Is there any way to test the rear-window defroster in my 1984 W123 300D
 

Funola has expressed interest in coming up here, giving me $$$, and pulling the glass himself if I can get it to work. This sounds like the perfect way to get lazy money, but I need to test it. How do I do that?

Answer
 

Pre dawn is best for this test.

Jump it with 12V for 2-5 seconds with a 16 amp (RED) fuse in circuit.
http://www.peachparts.com/shopforum/...-240d-fuse.jpg

As long as it is not in direct sun load, an Infrared Thermometer should tell you if it is working.

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I am an electric idiot... can you tell me how to jump it? The car is not startable, so I'd need to just do this right off the battery -- I assume that that is OK?

I'd prefer if you measure the current. set DVM on the highest scale (more than2 amps). remove fuse 10 and clip meter to fuse 10 tab (not the buss). clip other lead to batt + terminal. turn on the defroster and report the reading.

note: key must be on, battery good and connected.

btw your DVM is the fuse! there's one inside.

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That's the problem. I'm guessing that I won't be able to successfully put the key in the on position.

I am not 100 percent sure on whether that key has to be on on not. try it w/o key - the battery negative has to be connected to chassis though. if the defrost switch light comes on then you will probably get a measurement.

I have a running '82 300SD -- I'll see if that defroster comes on when I hit the switch w/ no key.

don't bother the SD will definitely require the key. The method I described may bypass the key and supply power directly to the defroster. try it with a jumper instead of the DVM. If the defrost switch light comes on then you can use the DVM to take measurement.

If you remove the trim around the window, you should be able to find the wires that go to the heating elements. On my SD they are on the driver's side behind the c panel and are simple screw terminals.

Set your meter on the ohm range and measure the resistance across the two wires.

Since its on a 16 Amp fuse all by it self, we can assume that its current draw is likely between 9-16 (117 watts to 208 watts) so we should expect to see a resistance value between 1.4 Ohms to 0.8125 Ohms or there abouts.

Most meters (even this Fluke 77 at my desk here) are unfused at 10 amps and cannot sustain much beyond that for a long period of time so using a meter of that caliber, you might damage it. My $90 Triplett 9045, which is fused, is only fused to 10A.

I just looked at the wiring diagram. fuse 10 is hot in run or start. the method I suggested should work by supplying power directly to the defogger relay/ measuring the current.