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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Savana rear heat comes on when towing uphill due to vacuum loss
Date: Thu, 15 Jul 2004 19:01:41 -0400
Message-ID: <>

All you need to do is install a vacuum reservoir (or make sure the installed
one is intact, if there is one).  This involves picking up a reservoir and a
check valve from the local salvage yard and plumbing the engine vacuum to the
HVAC controls to the reservoir.  Every reservoir I've ever seen had two vacuum
lines so all you have to do is cut the vacuum line and insert the
reservoir/check valve assembly in the line.

If you're cheap and/or a junk yard isn't handy you can make a reservoir out of
a suitably sized juice can and a couple of 1" long, 1/8" tubes (copper, steel
or brass.)  A quart size is convenient.  Just punch a couple of small holes in
the top of the can to drain the contents.  Rinse and dry.  Solder the tubes to
the holes.  Pick up a check valve for a buck or two at a car parts store (the
HELP line of small parts) and a bit of rubber tubing and you're set.

How big the reservoir will depend on how long you have to operate the engine
at near wide open throttle.  The HVAC system will slowly bleed in air so the
pressure in the reservoir will slowly rise until the valves don't seat
anymore.  The reservoir has to be large enough to handle that.

Other, more expensive and complicated solutions involve adding an electric
vacuum pump as used on most diesels.  Not worth the effort or money, though,
unless your HVAC system has a lot of vacuum leakage.


On Thu, 15 Jul 2004 13:32:26 -0400, Steve Parus <>

>Savana 2004 1500 5.4 L with YF7 convesion package and upfitter added
>rear heat and A/C.  The rear heat comes on when towing uphill.  I was
>told the YF7 package has a factory installed vacuum activated cooling
>water flow control valve that is open when there is no vacuum.  Thus
>when I tow uphill and the engine vacuum drops, the rear heat comes on.
>This happens even if the A/C system is on.  The loss of vacuum is also
>apparent as the front air flow changes from front panel to elsewhere
>(floor I think).
>Is this how things are going to have to work or is it possible the
>vacuum loos is too great ?
>In winter when rear heat is desired, the dash heat position must be in
>floor mode since that is the only position in which no vacuum is
>applied to the direction controls and thus the only position which
>will opens the rear water valve.  I've thought of connecting this
>valve to a vacuum line through a vacuum switch I could control on my
>own.  The connections are under the front right passenger carpet area.
>Where might I obtain an appropriate manual vacuum switch ?

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Savana rear heat comes on when towing uphill due to vacuum loss
Date: Fri, 16 Jul 2004 13:19:42 -0400
Message-ID: <>

On Fri, 16 Jul 2004 10:45:40 -0400, Steve Parus <>

>The engine is not being operated anywhere near full throttle, just a
>little more push on the gas pedal while climbing what is not anything
>resembling a mountain in Michigan, pulling a 4000 lbs trailer in
>"tow/haul" mode. it doesn't take but a minute or two for this effect
>to begin.

I knew I should have used a different term when I said "WOT".  The correct
thing was to say "at high manifold pressures".  On a heavily loaded engine in
a MH, the manifold pressure rises to near atmospheric at a little over a
quarter throttle.

Funny you'd mention that.  I just returned from a 2 week work/play trip to
Dearborn, 6 days of which were spent stranded in a transmission repair parking
lot while the hacks took three tries to rebuild my MH's transmission that
grenaded as I left the interstate.  Anyway, I had an OBD-2 data logger hooked
to the diagnostic port on the return trip so that I could keep an eye on the
transmission.  It was interesting to see just how fast manifold pressure rises
vs throttle position.  Even on the relatively mild hills on I-75 in Mi, the
engine spent long segments at greater than 700 inches of mercury (760 is
atmospheric pressure).  That is no vacuum to speak of, yet the HVAC behaved
properly, operating on the "stored" vacuum in the larger reservoir I fitted
after we got the rig.

I'm predicting that the reservoir/check valve combo will completely solve your
problem.  If not, then the best/cheapest way to get a vacuum  pump is to get
one from a junk yard.  Many (most) diesel light trucks use one to operate the
accessories and in some cases, vacuum brakes.  Aftermarket kits are available
but they're bit $$$$ compared to getting an OEM one from the junk yard.

That vacuum-operated valve is an on-off design.  It doesn't snap but it
doesn't modulate either.  The flow is either on or off.  The temperature is
regulated by an air mixing vane that mixes hot and cold air.

>Suggstions for electric vacuum pumps ?  I have easy acess to the
>valve's vacuum line and perhaps the electric pump would allow
>variation in its voltage which may allow partial opening of the valve
>to control the read temperature.  Assuming the valve is not a snap
>open/close type.

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