Home » Public Forums » GMCnet » Thermo Vacuum Switch (TVS) question
| Thermo Vacuum Switch (TVS) question [message #140243] |
Sat, 20 August 2011 18:58  |
larry.whisler
Messages: 356
Registered: August 2005
Karma: 8
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I have heard that doing away/by passing the TVS is the way to go on these coaches.
So what is the "proper" way to plumb these vacuum lines when doing so?
There are 3 ports on the switch ( carb, vacuum advance, and tee line that goes to manifold and trans ).
If I run the spark advance directly to the ported spark port on the carb, I am left with an open port on the switch.
Per the manual, the TVS allows vacuum advance after the engine has reached operating temp.
And if anyone knows what the other ports do, I'd appreciate the knowledge.
larry whisler
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| Re: Thermo Vacuum Switch (TVS) question [message #140252 is a reply to message #140243] |
Sat, 20 August 2011 20:06  |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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| larry.whisler wrote on Sat, 20 August 2011 19:58 |
I have heard that doing away/by passing the TVS is the way to go on these coaches.
So what is the "proper" way to plumb these vacuum lines when doing so?
There are 3 ports on the switch ( carb, vacuum advance, and tee line that goes to manifold and trans ).
If I run the spark advance directly to the ported spark port on the carb, I am left with an open port on the switch.
Per the manual, the TVS allows vacuum advance after the engine has reached operating temp.
And if anyone knows what the other ports do, I'd appreciate the knowledge.
larry whisler
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Sir: the ported directly to dist, take the T out of the manifold vacuun and replace with a straight splice to tranny and leave the tvs switch sitting there all by itself nakie, or remove and plug.
C. Boyd
76 Crestmont
East Tennessee
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| Re: Thermo Vacuum Switch (TVS) question [message #140280 is a reply to message #140243] |
Sat, 20 August 2011 22:48 |
Chr$
Messages: 2690
Registered: January 2004
Location: Scottsdale, AZ
Karma: 1
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I think this whole setup is why I failed emissions. I found my newly replaced vacuum line from the TVS to the distributor was cooked and cracked above that damn crossover. Who would have thunk that. The thing was probably on the Dyno running a retarded timing. Gahh!!! Well, at least I now have a Rockwell Manifold and New Carb. If the coach fails again, I have spent enough to get an exemption from Emissions testing, so not a bad thing.
So does the coach run better with the TVS out of the loop? Does it run cleaner? does it use less gas?
-Chr$: Perpetual SmartAss
Scottsdale, AZ
77 Ex-Kingsley 455 SOLD!
2010 Nomad 24 Ft TT 390W PV W/MPPT, EV4010 and custom cargo door.
Photosite: Chrisc GMC:"It has Begun" TT: "The Other Woman"
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| Re: Thermo Vacuum Switch (TVS) question [message #140296 is a reply to message #140280] |
Sun, 21 August 2011 04:03 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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| Chr$ wrote on Sat, 20 August 2011 22:48 |
I think this whole setup is why I failed emissions. I found my newly replaced vacuum line from the TVS to the distributor was cooked and cracked above that damn crossover. Who would have thunk that. The thing was probably on the Dyno running a retarded timing. Gahh!!! Well, at least I now have a Rockwell Manifold and New Carb. If the coach fails again, I have spent enough to get an exemption from Emissions testing, so not a bad thing.
So does the coach run better with the TVS out of the loop? Does it run cleaner? does it use less gas?
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It does not run any better with the TVS gone. The TVS only switches to manifold vacuum when the engine is over normal temperature. Dick Paterson told me he has seen many TVSs fail and he just discards them. I found mine a few weeks back when I was cleaning up my work bench. It has been down there in the basement for years.
It's purpose was to advance the ignition which increases the idle speed it the engine gets too hot. This happens primarily in city traffic or when stopped for extended periods of time. I have forgotten the temperature when it kicks but I believe it was something like 215 degrees. I know the value is in the maintenance manual but I'm too lazy to go look for it.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Thermo Vacuum Switch (TVS) question [message #140308 is a reply to message #140307] |
Sun, 21 August 2011 10:19 |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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| larry.whisler wrote on Sun, 21 August 2011 11:11 |
Ken,
I agree with your analysis of the spark advance function per the maint manual.
What was the purpose of tee connecting it to the manifold vacuum port and the transmission vac line?
