Home » Public Forums » GMCnet » [GMCnet] Vacuum Modulator
| [GMCnet] Vacuum Modulator [message #106009] |
Tue, 16 November 2010 10:51  |
paul h cashman
Messages: 176
Registered: May 2005
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Hi All,
I was going to replace my Transmission Vacuum Modulator today.I put a Vacuum
Gauge on the hose going to the old Modulator .
I Started the Engine to to get the excess Fluid, back in the Torque
Converter.I checked the vacuum Gauge and it read 0 Vacuum at Idle.I checked
the Vacuum line and it was clear.I checked where the line was hooked and it
was ported Vacuum .
I checked the emission Chapter in my Manual it did not show a hook up to the
Vacuum Modulator.
I checked the Transmission chapter it did not show a hook up either
My Transmission seemed to Shift normally, but this last Summer on two
occasions it didn't want to shift to high Gear..I backed off on the Throttle
and it shifted to High Gear.
Which Vacuum ,should the Modulator, be hooked to.?
Paul Cashman
Riverdale GA
78 Transmode
78 Bug Convertible
75 Corvette
89 Harley Sportster
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Paul H Cashman
Riverdale Ga
1978 Transmode
1975 Corvette
1978 Beetle Convertible
1989 Harley Davidson Sportster
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| Re: [GMCnet] Vacuum Modulator [message #106010 is a reply to message #106009] |
Tue, 16 November 2010 11:12  |
emerystora
Messages: 4442
Registered: January 2004
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Paul
check the steel line. It might be clogged. There is a restriction in it that can sometimes clog. Blow it out with compressed air and check to see if you can get flow through it.
On Nov 16, 2010, at 9:51 AM, Paul Cashman wrote:
> Hi All,
> I was going to replace my Transmission Vacuum Modulator today.I put a Vacuum
> Gauge on the hose going to the old Modulator .
> I Started the Engine to to get the excess Fluid, back in the Torque
> Converter.I checked the vacuum Gauge and it read 0 Vacuum at Idle.I checked
> the Vacuum line and it was clear.I checked where the line was hooked and it
> was ported Vacuum .
> I checked the emission Chapter in my Manual it did not show a hook up to the
> Vacuum Modulator.
> I checked the Transmission chapter it did not show a hook up either
> My Transmission seemed to Shift normally, but this last Summer on two
> occasions it didn't want to shift to high Gear..I backed off on the Throttle
> and it shifted to High Gear.
> Which Vacuum ,should the Modulator, be hooked to.?
>
> Paul Cashman
> Riverdale GA
> 78 Transmode
> 78 Bug Convertible
> 75 Corvette
> 89 Harley Sportster
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Emery Stora
77 Kingsley
Santa Fe, NM
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| Re: [GMCnet] Vacuum Modulator [message #106014 is a reply to message #106009] |
Tue, 16 November 2010 11:49 |
midlf
Messages: 2212
Registered: July 2007
Location: SE Wisc. (Palmyra)
Karma: 1
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| paul h cashman wrote on Tue, 16 November 2010 10:51 |
Which Vacuum ,should the Modulator, be hooked to.?
Paul Cashman
Riverdale GA
78 Transmode
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The maint manual just refers to the modulator supply as "engine vacuum". To me that means straight, unmodified, manifold vacuum.
Steve Southworth
1974 Glacier TZE064V100150 (for workin on)
1975 Transmode TZE365V100394 (parts & spares)
Palmyra WI
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| Re: [GMCnet] Vacuum Modulator [message #106053 is a reply to message #106009] |
Wed, 17 November 2010 02:23 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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It hooks to to MANIFOLD vacuum port just in front and slightly to the driver's side of the center of the carburetor.
To further expand on Emery's post.
If you still do not have vacuum after hooking it there, There is a crimp in hte steel line that runs across the passenger side of the manifold and down the back of the engine. The crimp is an intended restriction but occasionally becomes plugged with either rust or carbon. A few years back mine was plugged so badly that I could not open it up with 100 PSI of shop air. I ended up putting it in a vise and applying 100 psi of shop air to it while I heated the restrictor end with an acetylene torch. That finally opened it up. After that I bead blasted the entire line and sprayed it gray before reinstalling it again.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Vacuum Modulator [message #106070 is a reply to message #106053] |
Wed, 17 November 2010 08:49 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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Just curious, if the modulator is working correctly based on engine vacuum, why
does the steel line need a restriction ??
I was told by an extremely competent source that the steel line could be
replaced with a hose.
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: Ken Burton <n9cv@comcast.net>
To: gmclist@temp.gmcnet.org
Sent: Wed, November 17, 2010 2:23:06 AM
Subject: Re: [GMCnet] Vacuum Modulator
It hooks to to MANIFOLD vacuum port just in front and slightly to the driver's
side of the center of the carburetor.
