Need help troubleshooting heater, switch, relay, fuse, etc.

[chriswalkoe]

When I first bought my truck the "fan" worked in all settings, but until recently i didn't have it on the road.  Now that I am driving it on a regular basis, the fan doesn't work.

My first step was the check the fuse.  It is not blown.  I then replaced the fan motor and relay that is mounted right next to it. I also replaced the thingy that mounts by the AC, and has wires running to the relay. Still nothing.

I then used a voltmeter to see if there was power going to the switch on the control panel, the relay, and the fan motor.  The fan motor is getting power, as it read 12.45, same as the battery.  The relay is also getting power where it is supposed to.

I then checked the 4 "plug" connector that controls the fan from inside the cab.  3 of the 4 slots showed current.  The only one that didn't was the red wire.  Is this the culprit?

I have never done this before, so any advice would be great!  Just wondering what my next step should be... 

[bd]

Check the ground connection to the blower motor (there should be a ~6" long ground wire fastened between the cab sheetmetal and the blower motor mounting flange - make sure both ends of the wire touch shiny bare metal).

[chriswalkoe]

Will do thanks Rich!

[chriswalkoe]

No dice.  I cleaned everything up and replaced 2 leads.  Solid connections everywhere.  Should I replace the fan switch in the cap next?  On a side note it's 21 degrees today.  Went for a long drive this morning and the cab was very cozy, even without the fan 

[bd]

You posted that you measured 12 volts at the blower motor.  Check again.  If you verify 12 volts at the blower motor, measure voltage between battery negative and the blower case with the fan on - do you measure any voltage?  Is the blower motor terminal clean of any and all paint?  Any chance the squirrel cage in bound or hung up?

[PromiseKeeper]

Chris, check the "thingy" by the A/C again. I had it happen once where there was a burnt terminal on the PLUG end of that connection. Had to go to the junk yard to avoid buying a partial wiring harness to get the plug. For that matter, I'd check all the plugs you mentioned. I hope its something that simple!
Scott

[chriswalkoe]

BD - I checked to see if the blower motor case had current running through it (determine if there is a bad ground I suspect) and there was nothing.  I also checked the terminal for the blower motor, plus the boot, connector and wire.  They are in good shape.  12.47 v when tested.

As for the "thingy" I found out it's called a blower motor control module/resistor.  It is brand new, and there is current running to 2 of the 4 plugs on the harness that connects to it.

I also checked to see if the squirrel cage is being blocked by something to prevent it from spinning.  Both the new and old do not rotate when the fan switch is on.  I pulled the housing free with the power and ground still connected and there was no sign of life.

My only thought as to why it isn't working would be the fan switch .  Am I overlooking anything else? 

[bd]

Okay!  If you have battery power at the blower motor and the blower motor is grounded, the motor should run!  Try powering the motor with a jumper wire directly off of battery.  If the motor runs directly off of battery then we'll move under the dash to the blower switch.  On what year/model are you working?

Incidentally, do you have an "incandescent" test light?

[chriswalkoe]

 I will test both the old and the new motors with the jump wires like you said and let you know what's up.  As for the year, it's and 82' K10.  As for a test light, I don't have one, but I can pick one up tomorrow if you think it's a good tool to have.  I'm sure this won't be my last electrical work. 

[bd]

...As for a test light, I don't have one, but I can pick one up tomorrow if you think it's a good tool to have....

An 'incandescent' (filament) test light is often more useful for general electrical troubleshooting than a voltmeter, because the test light subtlely loads a circuit with low current flow (30-70 ma depending on the bulb), making the detection of poor connections much more pronounced and less problematic.  Troubleshooting with a voltmeter requires greater skill and knowledge to avoid diagnostic pitfalls.  ...You can do more with a voltmeter, but need to be conversant with Ohms Law to be proficient and not get into trouble.  Both tools have their definite advantages and uses - the trick is learning which to reach for under specific circumstances.  I highly recommend an incandescent test light for general electrical troubleshooting!

-----------------------------

But, there is a caveat: 

Definition:  impedance is a fancy word for 'effective electrical resistance.'

DO NOT USE an 'incandescent (low impedance) test light' to troubleshoot the low voltage/very low current circuits used in digital technologies such as OBDI and II systems or datalink circuitry, because there is a serious risk of instantly fusing expensive components with no indication that you have done so until something no longer functions!  For OBDI and II systems high impedance test equipment (e.g., LED test light, DVM, digital probe) is mandatory!!

-----------------------------

The technical why:

The difference in current flow between a low impedance incandescent test light (~300 Ω when lit) and a high impedance LED test light or DVM (~10,000,000 Ω) in a 12-volt circuit is a magnitude of ~4.  In other words an incandescent test light will draw about 10,000 times more current than a high impedance test light or DVM in a similar circuit.  That's the "electric chair" to the silicon components (transistors, ICs) on a printed circuit board!

[chriswalkoe]

So on a 12V circuit such as my 82' (no computer harness, airbags, etc.) an incandescent test light would be fine to test all connections?  Or are there still areas that I would want to stay away from , such as the instrument panel?

Thanks again for the help and education!

[bd]

You'll be fine using an incandescent test light on your '82.

[chriswalkoe]

It was the ground wire going from the heater motor to the firewall.  It looked good but it wasn't!  Glad that it was a simple fix.  Thanks for your advice everyone