Heating wiring question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Marshall50 · 01-30-2018, 05:09 PM

Hey guys, brand new to the forum. I've read many threads but can't seem to find what I'm looking for. Hoping someone can help me out.

I have a 79 K10 with a new 3 speed heater only fan selector. I recently swapped out the heater only box with an ac/heater box that someone donated. I don't care about the ac right now but only want the heater and defroster to work right now. Here's the problem I have... the fan selector is 3 wire and the resistor is 4 wire. First of all is this even possible to use the non a/c selector switch? I don't want to have to buy another new switch just yet unless I know for sure I will want ac in the future. Currently, I only have the blower motor working when the key is on and the selector switch is on hi. There is nothing on the 1st two speeds. Any help is appreciated.

Bandit130 · 01-30-2018, 06:08 PM

**hatzie** · 01-30-2018, 06:57 PM

You can run the non AC resistor and heater control with the heater sub-harness.

AC uses a relay to run the blower at full speed without melting the switch. You'll need to retrofit the AC wiring and relay along with the AC controls.

How are you handling the firewall ducting differences?

Marshall50 · 01-30-2018, 07:08 PM

Thanks for your response Hatzie!
That might be a hurdle I have yet to come to. I got the switch to operate correctly. Turn the key on, the blower motor comes on. I also got the blower motor to increase with the selector switch fan speeds respectively. I'm not using the number 4 connector on the resistor. I have hot air but I'm concerned the resistor might heat up to much. Is that possible?

**hatzie** · 01-30-2018, 07:37 PM

You should have airflow over it with the blower on. Not sure whether you'll get in trouble without...

01-30-2018, 05:09 PM	#1
Marshall50 Registered User Join Date: Jan 2018 Location: BRIGHAM CITY, UT Posts: 2	Heating wiring question Hey guys, brand new to the forum. I've read many threads but can't seem to find what I'm looking for. Hoping someone can help me out. I have a 79 K10 with a new 3 speed heater only fan selector. I recently swapped out the heater only box with an ac/heater box that someone donated. I don't care about the ac right now but only want the heater and defroster to work right now. Here's the problem I have... the fan selector is 3 wire and the resistor is 4 wire. First of all is this even possible to use the non a/c selector switch? I don't want to have to buy another new switch just yet unless I know for sure I will want ac in the future. Currently, I only have the blower motor working when the key is on and the selector switch is on hi. There is nothing on the 1st two speeds. Any help is appreciated.

01-30-2018, 06:08 PM	#2
Bandit130 Registered User Join Date: Dec 2014 Location: Euless, Texas Posts: 520	Re: Heating wiring question __________________ 1974 Cheyenne 10 LWB Stock 350 W/HEI /TH350/AC/4 BBL Quadrajet Mopar by Birth Chevy by Choice

01-30-2018, 06:57 PM	#3
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: Heating wiring question You can run the non AC resistor and heater control with the heater sub-harness. AC uses a relay to run the blower at full speed without melting the switch. You'll need to retrofit the AC wiring and relay along with the AC controls. How are you handling the firewall ducting differences? __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.

01-30-2018, 07:08 PM	#4
Marshall50 Registered User Join Date: Jan 2018 Location: BRIGHAM CITY, UT Posts: 2	Re: Heating wiring question Thanks for your response Hatzie! That might be a hurdle I have yet to come to. I got the switch to operate correctly. Turn the key on, the blower motor comes on. I also got the blower motor to increase with the selector switch fan speeds respectively. I'm not using the number 4 connector on the resistor. I have hot air but I'm concerned the resistor might heat up to much. Is that possible?

01-30-2018, 07:37 PM	#5
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: Heating wiring question You should have airflow over it with the blower on. Not sure whether you'll get in trouble without... __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.

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