Wiper Woes - PeachParts Mercedes-Benz Forum

wbrian63 · #1 07-10-2007, 10:19 PM

'92 300SE

Recently while sitting at a stoplight, my windshield wipers just stopped wiping in mid arc. I got them running again (I thought) by wiggling and turning the wiper switch on and off, etc.

This behavior would rear its ugly head every so often, but not to the point where I was worried about not having wipers.

I already had a new combination switch in house to fix the broken "flash to pass" headlamp circuit and figured that this would also fix the wipers - if the problem was bad contacts in the switch.

Didn't drive the car for about 10 days, and when I got in it yesterday - no wipers at all. Washer works fine, but no wipers.

Replaced the switch today - no change. (Did get the flash-to-pass function back)

I'm thinking there are two likely culprits: wiper motor or the (I think it's called) combination relay under the hood atop the fuse box. Neither of these items is cheap.

Can't find any diagnostic charts in AllData - any suggestions?

mbdoc · #2 07-11-2007, 08:06 AM

That system has 2 relays.

The main wiper relay is mounted to the top of the fuse box & controls all of the wiper functions. The combination relay only controls the low & intermittent functions.

wbrian63 · #3 07-11-2007, 02:15 PM

that's good to know - but I'm still looking for diagnostic methods to determine whether the problem is with the motor or the relay(s).

Any suggestions?

mbdoc · #4 07-11-2007, 02:22 PM

other than checking to see if you are getting power to the motor there isn't any testing.

wbrian63 · #5 07-11-2007, 02:38 PM

Thanks - I'll try that.

Are there any "likely failers" between the two relays - meaning does one fail more frequently than the other. If I determine that there's no power to the wiper motor, then the assumption is that it is one of the relays at fault.

Sure as the world as I pick one, it will actually be the other that's failed.

mbdoc · #6 07-11-2007, 05:04 PM

The main wiper relay is mounted to the top of the fuse box & controls all of the wiper functions. The combination relay only controls the low & intermittent functions.

Since the main relay controls ALL of the functions & the combination relay only low speeds does the high speed operate? IF not then the motor OR main relay would be the problem.

wbrian63 · #7 07-12-2007, 08:23 AM

Here's what I tried yesterday.

Engine not running, but key to #2 position.

Switch wipers on to intermittent - no movement.
Thinking there might be a dead segment in the motor, tried "helping" the wiper just a bit (less than 1/2" movement before resistance felt and I stopped) - no change.
Switch wipers to low speed - no movement.
Switch wipers to high speed - no movement.
Switch wipers back to intermittent.

Check for voltage at all 4 wires going into the main relay. 0 volts at red, yellow and green wires (brown is ground, from what I remember in the schematic).

Double/triple check everything. Pull fuse (#4) - all is OK. Replace same and check for voltage across fuse - all is OK.

Disconnect and reconnect wiper wire harness from main relay - no change.

Reach through driver's window to turn key off - first switch off wipers, but accidentally rotate switch wrong way - to low speed from intermittent, instead of to off.

Wipers start working....

Check all speeds - everybody's happy. On and off and on and off. Everything still copasetic.

Pull out the meter again. Check voltage at red wire. On low - around 7 or 8 (or was it 8 or 9) volts. On high - 12+ volts.

I think this points to the main relay. I'm going to call MB and see if they've got one in stock. I'll get it and keep it until the wipers fail again, since it's a 30-second swap job.

In the meanwhile, a good dose of RainX on the windscreen will at least keep me from wrecking the car should the wipers fail in a heavy downpour.

Please let me know if you think my logic is sound.

Regards