How to test W123 back glass defroster DIY the easy way

[funola]

I have been trying to buy a good used W123 back glass with a good defroster locally and trying to tell the seller how to determine whether the defroster is good or bad turns out to be not so easy. The electrical connections to the defroster grid on the glass is not accessible without a lot of work- the 2 panel trim pieces on the C pillars (seat belt runs through it) has to be removed and doing so may break the 3 plastic hook/ clips, especially in cold weather. Not advisable IMO.

The easiest way is to measure the current to the defroster. You will need an ammeter that can measure to at least 10 amps.

1. Remove fuse 10

2. Clip one ammeter lead to the lower spring contact of fuse 10. Make sure meter is set to the correct amp scale and the leads are plugged into the correct socket on the meter. My meter has a separate socket for the 20 amp input that is different for volt and ohm measurements.

3. Clip the other ammeter lead to battery + terminal

4. Push the defroster toggle switch to turn it on. If the light on the switch lights up, it is working. Now go read the meter! A good defroster should read a little over 7 amps.


This test takes just a few minutes. Key is not needed since you are supplying power directly to the defroster circuit, bypassing the accessory relay.