Weird electrical issue 84 300CD
 

A friend has an '84 300CD with a weird electrical issue. Whenever the right mirror is adjusted, any climate control button is pushed, or even the engine started, the wipers will cycle once. Every time. I have no idea what would cause this. Anyone had an issue like this before? I will be putting a Sanden retrofit in the car on Wednesday afternoon, so I would like to have some idea where to look to find the problem. Thanks in advance, Rich

Bad ground.

I’ve had 30+ years experience in electrical controls, weird combination faults are almost always caused by a bad connection in the common side of the circuit.

I seem to be fortunate to have few automotive electrical issues over the years. As general preventative maintenance on old cars. Just cleaning and treating all ground connection usually gives some benefit.

Sometimes it can be subjective like I think the headlights are a little brighter than they where. Certainly the grounding system was not designed nor intended to last thirty to forty or more years without maintenance.

That said on very modern cars it can be really difficult with some electrical problems. Some can be the intermittent ones that could drive you insane if you were not already there.

I have a long list of things to check and do before putting an old buggy into heavier service. It takes less time to lubricate everything that is needed. Than having to repair a broken up hood hinge that seized and tore the metal out. Or find and replace a power window and regulator.

Few probably even consider lubrication of the window channels with silicone. Then there are the sunroof components. Old dried up lubricants are doing nothing where there is need of lubrication.

Even just lubricating the door hinges etc also makes the car feel better to you. I feel on most examples it was years and years since things like this were done. Even the locking tumblers are probably neglected.

Rich, I has an issue that the wipers would cycle when I started the car sometimes. Then I noticed that the intermediate setting was not working. Changed out the wiper relay and have not had it cycle for no reason since changing out the relay.

Well that is cool. If that is all that is the problem, that would be great. I will be working on the car tomorrow afternoon, so I will look into the relay. I will also check the grounds as others have said.

I agree about the ground issue. I once had one on my W123 I am having a hard time remembering in detail, but it caused a bunch of weird issues. If I was running the radio, it would turn off briefly when I used the turn signal and also if I ran the cruise control, my tachometer would intermittently stop working. I believe this was because I had replaced my battery cables and the connectors (termination points) I used were not the quality I had expected.

Dkr.

You are not alone. My wipers activate for one cycle when I adjust the passenger side mirror. The range of motion of that mirror is limited as well. I have been meaning to take it apart and clean it up but have had bigger issues to tackle.

x2
 

My wipers have the same issue.
They always cycle once when I adjust the passenger mirror.
They will cycle once randomly (not always) when I use the climate control.
Has done this for years. I thought it might be the combination switch.

Wiper relay would be the easy fix.
Where is the wiper relay?



Jeff

When I had a random wiper issue on my '85 123 wagon, I took apart the multi-function stalk, cleaned up all the contacts with De-Ox-It or similar from radio shack, and that fixed the problem. The end cap can be carefully pried off and then the whole thing will come apart. I removed it from the steering column to do the work, but it probably can be done if left mounted.

How did this work out?

My 85 300d occasionally sweeps its wipers one cycle on startup. It’s really harmless but I’d like to fix it.

The car has a bunch of electrical gremlins. P.O. installed a Clifford alarm and clarion stereo poorly. I’m in the process of debugging my combination switch. Got the wipers to work by turning the wiper knob on and off rapidly several dozen times. Low beams don’t toggle on when I flip the lever, just have high beams all the time and the little blue indicator stays on. I think I narrowed that to an internal problem in the switch though.

I’ll look at my relay. The intermittent wipers actually work very well.

I don't know, the guy has not gotten back to me. I also did not have a chance to look at the issue fully while the car was here.

Hey, well I just cleaned up my combination switch and the double sweep of the wipers when I start the car is gone. I’m not sure how that all happens but perhaps there is some arcing that trips the circuitry in the wiper relay. There are some solid state parts in there that handle triggering two sweeps for the wiper washer action when the washer pump is activated. Maybe just maybe that circuit is easily triggered by noise on the wiper switch that happens as it gets energized by the ignition.

Mystery Wiper Sweeps
 

I've had this same issue (two wiper sweeps every time I started the engine) in my 1984 YM W123's and fixed it by replacing the wiper relay, it's way back behind the speedo binnacle, you jhave to remove that to access the relay .

Pelican Parts has a nifty little DIY 'How To' with good photos of what to do .

Yes, the more I think about it, the more I see the issue with either the relay or the combination switch. When you hit the washer it pulses the relay and makes it cycle.

My combo switch had really gummy contacts in the wiper section. There may be a parasitic short that carries just a bit of current over to the relay on startup. Just a pulse of high frequency noise as you close the key switch. Just enough to start the wiper cycle.

Moving the mirror puts big inductive loads on the 12v circuit and this may also be enough to bounce the relay through a dirty combo switch. Disconnecting and connecting a dc motor can cause big spikes.

So why does changing a relay help? My guess is that there is some filtering circuitry in there to prevent spiking/glitching from triggering the relay. Any sensible engineer would put something in to discriminate between false triggers and real signals. Perhaps a capacitor resistor circuit that degrades over time as the cap dries out.

The guck in my switch looked like a combination of grease and ground copper. A perfect medium to transmit arcing noise.

Just a theory. I guess my advice is clean the contacts then change the relay if things persist. Cheap fix first.

Edit - spoke too soon. My car still has this problem. On to the relay.

Hey I got something on this!

I went to LKQ this week and got a wiper relay for peanuts from a 300D. I decided to try to swap it into my car. Guess what? No more crazy wiper ghost. It is normal now. I can start it, stop it, honk the horn, move the mirror and the wipers don't act possessed.

The relays look different. The one with the red (my mark with a sharpie) mb logo is the old crazy one. The functional one is on the right.

https://farm2.staticflickr.com/1964/...a8c9abdc77.jpg

https://farm2.staticflickr.com/1934/...c32657c5fb.jpg

I accidentally grabbed the heated window relay on the same car a week ago. I came home and realized it wasn't the wiper relay. The Pelican DIY on the wiper relay is really messed up...it calls the window defrost relay the wiper relay in the how to. I got it mixed up after I saw that.