Help! How can I get my heater fan to stop running????

[kna4977]

I just installed a factory A/C setup in my 72 Blazer and got everything wired in. The problem I have is that when I turn on the ignition, the heater fan blows on high, and no matter where I place the fan switch, it continues to blow on high and I can't turn it off. Does anyone have any ideas on where I could begin to fix this problem? Thanks

[kna4977]

I just installed a factory A/C setup in my 72 Blazer and got everything wired in. The problem I have is that when I turn on the ignition, the heater fan blows on high, and no matter where I place the fan switch, it continues to blow on high and I can't turn it off. Does anyone have any ideas on where I could begin to fix this problem? Thanks

[hotrodhomi]

I sounds like you have the hot wire going to the wrong place. The switch should step down the current. It doesn't sound like this is happening. Is the switch bypassed?

[67chevy2wd]

Is the control new or used, it could be shorted, if not check all the wires again.

[jhow66]

Was it all new stuff?

[kna4977]

Everything is used, but appears to be newer. I'm thinking it's probably the control, but wanted to get some other opinions before I start replacing parts

[kna4977]

Everything is wired correctly. Could the control be bad?

[krue]

I would get a multimeter and start checking wires before I bought new parts.

[cdowns]

ac

[jhow66]

High speed blower relay is getting power all the time . If it is wired right then your fan switch is shorted inside. Unplug connector to fan switch-if fan does not run switch is bad-(or wired wrong).

[kna4977]

Originally Posted by cdowns

ac

Thanks for the diagram. The harness I have has the Anti-Dieseling Relay Wiring, but there is now were to plug it in as the original harness didn't include it. Is this a problem??

[kna4977]

Originally Posted by jhow66

High speed blower relay is getting power all the time . If it is wired right then your fan switch is shorted inside. Unplug connector to fan switch-if fan does not run switch is bad-(or wired wrong).

Is the fan switch on the back of the heater control?

[jhow66]

Originally Posted by kna4977

Is the fan switch on the back of the heater control?

Yes. When you put the fan control on high speed it trips the relay mounted on the heater box ( up behind glovebox on right end). Make sure the relay is mounted tight as the ground wire for relay goes under one of the screws.

[kna4977]

Originally Posted by jhow66

Yes. When you put the fan control on high speed it trips the relay mounted on the heater box ( up behind glovebox on right end). Make sure the relay is mounted tight as the ground wire for relay goes under one of the screws.

I actually ended up grounding it to the body. I know it has a good ground, but the fan is still blowing on high.

[jhow66]

Ok then unhook the fan switch. If blower does not run then your switch has to be the problem. Most likely you will find that the switch has been getting hot (melted plastic from loose connection). This causes the parts inside the switch to fail (springs, tabs, etc. loose their tension from the heat). The other two speeds on the switch are controled by a resistor on the back side of heater box. Do they work?

[mrein3]

I bought a used AC wiring harness when I did my non-AC to AC conversion. The plug for the fan speed switch was melted. I never even bothered to hook it up. I bought a new connector and a new switch.

Further I didn't bother to use the 30+ year old relay for high speed. I wired in my own relay and mounted it to the same screw.

I have a 10 gauge wire with its own fuse coming off the battery supplying a clean 12+ volts to the business end of the high-speed fan relay.

The only draw back is if I leave my set-up in the high speed fan position then try to turn off the truck. The HEI I installed gets a nice, clean 12 volts out of this circuit and keeps the truck running.

When I really get ahead of the game I'm going to install an extra fuse block to run all the things GM should have installed for me.

To make a long story short I'd look at that switch and connector. GM ran a lot of amps through that thing. After 30+ years and lots of furry connections you're going to get a lot of heat back there.

To prevent future melt-downs put in a new relay and run a seperate circuit. You'll never melt that connector again.

I have my headlight relay upgrade and hi-speed fan relay upgrade all running the same relay. I only need to keep one spare in the glove box.

