Well between watching football and being a dad, I found time to get in-front of the circuit drawings (F20), the -20, -4, and a computer. I have to say I find the manuals (both Chilton, Haynes, and our vaunted -20/-34's) to be lacking on this part of the vehicle.
Here's what I think I've learned:
If your blower motor goes out totally, or even if it just doesn't work on one setting here's what to trouble shoot:
1. Check the fuse if the motor doesn't work in any switch setting. If its blown, I wouldn't just replace it and keep running... I'd pull the black lead of the motor, then replace it and trouble shoot as below...
2. Pull the black lead (should be coming from a condenser/capacitor/noise suppressor etc plugged into an orange lead from the resister) that connects directly to the back of the motor:
3. Run a jumper from the + of the front battery to the motor's male terminal (don't put 24V from the rear battery on this fan!)
-if the fan spins the problem is in the switch or the resistor or the wires.
-if the fan doesn't spin, check for ground by grounding your volt meter to one of the rivets holding the fan system together and probing a hot terminal (either 24v or 12v will work)
--if the ground is bad, no motor will spin, so freshen it by working the mounting screws, and recheck the ground.
--if the ground is good, and the motor still won't jump and run, then you need a new motor. Best advice is to get one with a fan as the fans warp and wobble over time. But even if you need a new fan, you are not done trouble shooting... it may have killed part or all of the resistor and even the switch when it died so keep trouble shooting:
4. Check the voltage at the female terminal you pulled off the back of the motor (should be the output of the suppressor) check if you have voltage there at all settings of the fan switch in the cab. The voltage should increase as the position of the switch goes from off to full on.
-If any switch setting is with out voltage there is a problem in the switch or the resistor, and you need to trouble shoot them.
-If you have voltage that steps up with each up setting of the switch at all settings you probably just lost your motor, but if you put in a new motor and it still doesn't run except when jumped you need to trouble shoot the resister and the switch.
5. Pull the connector for the resistor (you'll have to remove the rear battery). Check for voltage on the respective terminals.. ie Yellow: low, Blue: Medium, Orange: High. The voltages to the terminals of the connector should be relatively the same (the switch doesn't slow the motor... the additive resistance of the coils of the resister do... the coils could go bad individually) REMEMBER WITH THE FRONT BATTER CONNECTED THE 12V system is still hot, and the + lead (and even the - of a 24V system) will be hot!
- If any one switch position has no or very different votage you need to pull the dash and check/replace the switch (and perhaps wires to the resister for shorts ie paths to ground with both the resistor and switch unplugged )
6. At the switch (you'll have to pull the dash) the brown wire is the hot 12V from the fuse box... Yellow: low, Blue: Medium, Orange: High setting to the resister.
The main problem is that a motor can fail and kill everything... or a resister failing can do the same... so isolating one bad component doesn't guarantee you fixed it... also from what I read, if something pulls enough current to blow the fuse it can kill the others... or if you just replace the fuse without figuring out what killed it you get multiple components killed.
Hope it helped. I'm still working my way through mine and will update this post as I learn more.
My truck 1986 M1009 used a 4 pole DR758 heavy duty resistor
And I used the 'monkey poop' i got in the electrical section of Lowes for sealing electrical conduits to seal my new motor and resistor... the manual will tell you to use new gaskets, but you probably won't find any...
monkey poop will not harden, and will be easily removed if you have to replace them again. It think the real name is "duct seal" or "conduit Seal".
