This blower motor is driving me insane. I connected the motor directly to a battery and it works. Replaced the blower motor resistor, checked the fuses, swapped the relay, tried a new control switch, wire harness seems intact - nothing is working. I’d tear my hair out, if I had any left. Any ideas?
I when you turn the switch on do you get current where the harness plugs into the motor? Years ago, in the early 80’s, my dad had a blower motor that would work when connected to 12 volt power supply but not when connected to the car. Got a new blower motor and it worked fine. Also, I have seen my fair share of bad out of box aftermarket fan relays.
Check for current where the harness plugs into fan.
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If it spins ok with +12 and ground (-) directly connected to the battery, then it should spin ok installed if the two inputs to the motor measure +12v and ground. So that’s the place to start. Are the two inputs to the motor 12 volts and ground? This measurement has to be done with everything connected, so may require some invention to get access to a place to place the DVM probes. If the voltage measures correctly but the motor doesn’t spin, there may be something preventing the fan blades from turning freely.
Both are excellent suggestions. When you connected the battery direct, was that using jumpers? You can pull it, set it on the floor, and see if it runs. If so, it’s probably something blocking the motor.
Next, I usually start with George_San_Jose1’s idea… check voltage at the motor input (key and blower switch on)… and test intermediate contact points until I find the problem. The video below is for a Saturn, but it’s similar steps. Your panel locations are probably different.
I’ve been troubleshooting a windshield washer pump recently. I did the same thing as the OP did, removed it and connected directly to the battery. It spun fine that way. When I put it back in the reservoir bottle, wouldn’t spin. Voltage measured ok installed. I eventually discovered it wouldn’t spin in the vertical orientation (like it is installed), but would if held horizontal. I invented a tool so I could rotate it manually, cleaned it out with a water-jet device, a bunch of gunk came out, and then it worked both orientations. Proof that most anything that could go wrong can go wrong when working on mechanical gadgets.
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Thanks for the tips guys - my dad gave me his extremely expensive multimeter from his construction days, but I’m ashamed to report I had no idea how to use it. That video was really hlepful though, so now I just have to find the damn thing. 
If you can’t find your dad’s DVM, or don’t want to risk damaging it, you can buy one that will do the job for less than $10. Harbor Freight sometimes offers coupons that gives them away for free when you buy something else. I have one of the free HF units I use for something almost daily. If I knew of a 2 or 3 page basic DC electrical tutorial posted on-line I’d provide a link, as that would be helpful to read before using the DVM. Unfortunately most of them first describe unnecessary details like the structure of atoms, the physics of electrical conduction etc, rather than getting quickly to what you need to know to use the meter correctly and accurately interpreting the result.
I’ll probably just go grab the cheap one then. That’s the issue I have experienced for using it for diagnosis purposes - the few videos I watched were like “This is how you use a multimeter for every single electronic purpose and all of the science you’ll never need to know!” and all I wanted to know was how to use it to do basic tests on automotive components.
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Here’s a tip to avoid damaging the DVM. It’s pretty difficult to damage a DVM when measuring stuff in cars in Volt mode, so start with that to get some experience. You can damage it if you measure resistance when the car’s battery is connected. So disconnect the car battery before measuring resistance. That will take care of minimizing most of the risks.
A good volt test is the basic alternator/battery check. Put the DVM in DC-Volt mode on a scale larger than 12 volts. 20 volts is the scale I use on my DVM. Connect the red probe to battery +, and the black probe to battery -. Before first start of the day the battery should measure about 12.6 volts. Immediately after starting the engine, 13.5-15.5 volts. Remember you are allowed to use alligator clip test leads to connect the probes to what you are measuring. Often makes the job easier.
https://www.harborfreight.com/18-inch-low-voltage-multi-colored-test-leads-66717.html
Beyond that, it’s possible to damage the DVM when measuring current. I guess the best advice is to always start measuring current with the highest current mode the DVM offers, usually 10 Amps. That often requires using a different hole for the red probe. If the current isn’t enough to register, then move to the lower current modes. And don’t try to measure big current draws, like the starter motor, rear window defroster, alternator currents, etc. Those have to be measured with special high current test equipment.
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Ok, so I know I asked about this like 40 years ago, but I had a million things going on and it got to be winter so the lack of A/C from the blower motor not working wasn’t so important. Now it’s getting hot again, so I need to get this working, before my wife kills me. I followed everything on the video and am pretty sure I did everything right. The one thing I noticed is that at the plug that goes into the blower motor, I did not get dc voltage on either side (I tested both, because I wasn’t sure which was ground at first…now I’m pretty sure it’s the yellow wire). I am wondering - does this mean I got a bad resistor, or is it the pigtail itself? It looks a little dried out on the end, but there is no obvious cracking or anything in the insulation.