Fan High Speed Issue

[gschroeder]

I have a 1970 CST-10 with factory A/C. The fan works fine on low, and medium. However, the fan sometimes does not work on high. Sometimes it works on high just fine, and sometimes not (when its not working on high the fan is not moving at all). I double checked the ground wire on the blower motor, and it is hooked up with a good ground on both ends. Any suggestions on how to trouble shoot this? I did a some research on this problem, and it appears the "Blower High Speed Relay" may be bad. I see LMC actually calls the part "A/C Blower Relay" for $12.95. Thank you for your help in trouble shooting this. Thank you

[davischevy]

The relay is on the passenger side behind the glove box. Turn the blower on and reach up in there and wiggle the wiring harness. Sometimes it's a poor connection. If that doesn't do it the relay is probably bad. It's a pain to change because the screws are hard to get to.

[HO455]

Here a link to an overview of the A/C fan circuit. It should help get you started.

https://67-72chevytrucks.com/vboard/...d.php?t=731399

Then if you do a search for A/C blower problem you will find many more threads on this issue.

Before buying a new relay make sure your current one has a good ground.

Good luck and please post your findings.

[davischevy]

Yes, I forgot about the ground. The ground wire is attached to one of the screws and the screw have a tendency to strip.

[Dieselwrencher]

Both of my 72s have the same issue. I have found that there must be a dirty/worn set of contacts in the high position of the blower switch. On both trucks I can wiggle the switch from mid to high and get high to work. Odd both are like this I know. I need to remove the switch on one and see if I can fix them.

[gschroeder]

Guys, thank you for your advice. I will turn the fan switch on high, wiggle the wires on the "A/C Blower Relay", and make sure the ground is connected properly.

Can I ask 2 more questions:
1.) What is the "Resistor A/C Blower"? I saw this component on LMC truck website (part number 36-0708), and wasn't sure what it is (or if this could possibly be the problem).
2.) How can I confirm my "Fan Switch" is OK? I tried wiggling the fan switch between medium and high, and that didn't make the fan speed change from medium to high. My problem seems odd as sometimes the fan works fine on high, and other times it does not work at all on high. Low and medium speeds always work just fine.

Thanks again guys for all your help.

Thank you

[Dieselwrencher]

Originally Posted by gschroeder

Guys, thank you for your advice. I will turn the fan switch on high, wiggle the wires on the "A/C Blower Relay", and make sure the ground is connected properly.

Can I ask 2 more questions:
1.) What is the "Resistor A/C Blower"? I saw this component on LMC truck website (part number 36-0708), and wasn't sure what it is (or if this could possibly be the problem).
2.) How can I confirm my "Fan Switch" is OK? I tried wiggling the fan switch between medium and high, and that didn't make the fan speed change from medium to high. My problem seems odd as sometimes the fan works fine on high, and other times it does not work at all on high. Low and medium speeds always work just fine.

Thanks again guys for all your help.

Thank you

1. This device uses resistance to change the voltage to operate the fan motor. It has resistors tied to the terminals on the back side. They put them in the air flow to cool them. If I remember correctly relay is on the right side of the plenum next to the pass side vent.

2. If you have power coming from the high speed terminal of the switch then it should be sending power to the resistor. You will need a test light or meter to check this.

[gschroeder]

Ryan, thank you for your help. I will trouble shoot the fan switch as you suggested. Do you have any advice on how to trouble shoot items 2 and 3 below:

1.) Fan Switch
Check for power at the high speed setting. If there is power, then switch is OK. If there is no power, switch is no good.

2.) Resistor A/C Blower
How to I confirm this part is OK or no good

3.) A/C Blower Relay
I will wiggle the wires and make sure the ground is OK. If this does not solve the problem, how do I confirm this part is OK or no good.

Thank you

[HO455]

Originally Posted by gschroeder

Ryan, thank you for your help. I will trouble shoot the fan switch as you suggested. Do you have any advice on how to trouble shoot items 2 and 3 below:

1.) Fan Switch
Check for power at the high speed setting. If there is power, then switch is OK. If there is no power, switch is no good.

2.) Resistor A/C Blower
How to I confirm this part is OK or no good

3.) A/C Blower Relay
I will wiggle the wires and make sure the ground is OK. If this does not solve the problem, how do I confirm this part is OK or no good.

Thank you

#2 If low and medium speeds work then the resistor is good. The resistor is not part of the high speed circuit.

#3 Move the fans switch from medium to high while touching the relay. You will feel the relay "Click" if it is working. No click then check the small orange wire that is not in the 3 prong connector for 12 volts to a good ground with the switch in high speed. If voltage is good then relay is likely bad. Do resistance check from that same orange wire to the terminal with the ground wire on the relay to confirm. No continuity the relay is bad.

With the fan switch in medium check for 12 volts at the orange wire. No voltage then trace the wire back to the fuse block.
If 12 volts on the orange wire then move fan switch to high and check the center purple wire. No voltage then the relay is likely bad.

[gschroeder]

HI455, I checked the ground and wire to relay and everything is OK. I also confirmed the orange wire is delivering power to relay on high fan speed every time as it should. The relay does click when I go from high to medium, but never clicks when I go from medium to high speed. I think will go ahead and replace the relay as it appears to be working erratically. I will let you know if the new relay fixed the problem. Thank you

[tdangle]

I had a similar issue and discovered the power for the high speed bypasses the resistor and is fed from a large wire at connection by the fuse block. Mine had a bad connection and when it did work you could smell burning plastic. Re-did the connection and all is good. I believe I found that power for the 2 low speeds comes from a different connection

[Ole Blue 71]

Originally Posted by tdangle

I had a similar issue and discovered the power for the high speed bypasses the resistor and is fed from a large wire at connection by the fuse block. Mine had a bad connection and when it did work you could smell burning plastic. Re-did the connection and all is good. I believe I found that power for the 2 low speeds comes from a different connection

Yes, I had the same problem. The power for high speed is a different wire off of the fuse block. Mine was a bad connection and melted insulation. I cut it back a ways to good wire and spliced it. I have consistent high speed now.