Skoda octavia mk3 hatch 2015 vrs 2.0 tsi with dsg

 

Hello guys, so I have encountered a problem that I cannot get to the root of:

 

Rear window defroster does not work from the panel button. Pressing the button causes it to light up for a few seconds and turns off. Does not activate the relay. The front window heater button does activate it's on seperate relay. Tryed switching both of these relays, and the situation stayed, pressing rear defroster turns on the button light for a few seconds, no relay noise again, front window heater activates relay, (so relay is ok I guess) 30 amp fuse is ok. Very confused.

 

Using a non-deticated VAG diagnostic tool (Thinkdiag) made the actuation test of the rear window defroster and that did activate the relay and I get power at the defroster wires. Very confused. Ground is ok, the heating element in the window is not open, proven by tester.

 

Can anyone help me with this issue?

 

The car has been rear ended slighty, the bootlid was changed, the original glass and wiring was transplanted to this new boot, everything is plugged in.

 

Thank you in advance.

Sounds like the diagnostics test is thinking the rear window heater resistance is out of spec and after a few seconds turns it off.

 

A scan with a proper VAG diagnostic tool would be the next step I suggest.

Can't get my hands on the proper diagnostic right now, but it is an option.

 

Does anyone have access to something like autodata electric schematics and could help out?

 

Would like to dig deeper and figure out how is the button and relay connected and what could be interfiering. Are there any special conditions for the rear glass defrost to work?

I don't have a  copy of the Mk 3 schematics but I seem to recall that the Mk 2 had a switch contact inside the lock mechanism in the tailgate that only enabled the rear wiper & demister if the tailgate was closed.

 

Assuming that the Mk3 works in the same way, could there be a problem with your tailgate switch or wiring?

5 minutes ago, KevinSt said:

I don't have a  copy of the Mk 3 schematics but I seem to recall that the Mk 2 had a switch contact inside the lock mechanism in the tailgate that only enabled the rear wiper & demister if the tailgate was closed.

 

Assuming that the Mk3 works in the same way, could there be a problem with your tailgate switch or wiring?

that is interesting and logical, although I do not get an error message while driving that my trunk is open

 

Edvinas

When I had the switch fail on my old Mk 2 I had no symptoms other than the non-working rear wiper and the fact that if you used the centre button on the remote to open the boot, the central locking would not automatically lock the boot when the tailgate was closed. The problem with my switch was corrosion, water had got into the connector and the terminal was virtually gone.

 

Without a wiring diagram you might be able to get at the switch by unbolting the lock or you'll have to remove the inner tailgate trim to get at it.

2 minutes ago, KevinSt said:

When I had the switch fail on my old Mk 2 I had no symptoms other than the non-working rear wiper and the fact that if you used the centre button on the remote to open the boot, the central locking would not automatically lock the boot when the tailgate was closed. The problem with my switch was corrosion, water had got into the connector and the terminal was virtually gone.

 

Without a wiring diagram you might be able to get at the switch by unbolting the lock or you'll have to remove the inner tailgate trim to get at it.

I will try this and return with results, thank you very much, Kev!

So I can confirm everything works as intended, with the remote to open the tailgate it unlocks only the tailgate, after you close it, everything remains locked.

 

Gotta mention that had to do this with two people, since the boot automaticlly unlocks even on a locked car, if you stand close to it and press the open button while having the keys in your pocket (keyless entry system).

Perhaps PetrolDave is correct in that the heater element is showing the wrong resistance. If I get a moment I’ll measure the resistance of mine for comparison. 

1 minute ago, KevinSt said:

Perhaps PetrolDave is correct in that the heater element is showing the wrong resistance. If I get a moment I’ll measure the resistance of mine for comparison. 

That would be awesome, just not sure how to interpret this information, how could the resistance change in the heater element (to make a diffrence)? If it had burned, would it not just be cut off somewhere, reminding that I have done the continuity test.

