Rear window defogger relay assessment

Alright guys, a few weeks ago, my rear window defogger stopped working. I have an idea of the cause, but still require a push in the right direction.

Problem:
Rear window defogger not functioning on my 1999 Golf MK4 1.9 TDI with heated mirrors.

Context:
This might have started roughly around the same time one of my heated mirrors came loose and was replaced (bacause some ** drove/walked/cycled into it), but not entirely sure this relates to it.
Also, this could be the mind playing tricks with me, but it seems that when the switch is turned off and instrument lighting on, the light in the switch is brighter than before. When the switch is activated, it still goes a lot brighter.

Assessment so far:

• With the switch on, checked voltage over the grid: 0V.
• With the power off, checked the resistance of the grid: 1.2 Ohm (which at 12V would imply 10A and 120W heating, which is the ballpark I'd expect)
• Checked incoming voltage at the defogger switch (pin 5): 12V.
• Checked connectivity of the switch (between pin 5 and 6): Works as it should. 0 Ohms when switched on, no connection when switched off.
• With the terminals of the grid disconnected at the tailgate, checked voltage at the tailgate terminals: 8.1V, which after connecting drops to 0V again.

Preliminary conclusion:
The component providing power to the heating grid seems to be underpowered, given the low measured voltage and the voltage drop upon connecting. Now this looks more like the behaviour I would expect from a transistor and not from a relay. So this doesn't fully add up.

Question:
Is there indeed a relay in between or other electrial circuit? (I would expect so, no one is his right mind would make these 120W go through that plastic switch I guess..) If so, is it in the relay panel in the compartment above the panels or somewhere else? And any tips on how to assess it?

Many thanks for the insights!

I don't know about a relay off hand, but I had the same problem with the dash light indicating that the rear defogger was on, fully bright, with the switch off. Also my brake lights were constantly. The latter issue blew fuse #13. The left boot on the hatch had a slight tear, and one of the wires was nicked. For now I taped over the tear and changed the fuse and all is well for now.

I know in my car I hear a distinct "click" when I turn the rear defrost on or off, and it sounds like it's coming from the switch itself, so I'd guess that the relay is part of the switch.

I don't see a relay involved.
Power goes from fuse 26 to pin 5 of the switch and from pin 6 to the grid.

Thanks for the input guys! The absence of a relay surprises me. Nevertheless, this means this votlage drop is caused in the switch or in the cabling I suppose. Back to probing with the multi-meter.

Probably a bad switch. It has large 12 gauge wires going to it because there is no relay.

Fuse 26 is for the defroster. There is no relay. It uses transistors which are integral to the dash switch for its control. I can't quite tell from the schematic, but from how I understand it, is if when you press the button and it illuminates orange, and you have no power to the glass element, assuming the wiring from switch to glass is OK, the switch is bad.

Nero Morg said:

Fuse 26 is for the defroster. There is no relay. It uses transistors which are integral to the dash switch for its control. I can't quite tell from the schematic, but from how I understand it, is if when you press the button and it illuminates orange, and you have no power to the glass element, assuming the wiring from switch to glass is OK, the switch is bad.

Click to expand...

As far as I can tell it's a switch, no circuitry.
2 LED's for night light up & defroster on.

Somewhat related question - did VW change the operation of the defogger over the years in the MKIV cars? My 04 is on/off and will stay on forever, even after restarting the car (it's been a while since I drove it, but I believe the switch stays depressed?). My 03 is momentary action on the switch and activates a timer. Rear defog stays on for about 10 minutes and turns off.

Yes they did, early MKIVs had a timer trigger like button, I think it would stay on for 10 minutes. Newer ones are on/off like you have. They can be swapped, I swapped mine out for the timer one.

Nero Morg said:

Yes they did, early MKIVs had a timer trigger like button, I think it would stay on for 10 minutes. Newer ones are on/off like you have. They can be swapped, I swapped mine out for the timer one.

Click to expand...

I'll have to check my buddies 03. Both are silver 5sp wagons with leather but his has the newer center console. I definitely like the on/off better than the timer. I've had several newer cars that have the timer and dislike it.

GuuZ said:

<<<<<<<<<<<<<<<<<snip>>>>>>>>>>>>With the terminals of the grid disconnected at the tailgate, checked voltage at the tailgate terminals: 8.1V, which after connecting drops to 0V again.<<<<<<<<<<<<snip>>>>>>>>>>>>

Click to expand...

Wanted to ask what you connected the volt meter ground to? If you used the chassis as a ground, you may still have a bad wire. I'm assuming the hatch stuff would need an independent ground circuit.

gforce1108 said:

Somewhat related question - did VW change the operation of the defogger over the years in the MKIV cars? My 04 is on/off and will stay on forever, even after restarting the car (it's been a while since I drove it, but I believe the switch stays depressed?). My 03 is momentary action on the switch and activates a timer. Rear defog stays on for about 10 minutes and turns off.

Click to expand...

My Tiguan also is a push button.
Turns off after a while and does not automatically go on again next start up. Never timed it.

wonneber said:

My Tiguan also is a push button.
Turns off after a while and does not automatically go on again next start up. Never timed it.

Click to expand...

I find it odd that the 03 was like that, 04 wasn't and then VW switched back later. I think going way back that my 89 Fox stayed on and my 90 Corrado was timed? At least both the 03 and 04 leave the cigarette lighter "hot".

BobnOH said:

Wanted to ask what you connected the volt meter ground to? If you used the chassis as a ground, you may still have a bad wire. I'm assuming the hatch stuff would need an independent ground circuit.

Click to expand...

I used the negative terminal that connects to the grid, not directly on the chassis. I plan on doing more multimeter work this weekend. If I find a difference there, the ground wire might be troubled.

All power for the rear defroster goes through the dash button. Early cars have a timer, then around 2003 they ditched it for a simple on / off button.

-J

The issue has been found. Turns out the switch was fine. When measuring I found the ground wire was bad. The right boot on the hatch turned out to be the previous owner's Pandora's box. The upside there is, besides finding a very poor connected and insulated grond wire for the defroster, I now also know why my tailgate doesn't lock with central locking.

In your investigation did you learn how to test if the switch is good without power? My car is having the same issue, and if it does end up being the switch (although I will check the ground and grid, etc) I was wondering if there was a way to tell if the switches in a pick a part yard are good as they have no power. Would you just measure resistance between pins?

An unpowered switch can generally be tested by simple continuity. A resistance test can also be revealing.

That’s what I did, indeed. Pin 5 and 6. You could measure resistance between the other pins to check your odds for the lights still working. Would have to look up the pinnumbers for that..

If you have a timed switch, you'll have to hook it up to 12v to test it.

-J