So in the never ending quest to get decent AC, I have found that my hot water shut of valve isn't getting vacuum when the control is in the AC position.
I have verified the purple line goes to the vacuum canister and I know I get vacuum there as I have a vacuum gauge.
A quick check shown things are mostly where they should be on the dash control.

Has anyone fallen down this particular GMC rabbit hole?
Should I pull the control unit and clean it up??

TIA

I have the 77 version and I think vac only closes the valve in recirc. Not sure if Norm and cold slider sends vac. Try yours on Rec to see if you get suction. Also vac lines under hood can be partly cracked and crispy giving some vacuum but not full signal. Look Ok but have cracks leàking. The wafer retaner can be cracked or loose and not sending full signal output.

JohnL455 wrote on Fri, 19 June 2015 16:58

I have the 77 version and I think vac only closes the valve in recirc. Not sure if Norm and cold slider sends vac. Try yours on Rec to see if you get suction. Also vac lines under hood can be partly cracked and crispy giving some vacuum but not full signal. Look Ok but have cracks leàking. The wafer retaner can be cracked or loose and not sending full signal output.

John,
You are correct!
That was 1/2 the problem, it only shuts off the hot water in recirc, good to know.
The other 1/2 of the problem was that the check valve was in backwards....

So now my AC will be Ice cold right?

I ate that rabbit but didn't care for the taste. On my 77 heat/AC control unit, there are two air switches. The bit one for the air door is readily available and about $25. The other one is for the hot water valve and is not available TO MY KNOWLEDGE.

I replaced it with a manual PEX valve like JR showed. The only problem is that in our part of the world, we may want heat in the morning and AC in the afternoon. On a January rally in Tampa, I found myself having to stop and open/close the valve in order to stay comfortable. I will replace the valve with a ball valve that has a longer handle and rig up a pull/push control (think choke cable on old cars and lawnmowers).

If you remove that valve in look inside it you will see that it does not fully shut off the hot coolant flow. It only reduces it some. I installed an manual valve year ago. I turn it off in the spring and back on when it gets cold. The manual (pex) valve is a quick install and can be done inn the parking lot of Lowes or wherever you buy it. You will need a razor knife, a screw driver and 2 hose clamps. I have mine under the front hood near the heater. It is easily accessible there.

...You just want to do what I said. Throw away the heater door and connect one of these to the temp control cable that currently operates that door:

http://www.crankshift.com/heater-core/heater-control-valve-3/

Mark, do you have a part number or make/model for that unit? It is EXACTLY what I'd prefer to do.

I have only looked at 2 of them and I believe both were OEM, but I'm not sure. Both had a loosely fitted butterfly for the valve with a hole drilled in the butterfly.

kerry pinkerton wrote on Sat, 20 June 2015 07:00

I ate that rabbit but didn't care for the taste. On my 77 heat/AC control unit, there are two air switches. The bit one for the air door is readily available and about $25. The other one is for the hot water valve and is not available TO MY KNOWLEDGE.

I replaced it with a manual PEX valve like JR showed. The only problem is that in our part of the world, we may want heat in the morning and AC in the afternoon. On a January rally in Tampa, I found myself having to stop and open/close the valve in order to stay comfortable. I will replace the valve with a ball valve that has a longer handle and rig up a pull/push control (think choke cable on old cars and lawnmowers).

You just want to do what I said. Throw away the heater door and connect one of these to the temp control cable that currently operates that door:

http://www.crankshift.com/heater-core/heater-control-valve-3/

I thought it was VAC= CLOSED. Though being if vac failed you would have heat and def.

Google is your friend. Well not really but it's danged handy even if they steal all your info and preferences and use them to load your screen with ads.

$15 bucks down the street

http:// shop.advanceautoparts.com/p/carquest-or-factor-air-cable-operated-non-bypass-open-heater-valve-74628/5652979-p?cm_mmc=PLA-_-Google-_-GPLA-_-5652979&am p;am p;ci_src=17588969&ci_sku=5652979&ci_gpa=pla&ci_kw=&iv_=__iv_p_1_a_214327102_g_12425515822_w_kwd-68673849142_h_9012692_ii__d_c_v__n_g_x _pla_y_6201684_f_online_o_5652979_z_US_i_en_j_68673849142_s__vi__&gclid=CIvTg9aqocYCFYc6gQod4hQArg#fragment-2

There are also listing for other chain stores or Amazon for about the same price. I prefer to buy local when I can.