 

Also you will probably have to remove the plastic trim around the perimeter to access these contacts (they are on clips, no screws). And there are more than two connections on the glass, so make sure if you are measuring to document in between which contacts the resistance is measured.

 

I am also in the process of getting AutoData license from my ex-University I have studied in, we had access to it while studying, hope the teachers are in the mood to lend a single login  the diagrams should be helpfull.

 

Thank you very much again, Kev.

A fault code scan with a proper VAG tool such as VCDS or OBDeleven would be useful as we're all really just making guesses at the moment.

Edited 26 December, 20223 yr by PetrolDave

You did not say whether during your output test with the coide reader the rear screen actually demisted.

 

You have not yet measured the resistance of the element and are trusting what the tester is telling you, the best thing would be the evidence of your own eyes (use hot breath or a kettle to create misting) failing that a multi-meter.

I will try to grab the codes and test the output to see if it is actually generating heat, both in one go.

 

As for the resistance measurement, since base values are not stated anywhere a reading won’t help me much, that’s why I did not bother. Won’t know if it is off base values.

Ok so for an update.

Managed to grab the codes which lead me to nowhere. Also measured the resistance in the heating element for comparision if anyone will measure theirs

Well, that's your problem then. Your heating element it virtually open circuit at 1 Mohm. Assuming a maximum 30A fuse for the circuit then the resistance is of the order of 1 ohm or less. This would result in about 150 watts whereas your 1 Mohm would only produce less than a milli-watt.

35 minutes ago, KevinSt said:

Well, that's your problem then. Your heating element it virtually open circuit at 1 Mohm. Assuming a maximum 30A fuse for the circuit then the resistance is of the order of 1 ohm or less. This would result in about 150 watts whereas your 1 Mohm would only produce less than a milli-watt.

I am just not sure if I have tested the heater as it should be tested, a quick google search lead me to various alternative ways of testing the grid (heating element), and finding a line that is open (it is easily repairable thankfully), the problem now is that all the methods actually check the voltage drop off between the grid, but I cannot proceed with this method since the car does not even supply the voltage to the grid since it senses the open circuit (I think? I am not sure, I think if one grid was broken, the defogger should continue to work and just the lines that are open would not be heated). Since the only way I managed to get voltage to the grid is via diagnostic test, I will probably do a throughout visual check and if that does not help, get a mate that could activate the test continously and then check the voltage drop off. Hope the problem lies here.

 

Hoping to reach a solution soon for future members encountering similiar problems.

 

Updating soon.

 

edit: also found a test for the rear defroster relay in the diagnostic tool test section, which activates the relay continously ten times and it passed with flying colours.

Edited 27 December, 20223 yr by Edvinac

How/where did you measure the resistance?

 

Did you try at the lower resistance ranges? It should be measured on the 200 ohm range.

You should be able to measure the resistance across the 2 connection points to the heating element. If that is where you have measured then you have a bad connection to all of the paths in the demister so the problem lies at the point where the terminal is connected. Loss of one or more of the individual elements would only increase the resistance slightly, certainly not into megohms.

12 minutes ago, J.R. said:

How/where did you measure the resistance?

 

Did you try at the lower resistance ranges? It should be measured on the 200 ohm range.

I did, at the connection points, the last option was the only one that gave a reading.

 

1 minute ago, KevinSt said:

You should be able to measure the resistance across the 2 connection points to the heating element. If that is where you have measured then you have a bad connection to all of the paths in the demister so the problem lies at the point where the terminal is connected. Loss of one or more of the individual elements would only increase the resistance slightly, certainly not into megohms.

those connections are soldered, might have to try and resoldering them.

 

But still, I am confused how can an open heater element cause the defroster button to not activate the relay. I would not be surprised if the fuse on the power line to the heater would be burned, but that is not the case. I am really intrigued with the problem in my system and hope it won’t lead me to a faulty module, and hope it has something to do with the grid.

