85 300SD blower motor fuse
 

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.

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).

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.

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. :confused:
6. I am going for a ride and try to blow a fuse, so I can figure out this mystery.

Don't waste your time attempting to blow the fuse again. Fuses are not all that accurate. One will blow at 16 amps and another will blow at 20 amps. It's not a big deal. The main reason for a fuse is to protect against a dead short (unlimited current).

If you continue to just check if you can possibly blow the fuse after a certain amount of time, this won't be a fruitful pursuit. We already know that you can, under the right circumstances.

So, time to check out the consumers. First is the blower motor. Then, possibly, the a/c clutch.

Did the fuse ever blow with the a/c not engaged? This will be an interesting piece of data, but, it still won't confirm anything.

Brian, you are right. I'll remove the motor and see what's going on with it. I guess I was hoping for a miracle cure. :o

Think nothing of it. That's what this board is for. You bounce ideas off us and we set you straight. ;)

Before pulling the blower motor, I checked the wiring to the compressor and found that one of the wires had a spot of insulation worn off from rubbing against the car frame during engine vibration. I could see copper. I think this was the problem. We shall see. :)

Well, wouldn't you be a lucky man today. ;)

Not the problem
 

If I recall correctly, the clutch power goes through the relay, not the blower fuse.

Drop the passenger side panel under glove box, remove blower motor, lube bushings with sewing machine oil, spin it, lube at least three times, then reinstall it and test. :)
Total time of repair for me = fifteen minutes. :)

Best oil to use on fans is Royal Purple, ISO 68 wt. Not that easy to find in small quantities. Next best is "Zoom Spout", a turbine oil found at most hardware stores, or 3-1 oil, blue and white can, NOT black and white can.
Trust me on this one.... :D :D

whunter, I am going to take your advice and lube the fan motor with one of the oils JimmyL suggested. However, it will be for preventative maintenance (thank goodness). The fuse chart lists the a/c compressor as one of the users of the blower fuse (see my original post). That's what led me to check the wires/connection. That and the fact that about a year ago, I was driving in the rain and hit a pothole, causing a loose a/c compressor wire to disconnect, causing a short circuit in the water (I guess), causing fuse #11 to blow. Anyway, I believe the fuse problem is fixed. One curious thing though. I have had the car two years. The a/c has always produced a marginal 50-55 degrees in the vent. Last night, after wrapping the worn wire, I went for a drive and the a/c took it down to 40 F in the vent. This is good :D but confusing :confused: , because I thought that when the worn spot touched the frame the fuse would blow, but maybe sometimes it just caused the compressor to disengage. Thanks to all for the great replies and help.

It depends if the wire touching the frame was the high side or ground side. If the ground side, all you would see is the symptoms you described of the compressor cutting out. If it was the high/positive side, you more than likely would see spots where arcing had occurred.

Sounds as if the blower motor is the culprit. www.**************.com has located the correct brushes for this, if they need replacing. Kit with instructions if you are unsure of doing it for the first time. However, a spot of lube may do the trick. Electrical contact cleaner, used very sparingly and protecting entrance to the bearings, may also be a good idea while you have it apart.