Rear window demister

[GTVeloce]

I have the same electrical gremlin in both the GTV and 75. My rear demister doesn't work. I get juice at the switch and through the relay etc but by the time I get to the rear window it shows about 5.5 volts across the two leads. It just seems strange it happens on both cars so I thought maybe others have come across this and better still a remedy for it!

[Al Campbell]

Low volts sounds like a high load dragging the voltage down over long leads or dirty contacts on the relay.

Is that 5.5 V with the demister in the circuit? I can't remember if you can unplug the wires from the demister grid or not, but if you can are you getting that 5.5V on an open circuit at the demister?

I'd expect something closer to 12V, but not quite full voltage as you'd expect a little loss under the high load of a demister.

I'll measure the volts across my GTV6's demister grid over the weekend.

AL.

[Al Campbell]

GT,

With my 1983 GTV6 idling I measured 10.95 V across the demister. Unplugging one side I got 13.8V.

The resistance of the demister element was 1.2 ohm, 10 of the 12 elements in the demister are still working i.e. 2 are broken. If all elements were working the resistance should be lower.

One side of the demister goes to earth and given the prevalence of rust in an ALFA, maybe you have corrosion where the wire attaches to the body. I've no idea where that is on the body.

Also have you tried swapping or replacing the relay? Given the high current involved, the contacts might be dirty after all these years.

AL.

[GTVeloce]

Thanks Al, that is very useful. I will have another go in light of this and try a few more things. I have managed to get 12V at the lead before the grid and the grid returns 1 ohm across the whole thing which impies it is working fine, so your theory about rust on the earth plug side could be relevant, especially the GTV which is getting rusty. The 75 has not a speck of rust so less likely. I will try creating another temporary earth to see if that works.
Cheers
Julian

[Al Campbell]

Yep I'd try a temporary earth.

Or first, with the demister turned on put the voltmeter between the earth side of the grid and use the point of the other probe to make contact with a good bit of metal on the body - you should get zero volts or close to, depending on the quality of your meter. If there's a lot of corrosion and a high resistance earth you might be get a low voltage.

Same thing if you can stick you probes up into the relay socket on the back of the two connectors across the relay contacts to see if your losing voltage across that.

AL.

[MD]

I agree with all of Al's suggestions and would like to just ad this minor one.

If the earth is good, the B+ rail to the demister is good, if the relay contacts are clean and sound then the only other reason why you would get a voltage drop under load is that the B+ supply to the relay has some corroded contacts that fail to deliver full voltage to your demister even though the relay is trying to get it there.

Check all B+ terminals supplying your relay.Also check the corrosion and tension of the fuse supplying the relay.

[colcol]

Check you see you have 12 volts coming out of the fuse, if not clean fuse, then if ok check for 12 volts at terminal no 30 on relay, if not bad connection between fusebox and relay, if 12 volts, at no 30 terminal on relay, then switch on heated rear window, check for 12 volts at terminal no 87 on relay, if not, relay contacts inside are burnt, get new relay, but make sure the terminals are the same, as each relay manufactuer puts the terminals in a different pattern to ensure you use their relays, if then you have 12 volts at terminal 87 on relay then check for 12 volts at wire on demister, if not, dirty connection between relay and demister, if 12 volts at demister, then you could have a bad earth, use multimeter to check ohms between clean earth on car and earth wire on demister, if you have a good clean earth, then it may be a break in the printed circuit on the back screen, use a multimeter to check that there are no breaks in the printed circuit, if there are breaks, go to Repco and get some circuit repair paint, that is paint with lots of metal in it, if it still does not work try painting the male terminal on the rear screen where it connects to the printed circuit on the screen, this worked on my old non Alfa, i hope it works for you, Colin.

[Al Campbell]

I thought about but didn't mention the voltage drop across the ignition switch. I was losing nearly 3 volts on my car. A couple of additional relays can make a big difference. One for the starter solenoid triggered by the start posi and another for the run position feeding the appropriate fuses. There was even a couple of unused relay sockets on the ALFA fuse panel.

AL.