What's wrong with my fridge.

Okay, here’s the short story since I’m working on this problem on another board as well.

My evap fan stopped working, after doing some tests I decided it was either a bad fan, bad motherboard or both. I replaced both and I’m still getting the same problem.
Now, this fridge has a test mode which, amongst other things, forces the fan to turn on.
When I put it in test mode, the voltage at the source reads 16v (which should be the correct voltage to run the fan), but, when I plug the fan in (and it’s not spinning), the voltage drops to about 2.5v.
In fact, with the fan plugged in, the voltage will start cycing from 2.5 down to 0, then it’ll jump back up to 2.5 and fade back to zero.
Any thoughts on this? What would cause that much of a voltage drop when a load is attached?

And in case anyone cares, there seems to be no resistance in either wire between the motherboard and the fan connector as well as infinite resistance between them, or between each of the wires and ground, so it probably isn’t a short.

Bump this one more time before I decide to cave and call in someone to repair it.
I’m at a loss here.

Sounds like a bad power supply. Check for bulging / leaking caps (do a Google search for “capacitor plague”).

I thought of that. I checked the old mobo and didn’t see any problem with any of the caps. I’ll take a look at the new one too, but I’m guessing that’s not it.

It just seems that with a new board and and a new motor, provided I didn’t replace a bad part with another bad part (which is possible, I know) it’s gotta be something else.
I suppose it could be an entirely different part of the system messing with the fans, but I just have no idea at all.

My thought would be a bad control board. When the source voltage drops like that under load it’s usually the supply that’s the problem not the load in my experience but I have another question. My fridge has one board controlling both fans. Maybe yours is different but is the condenser fan working OK and driven off the same board?

My fridge actually has three fans, a freezer fan, an icing fan (for the ice maker) and a condenser fan. All running off the same board. The other two run just fine.
In fact I just went and tried it again. I had the evap fan unplugged. Plugged in the fridge, got 16v, then I plugged the evap fan back in and checked the voltage again, it sat at 16v for a few seconds (the fan didn’t spin) and the dove down to about 5v. I still can’t help but wonder if there’s a small short or bad connection that doesn’t show with the low current of a ohm meter, but does show up when a bigger load is added to the circuit…is that even possible?

Yes, that’s possible. Are you able to spin the motor to feel for rough spots? I’ve had similar problems on motors where there was an issue with the air gap between the armature and the stator. They were bigger motors but still, might be a similar issue. Also, do you have any information on the motor itself. DC motors need a field, your’s is probably created by permanent magnets. If that field is lost the motor won’t be able to start.

The motor spins freely, no rough spots, no wobble, no nuthin’.

There are no brushes on those motors are there? I haven’t looked at one in a while.

Nope, all brushless DC motors.

Maybe the voltage is just dropping down because the controller isn’t sensing any counter-emf from the motor turning. Some of those brushless motors will fail if they’re handled roughly in shipping because the magnets become demagnetized IIRC. That won’t help you much if that’s the case though. It’s too bad those motors are so bloody expensive.

How likely is that? It seemed pretty well cushioned in the shipping box and I’d hate to order a new one to find out that it’s not the problem.

I don’t know, probably not very. I’m scratching my head trying to think of a better way to check one of those. The nice thing about working in a plant is it’s usually pretty easy to just try a new motor. I know that’s not such a great option for you.

I had a hell of a time testing them. When I replaced the freezer fan, I tried testing it and couldn’t get anything to make it spin, I even used a known good motor and couldn’t get it to spin. I tried using a 12v transformer, my car battery and even an 18v drill battery, nothing would make them spin.
Totally at a loss here.

How many wires does your motor have going to it?

Three, red, black, purple.
I should mention that everything I’m doing is based on the assumption that red and black are for power and purple is the ‘data’ wire which lets the board know that it’s spinning.
Actually, now that I think about it, I’m positive that’s correct. When I was testing for shorts, what I had done was cut the harness off the old motor, plugged it in, connect the red and black wires and then checked the resistance from the two leads off the board that (according to the service manual) supply voltage to the motors and got no resistance.

Ah. I missed this the first time through. Do you mean infinite between the two wires leading to the motor? That’s definitely not good news if that’s the case.

ETA because of the simulpost. The leads go to a coil, there should be a measurable resistance there 20, 30 ohms, something like that.

No, that was infinite resistance between them when the circuit was open. I was hoping maybe the shorted to each other, but no such luck.

What do you get between the motor power leads for ohmage?