To further expand on Emery's post.
If you still do not have vacuum after hooking it there, There is a crimp in hte
steel line that runs across the passenger side of the manifold and down the back
of the engine. The crimp is an intended restriction but occasionally becomes
plugged with either rust or carbon. A few years back mine was plugged so badly
that I could not open it up with 100 PSI of shop air. I ended up putting it in
a vise and applying 100 psi of shop air to it while I heated the restrictor end
with an acetylene torch. That finally opened it up. After that I bead blasted
the entire line and sprayed it gray before reinstalling it again.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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| Re: [GMCnet] Vacuum Modulator [message #106084 is a reply to message #106070] |
Wed, 17 November 2010 12:44 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Ray, that restriction is in the steel line to attenuate or moderate the
vacuum pulses caused from changing throttle positions, and exist to protect
the modulator diaphram from these sudden changes in vacuum.
Jim Hupy
Salem, Or
78 Royale 403
On Wed, Nov 17, 2010 at 6:49 AM, Ray Erspamer <78gmc-royale@att.net> wrote:
> Just curious, if the modulator is working correctly based on engine vacuum,
> why
> does the steel line need a restriction ??
>
> I was told by an extremely competent source that the steel line could be
> replaced with a hose.
>
>
>
> 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: Ken Burton <n9cv@comcast.net>
> To: gmclist@temp.gmcnet.org
> Sent: Wed, November 17, 2010 2:23:06 AM
> Subject: Re: [GMCnet] Vacuum Modulator
>
>
>
> It hooks to to MANIFOLD vacuum port just in front and slightly to the
> driver's
> side of the center of the carburetor.
>
> To further expand on Emery's post.
>
> If you still do not have vacuum after hooking it there, There is a crimp in
> hte
> steel line that runs across the passenger side of the manifold and down the
> back
> of the engine. The crimp is an intended restriction but occasionally
> becomes
> plugged with either rust or carbon. A few years back mine was plugged so
> badly
> that I could not open it up with 100 PSI of shop air. I ended up putting
> it in
> a vise and applying 100 psi of shop air to it while I heated the restrictor
> end
> with an acetylene torch. That finally opened it up. After that I bead
> blasted
> the entire line and sprayed it gray before reinstalling it again.
>
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
> _______________________________________________
> 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
>
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| Re: [GMCnet] Vacuum Modulator [message #106113 is a reply to message #106084] |
Thu, 18 November 2010 05:55 |
Steven Ferguson
Messages: 3447
Registered: May 2006
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Thanks for that Jim. I have always wondered why the odd shaped
termination.
On Wed, Nov 17, 2010 at 11:44 AM, James Hupy <jamesh1296@gmail.com> wrote:
> Ray, that restriction is in the steel line to attenuate or moderate the
> vacuum pulses caused from changing throttle positions, and exist to protect
> the modulator diaphram from these sudden changes in vacuum.
> Jim Hupy
> Salem, Or
> 78 Royale 403
>
> On Wed, Nov 17, 2010 at 6:49 AM, Ray Erspamer <78gmc-royale@att.net>
> wrote:
>
> > Just curious, if the modulator is working correctly based on engine
> vacuum,
> > why
> > does the steel line need a restriction ??
> >
> > I was told by an extremely competent source that the steel line could be
> > replaced with a hose.
> >
> >
> >
> > 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: Ken Burton <n9cv@comcast.net>
> > To: gmclist@temp.gmcnet.org
> > Sent: Wed, November 17, 2010 2:23:06 AM
> > Subject: Re: [GMCnet] Vacuum Modulator
> >
> >
> >
> > It hooks to to MANIFOLD vacuum port just in front and slightly to the
> > driver's
> > side of the center of the carburetor.
> >
> > To further expand on Emery's post.
> >
> > If you still do not have vacuum after hooking it there, There is a crimp
> in
> > hte
> > steel line that runs across the passenger side of the manifold and down
> the
> > back
> > of the engine. The crimp is an intended restriction but occasionally
> > becomes
> > plugged with either rust or carbon. A few years back mine was plugged so
> > badly
> > that I could not open it up with 100 PSI of shop air. I ended up putting
> > it in
> > a vise and applying 100 psi of shop air to it while I heated the
> restrictor
> > end
> > with an acetylene torch. That finally opened it up. After that I bead
> > blasted
> > the entire line and sprayed it gray before reinstalling it again.
