Carpokes Porsche Forum

I'm in the process of adding a rear wiper to my 1986 944 that didn't come with one originally.

I purchased all of the parts (motor, arm, blade, switch). The wiring for the console switch already exists in my car. I had to make my own harness that runs from the 4-pin barrel connector near the sunroof motor up through the grommet at the top of the hatch down through the little tube that runs down the side of the hatch and to the motor.

The part I'm confused about is when I bench tested the motor - if I gave 12v+ to pins 53 and 53a the wiper would start wiping back and forth, and when I gave 12v+ just to pin 53a the wiper would return to it's resting position and quit. All good so far, but when I introduce the switch to the circuit bad things happen. Luckily I put some small fuses inline on the two non-ground pins before testing it in the car (I didn't want to fry any wiring in case something wasn't right).

Anyway, with all components installed on the car, when I turn the key and the wiper switch is in the "off" position all is fine. With the key on and the wiper switch in the "on" position the wiper goes back and forth as it should. But with the key on and moving the wiper switch from the "on" position to the "off" position it pops my fuse on pin 53.

The wiring diagram shows that the switch operation is as follows:
- rear wiper switch in "on" position: sends 12v+ to pin 53
- rear wiper switch in "off" position: sends ground to pin 53

So it seems that even though it is grounding pin 53 in the "off" position - that is fine as long as the motor is at rest, but when the motor is in the middle of a "wipe" motion and you turn the switch off, that is when the issue comes up.

Now, I think the motor I got was an early car motor and I installed it on a late model car, but I wouldn't think that would matter as the wiring diagrams for early year cars seem to be identical to the wiring diagrams on late years.

So what I ended up doing to temporarily remedy this was to take the console switch connector apart and move the ground pin (pin #5) to an empty hole so that the switch doesn't make any contact with it. Essentially taking the ground out of the circuit at the switch.


With this configuration, the wiper switch works as I would expect it to:
- on makes the wiper start waving back and forth
- off returns it to its proper resting spot and that's where it stays
No blown fuses, no issues.

Just wondering if anyone has any extensive knowledge about these things and could explain why I'm having an issue. Or maybe the motor I bought was bad???

I suppose it's possible the harness connector was assembled incorrectly at the factory. Or perhaps it popped apart at some point and a previous owner put it together haphazardly?

The connector pin locations ought to be identified on the writing diagram from the factory service manual. Maybe that pin is back where it belongs?

The pins were in the correct spots originally (according to the wiring diagram).

For this switch all 3 connectors on the switch are in a straight line. Moving the one connector across to the other side takes it out of the circuit.

I just moved the ground because I presumed it would make my setup work (confirmed). I'm just wondering if my wiper motor is not as it should be I guess.

I honestly can't figure out the reasoning for the switch to ground that pin when it goes into the "off" position - very confusing...

Hmmm. It's been a long time since I've had a wiper motor apart. I know they have a mechanism that allows the wiper to complete its cycle after the switch is turned off, so the blade(s) can park. It's possible that behavior involves grounding some part of the circuit?

Or... Does the rear wiper switch have illumination? (I don't have one so I don't know.) Switch illumination would require a ground. Though it blowing the fuse only in certain situations doesn't really make sense. Just thinking out loud here.

No, the switch is not illuminated. I guess I'm good with the solution I have come up with. As long as it works...

I had no idea this could be done. How did you cut the glass for the wiper base to be installed??

Mine actually had a hole there with a plug in it from the factory I believe.




I have cut holes in glass before using diamond coated core bits - I use this method to put christmas lights in wine bottles. But I haven't tried using one of those to cut thru tempered or safety glass...

@notny41

Fantastic Christmas decoration idea. And that's your video too!

Very cool!

notny41 wrote: Thu Dec 21, 2023 8:36 am
Just wondering if anyone has any extensive knowledge about these things and could explain why I'm having an issue. Or maybe the motor I bought was bad???

Similar to my post here

https://carpokes.com/viewtopic.php?t=2206#p23957

The way the motor stops in the park position is by shorting out the motor. The motor has continuous power everywhere but about 10 degrees around the park position, where is is either powered or grounded by the switch. When the power is removed, the motor in motion is acting as a generator. If it does not have a load on it, no energy is removed by the generator action. Energy is then lost only through friction of the bearings, gears and the wiper blade on the glass. Typically, if the braking ground is removed, there is not enough friction and the motor coasts on through the parking position and picks up the continuous power again. The cycle repeats with the motor running indefinitely.

When the motor is in motion, and the windings shorted, all the energy is dissipated very quickly by the resistance in the switch wiring and the motor's internal windings. This low resistance means a high current surge happens. This is generally not harmful because the surge is quick, and only happens once in a great while, i.e. when you shut the wiper off. The wiring does not overheat. The fuse you put in the circuit, however, reacts very quickly to this surge and blows. You may safely put the ground wire back into the correct position. The fuse is not part of the design.

Cheers!