Rear screen heater - broken line. Fix?

[Dr Mike Oxgreen]

My Saaaab 9-5 estate has acquired a broken line in its rear screen heater, which I’ve tried to diagnose without success.

I know you can get kits to repair them, consisting of metallic paint and a little brush, with a masking template to help you paint across the break. But that assumes you know where the break is.

A visual examination drew a blank: I can’t see a break in the faulty line. This surprised me, as often it’s obvious.

I then thought I could simply use my multimeter in continuity mode and buzz the faulty line to identify where it doesn’t buzz, but quickly had an attack of the bleedin’ obvious: the lines are all in parallel, so any two points anywhere in the pattern will always have continuity between them. Duh!

So I started measuring the resistance of short sections of the faulty line in roughly two inch sections, overlapping slightly to ensure I didn’t miss the fault. Each section gave around 0.7 to 0.8 Ohms all the way across. I was expecting to find a section that had much higher resistance where the current has to take the long way round via the other lines, the resistance of a complete line being more like at least 2 to 3 Ohms.

But I went across the whole length and got the same resistance for each 2-inch section.

So was my methodology faulty, or is there another failure mode that I haven’t considered?

The only other thing I can think of is to deliberately scratch a break in the line close to one end, thus breaking the circuit via the other lines. Then I could go along the line testing for continuity, and repair both the fault and the deliberate break. But this seems a slightly rubbish idea.

Any other ideas to pinpoint the fault?

[MrHappy]

I repaired my rear screen (Saab 9-5 estate as well!) last year with some of this conductive paint:

https://uk.rs-online.com/web/p/electronics-varnish...

From memory, breaks in the lines are fairly easy to repair, whereas there may be issues at the edges of the glass and that needs some trim removing to get to them. Mine were just breaks in the lines where a previous owner had cut them with something sharp in the boot. I used a bit of frog tape either side of the line to give a neat finish. There’s some decent videos out there on YouTube on how to diagnose the problem.

[GreenV8S]

The kits you're describing are designed to fix problems where some lines in the element are broke. This problem is usually obvious to spot because it does not defrost around the broken element(s) although the intact elements defrost as normal. Is this the problem you have?

If you aren't getting any defrosting at all and you're seeing connectivity across the element, the element may not be the problem. If you can get electrical contact with the heating element then turn it on and see whether you have a voltage across the element. If not, use a voltmeter to find whether the +ve side is being pulled up to 12V and the -ve side is being pulled down to 0V. That will tell you which side of the circuit you need to investigate next.

[Panamax]

OP seems to have said that one line in the element isn't working but the rest are. He says the lines are wired in parallel so testing always shows continuity. He's talking about making a break in a known position on the faulty line so as to be able find the "other" break with continuity testing. i.e. only the section between the two breaks will be dead.

[Dr Mike Oxgreen]

Thanks guys. Yes, the heater works, but has one duff line.

As I mentioned, I do know about the repair kits but they rely on knowing where the break is, and that’s what I was failing to locate.

I found a YouTube video that suggested using a test lamp with the heater switched on. I don’t have a test lamp, but used my multimeter in voltage mode, with the engine running and the heater on, looking at the voltage along the line using a nearby unpainted grounding point.

Using this method, I found a point where it suddenly goes from about 10-11 Volts to about 0.2 Volts. And whilst I can’t see a break in the line, there is a visible hairline scratch in the glass crossing the line at that point.

Still not sure why my resistance measurements didn’t find it, but it looks like it’s more reliable to check the voltage with the heater switched on.

I’ve now bought a thing of conductive paint and will have a go at it tomorrow when it’s a bit warmer. Meanwhile I’ve marked the point with a bit of masking tape and a pencil mark.

[Polly Grigora]

Good test method that you carried out using ohms readings, yes volts readings are better but as you comment the resistance should have changed when you measured across the break

Have you now measured the resistance again since now knowing where the break is?

I hate mysteries

[Dr Mike Oxgreen]

Polly Grigora said:

Have you now measured the resistance again since now knowing where the break is?

I hate mysteries

Will do - me too!

[finlo]

Your resistance test would also go the long way around the same as your continuity test did.

[Dr Mike Oxgreen]

finlo said:

Your resistance test would also go the long way around the same as your continuity test did.

That’s true, but going the long way should have given much greater resistance than the two inches.

[finlo]

Putting the meter on low range DC voltage would register 0 volts until you bridged the break which would then show 12 ish volts.