Heater Issues

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dangonzo

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While replacing the heater core I decided to try to figure out why my blower motor doesn't work. I tried to narrow down the issue but am having trouble so maybe somebody can help me figure it out.

The fuse appears to be fine.
There is power from the switch in every position except for low. I tested this with a test light and when the switch is on high, medium high, and medium low, I get a signal at the corresponding wire.
There is power to the blower motor from the wire coming off the relay.

So from that it felt like everything was in order (with the exception of the switch not giving power to the low wire). So I hooked the blower motor up directly to the battery positive and grounded it to the battery as well and the blower motor ran fine.

I figured that meant that the motor itself was fine, and the issues were coming from the power from the relay or the ground that the motor uses. But after hooking up the multimeter to the power wire coming from the relay and the ground the motor uses I got 12.34V with the switch in the on position.

Next, I ran a jumper wire from the battery positive to the motor and used the ground that the motor normally uses and received no response from the motor.

Then I did the opposite and used the power from the relay and grounded the motor with the battery negative terminal. I saw a spark at the ground on the motor but nothing else. I don't really know what this should be telling me.

Can anybody help me with what to do next?
 

1987 GMC Jimmy

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Have you checked the resistor to see if it's burned?
 

dangonzo

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No I haven't but only because I thought that when the switch is on high that the current doesn't flow through the resistor, only through the relay.
 

1987 GMC Jimmy

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You're right, it doesn't. I took it as your blower wasn't working at most speeds. Is it only giving you issues on high?
 

dangonzo

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No it doesn't work on any speed.
 

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Okay, I think I'm following you now. Sorry, I've been a little slow today. It still wouldn't work when you supplied alternate power using the normal ground, which is what stuck out to me. If the resistor itself was the only issue, you should still have the high speed. What it sounds like to me is there's a ground issue. Most of my HVAC experience comes from working on my B Body, which is identical in some respects and totally different in others. My understanding is that the ground wire on a square body goes from the flange of the motor to the firewall. Rust on either end or a rodent severing the wire altogether could be the issue. I'd trace the wire and see if you can find a break or a dirty, rusty, or poor contact. From your diagnostics, it sounds like your fan speed selector switch is messed up, too, if you're not getting power on the low setting. That'll be confirmed when you get the blower working again, and if it is bad, no biggie. Just pull the HVAC controller and replace the switch. It just screws in there, and your local parts store should be able to get it no problem. I'd also check the resistor just to do it. It's just unplugging the connector, which it sounds like it already is, and unbolting the two little bolts. They can rarely fail internally, but they typically look totally ugly when they burn, and you'd be able to tell instantly.
 

Honky Kong jr

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Is this an A/C truck? If it is or not run a new ground wire from the motor attaching screw to the cab and off of that run a wire to the frame. I added 5 additional grounds from the frame to the cab, to the fuel tank and to the bed.
 

RustyPile

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I've typed my responses in bold and underlined to each of your statements..

The fuse appears to be fine.
There is power from the switch in every position except for low. I tested this with a test light and when the switch is on high, medium high, and medium low, I get a signal at the corresponding wire.
There is power to the blower motor from the wire coming off the relay.
This all means the fuse IS fine..

So from that it felt like everything was in order (with the exception of the switch not giving power to the low wire). So I hooked the blower motor up directly to the battery positive and grounded it to the battery as well and the blower motor ran fine.
This all means the motor IS fine..

I figured that meant that the motor itself was fine, and the issues were coming from the power from the relay or the ground that the motor uses. But after hooking up the multimeter to the power wire coming from the relay and the ground the motor uses I got 12.34V with the switch in the on position.
This means the ground is sufficient only for the multimeter..

Next, I ran a jumper wire from the battery positive to the motor and used the ground that the motor normally uses and received no response from the motor.
This means the ground is not sufficient to carry the high current demand of the motor..

Then I did the opposite and used the power from the relay and grounded the motor with the battery negative terminal. I saw a spark at the ground on the motor but nothing else. I don't really know what this should be telling me.
this means your "make shift" ground isn't a proper ground for the fan motor.

Can anybody help me with what to do next?

Check the ground that runs from the body to the fan motor frame.. Make sure the terminals are clean and making good contact with clean bare metal -- no rust, no paint..

Once you have established a good ground for the fan motor and you still have no low speed, the speed selector switch is probably bad or the low speed portion of the speed resistor is burned... Although it is always a good idea to have redundant grounds on these rusty old trucks, I don't believe you have a ground issue between the body and battery and/or the frame.. If you did, you'd be experiencing other electrical issues...
 
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dangonzo

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Thanks thanks everybody I appreciate the help! I'll try this when I get home tonight.
 

dangonzo

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It was a grounding issue. I cleaned the contacts on the fire wall and the blower motor body and that took care of it.
 

RustyPile

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It was a grounding issue. I cleaned the contacts on the fire wall and the blower motor body and that took care of it.

donja just luvit when a good plan comes together...:cheers:
 

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