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Old 06-16-2020, 06:02 PM   #1
sick472
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Fan Switch Testing Question/s

I am troubleshooting my wiring harness and have disassembled the fan speed switch and cleaned it up because is was transferring little to no power through the switch. It was pretty gummed up inside. This is on a 72 with A/C (A/DC to be deleted). The switch is now showing life. This is what I get with my conductivity meter...

Using "B" for the power terminal (brown wire),

No power throughput to any of the other terminals in the OFF position (good).

Power at the "L" terminal in Low position (seems right),
Power at the "L" and "M" terminal in Medium position (seems odd to be using both resisters for medium speed - I'd thought that might be a Low function),
And, Power at the "L" and "H" in the High position (really seems odd, maybe the Low feed is denied at the relay?).

Is this right? I am having trouble wrapping my head around how the fan speed switch, relay, and resister set works together. I've got the wiring diagram, but still miffed. Re-pop switches are cheap, but I am just stubborn that way...a good challenge and learning experience = priceless. lol

Anybody teach an old dog new tricks?
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Old 06-16-2020, 06:20 PM   #2
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Re: Fan Switch Testing Question/s

What problem are you troubleshooting?
This link should help.

http://67-72chevytrucks.com/vboard/s...wiring+diagram
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Old 06-17-2020, 08:45 AM   #3
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Re: Fan Switch Testing Question/s

Thanks for the link HO! It was some good reading. The trouble is that the truck's wiring harness has been in the form of a rat's nest in a box since '97 and it's time to start putting it back on the truck. So, I am checking it over inch by inch looking for bad areas and checking the functions of all the "things". The fan switch being one of those things. I don't know if it has a problem, just trying to test it out before installing the rat's nest and having to troubleshoot problems in that awkward position laying upside down in the seat with my head in the floor. lol

I could see how the "Low" setting could use one or both of the resisters while the "Medium" speed would only use one resister leg...the "High" speed should use neither. BUT, my switch is not agreeing with my logic unless the relay drops out the unneeded circuit. It just does not make sense to me and I am looking for the truth...whether I'm right or wrong.
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Old 06-30-2020, 02:56 PM   #4
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Re: Fan Switch Testing Question/s

I hooked all the wiring up to the blower motor (along with the applicable wiring to test the blower and switch). The blower runs low in the "low" position, but runs much higher in the "med" position, and back to a medium speed in the "high" position.

I guess I will be buying a new switch and trying it out. The wiring seems to be hooked up correctly compared to the harness and wiring diagram. I'll get a new switch and check continuity between the switch terminals and compare to my old switch.
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Old 06-30-2020, 03:28 PM   #5
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Re: Fan Switch Testing Question/s

Your blower speed switch is working correctly. You have a good idea of what the circuit is doing. I would test the relay. It has been some time since I had hands on messing with this circuit. Pretty sure the only time relay is activated is when high speed is selected. It bypasses the resistor and provides power direct to fan. Make sure the +12v from cig circuit is in place. That wire being disconnected or missing is the main cause of the connector at fan switch melting.
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Old 06-30-2020, 06:15 PM   #6
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Re: Fan Switch Testing Question/s

Thanks for the confidence Richard, I'll test further this weekend and see where it takes me. The CIG wire "thing" may explain why my heater plug was melted (may years ago). I will have to noodle on how the CIG wire could cause such trouble...I have it in the harness and have tested that circuit and found it to be good.
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Old 07-01-2020, 12:57 AM   #7
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Re: Fan Switch Testing Question/s

Just an fyi. I tested my nos gm blower switch and got the same readings you did on your switch. Will see if I can locate the relay tomorrow in my pile of stashed parts to help you in testing it. I would bet that it is not closing. The feed from the cig circuit to the relay just ensures all the amperage on high speed fan does not pass through the switch.
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Old Yesterday, 10:54 AM   #8
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Re: Fan Switch Testing Question/s

Thanks for the help everyone! I thought I'd wrap up the thread with some concluding info for future "heater switch/relay troubleshooters".

I found the attached schematic that explained it all (Thanks Vetvette for all your previous electrical posts - you have made my life much easier). My original tests are confirmed in the CONTROL SWITCH chart below (bottom right).

I bench tested the relay out last night to find that the relay has a Normally Open and Normally Closed circuit. The NC circuit passes power though to the blower in the LOW and MED switch positions (from the smaller gauge feed wires) and once the HI position is triggered, the relay is activated...cutting off the smaller feed and passing the full power (from the 12 ga orange wire) through to the blower. My earlier problem with the HI position not being higher than the other positions was that the relay was not grounded. It needs to have the relay bracket grounded. Otherwise, it does not trigger, but still allows the LOW power through on the NC circuit. This will turn the blower motor, but only in LOW mode.

Sorry for the quality of this schematic, it was the best I could find...
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Last edited by sick472; Yesterday at 11:00 AM. Reason: forgot to upload schematic
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