[kna4977]

Originally Posted by jhow66

Ok then unhook the fan switch. If blower does not run then your switch has to be the problem. Most likely you will find that the switch has been getting hot (melted plastic from loose connection). This causes the parts inside the switch to fail (springs, tabs, etc. loose their tension from the heat). The other two speeds on the switch are controled by a resistor on the back side of heater box. Do they work?

You are correct about the melted plastic. The old harness was melted at the control connection. As far as the other 2 speeds, I can't tell because it only blows on high.

[kna4977]

So, I guess I need to replace the relay and control switch???

[hotrodhomi]

Do you have a DMM? you could test these before you waste any money on something you don't need. If not, buy away, it should be too much $$.

[jhow66]

Originally Posted by kna4977

So, I guess I need to replace the relay and control switch???

Just try the switch first. Did you check it?

[kna4977]

Originally Posted by jhow66

Just try the switch first. Did you check it?

I unhooked the control switch --> the fan still blows

I unhooked the relay---> fan doesn't blow

Bad relay???

[kna4977]

Originally Posted by mrein3

I bought a used AC wiring harness when I did my non-AC to AC conversion. The plug for the fan speed switch was melted. I never even bothered to hook it up. I bought a new connector and a new switch.

Further I didn't bother to use the 30+ year old relay for high speed. I wired in my own relay and mounted it to the same screw.

I have a 10 gauge wire with its own fuse coming off the battery supplying a clean 12+ volts to the business end of the high-speed fan relay.

The only draw back is if I leave my set-up in the high speed fan position then try to turn off the truck. The HEI I installed gets a nice, clean 12 volts out of this circuit and keeps the truck running.

When I really get ahead of the game I'm going to install an extra fuse block to run all the things GM should have installed for me.

To make a long story short I'd look at that switch and connector. GM ran a lot of amps through that thing. After 30+ years and lots of furry connections you're going to get a lot of heat back there.

To prevent future melt-downs put in a new relay and run a seperate circuit. You'll never melt that connector again.

I have my headlight relay upgrade and hi-speed fan relay upgrade all running the same relay. I only need to keep one spare in the glove box.

Do I have to get a relay from a GM restoration parts place, or is it something I could find at Advance Auto Parts or Auto Zone??

[mrein3]

Originally Posted by kna4977

Do I have to get a relay from a GM restoration parts place, or is it something I could find at Advance Auto Parts or Auto Zone??

I bought one in a bubble pack at Checkers.

However I wouldn't go buying parts yet if I was you.

IIRC, the voltage coming out of the switch is changed to give you different fan speeds. It has been a while since I got mine working but the fan speeds were something like 3 volts, 6 volts, then 12 volts. You really need a Digital Multi Meter (DMM) to figure out your problem.

If I was shotgunning this thing, since your fan works on high no matter what position the switch is in, until you unplug the relay, I'd replace the plastic connector behind the switch and the switch itself.

Think about it. For a relay to work you need a voltage to get to the relay. This then closes the connection in the relay which then sends power to the fan through a different, beefier circuit.

It sounds like the lower voltage wires are all shorted together and turning on your relay. If your plug is all melted this is a very strong possibility.

[kna4977]

Originally Posted by mrein3

I bought one in a bubble pack at Checkers.

However I wouldn't go buying parts yet if I was you.

IIRC, the voltage coming out of the switch is changed to give you different fan speeds. It has been a while since I got mine working but the fan speeds were something like 3 volts, 6 volts, then 12 volts. You really need a Digital Multi Meter (DMM) to figure out your problem.

If I was shotgunning this thing, since your fan works on high no matter what position the switch is in, until you unplug the relay, I'd replace the plastic connector behind the switch and the switch itself.

Think about it. For a relay to work you need a voltage to get to the relay. This then closes the connection in the relay which then sends power to the fan through a different, beefier circuit.

It sounds like the lower voltage wires are all shorted together and turning on your relay. If your plug is all melted this is a very strong possibility.

The plugs on the old harness are melted. This is a brand new harness.

[kna4977]

I replaced the relay because it was only $10. It now has a clicking sound under the hood when I have it on A/C and move the fan switch through the positions. I still have no difference in fan speeds though. Only high regardless of the position of the control.