I could not find any other info on these functions by the switch.
larry
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Sir: not Ken, but the tvs switch allows the ported vacuun to flow thru to dist till the engine temp reaches 215* and then the tvs switches to the manifold vacuum to go the distributor. This allows for faster idle only cause you are probably in traffic with the AC going. When the engine temp cools to below 215 it switches back to ported vacuum. Ken can probably splain it better.
C. Boyd
76 Crestmont
East Tennessee
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| Re: [GMCnet] Thermo Vacuum Switch (TVS) question [message #140314 is a reply to message #140312] |
Sun, 21 August 2011 13:46 |
Dave Mumert
Messages: 272
Registered: February 2004
Location: Olds, AB, Canada
Karma: 0
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Hi Larry
Cars ran full manifold vacuum to the distributor for decades without any
problems. As I understand it the ported vacuum can about as an attempt to
lower emission in the 70's.
Check this site for a graph of ported vacuum vs. manifold vacuum
http://tinyurl.com/3vp5a7c
As soon as the throttle is cracked the ported vacuum is the same as manifold
vacuum. I'm guessing by the data on the graph that you can safely ignore
those that insist using manifold vacuum will destroy your engine.
Check here for an explanation of the difference and how it affects your
engine
http://tinyurl.com/ok7pfy
Basically the choice is yours, you may want to try both. You will likely
need to adjust your timing when you change, be sure to disconnect and plug
the vacuum line to the distributor in either case.
Dave
> -----Original Message-----
> Subject: Re: [GMCnet] Thermo Vacuum Switch (TVS) question
>
> okay, so if the TVSwitch is bypassed, would it be better to run the
distributor
> spark advance to the tee at the mainifold vacuum port or to the ported
park
> port on the carb?
>
> larry w
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| Re: [GMCnet] Thermo Vacuum Switch (TVS) question [message #140316 is a reply to message #140312] |
Sun, 21 August 2011 14:17 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Larry, when vacuum is applied to the diaphram in the vacuum advance, the
timing is advanced beyond what the basic timing setting is, plus the amount
that the centrifugal advance weights inside the distributor also do. The
total of all three is somewhere around 32 -36 degrees, depending on the
basic timing adjustment. When the throttle is depressed, vacuum is reduced,
and the spring in the vacuum advance mechanism, being stronger than the
reduced vacuum effect of the diaphram is, retards the timing by the amount
of the limits of the cannister. This is the condition where you would be
accellerating or climbing a grade where "pinging" would be more likely to
occur. That is what the purpose of the vacuum advance system is. Allows more
total advance, and reacts far more quickly than the centrifigal weights do
when the change in load occurs. As far as where you hook the hose, once the
throttle plates are open, it probably does not make much difference. If you
select the manifold tap, be sure to block the ported vacuum on the carb.,
and vise versa. It isn't rocket science, unless you have a fixed advance
system where all the functions of the advance/retard systems are handled by
the Engine management computer, as would be the case in the most advanced
Fuel injection system. Hope this helps you understand, rather than confuse
you.
Jim Hupy
Salem, OR
78 GMC Royale 403
On Sun, Aug 21, 2011 at 11:16 AM, larry.whisler
<larry.whisler@hotmail.com>wrote:
>
>
> okay, so if the TVSwitch is bypassed, would it be better to run the
> distributor spark advance to the tee at the mainifold vacuum port or to the
> ported park port on the carb?
>
> larry w
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: [GMCnet] Thermo Vacuum Switch (TVS) question [message #140329 is a reply to message #140314] |
Sun, 21 August 2011 16:38 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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I'll take the middle road. If you have a 26' coach and the original 3.07 final drive, I'd run it ported. If you have a lower gear final drive or a lighter load, run it off the manifold. What you don't want to do is go up the advance curve faster than the engine can accelerate loaded, cos that leads to detonation - which leads to damage. With an automatic behind it, it probably doesn't matter. The intent of ported vacuum is the same as VW's all mechanical station wagon/kombi distributor - let the advance track engine acceleration rather than anticipate it.
--johnny
--- On Sun, 8/21/11, Dave Mumert <dave@mumert.com> wrote:
From: Dave Mumert <dave@mumert.com>
Subject: Re: [GMCnet] Thermo Vacuum Switch (TVS) question
To: gmclist@temp.gmcnet.org
Date: Sunday, August 21, 2011, 6:46 PM
Hi Larry
Cars ran full manifold vacuum to the distributor for decades without any
problems. As I understand it the ported vacuum can about as an attempt to
lower emission in the 70's.
