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04-06-2025, 09:38 AM
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#1 |
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Registered User
Join Date: Dec 2004
Location: Highland, IN
Posts: 2,353
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Heat switch
I lost low and medium speeds on my blower motor on my heater on my 79. I replaced the resister on the heater box under the hood. No change. Think the heater switch is bad. Is there any way to test this. Looked on line. Only show you how to replace it. Need some help. Thanks
Chris
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Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart  My build 2008-2016. http://m.imgur.com/a/bURrE |
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04-06-2025, 11:07 AM
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#2 |
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Moderator
 
Join Date: Oct 2008
Location: Wentworth, NH
Posts: 5,049
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Re: Heat switch
Low & Med positions feed 12v to terminals on the blower resistor.
Your blower resistor will have four terminals or three terminals depending on how many medium positions you've got on the switch. High triggers a relay that switches battery voltage to the blower motor on AC trucks. On the regular heat-only trucks the high position goes to the motor output terminal on the blower resistor.
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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...
And please let us know if and how your repairs were successful. |
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04-07-2025, 07:34 AM
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#3 |
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Registered User
Join Date: Dec 2004
Location: Highland, IN
Posts: 2,353
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Re: Heat switch
Thought i would try a new relay. Blows the fuse and does not work at all. Are the top three of these relays the same?
https://www.oreillyauto.com/shop/b/l...cDDoDppaQoHpow
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Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE |
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04-08-2025, 07:38 AM
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#4 |
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Registered User
Join Date: Dec 2004
Location: Highland, IN
Posts: 2,353
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Re: Heat switch
I pulled the switch last night and checked that seems to work, any other thoughts? could I have bought a bad relay?
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Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE |
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04-08-2025, 07:38 AM
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#5 |
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Registered User
Join Date: Dec 2004
Location: Highland, IN
Posts: 2,353
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Re: Heat switch
I recently replaced the resistor and I pulled that and checked it is all good
__________________
Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE |
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04-08-2025, 07:02 PM
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#6 |
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Moderator
Join Date: Oct 2008
Location: Wentworth, NH
Posts: 5,049
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Re: Heat switch
That SPST Micro ISO 280 relay in the O'Riley link is a modern GM part. My Impala & GMT800 use a bunch of them.
The blower relay on the side of the big Harrison Frigidaire AC box in your 1979 CK is a proprietary GM AC Delco 1581743. https://www.rockauto.com/en/moreinfo...t=3088&jsn=651 It's electrically the same as a 1" cube 40A Bosch SPDT Mini ISO relays. If the AC Delco part suddenly became NLA you can move the Packard 56 terminals from the Proprietary GM plug to a five terminal Bosch ISO relay socket. Download the 1979 wiring diagrams and look at the RPO C60 wiring detail.
__________________
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...
And please let us know if and how your repairs were successful. |
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04-14-2025, 09:13 PM
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#7 | |
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N9PHW
Join Date: Jan 2003
Location: PNW USA
Posts: 162
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Re: Heat switch
Quote:
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"I got it one piece at a time and it didn't cost me a dime..." |
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04-14-2025, 11:03 PM
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#8 |
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Registered User
Join Date: Dec 2004
Location: Highland, IN
Posts: 2,353
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Re: Heat switch
I pulled the relay apart and cleaned the contacts. All good now.
__________________
Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE |
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Linear Mode
