85 300SD blower motor fuse

[Bruce Bonds]

When I turn on the a/c and fan the fuse blows within minutes. It is position number 11, 25 amp. The yellow card says, "blower motor, air cond. compressor, switch-over valve idle stabilization" for # 11. Any ideas on where to start looking for the problem? The blower has never given any trouble before.

[Brian Carlton]

Sometimes the blowers get harder to rotate with age and cause excess current to be drawn. This is likely if it takes several minutes for the fuse to blow. You don't have a dead short, so, the wiring is probably fine.

If you run the system without the a/c, does the fuse remain intact?

The a/c clutch uses another 3 amps or so. It's also possible that the clutch is pulling more current than it should, but, I would think that this scenario is less likely.

The blower motor is the first line of attack.

Be careful operating the blower if the motor is really dragging. The increased current can toast the climate control unit so it's not even useful as a core for a rebuild (learned that one the hard way).

[Bruce Bonds]

Thanks Brian and Craig. I'll be checking the motor this afternoon. I plan to remove it and try to jump it to the battery. Could worn out brushes be the problem? Probably not, since I didn't have the usual symptoms where you have to tap the motor to get it started. I wish it was the same as the 123 motor, because I have two spares. I'll know more in a little while.

[Brian Carlton]

Originally Posted by Bruce Bonds

Thanks Brian and Craig. I'll be checking the motor this afternoon. I plan to remove it and try to jump it to the battery. Could worn out brushes be the problem? Probably not, since I didn't have the usual symptoms where you have to tap the motor to get it started. I wish it was the same as the 123 motor, because I have two spares. I'll know more in a little while.

Jumping it to the battery won't tell you anything because you already know that it works. Try and rotate the armature by hand. See if it rotates freely or is indicating that it is sticking. I recently had a problem with the aspirator motor. After 18 years it was screaming in its bushings. Just had to remove the armature, clean it up, and put a couple of drops of oil onto it. I'm not sure if you can take the blower motor apart. But, it's probably worthwhile, if its possible.

Worn out brushes are not the problem if the motor works. Of course, the brushes are probably worn down pretty low by now, but, that has nothing to do with the original problem.

[Bruce Bonds]

1. Friday night I started the car with the a/c on and the high blower on. W/i two minutes the blower stopped. I changed the fuse and it ran fine when testing.
2. Saturday night I started the car and it blew another fuse.
3. Sunday morning I posted on shopforum.
4. Sunday afternoon, before trouble shooting the blower, I replaced the fuse and cranked the car with everything off. I pushed all the buttons, a/c, high blower, on and off. I turned the car off and restarted with everything on and I could not blow the fuse.
5. The only difference is that the two blown fuses were the copper fuses and the fuse that is holding up has a fatter aluminum strip. I thought the copper fuses were better.
6. I am going for a ride and try to blow a fuse, so I can figure out this mystery.