Check this site for a graph of ported vacuum vs. manifold vacuum
http://tinyurl.com/3vp5a7c
As soon as the throttle is cracked the ported vacuum is the same as manifold
vacuum. I'm guessing by the data on the graph that you can safely ignore
those that insist using manifold vacuum will destroy your engine.
Check here for an explanation of the difference and how it affects your
engine
http://tinyurl.com/ok7pfy
Basically the choice is yours, you may want to try both. You will likely
need to adjust your timing when you change, be sure to disconnect and plug
the vacuum line to the distributor in either case.
Dave
> -----Original Message-----
> Subject: Re: [GMCnet] Thermo Vacuum Switch (TVS) question
>
> okay, so if the TVSwitch is bypassed, would it be better to run the
distributor
> spark advance to the tee at the mainifold vacuum port or to the ported
park
> port on the carb?
>
> larry w
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Thermo Vacuum Switch (TVS) question [message #140330 is a reply to message #140316] |
Sun, 21 August 2011 16:39 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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You 'splain it bettern' me.
--johnny
--- On Sun, 8/21/11, James Hupy <jamesh1296@gmail.com> wrote:
From: James Hupy <jamesh1296@gmail.com>
Subject: Re: [GMCnet] Thermo Vacuum Switch (TVS) question
To: gmclist@temp.gmcnet.org
Date: Sunday, August 21, 2011, 7:17 PM
Larry, when vacuum is applied to the diaphram in the vacuum advance, the
timing is advanced beyond what the basic timing setting is, plus the amount
that the centrifugal advance weights inside the distributor also do. The
total of all three is somewhere around 32 -36 degrees, depending on the
basic timing adjustment. When the throttle is depressed, vacuum is reduced,
and the spring in the vacuum advance mechanism, being stronger than the
reduced vacuum effect of the diaphram is, retards the timing by the amount
of the limits of the cannister. This is the condition where you would be
accellerating or climbing a grade where "pinging" would be more likely to
occur. That is what the purpose of the vacuum advance system is. Allows more
total advance, and reacts far more quickly than the centrifigal weights do
when the change in load occurs. As far as where you hook the hose, once the
throttle plates are open, it probably does not make much difference. If you
select the manifold tap, be sure to block the ported vacuum on the carb.,
and vise versa. It isn't rocket science, unless you have a fixed advance
system where all the functions of the advance/retard systems are handled by
the Engine management computer, as would be the case in the most advanced
Fuel injection system. Hope this helps you understand, rather than confuse
you.
Jim Hupy
Salem, OR
78 GMC Royale 403
On Sun, Aug 21, 2011 at 11:16 AM, larry.whisler
<larry.whisler@hotmail.com>wrote:
>
>
> okay, so if the TVSwitch is bypassed, would it be better to run the
> distributor spark advance to the tee at the mainifold vacuum port or to the
> ported park port on the carb?
>
> larry w
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
|
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| Re: [GMCnet] Thermo Vacuum Switch (TVS) question [message #140379 is a reply to message #140312] |
Mon, 22 August 2011 00:37 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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| larry.whisler wrote on Sun, 21 August 2011 13:16 |
okay, so if the TVSwitch is bypassed, would it be better to run the distributor spark advance to the tee at the mainifold vacuum port or to the ported park port on the carb?
larry w
|
NO
The TVS normally does noting. It only gets involved if the engine is running overheated. The TVS is nothing more than a switch. It switch between two vacuum sources. Normal (ported) vacuum and overheated (manifold) vacuum.
The TEE is simply a way for the TVS to get to a manifold vacuum source.
These two sources are NOT the same.
They do not track the same. I have looked at both of them simultaneously at various throttle settings using two vacuum gauges. The ported vacuum runs a couple of inches lower at most throttle settings and coach speeds.
When GM started using ported vacuum they also used a different vacuum advance with a little different curve. If you switch to manifold vacuum you have two choices beside having to adjust the carb idle stop screw.
I switched mine at one time and did not make either of the following changes. It would ping on manifold vacuum. The pinging went away on the normal ported vacuum. I had to reduce the static timing 4 to 5 degrees to eliminate the pinging on manifold vacuum. Mine is back on ported vacuum and the static timing is back where Dick Paterson recommended it should be.
