Blower fan not running

[jlmiller5]

This is a follow up to this old post from about 5 years ago.  I seem to be in about the same situation as this guy was:
http://forum.73-87chevytrucks.com/smforum/index.php?topic=16395.0

Dang, he didn't answer, and here I am at the same place almost 6 years later.  I have the same GMC Sierra as the OP, same color even.  Its a 454 w AC. 

I get nothing out of any of the wires in the Blower fan relay harness except for the 12v at the heavy gauge red wire, no matter the switch position, key is on and slider is on vent.  When I jump the 12v over to the purple, the fan runs but speed cant be changed with the switch, no matter where its set.  If I give the relay 12v from someplace else to make it close, it'll close the relay and the fan runs, but you still cant change the speed with the switch on the dash.  The resistors all have continuity if tested by a meter, and they seem to have reasonable resistances to give three separate speeds, so maybe that can be ruled out?

Sound like a bad switch?  Is that hard to swap out?  Its about $20 so right on the edge of too expensive for an experimental parts swap.

Thanks guys, I'm Jimmy by the way.

[bd]

Assuming your 78 truck has factory A/C... with the ignition switched on, is there 12-volt power at the yellow wire that connects to the blower resistor?

[jlmiller5]

Thanks for helping, yes factory A/C.  It shows nothing on that yellow wire in the resistor plug with ignition on and sliders on vent, high fan. 

I've checked the fuse under the dash, is there another one somewhere?  The text for the fuse location on the fuse box has a little arrow pointing diagonal to a location with a good fuse in it, and then another arrow that points down to a socket that is not a fuse location, should something be in that socket?

[bd]

The A/C fuse should be in the cavity located at the top right corner of the fuse box.  The socket below the fuse is just an accessory power tap from the ignition switch that is not protected by the fuse.  The blower circuit is hard-wired to the upper fuse terminal clip behind the fuse box via a dark brown wire. 

With the ignition switched on, probe both ends of the 20-amp cartridge fuse AND the fuse clips to verify continuity between the fuse and its clips.  If there is power to the upper fuse clip, look for an unplugged in-line connector with a single dark brown wire, hanging out of the harness somewhere between the fuse box and the dash selector switch - it's typically located under the dash, to the right of the steering column.

[jlmiller5]

I got it, I found another fuse up even higher than the one I thought was the one I needed to fix. It was a 30A and blown.  I was only able to find it because of the fusebox diagram here:
http://www.chuckschevytruckpages.com/77fusebox.html

The prev. owner had installed a brake controller, and its wiring is blocking the fuses, there are two up there that I couldnt see.  Thanks so much for your help!

[bd]

http://forum.73-87chevytrucks.com/smforum/index.php?topic=16395.0

...I have the same GMC Sierra as the OP, same color even.  Its a 454 w AC....

   

        ...Huh-ummm!   

               ...the linked vehicle is a 1978....   


In 1979 the fuse box migrated to ATO (blade) fuses.

The blower is powered off the "HTR" fuse at the top of the box.  Check it.  Same rules apply.  If the fuse is good and there is no power on the brown wire at the selector mode switch, then there is an open in the... brown wire... between the fuse box and the switch.   

[bd]

Here is the 1979 Wiring Manual.

[bd]

Incidentally, the heater fuse should be 20 or 25 amps.  The fuse box should be marked with the correct size.  A blown 30-amp heater fuse means you have more work to do.  Begin by unplugging the blower motor to see if the fuse still melts.

[jlmiller5]

Sorry about that, '79 my bad. Turns out that it makes a difference!

Unplug the blower motor under the hood?  Do I need to do this while its running?

[bd]

Begin by installing the correct heater fuse.  Then if it continues to blow the fuse, unplug the power wire that connects directly to the blower motor and see whether the fuse stops blowing - kind of a grass roots approach. 

If it continues to blow the heater fuse with the blower motor unplugged, unplug the harness from the blower relay and try again.  If it stops blowing the fuse with the relay unplugged, read through Blower Motor Relay Info (Keeps blowing fuses) in the Tech Pages.

Let us know how it goes.

[jlmiller5]

So I finally got time to mess around with the old truck.  It does keep blowing fuses.  As long as I leave the slider on "Vent" it will run for a while, I suppose indefinitely.  If I move the slider to defrost, or any of the AC modes it immediately blows. I went through a whole pack of 20 and 25A fuses testing it out.
Here are the attempts that I ran:

• Fan unplugged
• Fan Plugged in and Relay unplugged
• Fan and Relay unplugged
• Fan and Relay and Blower Motor resistors unplugged

So does this mean that the control unit itself, in the dash, is the culprit? Basically everything else is unplugged.  I suppose a hidden wire short could be at fault, but why would changing the modes have anything to do with it then? A shorted power is a shorted power no matter where the slider is.

Anyway, thanks so much for your help and advice.

[bd]

Unplug the A/C compressor and try again.  If that has no effect, unplug the A/C cycling switch.

[jlmiller5]

Okay, I think I found it!  I was doing like you said, checking after every plug was disconnected, and the A/C compressor plug fixed it.  I was still disappointed being that this is a pretty expensive component.  The seller had told me everything was working, it just needed a "recharge".  of course I didn't put too much faith in those kind of statements, but I was hopeful.  Then I remembered the ugly solenoid on the side of the carb, that's also on the same plug circuit. I saw it and figured it didn't work when I rebuilt the carb last month right after I bought the truck.  It looks like it holds idle up when the A/C is running.  So I plugged everything in except for that part, and now everything works fine.  Ill just go get a new one of those solenoids from Advance, and be on my way.

Thanks bd!

[bd]

If you would like to test it and have a DMM, unplug the A/C compressor and measure the resistance of the clutch coil, terminal-to-terminal.  It should measure very close to 3 Ohms.  If it measures less than 2.5 ohms the clutch coil winding is shorted and needs to be replaced.  Next, measure between either clutch coil terminal and the compressor case; the meter should indicate infinite or overload - if not, the coil is grounded and needs to be replaced.

Perform a similar measurement of the idle solenoid with it unplugged.  The solenoid should measure between 10 and 20 Ohms between its single terminal and its case.