How to test W123 back glass defroster DIY the easy way
 

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.

http://i31.photobucket.com/albums/c3...609_125554.jpg

Another way, if you don't have an ammeter:

Remove the driver side lower dash cover
Unplug the big C104 connector.
The C104 is the one with the tail going toward the back of the car, don't confuse it with the connector for the turn signal switch which looks similar. It is easy to tell them apart because the tail of that connector goes up the steering column.

Once you have C104 apart, check between the pin side, pin 14, to ground. You should have about 2 ohms if the element is good.

Thank you. Pictures really, really help. I'll go to Autozone tomorrow to see if I can rent something like this (I need to have the Corolla scanned anyway... the low idle problem persists... sheesh...).

I'm also looking at checking out an automotive electronics book out of the library so that I don't end up in this situation again, trying to talk stupid to someone who speaks knowledgeable :o.

Steam up the glass on a cold morning & power the defroster, you will soon see if all the elements are working. Take a pic as proof.

Not sure if Autozoo rents DVMs. Go to Harborfreight and buy this for $5.29. It has a 10A input.

http://www.harborfreight.com/7-function-multimeter-98025.html

http://www.harborfreight.com/media/c...mage_12568.jpg

nice tips, funola and jay_bob. an ohmmeter uses much less power than an ammeter so for convenience and safety i think the C104 suggestion is a good one.

Layback suggestion of steaming up the window is good. this way you know if the whole Grid is working.

I have a spare window I was thinking of steaming up and applying 12v directly to it to see if the whole Grid works.

There is/was a kit with the paint to touch up any Grid lines that do not work.

The HF DVM is easy to get for free. just look for their sales flyers and there is always something for free. One being a DVM.

I now have 26 sitting on a shelf.:D Hey, they are free.
this is not counting the ones I have in the vehicles and gave away.

Charlie

Can you post a picture of the connector and also the relay? The reason I made this DIY is that I want to make it easy.

Sorry I don't have my 123 any more, sold it to Greazzer...

There is a photo of it in the 123 ETM but it looks like those pictures of UFOs as seen in the rag papers in the checkout line.

Maybe vstech can get a shot of the under dash of a 123. He's got a few available...

You're worried about how much power the meter use?:confused:

As for safety the meter should be fused. The only thing you should be careful of using the amps scale is to not short the probe to ground when the other probe is connected to battery positive.

I drove up to Joosepi Luna's this past weekend to buy his back glass with a good defroster and took this pic before cutting the glass out with a knife. The current is 7.41 A for a good defroster.

http://i31.photobucket.com/albums/c3...609_125554.jpg

FWIW, I got a reading of 7.04 A.

My meter could be off, or yours. Probably mine since it is old.;)