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1986 420SEL/1995 E300D/4 BMW’s/2 Vanagon’s
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Discussion Starter #1
So I did some searching and tried some things but I am still stuck.

Here are the relevant details:

Installed a new rear window this summer. It is pristine. No delam, no corrosion of the defrost connections or wires.

Before doing so, my badly corroded and delaminated window still retained some defrost function. A few inches in the middle of the window still heated. That was last winter.

Now, six months after I installed the new window, the weather is finally giving me opportunities to use the rear defrost.

When I push the defrost button, the light comes on. However, no defrosting action occurs. Even after 10 minutes. After about 10 minutes, the defrost will automatically turn off just like normal...at least the light goes off on the button. Does anyone know what tells the defrost to go off? Is it strictly time?

I used my multi-meter to test some things. According to the fuse box diagram in my car, the #3 strip fuse is for the rear defrost. If I ground to the engine and touch that strip, I get 13.98V.

I tried seeing if power was being sent to the rear window. I am not 100% sure how to check this. I touch my MM to both sides of the defrost connections on the RW, nothing. I touched one and grounded to the door catch, nothing. I check continuity from one terminal on the window to the other, looks good. However, now I am remembering that I did not detach the defrost wires to do this so it may just be finding continuity on the body of the car.

I read on the forum while searching that there is a generic relay that works in conjunction with the rear defrost. I have like six or so of those ice cubes and I am not sure which one is which. I pushed and wiggled all of them to make sure they were seated. Plus, if I remember correctly, the one that controls the defrost also is for the hazards, which still work.

Anybody else have any ideas for me? Thanks!
 

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w126
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According to the schematic the combination relay(turn signal/hazard warning/wiper) feeds the defroster.
 

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1986 420SEL/1995 E300D/4 BMW’s/2 Vanagon’s
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1,026 Posts
Discussion Starter #3
More Info + Photo

So, guys, thanks for the info so far...

I did some more poking around/testing. I am now sure that no power is going to the rear. I disconnected all the wires in back, made sure I was properly grounded, and tested it.

It turns out the relay for the rear defrost is not one of the ice cube relays. It, the larger black one. I found that on parts.com they can be had for $43...not bad. With that in mind I decided to look at the innards of my relay in case there was something that could be fixed/soldered. Anyway, take a look at the attached pic. There is a dark almost what I would say is a burned spot on the PCB right near the actual relay that is for the rear defrost. I verified this by looking at the circuit diagram in the maintenance manual.

For any of the electronics experts out there...what would cause this to happen? Can it simply occur due to age? If there was a cause other than age, I am concerned about spending the coin on a new one only to have it burn out as well. I got out my trusty power supply and applied 13.98V to the poles of the relay and the relay component itself is still functional.

Any tips or advice is welcome.
 

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1986 420SEL/1995 E300D/4 BMW’s/2 Vanagon’s
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1,026 Posts
Discussion Starter #4
Rear Defrost FIXED!

Okay, after my last post I did some more tinkering. I used my multi-meter to test the burned trace on the PCB and there was no continuity. Also, when I was touching it with my probes, I noticed it was sort of loose and flaky. To fix it, I got an old coax cable and cut it apart to get the solid copper core out. I then soldered the copper core across the three component terminals on the burned trace. I now had continuity. The trace only connected these three terminals and nothing else. I put everything back together, put the relay back in the car, and turned the ignition to accessory. I had my girlfriend push the defogger button while leaned in and felt and listened for the click of the relay when I told her to push the button...CLICK! Next, I got my multi-meter out and tested the power at the rear window...bingo. Just for effect, I put a steamy pot of water on the rear shelf to fog up the window and make sure it cleared. Clear it did. The below picture shows the fixed trace and a scrap piece of the copper core I used. I don't know what is more rewarding...fixing this myself or saving the $43 on a new relay. I am damn pleased with myself though.
 

