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Home > Puma (Tdci) > Rear screen heater switches brake lights on, hmmmmmm

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blurry

Member Since: 20 Oct 2020
Location: Wales
Posts: 5

Rear screen heater switches brake lights on, hmmmmmm

I have been trying to figure out why the rear screen demister doesnt work, 2011 2.4 puma.

The switch, relay, dash light all work.

A multimeter continuity test of the heater element is perfect.

Testing at the left hand 'ground' terminal and the righthand positive terminal wires that connect to the glass shows 14.47V with the engine running.

If i connect the positive cable, then run the multimeter through the ground , i get 13.6v with engine running, so there is an expecte voltage drop, and shows that electricity is going round the heater element.

However, when I plug both ground and positive cables onto the element, and then test the voltage across the terminals on the glass, the result is zero, the screen does not heat up even though it should.

By chance I noticed a flickering brake light (not the high level brake light), if the negative terminal of the rear heated screen is removed, within a few seconds,the rear brake lights light up. If i reconnect the rear screen black cground cable the brake lights go out.
If i then unplug both screen cables and plug the positve back on, the rear brake lights come on, if I momentarily reconnect and then remove the rear screen ground cable they go out.

When the rear brake lights come on in this manner, turning off the rear screen at the dash button also turns the brake lights off.

If the rear screen wires are reconnected, and regrdless if the rear screen is turn on or off, the brake lights work as they should.

The rear brake lights are wipac Led versions that I did not fit.
I have checked the supply plugs and cables fromt he rear door, also the ones behind the rear speaker cover.

Has anyone experienced this, or have an idea where to trouble shoot next?
Thanks in advance.

22nd Dec 2020 10:27am

Martin
Site Admin

Member Since: 02 Apr 2007
Location: Hook Norton
Posts: 6605

2015 Defender 90 Puma 2.2 XS CSW Montalcino Red

Removing the ground from the rear screen element causes current to flow to ground through the brake lights circuit, as the centre brake light and the element share a ground track on the window, which you've disconnected. This is expected.

What resistance are you measuring for the element. You say there is continuity, but it shouldn't be zero Ohms.

Check the resistance of the ground wire itself (not the terminal on the element) back to the battery negative, in case this is broken or high resistance.

You're getting voltage supplied, but haven't checked the path to ground.

1988 90 Td5 NAS soft top

2015 D90 XS SW

22nd Dec 2020 10:49am

blurry

Member Since: 20 Oct 2020
Location: Wales
Posts: 5

Hi Martin, thank you, after reading your reply about rear screen to ground brake lights that makes total sense now, as it goies through the high level brake light bulb.

How to check the ground cable all the way back to the battery? multimeter ground to the battery terminal and multimeter positive to the end of the ground cable at the rear window end?
Set to what ohms?
Thanks again.

22nd Dec 2020 2:07pm

llessur

Member Since: 15 Nov 2011
Location: Hereford
Posts: 151

I had exactly the same happen, and thanks to Martin, I found a broken earth wire in the convoluted rubber grommet that goes to the door.

22nd Dec 2020 4:04pm

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