> >
> > --
> > Ken Burton - N9KB
> > 76 Palm Beach
> > Hebron, Indiana
> > _______________________________________________
> > 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
> >
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Steve Ferguson
Sierra Vista, AZ
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| Re: [GMCnet] Vacuum Modulator [message #106121 is a reply to message #106070] |
Thu, 18 November 2010 07:42 |
 |
mike miller
Messages: 3576
Registered: February 2004
Location: Hillsboro, Oregon
Karma: 0
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| Ray Erspamer wrote on Wed, 17 November 2010 06:49 |
... I was told by an extremely competent source that the steel line could be
replaced with a hose.
|
No matter what you use to get vacuum to the modulator, I understand it should have a low spot in the line:
<http://www.gmcmhphotos.com/photos/showphoto.php?photo=3989&cat=3382>
(Just something I have read about… Not my personal experience.)
Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
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| Re: [GMCnet] Vacuum Modulator [message #106127 is a reply to message #106121] |
Thu, 18 November 2010 08:04 |
 |
USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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G'day,
Question for both Ray and Mike:
Why?
Regards,
Rob M.
USAussie
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org
[mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Mike Miller
Sent: Thursday, November 18, 2010 7:42 AM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Vacuum Modulator
Ray Erspamer wrote on Wed, 17 November 2010 06:49
> ... I was told by an extremely competent source that the steel line could
be
> replaced with a hose.
No matter what you use to get vacuum to the modulator, I understand it
should have a low spot in the line:
<http://www.gmcmhphotos.com/photos/showphoto.php?photo=3989&cat=3382>
(Just something I have read about… Not my personal experience.)
--
Mike Miller
#01 - `73 26' X Painted D. "The Spare"
#02 - `78 23' Birchaven Rear Bath "The User"
#03 - `77 23' Birchaven Side Bath "The Keeper"
Hillsboro, OR
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Vacuum Modulator [message #106129 is a reply to message #106128] |
Thu, 18 November 2010 08:38 |
Larry
Messages: 2875
Registered: January 2004
Location: Menomonie, WI
Karma: 10
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| CBWoodSR wrote on Thu, 18 November 2010 08:11 |
Rob..
2nd one
Try the link, it explains it there..
So water doesnt condense into the modulator and freeze and let the trannie react accordingly as explained
Thankx
|
Yup, that's it. A fairly common problem in the northern states. When water gets into the modulator and freezes, the modulator no longer "modulates", letting the line pressures run wild. When that happens, it is usually on a cold morning. You start the car, put it in reverse to back out of the driveway, and "BAM"...blows out the reverse servo. Actually puts a large chunk of cast iron from the valve body on the bottom of the pan. All other gears work, but obviously no reverse. The loop in the line allows water to gather in low spot of the rubber line instead of the modulator. The fix is just putting in a new valve body.
Larry 
78 Royale w/500 Caddy
Menomonie, WI.
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| Re: [GMCnet] Vacuum Modulator [message #106170 is a reply to message #106129] |
Thu, 18 November 2010 21:31 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
Karma: 0
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| Larry wrote on Thu, 18 November 2010 08:38 |
| CBWoodSR wrote on Thu, 18 November 2010 08:11 |
Rob..
2nd one
Try the link, it explains it there..
So water doesnt condense into the modulator and freeze and let the trannie react accordingly as explained
Thankx
|
Yup, that's it. A fairly common problem in the northern states. When water gets into the modulator and freezes, the modulator no longer "modulates", letting the line pressures run wild. When that happens, it is usually on a cold morning. You start the car, put it in reverse to back out of the driveway, and "BAM"...blows out the reverse servo. Actually puts a large chunk of cast iron from the valve body on the bottom of the pan. All other gears work, but obviously no reverse. The loop in the line allows water to gather in low spot of the rubber line instead of the modulator. The fix is just putting in a new valve body.
|
what is the difference between that and a rusted tube that blocks any vacuum from reaching the modulator?
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: [GMCnet] Vacuum Modulator [message #106238 is a reply to message #106170] |
Fri, 19 November 2010 19:13 |
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mike miller
Messages: 3576
Registered: February 2004
Location: Hillsboro, Oregon
Karma: 0
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Senior Member |
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| fred v wrote on Thu, 18 November 2010 19:31 |
| Larry wrote on Thu, 18 November 2010 08:38 |
... The loop in the line allows water to gather in low spot of the rubber line instead of the modulator. ...
|
what is the difference between that and a rusted tube that blocks any vacuum from reaching the modulator?
|
I was hoping that someone with actual experience with this issue would respond. ... 
Here is my theory.
Water freezing in the vacuum LINE would give you the same symptoms as a rust plugged line... until it warmed up.
Water freezing INSIDE the MODULATOR would expand the diaphram and/or other wise gum up the works. ... leading to the BAD things described.
Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
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