-745-3188
Web Site: http://ray-lisa.page.tl/
________________________________
From: Larry <weidnerl@wwt.net>
To: gmclist@temp.gmcnet.org
Sent: Wed, September 8, 2010 9:44:20 PM
Subject: Re: [GMCnet] Shifting and Vac Modulator
Ray,
I had exactly the same problem with mine about 2 yrs ago. Suddenly one day
without prior symptoms or warning it would shift out of 1st at 5mph and into
high by 15mph. After f#%king around with the vac modulator for a week or so,
and dropping the pan twice to check the SP and kick down solonoid, it occured to
me that this was a speed problem not a vacuum problem. I swapped out the
governor, and the problem went away. If all else fails try the governor. JWID
--
Larry :)
78 Royale w/500 Caddy
Menomonie, WI.
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
|
|
|
|
Goto Forum:
Current Time: Sat Sep 21 10:29:27 CDT 2024
Total time taken to generate the page: 0.00887 seconds
|
GMCforum
Search
Help
Register
Login
Home