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For any of the electronics experts out there...what would cause this to happen? Can it simply occur due to age? If there was a cause other than age, I am concerned about spending the coin on a new one only to have it burn out as well. I got out my trusty power supply and applied 13.98V to the poles of the relay and the relay component itself is still functional.

Any tips or advice is welcome.[/QUOTE]

--------------------------------
That is heat damage from the coil of the relay getting hot. This has nothing to do with the load. It has to do with poor BTU calculation or lack thereof by the circuit board designer.
 

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1993 300TE, 1995 E320 Wagon
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I don't know what is more rewarding...fixing this myself or saving the $43 on a new relay. I am damn pleased with myself though.
Oh, definitely the "fixing it myself" thing. SOOO satisfying!!

Excellent tinkering, foolio2!

And happy holidays....:thumbsup:
 

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1991 S Class (W126) 300 SEL - 15W-40 Valvoline Max Life Diesel Oil, Mann Filt. Zerex G-05 Coolant,
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Epic WIN! Well done. I love it when you tinker, get creative and things actually get fixed. I had the same problem with my rear defog when I got the car, but the relay fix was also complicated with the wipers going off anytime they damn well pleased. I bit the bullet for a new relay and all was good. Glad you didn't have to do that.:thumbsup:
 

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A brown spot on a circuit board is not necessarily indicative of a failure, the fiberglass just gets toasty after a while. However, a high-resistance (i.e., flaky) connection can indicate a problem by overheating and turning brown. What you did is the "right" thing to do - whenever you see a break in a PCB trace (not just a vaporized trace), it should be jumped over with wire, not "slathered over" with solder. Solder is very brittle and has very poor mechanical strength. The only little caveat on this is that some circuits are designed with a PCB trace acting as a fuse, which is designed to fail in the event of a gross short - and replacing it will probably increase the current capacity of that area, allowing high current to go somewhere else. In this case, I wouldn't worry about this though.
 

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1986 420SEL/1995 E300D/4 BMW’s/2 Vanagon’s
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Discussion Starter #10
Thanks, fellas. I also saw a video on youtube where a guy used some sort of conductive marker that you could use to just draw on the trace. However, it was 10:00 PM and I was to into it to wait until the next day to go look for such a thing. We'll see how long this fix holds up. If indeed it fails again, I will probably just get a new relay as I would like to think the newer ones have been re-designed to compensate for this failure...unless it was just due to age.
 

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82 300sd, 84 500sec
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Thanks for posting your repairs, and for identifying which of the numbered but not named relays powers the defogger.

As to the conductive pen, you're better off having done what you did to replace that circuit, especially in a relatively high current application.
The conductive paint pens are not particularly cheap, nor do they put down all that thick a coating of the conductive paint.
They're primarily intended for touching up signaling circuits, rather than heavy power draw traces, from all I've been able to tell.
 

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Old thread here, but can anyone tell me where this relay is located (driver side pedal area, passenger's footwell fuse box, behind glove box)?

Does anyone have the part #?

All sites seem to lead to the ice cube fuse, which is wrong.

Thanks for your help.
 

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1989 SEL 560
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581 Posts
Old thread here, but can anyone tell me where this relay is located (driver side pedal area, passenger's footwell fuse box, behind glove box)?

Does anyone have the part #?

All sites seem to lead to the ice cube fuse, which is wrong.

Thanks for your help.
See if this helps. 126-820-12-10
 

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Premium Member
1989 SEL 560
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581 Posts
Old thread here, but can anyone tell me where this relay is located (driver side pedal area, passenger's footwell fuse box, behind glove box)?

Does anyone have the part #?

All sites seem to lead to the ice cube fuse, which is wrong.

Thanks for your help.
The relay is located inside the fuse box. Mine is black and oblong. It sits toward the back of the fuse box about the middle. It is on the right hand side of the several aluminum cubes. Mine is the original part number 201 821 00 47.
 
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