blower motor

[smartcraig]

I have a 1970 c20 with a/c, the blower works on low and medium but not on high, would this be a switch problem or a relay, going to make an order for other parts and was want to get this fixed too.

[Longhorn Man]

it could be the switch, the relay, a wire, or possibly the resistor. (not 100% sure if it goes through the resisitor on an A/C truck or not)
Gonna have to troubleshoot this a little.

[stope4]

Replace the relay first, resistor second, switch third. Check all wires before replacing anything.

[Pl4yboy]

10 to 1 says it's the switch. I've replaced three of them in my truck. For now, I just gave up.

[72BLUZ]

[QUOTE=stope4;2564273]Replace the relay first, resistor second, switch third. Check all wires before replacing anything.[/QUOTE

resistor first, switch second. the relay is obviously working because it has power to the switch. "thats my expert opionion and what i encounter daily in the shop"

[Longhorn Man]

the relay is only for high, and if it is bad, it will do exactly as described.
The relay may still be bad, esp if water leaks through the cowl on the passenger side. The water will run right onto the relay causing it to rust out.
Both of my A/C trucks had this problem.

[smartcraig]

Ive put in a new switch and a new relay and still the same problem, if it is a wire where should I start looking

[joedoozer]

Its the resistor. Take the diverter box off, you will see the plug and resistor inside.

[Hutch]

Mine did the same thing
The symptoms were
Low=low
Med= Med
High=very low
It was a broken ground from the relay up high behind the glovebox
It was only several inches long and grounded to one of the ac box studs
For a test just ground the black wire while the fan is on high
I think that these get left ungrounded when folks replace/repair their heater cores

[hootz]

If high speed is the only speed not working, the resistor can not be the problem, it is bypassed through the relay for high speed. The relay has a separate feed for the high speed, usually an orange or red heavy gage wire at the relay. This feed wire was usually fused and the fuse holder can fail due to the heat from the current draw.Normally you should find 12 volts on this feed wire with the ignition on.

[smartcraig]

Hutch you hit the nail on the head, I ran a new ground over and Presto, the high speed works . Thanks alot. Have a beer on me.

[GMC Jim]

Pl4yboy--- Regarding your blower switch probem.... Is your truck equiped with heater only or an air conditioning system..? As I recall, those vehicles that were heater only installs did not use a relay as the blower motor amperage draw was well within the capacity of the switch. However, if the blower motor was replaced with the one designed for air conditiong systems, the amperage draw will exceed the capacity of the switch causing premature failure. The blower designed for air conditioing systems is almost twice the CFM capacity as the standard heater only units and is much prefered.... The purpose of the relay is for the blower switch to operate the relay thus avoiding the high amp load causing switch failure at high speed blower operation.

[Hutch]

Hutch you hit the nail on the head, I ran a new ground over and Presto, the high speed works . Thanks alot. Have a beer on me.

Nothing like a cheap and easy fix
I think I will have one of these

[GMC Jim]

Additional info on blowers-Heater only blower motor-- 6.25 amp draw (2550/2950 RPM)
Air conditioning blower motor -- 16.5 amp draw (3700 min RPM)