Rear window defrost problem

[J.R.]

58 minutes ago, Edvinac said:

the CEM also senses a temperature sensor, so we decided to try it in a cold morning which yielded no results.

 

Check the temperature sensor output, preferably using VCDS, if it thinks the ambient temperature is high then it will switch off the rear screen heater prematurely or not switch it on at all, it may also not work if it is not getting any signal or an implausible one from the sensor.

 

It could also be a battery management problem, if the cell voltage is low or it thinks it is then high current devices like that will be inhibited especially on a stop-start vehicle.

 

I would have hot wired the relay coil by now but that might bring up other fault codes.

[KevinSt]

The only words in the owners manual about the rear window heater are:

 

The windscreen and rear window heater only operates when the engine is running. The windscreen and rear window heater automatically switches off after approximately 10 minutes.

 

There is no mention of only operating when the ambient temperature is below a particular limit.

 

Looking at the schematic the rear heater selection button (S218) is shown as sending a digital message on the VAN bus. Could there be a problem with the bus connections from terminals 5 & 6? Maybe this results in the warning light coming on until no message is fed back to the AC Control Module (A63) by the Multifunction Control Module (A11) to keep the warning light lit. This might explain why the relay can be controlled by the diagnostic control since it might communicate with A11 via a different pair of cables on the VAN bus.

 

I would be tempted to resistively check the continuity of the VAN bus wires from A63 pins 5 & 6 to A11 pins 16 & 17.

Edited 11 January, 2023 by KevinSt

[J.R.]

19 minutes ago, KevinSt said:

There is no mention of only operating when the ambient temperature is below a particular limit.

You surely dont expect the manual to tell you relevant information do you!

 

What other reason would there be for the controller to have a temperature sensor?

 

I think that you have been overthinking this and looking at the most complex causes without getting down to the absolute basics, you have now found that the screen is not in fact open circuit, I think you should check the input recieved from the temperature sensor before going down the Canbus wormhole.

[KevinSt]

I suspect that the temperature sensor is required to determine when to turn on the screen washer nozzle heaters (R37 & R38). There is unlikely to be any temperature override on the rear window heater since it could need demisting no matter what the external temperature.

 

Without knowing the full heater control logic it is difficult to target the fault-finding.

[ords]

40 minutes ago, KevinSt said:

I suspect that the temperature sensor is required to determine when to turn on the screen washer nozzle heaters (R37 & R38). There is unlikely to be any temperature override on the rear window heater since it could need demisting no matter what the external temperature.

 

Without knowing the full heater control logic it is difficult to target the fault-finding.

+1

[Edvinac]

19 hours ago, J.R. said:

 

Check the temperature sensor output, preferably using VCDS, if it thinks the ambient temperature is high then it will switch off the rear screen heater prematurely or not switch it on at all, it may also not work if it is not getting any signal or an implausible one from the sensor.

 

It could also be a battery management problem, if the cell voltage is low or it thinks it is then high current devices like that will be inhibited especially on a stop-start vehicle.

 

I would have hot wired the relay coil by now but that might bring up other fault codes.

I actually really have to take a look at the temperature sensor, because there was a lot of work done in the front of the vehicle which included removing the bumper. Also the electrician mentioned something about bypassing the temp sensor and the defogger still not working, not sure if this is possible. I have a hard time remembering if the sensor is in it's place, hopefully I can check by removing front bottom grill. (atleast that's the location that google states). Although the temp is displayed correctly on the dash.

 

2 hours ago, KevinSt said:

The only words in the owners manual about the rear window heater are:

 

The windscreen and rear window heater only operates when the engine is running. The windscreen and rear window heater automatically switches off after approximately 10 minutes.

 

There is no mention of only operating when the ambient temperature is below a particular limit.

 

Looking at the schematic the rear heater selection button (S218) is shown as sending a digital message on the VAN bus. Could there be a problem with the bus connections from terminals 5 & 6? Maybe this results in the warning light coming on until no message is fed back to the AC Control Module (A63) by the Multifunction Control Module (A11) to keep the warning light lit. This might explain why the relay can be controlled by the diagnostic control since it might communicate with A11 via a different pair of cables on the VAN bus.

 

I would be tempted to resistively check the continuity of the VAN bus wires from A63 pins 5 & 6 to A11 pins 16 & 17.

Thanks for getting into the schematics.

 

S218 is actually windscreen heater button, and it uses the same cables as rear defogger and is working correctly so everything should be alright, dont you think? Or am I reading the schematic wrong?

 

EDIT: IF ANYONE NEEDS ANY INFO/SCHEMATICS ON THEIR CARS PLEASE FEEL FREE TO CONTACT ME, I WILL BE ABLE TO HELP OUT UNTIL AUTODATA LICENSE ENDS.

 

 

Edited 11 January, 2023 by Edvinac

[KevinSt]

If the front screen heating works then that would suggest that VAN bus between A63 and A11 is working correctly. What if there is an internal fault with A63 where it doesn't actually send a message on the bus?

 

It is interesting how the front works but not the rear. Both are controlled from A11. Either it is not receiving the command to operate the relay or it is seeing or being told something to stop it from operating the relay even if it is receiving the message from A63.

 

You should be able to measure the resistance of the temperature sensor from the connector of A11, if there is a sensible value then it would eliminate that as a potential cause.

[Edvinac]

1 hour ago, KevinSt said:

If the front screen heating works then that would suggest that VAN bus between A63 and A11 is working correctly. What if there is an internal fault with A63 where it doesn't actually send a message on the bus?

 

It is interesting how the front works but not the rear. Both are controlled from A11. Either it is not receiving the command to operate the relay or it is seeing or being told something to stop it from operating the relay even if it is receiving the message from A63.

 

You should be able to measure the resistance of the temperature sensor from the connector of A11, if there is a sensible value then it would eliminate that as a potential cause.

It might be actually better thing to check the resistance at the CEM connector rather than the sensor itself since it will cross off the sensor and cables as faulty.

 

Has anyone actually tryed digging to the CEM of the octavia? Is it behind the fuse box?

[KevinSt]

Could there be a short to ground on one of the temp sensor wires that is making the controller behave in a strange way? Also, do the windscreen washer nozzle heaters come on when the weather is cold?

[KevinSt]

The manual says:

 

The windscreen washer nozzles for the windscreen are heated when the engine is running and the outside temperature is less than approx. +10 °C

 

So if they are not working then that might be an indication that there is something strange going on with A11 or the temp sensor.

[zore89]

Did anyone fix this? I have the same problem when i push defrost buton it turns off after 1 second ,also relay does not click, when i turn it on with scanto relay is normal working, 

i also tried with new rear lick mechanism but ita the same?