Another Blower fan switch/relay/resistor thread

beyerku · Aug 21, 2024

Hi all: I read I think all of the threads here about blower fan speed and switch problems, and I am still a bit confused. It looks like the "lack of high" fan speed problem is clearly the relay in the dash, but I want to clarify if it's correct that not all trucks have that relay at all?

My 1975 K20 does not have A/C, and has only a three-speed fan switch with only the resistor in the engine compartment firewall next to the fan. I don't have any relay at all. Is it correct then that trucks with A/C have a four-speed fan switch, relay in the dash, and the resistor in the engine compartment? I have an extra switch that my dad gave me that has four "clicks" in it, so I'm trying to find out if my truck is weird or if it's just that no one on here talks about three-speed fan switches without a relay. Thanks!

Snoots · Aug 21, 2024

Whoa, back up. There is no relay for the fan in the dash.
As far as 'clicks' go, you'll have to pull the switch connector and see if it matches the switch in your dash and/or the 4-speed switch.
The high speed WILL be controlled by a relay.

PrairieDrifter · Aug 21, 2024

beyerku said:
Hi all: I read I think all of the threads here about blower fan speed and switch problems, and I am still a bit confused. It looks like the "lack of high" fan speed problem is clearly the relay in the dash, but I want to clarify if it's correct that not all trucks have that relay at all?

My 1975 K20 does not have A/C, and has only a three-speed fan switch with only the resistor in the engine compartment firewall next to the fan. I don't have any relay at all. Is it correct then that trucks with A/C have a four-speed fan switch, relay in the dash, and the resistor in the engine compartment? I have an extra switch that my dad gave me that has four "clicks" in it, so I'm trying to find out if my truck is weird or if it's just that no one on here talks about three-speed fan switches without a relay. Thanks!

Non a/c trucks have 3 speeds. A/c trucks have 4 speeds. The resistor next to the blower is supposed to suppress radio noise. I can't speak on if non a/c trucks have the relay or not. But they all should have came with that "resistor"

The relay will be located on the firewall.

beyerku · Aug 21, 2024

I have the resister in the firewall. I thought the relay was supposed to be behind the glovebox in the cab. I don't have one.

The resistor for me modifies the current coming from the switch before it gets to the fan. I don't think it is a radio-noise thing.

My fan switch has only three positions.

PrairieDrifter · Aug 21, 2024

beyerku said:
I have the resister in the firewall. I thought the relay was supposed to be behind the glovebox in the cab. I don't have one.

The resistor for me modifies the current coming from the switch before it gets to the fan. I don't think it is a radio-noise thing.

My fan switch has only three positions.

Well there's the resistor block. Then there's a "capacitor" whatever it's called, that suppresses radio noise just before the connection to the blower motor.

The high speed relay would be on the firewall or on the outside heater box.

75gmck25 · Aug 21, 2024

On my ‘75 with A/C the high speed relay and the resistors are mounted on the big evaporator plenum under the hood. I’m not sure where they are located if there is no A/C.

Another Blower fan switch/relay/resistor thread

beyerku

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Snoots

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PrairieDrifter

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beyerku

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PrairieDrifter

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