TVS valve/switch

[donald w. ogden]

Could someone please explain the function and operation of the TVS (thermal
vacuum switch ?) located on top of the engine (403) on the starboard/foward part
of the manifold. I want to be sure it's working properly and hooked up
correctly.
It has three ports. Bottom port is connected to carb, front lower left. Middle
port is connected to vacuum advance on distributor. Top port is connected to
carb, front upper center/right.
Vacuum readings taken at either top or middle port show 0" at idle and increases
to 10-15" at higher rpm (no load). Bottom port seems to be for vacuum source.
Don

 

[tim dawson]

The purpose of this guy is to adjust timing if the engine is overheating, to
attempt to get a handle on the overheating. It does this by altering the
vacuum to the distributor vacuum advance, as you probably have already
determined by tracing the hoses. Don't have my manual handy (at the office),
so I can't tell you how large the adjustment is, just that there is one.

- - Tim

[Charset iso-8859-1 unsupported, filtering to ASCII...]
> Could someone please explain the function and operation of the TVS (thermal
> vacuum switch ?) located on top of the engine (403) on the starboard/foward part
> of the manifold. I want to be sure it's working properly and hooked up
> correctly.
> It has three ports. Bottom port is connected to carb, front lower left. Middle
> port is connected to vacuum advance on distributor. Top port is connected to
> carb, front upper center/right.
> Vacuum readings taken at either top or middle port show 0" at idle and increases
> to 10-15" at higher rpm (no load). Bottom port seems to be for vacuum source.
> Don
>
>

- --
================================================================================
Tim Dawson (tadawson) Owner/Engineer
TPC Services Bellnet: (972)-221-7385
Lewisville, Texas 75067 FAXnet: (972)-221-0393
"The world is complex. Sendmail.cf reflects this...."

 

[gene]

from my web page on vacuum

As Emery points out the GMC manual says the original equipment distributor
is connected to PORTED
VACUUM when the engine is cold and MANIFOLD VACUUM when the engine is hot
using the TVS valve.

Gene

>Could someone please explain the function and operation of the TVS (thermal
>vacuum switch ?) located on top of the engine (403) on the
starboard/foward part
>of the manifold. I want to be sure it's working properly and hooked up
>correctly.
>It has three ports. Bottom port is connected to carb, front lower left.
Middle
>port is connected to vacuum advance on distributor. Top port is connected to
>carb, front upper center/right.
>Vacuum readings taken at either top or middle port show 0" at idle and
increases
>to 10-15" at higher rpm (no load). Bottom port seems to be for vacuum
source.
>Don
>
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/