Rear screen demister

Hi

Just a question on the demister C3 hdi 1.4 2004. Has 12v running at positive and at 2 ohms reads 0.067 on each resister line. Earth reads 0.0017 at zero ohms. Switch lights up relay clicks. It also runs for the ten minutes then safety cut off.

The issue I have is the demister barely has power to make a line in the moisture on the rear screen. Is this a relay issue if so, which relay is it. Can I change it. What part number is it.

Is it a separate relay for rear wash wipe as this is working ok.

Thank you for your time on this

Hello c3po03

The value you have for the resistance of the heater elements looks about right. As they're all in parallel you can't measure each part individually, however 0.67 ohms overall would give a current of approx 20 amps at 12 volts which sounds reasonable.

How did you measure the 12 volts and was it exactly 12 volts. I'm always suspicious of readings from digital volt meters as they are high impedance, that means they draw a tiny current when making measurements which can give false readings. I would always use a 12 volt test lamp when looking for voltage. Two wires connected to a 21w 12v bulb will do if you haven't got a dedicated tester.

Connect the bulb between the two ends of the element with power on and see if it lights up brightly. I suspect it won't.

If you're not actually getting 12 volts to the heater then my first place to look at would be the wiring going from the body to the tailgate. Where there's movement there's a chance of breakage.

Hi
Firstly thank you for the reply. When car running it's 13 to 14 volts, tested when running. I did ohms test on the factory earth and it was wondering around 300 225 185 90. So put my theory hat on ran earth to one of the number plate light bolts on boot, two bolts in trail gate and got a good static reading of 0.017 anyway less than 2 ohms resistance. Whole grid still low on power to move the condensation on rear window.

c3po03 wrote: ↑Wed Jan 09, 2019 1:41 pm The issue I have is the demister barely has power to make a line in the moisture on the rear screen. Is this a relay issue if so, which relay is it. Can I change it. What part number is it.

The 'Relay' is inside the BSI (the engine bay fuse box). It is not a classic relay but looks like a solid state control and a soldered onboard relay. Not a plug-in thing.

Your fault doesn't sound like a relay fault - one if those either it works or it doesn't as you seem to be getting some good voltages there.

The diagram shows a single supply wire on pin number 9 of the 16 way black plug for 12v supply and earth is marked E12 (left hand of tailgate).

c3po03 wrote: ↑Wed Jan 09, 2019 4:22 pm So put my theory hat on ran earth to one of the number plate light bolts on boot, two bolts in tail gate

Adding an extra earth sounds like it would have been a good idea if the 12v is ok. Looks like you need to try Missing Lincs putting the system under load to fully test it.

Hi
Here is my update on the rear demister.
So if you have good working relay, good power in 14 v with car running and a good ground to earth, less than 0.02 ohms. A complete rear grid no damage. A rear demister that works but does not clear the window.
Check the connector to the window. Both mine were connected,not loose. When re fitting the earth wire mine came off. If your car is a few years old I guess the original fitting will deteriorate. I will report back when new fixing paste applied.
My theory at the moment is a slight connection here will provide a low power working rear grid.

c3po03 wrote: ↑Sat Jan 12, 2019 6:13 pm My theory at the moment is a slight connection here will provide a low power working rear grid.

HI c3po03, this sounds plausible. You are running out of things it can be.

Thanks for the update on your progress.

The rear demister on the C3 usually clears the screen ok , so hopefully you will find the connection to be at fault.

Hello

I have found some super conductive epoxy resin. This is temperature sensitive on curing. On successful working demister I will update as to assist anyone else that may have this problem.

Thank you and happy motoring.