Need help with cabin heat ASAP

[1976GMC]

Hello fellow GMC'rs,
I need help and I need advice regarding cabin heat. I have a trip coming up in a few days. The temps outside have dropped, not like in the mid-west
but still cold, and my cabin heat stopped working. I had some issues with it before, but I thought I had it solved with re-connecting all the vacuum
lines on the controller unit.

Right now I have the fan blowing, but not getting any heat at all - just cold air. I found out that my vacuum valve on the line to the heater core
was not operating (not getting any vacuum). I put a zip-tie on it to keep it in fully open position. This allowed fluid flow and it actually
produced heat in the cabin but only to the center outlet. Also, when controls were in the DEFROST position, I was also getting warm air delivered to
the windshield. That sounds great and I though I had the issue solved... Unfortunately, after playing with controls some more, the heat stopped
completely. The valve is still in OPEN position, the heater blend door seem to be in proper position for heat (lever position on the box does change
when I switch from COLD to HOT). I do have vacuum feed to the vacuum canister (about -16-17 in Hg) and out of the canister and to the controller in
the cab (about -15 in Hg). I'm getting vacuum to some of the selector valves (center one on defrost and the one on the passenger side for sure).

What am I missing and what can be causing the lack of heat?

One thing that puzzles me is this: the rubber hose going to the heater box warm past the vacuum valve (not hot, just warm). However, return hose
coming out of the box is ice cold. If I touch the same outlet hose further down, closer to the radiator, - it is as warm as the Input hose. Could my
heater core be completely plugged and not allowing coolant circulation at all? What else do I need to look for?

Any advice is appreciated. For this trip I might have to bundle up in blankets but still need to get this solved eventually. Especially because
I can tolerate cold temps much better than my significant other
--
Vadim Jitkov
'76 Glenbrook 26'
Pullman, WA

 

[john shutzbaugh]

I might try bypassing that water valve. The fact that you have feel warm but not hot past the valve suggests a restriction at that point. if that
does not help, I would suspect a plugged heater core.

Hoping for a bad valve! John S.
--
John Shutzbaugh, Vacaville, CA, ncserv;
77 Eleganza, bought it new, can't blame PO, and
78 Buskirk stretch, "What were we thinking?"

 

[1976GMC]

> I might try bypassing that water valve. The fact that you have feel warm but not hot past the valve suggests a restriction at that point. if
> that does not help, I would suspect a plugged heater core.
>
> Hoping for a bad valve! John S.

Question - is this valve in CLOSED or OPENED flow position when vacuum is not supplied to it?
--
Vadim Jitkov
'76 Glenbrook 26'
Pullman, WA

 

[john r. lebetski]

Valve is NORMALLY OPEN. Vacuum closes the valve on MAX AC You are looking at the puzzle backwards.
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[john r. lebetski]

The 2 GM defaults are
No vacuum to valve = OPEN
No vacuum to selectorr doors= DEF
Both for saftey reasons.
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[Matt Colie]

> The 2 GM defaults are
> No vacuum to valve = OPEN
> No vacuum to selectorr doors= DEF
> Both for saftey reasons.

Vadim,

John is correct. But, what has not been said is that the vacuum controlled valve can fail in a mid-position such that even if you were to wire it so
the visible operating arm was in the open position, it still might not actually be wide open. The Warm/Cold lines you are finding would suggest that
you have this issue.

It also could be that the heater core is air-bound. Have you had the coach out for an extended highway run with the heater on since the last coolant
service. This happened to me, and we were uncomfortable until - Blast of Heat - then we were uncomfortable again until I got it shut down.

Your heat "only out the center defrost" also sounds a like so many little vacuum leaks. There is no easy way to diagnose this, just inspect every
little vacuum line that you can find. If you are lucky (as I was), you might just find one little rubber line cracked where it connects to an
operator.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

 

[Carl Stouffer]

I had a similar issue a few years ago when we REALLY needed the heat. I checked everything and wasn't able to make repairs on the road and we just
bundled up and suffered during the rest of the trip. I subsequently back-flushed the heater core with a garden hose and got a lot of dirty looking
water out. That solved the problem for me. It sounds like you may have a different issue, but I'm just putting this out there just in case you want
to try it.

