Very odd blower motor problem

[Wired]

Recently the blower motor on my 79 K30 (no A/C) randomly stopped working without notice. I decided to start at the motor itself and work through the system and it is stumping me. I'm hoping some of you guru's can help, as my experience is mainly from modern GM vehicles.

First I checked the motor. I supplied it 12v from the battery while it was still in place and is spins fine. I was also able to operate it with a 12v duracell.

Next I measured voltage from the lead that connects to the blower motor terminal. It reads anywhere from 8-10 volts when the truck is on, but does not power the motor when I plug it in. Voltage would stay constant regardless what setting the speed switch was set to.

The resistor is plenty rusty on the inside, but still has continuity between all appropriate connectors.

Next I took the fan speed switch off and took it apart. It was dirty but I cleaned all the contacts and check continuity; it was fine.

I then measured voltage coming from the brown wire that leads into the fan speed switch and plug. It would read 8-10 volts. But get this, as soon as I plugged in the blower motor, the voltage would drop to 0. This made me assume a circuit breaker was tripping (in addition to it only being 8-10 volts instead of 12+).

I checked the heater/A/C fuse and it was fine (20amp)

To top it off, while working on this problem, the wipers stopped functioning. Fuse is fine, and power is getting to the wiper motor.

What does it take to kill a gremlin? I can take photos of anything anyone would need to help solve this. I plan on checking continuity between the resistor and the noise suppression coil to see if a wire is corroded/barely contacting as another member here suggested.

THANKS for any help you guys can give.

 

[chengny]

In light of the fact that the blower motor and the wipers are affected, I would take a good look at the primary part of the system.

Start with the battery ground cable connections (battery and engine ends) and also the cable itself. A cable can appear fine from outside the insulation but on the inside, enough of the conductors may be broken to cause an intermittent power loss.

Next do the hot side. If the starter is cranking strong, the primary section of cable (battery to solenoid) is probably fine. But check the connection for the two red wires that branch off (at the solenoid "B" terminal). Check the connection at the junction block on the firewall. Also check the fusible links (the two at the starter and the one at the junction block). The actual fusible part is the wire just before the plastic splice cover (with the amp rating embossed on it). Try wiggling the sort sections of wire with your hands and observe whether the fan speed seems to be affected.

A voltage reading isn't always a good indication of the current carrying capacity of a conductor. A connection that is just barely hanging together will often show 12 volts under no load conditions but as soon as a load is applied, the voltage takes a nosedive.

This is exactly what you describe here:

It would read 8-10 volts. But get this, as soon as I plugged in the blower motor, the voltage would drop to 0.

But I am more inclined to suspect a grounding issue because voltage remains at 12 when you check the wiper motor.


This does appear to be two unrelated issues (hot side for the blower and ground side for the wipers) but I would check the primary grounding system first.

 

[Wired]

Thanks for the ideas! I was thinking a conductor might be damaged somewhere due to the drop in voltage when a load is applied. The starter has been hesitating lately but then after a second, the engine instantly fires up. The wipers stopped working about 3 hours into working on the truck for the blower motor.

There is a homemade heavy gauge ground between the battery negative and the body in addition to the regular lead going to the engine block. I will inspect the whole primary system tomorrow. Being several owners away from original, lots of people fixed things at a "custom" level.

To note, the fuse block/junction box on the firewall looks pretty nasty, lots of debris and goo, almost as if it was exposed to high heat. Looking at the heater/AC fuse socket, the inside also appears to be exposed to heat, but not the rest of the interior fuse block.

 

[Wired]

Also (I don't see an edit button, sorry) I ran a jumper cable between the negative battery terminal and the blower motor housing to make sure there wasn't a grounding issue.

Is there a relay for the blower motor? I have a repair manual on order.

 

[J Knight]

I had a similar problem with my fan a couple of weeks ago. Everything checked out fine, even the fuse was good. Come to find out the clips that hold the fuse(glass type) in place was a little corroded. Cleaned the clips and put in a new fuse....BAM! fixed.

 

[rich weyand]

You need to check the wires themselves. I found a dead conductor in the wire from the resistor connector on the firewall to the blower motor. The weather-proof connector was oriented "cup up", which channeled water into the conductor and caused it to corrode internally. You can check this by jumpering from the blower connection on the resistor connector direct to the blower motor, skipping that wire.

See adjacent thread: http://www.gmsquarebody.com/forum/showthread.php?t=11498

 

[Wired]

You need to check the wires themselves. I found a dead conductor in the wire from the resistor connector on the firewall to the blower motor. The weather-proof connector was oriented "cup up", which channeled water into the conductor and caused it to corrode internally. You can check this by jumpering from the blower connection on the resistor connector direct to the blower motor, skipping that wire.

See adjacent thread: http://www.gmsquarebody.com/forum/showthread.php?t=11498

I just ran a battery to the speed switch and the blower runs, so it looks like the harness is fine. The fuze block looks like it is the culprit as I only get 8 volts at the switch.The hot side of the fuse block is pretty nasty looking.

Anyone know how the fuse block is attached to the firewall?

I haven't gotten anywhere with the wipers, but I removed the dash cover and instrument panel to get a better look around.

 

[Wired]

FIXED both the wipers and the blower motor.

After making my own cable with a inline fuse, I was able to bridge the heater socket on the fuse block. The block receiving tabs were so corroded that they couldn't carry a load. After some solvent and using a fingernail file, I was able to clean the socket so that a normal fuse would work. There was also a lot of plastic debris stuck in the heater socket, preventing full contact with the fuse. I decided to give the wiper socket the same treatment and they are back up and running.

Thanks for the ideas guys! Glad to have it working again. While I was behind the instrument cluster I monkeyed with the speedo cable and I think I finally fixed the speedometer as well.

 

[89Suburban]

Good job.

 

[Dadof2]

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