Need help testing heat and a/c control - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**jlmerie** · 02-13-2014, 01:03 PM

I need to replace the heat and a/c control on my 1972 Sierra Grande. I found a good one at the local pull and pay. It does not have any broken levers and looks good. I am using a circuit test to see what wires have power at different switch positions. After testing I realized I am not sure what the results are supposed to be. Any help would be appreciated. I tested the control with the wiring bundle attached to the switch. There is a power in wire and three power out wires (Yellow, Blue, Orange). Here are the results I got from my test:
Switch off: All out wires off.
Switch 1st position. Yellow wire on (blue and orange wires off)
Switch 2nd position. Yellow and Blue wires on (Orange wire off)
Switch 3rd position. Yellow and Orange wires on (Blue wire off).

Thanks

**jlmerie** · 02-14-2014, 10:04 AM

Nobody knows if this is correct?

mrein3 · 02-14-2014, 02:11 PM

I put AC on a non-AC truck. When I got to wiring it I used the drawings in the service manual - Somebody on here has color ones available which make it much easier.

I no longer remember which color was which but my memory says I measured 3, 6, and 12 volts from the different fan switch positions. Or maybe it was 6, 9, and 12. I believe the resistor block in the HVAC box is what gives you the different voltages. Sorry I can't be of more help than that. Do however use a volt meter and not a test light.

I do know that EVERY plug on the back of EVERY switch I looked at in junk yards when piecing my system together was melted. I'm also pretty sure that none of the wires should be crossed. So take a look at the plastic plug behind the switch before you go too far.

**jlmerie** · 02-14-2014, 06:24 PM

Thanks mrein3. I visually inspected the switch and the plug on the back. I actually have two of each of them. My test came back with the same results both times I tested.(once with each switch and plug). You suggest to only use a volt meter to test the switch. Don't I have to have a 12 volt input source along with whatever voltage regulators might be in front of my switch? Clearly the purpose of the switch is to direct power to the right wires. I am guessing it is the combination of those wires that end up with the voltage measurements you included. Am I missing something here? It sure would not be the first time

02-13-2014, 01:03 PM	#1
jlmerie Registered User Join Date: Sep 2005 Location: Colorado Posts: 1,074	Need help testing heat and a/c control I need to replace the heat and a/c control on my 1972 Sierra Grande. I found a good one at the local pull and pay. It does not have any broken levers and looks good. I am using a circuit test to see what wires have power at different switch positions. After testing I realized I am not sure what the results are supposed to be. Any help would be appreciated. I tested the control with the wiring bundle attached to the switch. There is a power in wire and three power out wires (Yellow, Blue, Orange). Here are the results I got from my test: Switch off: All out wires off. Switch 1st position. Yellow wire on (blue and orange wires off) Switch 2nd position. Yellow and Blue wires on (Orange wire off) Switch 3rd position. Yellow and Orange wires on (Blue wire off). Thanks __________________ James

02-14-2014, 10:04 AM	#2
jlmerie Registered User Join Date: Sep 2005 Location: Colorado Posts: 1,074	Re: Need help testing heat and a/c control Nobody knows if this is correct? __________________ James

02-14-2014, 02:11 PM	#3
mrein3 Registered User Join Date: Feb 2000 Location: Center City, MN, USA Posts: 3,253	Re: Need help testing heat and a/c control I put AC on a non-AC truck. When I got to wiring it I used the drawings in the service manual - Somebody on here has color ones available which make it much easier. I no longer remember which color was which but my memory says I measured 3, 6, and 12 volts from the different fan switch positions. Or maybe it was 6, 9, and 12. I believe the resistor block in the HVAC box is what gives you the different voltages. Sorry I can't be of more help than that. Do however use a volt meter and not a test light. I do know that EVERY plug on the back of EVERY switch I looked at in junk yards when piecing my system together was melted. I'm also pretty sure that none of the wires should be crossed. So take a look at the plastic plug behind the switch before you go too far. __________________ '70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN

02-14-2014, 06:24 PM	#4
jlmerie Registered User Join Date: Sep 2005 Location: Colorado Posts: 1,074	Re: Need help testing heat and a/c control Thanks mrein3. I visually inspected the switch and the plug on the back. I actually have two of each of them. My test came back with the same results both times I tested.(once with each switch and plug). You suggest to only use a volt meter to test the switch. Don't I have to have a 12 volt input source along with whatever voltage regulators might be in front of my switch? Clearly the purpose of the switch is to direct power to the right wires. I am guessing it is the combination of those wires that end up with the voltage measurements you included. Am I missing something here? It sure would not be the first time __________________ James



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