--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member

 

[john r. lebetski]

The partial open/ restricted water valve that Matt mentioned, along with low coolant or dirty core can cause warmer air from some vents and colder
from others as only part of the core is warm and the air moves in somewhat a linear fashion after leaving the core. I had fairly good heat out of the
VENT position and less out of the HEAT position (seems reversed) until all the air worked out of the core. Now it works correctly. Added challenge is
the high mounted position of the core relative to the 9 PSI rad cap. Your cap must be 100% as far as both rubber seals and the tiny return poppet
spring must close the return valve. This tiny spring fails long before the main pressure spring and is hidden. Also be sure the overflow hose is not
aged out and clamps snug if not GM spring type . Hose can look ok and be porous with micro checking. It will duct the coolant to the tank, but can
return air instead, lowering the system level instead of replenishing it with each hot/cold cycle
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[john r. lebetski]

Also be sure you press down on the cap reinstalling it so it turns to the final position not just to the saftey position. This may be obvious, but
more difficult than on a typical pass car due to location.
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[roy keen]

Is the engine at 180 Deg.or more? If not you may not have a good heat source.
--
Roy Keen
Minden,NV
76 X Glenbrook

 

[1976GMC]

First of all a BIG BIG THANK YOU to everyone for the replies. This forum and this community is truly amazing.
I had a chance to fiddle some more with the system. It turns out the vacuum valve was stuck partially closed. With the bypass - the heat in the
cabin works again. Both hoses to and from the heater core are now nice and warm to touch indicating that the coolant is flowing through it. I didn't
run the engine for too long, but it was long enough that the heat output to the cabin was noticeable even before the DigiPanel registered the engine
temp at the low end. My heat to defrost works too and seems to be uniform across all the windows. When directed to the cabin, I still get heat only
to the center outlet. The passenger side might be getting some heat too, but as I said - didn't run the system long enough to confirm it 100%. I'm
just glad to have heat at least to the center and to defrost.

Somewhat related question - I have a large floor vent at the center of the cabin (right below where two center outlets working). It seem like that
vent is blowing just cold air all the time and blowing it pretty intensely. The flow of air to that outlet does not change at any controller
position. Is that an "aftermarket" addition or is it likely to relate to come vacuum controller not working? Is that suppose to be intake for
recirculated air? I'm tempted to block off that vent with tape until i have time to explore what all control valves are doing/not doing. Is that a
good plan? Is it likely to cause some unintended harm elsewhere?
--
Vadim Jitkov
'76 Glenbrook 26'
Pullman, WA

 

[terry1]

That rectangular hole is the recirculate intake. Don't block it off. Stick a flashlight in the opening, go out front under the hood, and look for that
light in the outside air intake. I could see right through to the inside on mine, and subsequently blocked the outside off with a sheet of rubber.
Duct tape will do on the side of the road. It was a cold ride back from Danville in February when we bought the coach before I fixed that obvious,
incredible, design defect.
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.

 

[Ken H.]

Vadim,

That ~3"x~12 hole in the center of the firewall is the "Max Air Duct". '75
& '76 coaches (Type II air conditioning) came equipped with a plastic duct
which fit into that hole, then lead to 4 louvered outlets below the dash.
I think there was even a retrofit kit for earlier coaches. There should be
flow from there ONLY when the "Max" position is selected on the HVAC
control. If there's always flow, something's wrong with the max air
deflector door in the HVAC box. If you don't have Zay Brand's "Separating
Hot from Cold" booklet from GMCWS.org, you need to get it ASAP.
http://gmcws.org/Tech/air_conditioning/air_conditing.html and
http://www.gmcmhphotos.com/photos/g4637-type-ii-dash-air-condition-modification.html

HTH,

Ken H.

> First of all a BIG BIG THANK YOU to everyone for the replies. This forum
> and this community is truly amazing.
> I had a chance to fiddle some more with the system. It turns out the
> vacuum valve was stuck partially closed. With the bypass - the heat in the
> cabin works again. Both hoses to and from the heater core are now nice and
> warm to touch indicating that the coolant is flowing through it. I didn't
> run the engine for too long, but it was long enough that the heat output
> to the cabin was noticeable even before the DigiPanel registered the engine
> temp at the low end. My heat to defrost works too and seems to be uniform
> across all the windows. When directed to the cabin, I still get heat only
> to the center outlet. The passenger side might be getting some heat too,
> but as I said - didn't run the system long enough to confirm it 100%. I'm
> just glad to have heat at least to the center and to defrost.
>
> Somewhat related question - I have a large floor vent at the center of the
> cabin (right below where two center outlets working). It seem like that
> vent is blowing just cold air all the time and blowing it pretty
> intensely. The flow of air to that outlet does not change at any controller
> position. Is that an "aftermarket" addition or is it likely to relate to
> come vacuum controller not working? Is that suppose to be intake for
> recirculated air? I'm tempted to block off that vent with tape until i
> have time to explore what all control valves are doing/not doing. Is that a
> good plan? Is it likely to cause some unintended harm elsewhere?
> --
> Vadim Jitkov
> '76 Glenbrook 26'
> Pullman, WA
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[1976GMC]

