Hi,

I tried a search but could not get anything on this: the rear window defroster seems to be no longer working (1989 560 SEL). The switch lights up when I turn it on, but there is no heat on the rear windshield. I checked the fuse - a ok. So now I suspect maybe a relay?

Any help would be appreciated!

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

Henry, I checked the schematic; there is a relay (N10) that controls the wiper motor, turn signals, and rear defroster. The black wire on terminal 6 (pin87) goes to the rear defroster.
The wife has the car today so I can't check; but I assume the relay is in the main fuse box near the brake booster.

Maybe this will get you started.
1) Check to see if the wipers and turn signals work.
2) If they do, pull the relay and try the wipers/signals again. this confirms you have the right relay.
3. Lift the relay socket up and jump a meter onto the pin 1-30 (red/grn wire). This should be hot, coming from the fuse. Then check pin 6-87 black wire (with the rear defroster turned on). This is the wire going directly to the rear window.
4. if you get voltage on 6-87 then the relay is good, the problem is in the wiring to the window, or the window resistors themselves. (I'll bet it's a broken strip in the heater).
5. If you don't get voltage at pin 6-87, check the wires from the rear defroster switch (pin 3-blue, pin 5-black). But since the switch lights up, it's probably sending voltage to the the relay coil).

The fuse for the rear window is a 30 amp, position #3, with a red/grn wire.
You checked this, but might put a meter on it also.

Hope this helps.

DG

Your advice just hit it straight on the nail.

I suspect it's the combi relay, and I ordered another used unit for $20 to mess with.

The rear windshield is perfect, so I don't suspect it's the heater element.

What I did not think about is removing the fuse and putting the meter to the terminals - that would tell me if there is current going to the element (or would at least isolated the relay).

Now here is the interesting thing: Trying to pull out the old combi relay. It's a larger black box inside the fuse box, and it just will not budge! Is there any trick to it?

On one site I read of one incident where a guy pulled the relay, re soldered the connecting pins, and it worked fine again. So I think between the old relay, the $20 unit, and a soldering iron I just bought I might have the problem solved, without having to dish out over $100 for a new relay....

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

Well, I haven't had issues with that particular relay, so I haven't tried to pull it out - Ha.
My memory is not what it was, but awhile back I worked on the cooling fan relay wires in the back of the fuse box. It's pretty easy to unscrew and lift up the entire fuse/relay board and get underneath. Just take out a few phillips screws and gently lift it up enough to get under there. Then the individual relay holders can be pressed up, and you can see how the relays are inserted into the sockets, and access the wires and pins.

The wife has the car out again, so I'm going by memory again.

DG

The good news: It was the relay.

The bad news: It's the same relay I already replaced with a new one in 2004, bought here at fastlane. On the original, the blinkers started malfunctioning periodically. I replaced it with the new one, and everything worked fine.

But me, being the pack rat, DID NOT THROW AWAY the old bad relay. I found it, swapped them out, and - ahem - the rear defroster started working again.

I am going to retest with the newer relay, just to make sure it was not a bad contact... Fortunately, it's only about $65. The bad news is I expected it to last longer than six years...

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

Glad you found it.
Well, since you have a spare, you could maybe wedge one open and see if you can clean the contacts, or check if the coil winding is open; or maybe actually switch out a working one from your spare. Nothing to lose.
But do the surgery on the window heater relay, not the wipers or turn signals. I supect all three relays in the housing are of equal current rating.

DG