10 hours ago, Edvinac said:

But still, I am confused how can an open heater element cause the defroster button to not activate the relay.

AFAIK the BCM monitors the element to check for open or short circuit when in normal use (not when doing output test) so if it sees 1M ohm then it won't operate the relay.

It does indeed, like most monitored circuits it is looking for a calculated volt drop across a parallel shunt resistor in the switched contact supply.

 

It's a very good protection against short circuits and blown fuses.

Edited 28 December, 20223 yr by J.R.

I was hoping to find a wiring diagram for a VW or Audi that would have a similar circuit to the Octavia, but I can't find one. If there is a control module then it is quite possible that it monitors the current flow to determine if there is a fault.

 

The connection terminals on the window glass are not soldered, I believe they use a silver-loaded epoxy to make the connection. Unless the epoxy is broken then it should still be making good connection to the heater element. If these connections are intact then there must be breaks in all of the heating elements.

 

Maybe you can make connection to the individual elements to check the resistance along their length?

Edited 28 December, 20223 yr by KevinSt

1 hour ago, PetrolDave said:

AFAIK the BCM monitors the element to check for open or short circuit when in normal use (not when doing output test) so if it sees 1M ohm then it won't operate the relay.

 

It has to operate the relay for a short period to measure the volt drop at which stage it either maintains the contact or drops it out.

Breaks in the element can be seen, it's unlikely that all the parallel conductive elements are scratched through.

 

It will be either the tab connection or the "tails"

Edited 28 December, 20223 yr by J.R.

Hello again everyone, I was feeling that I own everyone a reply, was just postponing it because TL;DR have not yet fixed the issue.

 

For clarification, there was a mistake in my previous post. The heating element is not opened. It is acutally reading 0,8 ohms (had to change the test leads because they were ****e). Have also voltage drop tested every line of the heating element, every line is in tact. no luck. (used a partner that forced the activation of the element through a diagnostic tool.)

 

Also have checked if the auto hvac option is ticked in CEM through VCDS. All good.

 

Have tryed another Climatronic(ac control unit) from friends mk3 skoda, nothing changed, pressing the button illuminated the LED on the button for 2seconds and turns off, does not activate the relay. Mine on the other hand worked on his car, so we can cross that out.

 

Gave up, took it to an electrician that had loads of diagnostic tools, included VAG OEM one, after an hour, he still did not have an answer to this issue. I've suggested maybe disabling the start stop system could've disabled this heating function aswell, since the start stop is not working (disabled) and the car is seeing "high power consumption". So we decided that I should turn it on and try again. No results, start stop working as it should. the CEM also senses a temperature sensor, so we decided to try it in a cold morning which yielded no results. He also did some adaptations as he mentioned, but I have no idea what exactly. Totally lost at this point, maybe I should try and take it to a VAG-specific electrician.

 

Also, managed to grab a few licenses of AutoData from my university and have the electric schematic in my hand. Attaching it right here if anyone wants to analyze it. "vrs" is the vanilla one from AutoData, "vrsdiagrama" is the one I used for testing, basiclly i've marked cables in these colors:

Red-Constant power;

Blue-Switched power;

Green-Constant ground;

Yellow-Constant ground.

 

Main components (written in PDF, but going to mention them anyway):

K13-Rear window defrost relay;

A63-buttons for the defroster for front and rear windows;

A11-CEM (central electric module);

F53-30amp fuse for constant power to defroster

F7-10amp fuse for constant power to relay activation (this fuse is responsible for a lot of multiple things)

30-battery +;

31-ground.

 

If I am correct, I am currently having an issue where the click of the rear defroster button does not activate ground from CEM to the rear defroster relay to activate it (yellow line on my poorly drawed diagram). The electrician also mentioned it's too soon for a failing control module (?)

 

Appreciate all ideas.

 

Edvinas.

 

 

 

vrs.pdf vrsdiagrama.pdf

Edited 10 January, 20233 yr by Edvinac