If you do change to manifold vacuum do one of the following:
1. Retard the static timing to make up for the difference in the vacuum curve. Also be aware that the total advance (static + centrifugal + static) will now be less so in theory your fuel mileage should be less. It may be an insignificant amount.
2. Buy and install a new vacuum advance that has the correct curve for manifold vacuum. These are not too expensive because many, many vehicles ran these prior to the use of ported vacuum. When you go looking for one be sure you understand the maximum advance available and at what vacuum levels it is designed run at to get those advance numbers.
To me it is not worth all of the trouble for no gain and a potential net loss.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Thermo Vacuum Switch (TVS) question [message #140393 is a reply to message #140316] |
Mon, 22 August 2011 07:31 |
Jim Bounds
Messages: 842
Registered: January 2004
Karma: 0
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Senior Member |
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Concerning the "ported vacuum tree" component on the motor, I see it as an apendix that fails in the bad position and being over 30 years old we disconnect it first thing. You must pull "ported vacuum" in most all conditions from the carb port directly to the dist. for the motor to get what it wants. the vacuum tree fails usually in the manifold vacuum condition when they go bad. In idle condition, you do not want any vacuum to the dist., the idle circuit in the carb controlls vacuum ouput of the port and when the carb is in it's oidle jects, the ported output should have "0" vacuum for the dist. As the carb comes off idle, the idle circuit gives way to the main jets and vacuu m on the ported output comes up pulling the vacuum advance in the dist.. This is the condition you want. At idle, you have only the initial advance the dist. is set at-- mech and vacuum advance in the dist. will be off. Just off idle, the vacuum adv. kicks in then
as you spool up the R's, the mech. advance does it's thing to give you total advance. When you "gag the maggot' vacuum adv. drops off retarding the advance to allow for better full throttle operation when the power valve kicks in a 4" where we set up the carbs at.
All this is done without relying that the ole ported vacuum tree doing it's thing. If you have emissions and vacuum routing inspection, you can plug the intake vacuum hose and do the same thing. This is what we do here. BTW, think about this-- adjust your throttle minkage using a vacuum gauge on the ported output to idle at "O" vacuum at idle-- in park-- AC off. Twist the dist. till you have 950 RPM on the motor. You are now giving the motor what it wants not what the specs say you should. Now you can fine tune the motor to not ping under load "Power tuning on the fly". This, to me, is the best way to tune an old motor that has been modified. Works for me.
Jim Bounds
--------------------------
----- Original Message -----
From: James Hupy <jamesh1296@gmail.com>
To: gmclist@temp.gmcnet.org
Cc:
Sent: Sunday, August 21, 2011 3:17 PM
Subject: Re: [GMCnet] Thermo Vacuum Switch (TVS) question
Larry, when vacuum is applied to the diaphram in the vacuum advance, the
timing is advanced beyond what the basic timing setting is, plus the amount
that the centrifugal advance weights inside the distributor also do. The
total of all three is somewhere around 32 -36 degrees, depending on the
basic timing adjustment. When the throttle is depressed, vacuum is reduced,
and the spring in the vacuum advance mechanism, being stronger than the
reduced vacuum effect of the diaphram is, retards the timing by the amount
of the limits of the cannister. This is the condition where you would be
accellerating or climbing a grade where "pinging" would be more likely to
occur. That is what the purpose of the vacuum advance system is. Allows more
total advance, and reacts far more quickly than the centrifigal weights do
when the change in load occurs. As far as where you hook the hose, once the
throttle plates are open, it probably does not make much difference. If you
select the manifold tap, be sure to block the ported vacuum on the carb.,
and vise versa. It isn't rocket science, unless you have a fixed advance
system where all the functions of the advance/retard systems are handled by
the Engine management computer, as would be the case in the most advanced
Fuel injection system. Hope this helps you understand, rather than confuse
you.
Jim Hupy
Salem, OR
78 GMC Royale 403
On Sun, Aug 21, 2011 at 11:16 AM, larry.whisler
<larry.whisler@hotmail.com>wrote:
>
>
> okay, so if the TVSwitch is bypassed, would it be better to run the
> distributor spark advance to the tee at the mainifold vacuum port or to the
> ported park port on the carb?
>
> larry w
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
_______________________________________________
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