How can I test a small DC motor?

I’m having some problems with my fridge. The problem is that one of the evap blower motors isn’t spinning. At the moment, I’m trying to figure out if it’s the fan or the mother board not sending power to the fan. Now, the easy way would be to check the voltage at the power source, but for various reasons I’m having an issue with that. Right now I’m just looking for a good way to check a 12v motor. My first thought was to run some wires from a 12V wall wart type transformer. When I did that it twitched a bit but that was it. Hooking the leads up to a known good motor didn’t make it spin either.
Is there some way I can check this with a multimeter before I replace it (or any other way)?

Car battery is 12v…

I’ll try that perhaps.

That’ll give you 12 volts AC. For DC you need a wall wart with a diode or four in it. It should say on the wart itself.

Really? I always thought most small electronics transformers put out DC. I’ll have to check that when I get back. I suppose that could explain why the motor twitched a little at first. I hope I didn’t ruin it with my test. That would really screw up my diagnostics.

I’m gonna hook it up to my car battery when I get home and try that.

A transformer puts out A.C. The power rectifier section of the unit changes the outout to D.C. Most small electronics do require D.C. The black box itself will tell you what the output is. My guess is that it is in fact D.C. Perhaps it can’t source enough current to turn your motor though. I assume you tried both polarities?

I’d be a little leery about hooking it directly up to a car battery without a fuse in between-- what I’d do is try hooking it up through the cigarette lighter (you can buy cigarette lighter pigtails from radio shack, or you can just strip the wires back from some car adapter you’re never going to use again).

What exactly are the issues with checking the voltage? Another thing I was going to suggest was that depending on what kind of wiring connectors everything has you could try just plugging some sort of car light bulb (tail light, headlight, whatever) and seeing if it lights up. Computer case fans also run on 12v DC, so if you have any of those lying around that could be another thing to try plugging in.

ETA: Also if you have any old power supplies (or old computers) lying around, this could be another source of 12V power to test it out with.

The issue I’m having with checking the voltage is that at home I only have my digital multimeter which I HATE (except for checking continuity). It’s soooo touchy and it jumps on it’s own between Volts and millivolts that it’s really hard to read.
I’ve got my analog one at work and I’m taking that home with me.
The other things is that each fan has three wires, I assume the third is for checking the speed (or making sure it’s still running) for the MB’s diagnostics. What’s odder is that there’s a red, black and purple wire and from what I can tell, it sends power through purple/red. Not that it’s a big deal, just unexpected, though I could be wrong, as I said the multimeter I had at home is a pain to use.

Also, I’ll double check when I get home, but I think the transformer and the fan were slightly different voltages, so that could be part of the problem. Hey, it was 1:00am and I just wanted to get my fridge working again.

Okay, so the motor is 13V, I would have guessed that a 12V source would at least get it to spin up. It would, wouldn’t it?

Tried hooking both fans to my car, neither worked which means either the car just won’t work to test them, or their both shot.

Hmmmm, just turned the fridge back on and found that the wiring harnesses, for both fans, are putting out voltage across two of the terminals (about 12v, could be 13v), but the fans aren’t spinning. I’m guessing one is dead and somehow I killed the other one while trying to test it.

Just went and double checked, yup, seems as if the fridge is sending the required voltage and both motors are dead.
There goes $100 I wasn’t planning to spend this week.:frowning:

Both motors going bad doesn’t seem likely. Check the voltage from the control board with the fan running. The board might be showing 12 or 13 volts with an open circuit but bogs down under load. If you only get a few volts with the motor running you’ll know the control board needs to be replaced not the motors.

Well, the fan doesn’t run, so I can’t technically check it while it’s running, but I did push my multimeter probes into the wiring harness while the fan was plugged in and it read 12v, so as odd as it is, I’m thinking both fans are shot. My guess, like I said before, is that one was toast and I killed the other one in the process, somehow. My only guess is that using that transformer to test it must have fried something. I think I’m going to order two new fans* and if that doesn’t do it, I can order a new board…and wait another two days.
*There’s a shitload of people out there that have had one or the other of these fans go bad, so even if only one really is bad, or even if neither of them are, replacing them now will at least push off the time until I have to take it apart again. And since the board is in another part of the fridge I can get this all put together when the fans are in without worrying about taking it all apart again in two days.