> Vadim,
>
> That ~3"x~12 hole in the center of the firewall is the "Max Air Duct". '75
> & '76 coaches (Type II air conditioning) came equipped with a plastic duct
> which fit into that hole, then lead to 4 louvered outlets below the dash.
> I think there was even a retrofit kit for earlier coaches. There should be
> flow from there ONLY when the "Max" position is selected on the HVAC
> control. If there's always flow, something's wrong with the max air
> deflector door in the HVAC box. If you don't have Zay Brand's "Separating
> Hot from Cold" booklet from GMCWS.org, you need to get it ASAP.
> http://gmcws.org/Tech/air_conditioning/air_conditing.html and
> http://www.gmcmhphotos.com/photos/g4637-type-ii-dash-air-condition-modification.html
>
> HTH,
>
> Ken H.
>
>

>
> > First of all a BIG BIG THANK YOU to everyone for the replies. This forum
> > and this community is truly amazing.
> > I had a chance to fiddle some more with the system. It turns out the
> > vacuum valve was stuck partially closed. With the bypass - the heat in the
> > cabin works again. Both hoses to and from the heater core are now nice and
> > warm to touch indicating that the coolant is flowing through it. I didn't
> > run the engine for too long, but it was long enough that the heat output
> > to the cabin was noticeable even before the DigiPanel registered the engine
> > temp at the low end. My heat to defrost works too and seems to be uniform
> > across all the windows. When directed to the cabin, I still get heat only
> > to the center outlet. The passenger side might be getting some heat too,
> > but as I said - didn't run the system long enough to confirm it 100%. I'm
> > just glad to have heat at least to the center and to defrost.
> >
> > Somewhat related question - I have a large floor vent at the center of the
> > cabin (right below where two center outlets working). It seem like that
> > vent is blowing just cold air all the time and blowing it pretty
> > intensely. The flow of air to that outlet does not change at any controller
> > position. Is that an "aftermarket" addition or is it likely to relate to
> > come vacuum controller not working? Is that suppose to be intake for
> > recirculated air? I'm tempted to block off that vent with tape until i
> > have time to explore what all control valves are doing/not doing. Is that a
> > good plan? Is it likely to cause some unintended harm elsewhere?
> > --
> > Vadim Jitkov
> > '76 Glenbrook 26'
> > Pullman, WA
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

I do in fact have the Type 2 controller system, but the additional outlets have been removed. I only have the 3x12 opening down there. I've checked
door operation behind that opening. It does move and operate with the vacuum controller. However, even when fully closed, the air is still blowing
past the edges of the door. My guess the door just doesn't seal fully, perhaps need some foam tape there to seat against. With anticipated cold temps
during the drive, I've decided to block off that outlet for now. When summer comes, I'll open the heater/cooler box and try to get all things to
function as intended. Perhaps with installation of a ball valve on the heater core hose, I'll even get the AC to blow cold air I the summer
--
Vadim Jitkov
'76 Glenbrook 26'
Pullman, WA

 

[Ken B]

If you want it is a simple project to just replace that vacuum valve with a ball valve. Last one that I did was on a coach that I was transporting
cross country from Salt Lake City in the winter. II had no heat so I went Home Depot and bought a ball valve and swapped the old one out is about 10
minutes in the HD parking lot. It was a one screwdriver project. I delivered the coach to Florida where the owner said "good, I'll turn the ball
valve off and be done with it. I'll never need the heat anyway. If I ever do I'll turn the valve back on again."

The Valve is located in the heater hose in the center. It is accessed by opening the driver's side hood. Oh yes, to prevent a vacuum leak, be sure
to stick something in the vacuum line like a screw when the old one valve is removed.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

 

[Matt Colie]

> I do in fact have the Type 2 controller system, but the additional outlets have been removed. I only have the 3x12 opening down there. I've
> checked door operation behind that opening. It does move and operate with the vacuum controller. However, even when fully closed, the air is still
> blowing past the edges of the door. My guess the door just doesn't seal fully, perhaps need some foam tape there to seat against. With anticipated
> cold temps during the drive, I've decided to block off that outlet for now. When summer comes, I'll open the heater/cooler box and try to get all
> things to function as intended. Perhaps with installation of a ball valve on the heater core hose, I'll even get the AC to blow cold air I the
> summer

Vadim,

That door has probably lost the foam cover than it had just a few decades ago. The foam cover was both to seal the doors and as a noise abatement.
You can replace it with any soft foam. If you can find really soft self stick foam weather striping, that is what I used and it was effective when I
finally got it to stick.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

 

[1976GMC]

> ...
> That door has probably lost the foam cover than it had just a few decades ago...
> Matt

That is exactly what is going on there. Just bare metal flap at this time.
--
Vadim Jitkov
'76 Glenbrook 26'
Pullman, WA