Introduction: Mercedes W123 Windshield Wiper Relay Repair

How to fix the intermittent setting for the windshield wipers on your Mercedes-Benz W123.  Usually if the intermittent setting stops working, this is due to the windshield wiper relay (P/N 0 335 320 018) "going bad".  This instructable will show you how to remove the relay, test the switching of the relay, and fix what is really failing on this part (the capacitors).

When I purchased my 1985 Mercedes 300D, the windshield wiper intermittent setting (detent 1 on the wiper selection bar) did not work properly.  When selected, the windshield wipers would either work exactly like the slow and continuous setting (detent 2) or they would be intermittent for a cycle or two and then go back to slow and continuous. After some googling, I found out that a relay is responsible for turning the wipers on and holding them off for the short delay and was the source of my problem.

To purchase a new (used) relay on ebay costs approximately $30.  This seemed too much for me and I don't know of any good junkyards around me, so I decided to take the relay out, open it up and attempt to fix it.  After only a little bit of work and some light soldering, I was able to fix the relay and now it works just like new.

Step 1: What You Will Need

What you will need:

No. 2 Phillips Screwdriver

Soldering Iron


Vacuum Desoldering Tool (Not strictly necessary, but incredibly helpful)

47 uF/16V Electrolytic Capacitor (QTY: 2)
*Note: These can be rated for higher than 16V.  The originals are 16V, but a higher voltage rating (25V, 35V) is fine.  However, these MUST BE 47uF so that the timer delay (RC Timer) in the timer circuit is preserved. These can be purchased at RadioShack.


Something that can give you at least 12V (2 9V batteries, One 12V Battery, 12V Power Supply, etc.)



Step 2: STEP 1: Remove the Relay

First you will need to take out the lower kick panel on the drivers side. Use the Phillips screwdriver to remove TWO plastic screw-type clips from either side of the bottom of the kick panel.  These require a 90 degree turn in one direction and then should pop right out.  Then, take out the THREE screws that hold the top of the kick panel to the bottom of the instrument cluster dash panel.  With a small amount of jiggling and a little muscle, the kick panel should come out.  

Now, use your flashlight to light up inside the dash where the kick panel used to be.  The relay is hidden behind a heater tube against the back wall behind the instrument cluster.  If you were to see through the instrument cluster, it would be almost directly behind the fuel gage. It will take a little bit of stretching but should pop right out.

Step 3: STEP 2: Open Up the Relay

Use a screwdriver to pry the shell off of the main relay circuit board.  Be gentle here, you do not want to break anything.  The relay should look like the photos above.

Step 4: STEP 3: Test the Relay Itself

Now take your power supply (it can be 2 9V batteries, a 12V battery, or an actual power supply).  Use a small length of wire to connect the positive terminal of the battery to one of the relay posts and the negative terminal to the second relay post. When you touch the second terminal of the relay, the relay should switch. It may seem like you need 3 hands here, but try to keep the body of the relay circuit board steady because this relay is pretty heavy-duty and carries a lot of force when it switches.

If the relay does not switch, then the relay itself is bad and it may make the most sense at this point to go out and purchase a new Bosch part.  If it does switch, move on to the next step.

Step 5: STEP 4: Check Out the Electrolytic Capacitors

In the photo below you will see some light brown residue on the top capacitor.  On my relay, my capacitors had cracks in them and were leaking electrolyte.  This is an indication of a defective capacitor.  The original capacitors are 47 uF/16V.  We will replace these capacitors with new 47 uF / 25V capacitors.  

Note the location of the positive and negative terminals on the capacitors.  These are important.  The second photo is a slightly more clear sketch showing the location of these terminals.  On newer capacitors, the positive side is generally not identified, but the negative side is indicated with a lighter colored stripe down the side of the capacitor.

Step 6: STEP 5: Remove the Electrolytic Capacitors

Turn the relay over.  It should look like the photo below.  I have labelled the terminals of the capacitors below.  Use your soldering iron and vacuum solder removal tool to remove the solder from these points and uninstall the capacitors.

The second photo shows the relay module with the capacitors removed.

Step 7: STEP 6: Solder in the New Capacitors

Solder the new capacitors in the place of the old.   The capacitors must be 47 uF (microFarad) electrolytic type and can be of any voltage as long as they fit in the board space.  I used 25V capacitors.  Be careful to keep the polarity the same as the originals.

Step 8: Clean Terminals Re-Install and Test

Reinstall the relay cover on the module.  Use Brasso to polish the connectors on the bottom of the relay.  Once they are polished, use an Isopropyl alcohol (IPA) solution such as windex or rubbing alcohol to make sure the Brasso is completely cleaned off of the connectors.  Reinstall the relay module in the car, and test your wipers.  REMEMBER: Do not reinstall the kick panel until you know that the relay is working as it should!  There is nothing more frustrating than putting something completely back together and then realizing it doesn't work.  This instructable is meant for someone like me who knows enough about electronics and soldering to be make a quick fix like this, and is too cheap to buy a $30 part, especially if I don't have to.  Happy Driving!