A little electrical help for the new year?

Jay Zollinger · 01-01-2005, 08:59 AM

So, I'm driving my '73 911 in the rain last night and the wipers stop. I replace the fuse, the wipers move slightly, and the fuse blows again. Now the fuse blows as soon as I turn the ignition key--before the wipers move and regardless of switch position (off, low, medium, or high speed).

I have removed the wiper motor from the wiper arm assembly to make sure the motor was not overloaded--nothing. I have also inspected the wires at the switch, motor, and fuse box, and do not see any loose connections or worn sheathing that might cause a short. The fuse is the right amperage (25A), and the wipers have worked perfectly until now. All other electrical components are working fine (except for the rear defrost, which didn't work before). I don't see any burnt or blackened areas near the motor or switch.

Any ideas? If you think you can help me, please talk (or post) slowly and simply--I am an electrical moron. It's winter in Oregon, which means I need my wipers. Thanks in advance for any suggestions.

Rot 911 · 01-01-2005, 11:30 AM

Did you check the switch in the steering wheel column? Did you check to see if the motor turns freely and is not frozen?

Early_S_Man · 01-01-2005, 12:03 PM

How about worn bushings or binding of the wiper linkage???

Wiper motor is part of a system ... testing it removed from the car alone doesn't tell you much ...

Sounds to me like it is time to renew every bushing and lubricate every piece of likage in the system ...

fintstone · fintstone's Garage

Warren is right. While you have the motor out, it should be easy to manually move the wiper linkage by hand to observe if there is binding or if the bushings are bad.

The first thing I would check is your washer pump and other wiring in the circuit on the same fuse. If your wiper motor is removed or unplugged....just put a new fuse in and turn the key on to see if it still blows.

Jay Zollinger · 01-01-2005, 01:02 PM

I have detached the motor from the wiper arm linkage without removing the motor from the car in order to isolate any problems with the linkage. This makes no difference, so I don't think that the linkage is the problem.

The motor does not turn freely, but should it? I don't remember ever being able to move the wiper arms like you would be able to if the motor turned freely when it is not receiving power. Also, the fact that the fuse blows with the switch in the off position makes me think that the motor is not the issue.

As for the switch itself, I have pulled the switch from the steering column without desoldering any of the electrical connections so that I can inspect it. I don't see anything wrong with the switch, but I'm not sure what I'm looking for other than loose or exposed wires or burn marks. Is there a second switch on the motor that I should also be inspecting?

Jay Zollinger · 01-01-2005, 03:12 PM

OK, if I disconnect the washer pump (which I think is the only other accessory on this circuit), the fuse still blows. If, however, I disconnect the hot lead to the wiper motor itself, the fuse does not blow. Does this mean for certain that I need a new motor, or could there be some other problem that I am bypassing by disconnecting the motor?

911pcars · 01-01-2005, 04:31 PM

Let's summarize:
- The fuse blows with the source wire connected (switch ON).
- The fuse is okay with the source wire disconnected (switch ON or OFF).
- Linkage disconnected doesn't make a difference.

Sounds like the wiper motor itself is the culprit.

W/o the linkage attached, the motor shaft should be free to rotate. Either the bearings are shot (frozen) or there is a short in the motor circuit. If the motor stalls due to binding that could cause the circuit to overheat and blow the fuse.

Remove the motor and see if you can disassemble it. I haven't taken one apart, so I don't know if bearings are available; maybe just a broken wire. A boneyard motor shouldn't be too hard to locate. Just to be sure, connect another accessory onto the disconnected wiper source wire (washer pump motor?) and verify the circuit wires are okay too.

Hope this helps,
Sherwood

Superman · 01-01-2005, 04:53 PM

Check the switch. See if you get continuity when you close the switch contacts, and see if you get no continuity when the contacts are open. In other words, is it sending power to the motor when it's supposed to, and not sending power when it's not supposed to?

Then, depending on your findings the road forks. If the switch is working fine, then what happens when you give the motor twelve volts?

I'd guess the motor shaft should turn fine by hand. If it does not, then I would be very suspicious of the motor. But if the fuse blows when the motor is disconnected (and the hot wire to it is not grounded, of course), then I am more suspicious of the switch.

fintstone · fintstone's Garage

Sounds like the motor is bad. Even so, check the linkage good while the motor is out. Although the motor is probably just worn out through long use...a binding linkage will also burn one up.

Jay Zollinger · 01-02-2005, 02:47 PM

Thanks for everyone's very helpful input. After confiming that the fuse still blows with the leads from the switch disconnected at the motor (but with the hot and ground leads to the motor still connected), I took the backing plate off the bottom of the motor. There is no question that this is the cause of the short. There is a 1/2" hole burned into the top of the plastic cog attached to the bottom of the rotating shaft (above shaft in photo--sorry for poor lighting). I will check with my local rebuilder to see if a replacement cog is available and, if not, decide whether to look for a used motor from a boneyard or to spend the close to $300 needed for a new one. Thanks again, all.

fintstone · fintstone's Garage

Run an ad on the parts thread. Not too much demand so they are probably available and reasonable...and if you get your old one fixed...you have a spare. There is a guy parting a '75 front clip posting there now...with a wiper motor.