Blower Motor Fan Speed does not work on High

[74chevyc30]

I have a 1974 c30 and my blower motor fan speed does not work on the "High" setting. My blower motor is on its last leg, with it taking a few seconds to wind up, and the squealing noise. I'm not sure if that's a part of the problem or not. Again, I am not familiar with square body's so any information will help. Thanks.

 

[Grit dog]

My 77 is the same way but I haven’t looked into it, it is healthy at the lower speeds. I read (probably here) that the 3 lower speeds run direct power thru the switch in the cab but hi speed runs thru a relay with a different power source. So likely it’s that source or the relay is bad.

 

[PrairieDrifter]

It's the blower motor relay on the firewall/hvac box on the engine side.

Need to make sure you're getting power in, but most likely a bad relay.

 

[Strick]

If it's an A/C truck then it has a blower high relay mounted on the evap housing.
HS

 

[75gmck25]

In a ‘74, that relay is powered by a separate power wire that runs from the power junction next to the brake booster over to the relay. Midway in the wire there is a glass fuse holder and 30 amp fuse. I had to replace my fuse holder with a newer style fuse holder and 30 amp fuse because the old fuse holder kept melting.

 

[chevdude]

It's actually called the blower resister.
Typically, the high speed quits first.
Also, a good indicator that your blower motor needs replacing

 

[Ricko1966]

It's actually called the blower resister.
Typically, the high speed quits first.
Also, a good indicator that your blower motor needs

In some IDK about all squares, to the best of my knowledge all,they used 2 seperate circuits power went through the resistor for lower speeds. Then completly bypassed the resistor and used a seperate circuit with a relay for high speed. So they have a blower resistor and a blower relay.

 

[75gmck25]

Ricko1966, you are correct.

Lower speeds do run through the resistor, but high speed is separately powered directly through the relay (as I explained a couple posts back). If high speed is the only one not working, its the direct power circuit and fuse under the hood and it does not go through the resistors.

 

[chevdude]

Good to know

 

[Ricko1966]

Ricko1966, you are correct.

Lower speeds do run through the resistor, but high speed is separately powered directly through the relay (as I explained a couple posts back). If high speed is the only one not working, its the direct power circuit and fuse under the hood and it does not go through the resistors.

Yep. I was clarifying it for the post 6

 

[Snoots]

Change your blower motor first.

 

[74chevyc30]

Thanks guys. I'm going to check it out tomorrow and get back with more info.

 

[85 red GMC]

I have a 1974 c30 and my blower motor fan speed does not work on the "High" setting. My blower motor is on its last leg, with it taking a few seconds to wind up, and the squealing noise. I'm not sure if that's a part of the problem or not. Again, I am not familiar with square body's so any information will help. Thanks.

Good luck finding the problem Maybe it is like my 1979 Olds 98 where the high speed fan needs a special relay under the hood. I have had the car 2.5 years and had to replace that relay when I got the car and recently again as it was cutting off at times. I am also fine with the other speeds as the fan is strong in any of them but just less volume in lower speed.

 

[neiltee]

I have a similar problem on my 75 K10. The high speed fan doesn't work, and I now understand that's a separate circuit. The lower speeds all blow so softly there isn't any effective air flow. Additionally, the R134 freon has mostly leaked out. In the beginning, the high pressure limit switch would blow off when stopped at a traffic signal. There are dual electric fans. But the whole system ended up near empty. The system does blow mildly cooler air at very low fan speed.

 

[neiltee]

I should add that I had a new wiring harness installed and the fan problem popped up shortly after that.