Dodgy Demister

[mmi]

A challenging one...
I have thought that the 2 contacts are one to the ground and the other is the +connection. But as the tailgate heater draws about 22 Amps means the resistance is "too" small if simply measured by a multimeter. Anyways you should definitely have about 12 Volts on the other when switched on. I'll check mine tonight to see which one is the +connection - unless resolved before that.
And, I'm pretty confident that engine has to be running for the heater to work - but will double check that too.

 

[andys]

A challenging one...
I have thought that the 2 contacts are one to the ground and the other is the +connection. But as the tailgate heater draws about 22 Amps means the resistance is "too" small if simply measured by a multimeter. Anyways you should definitely have about 12 Volts on the other when switched on. I'll check mine tonight to see which one is the +connection - unless resolved before that.
And, I'm pretty confident that engine has to be running for the heater to work - but will double check that too.

Are we talking about these below? On mine they both have continuity to ground. If there's a short across the element wouldn't the BCM sense that and generate an error?

I've had to leave it for now, I have a couple of runs to the tip and then some plumbing repairs to do.

 

[Dellmassive]

One side will be the positive feed and one side the ground connection.

You will need to disconnect the wires to test the supply.(both)

One of the wires will be a solid ground.

The other should be the switched 12v. ... with engine running like MMI mentioned.

You could also check the wires at the door loop connector. Carfully pull out the rubber door loop bung at the top hinge van side. And u will see the loom and connector.

 

[andys]

One side will be the positive feed and one side the ground connection.

You will need to disconnect the wires to test the supply.(both)

One of the wires will be a solid ground.

The other should be the switched 12v. ... with engine running like MMI mentioned.

You could also check the wires at the door loop connector. Carfully pull out the rubber door loop bung at the top hinge van side. And u will see the loom and connector.

Ok, unplugging both sides of the heating element produces the expected error

00975 - Heated Rear Window (Z1)
011 - Open Circuit
Freeze Frame:
Fault Status: 01101011
Fault Priority: 3
Fault Frequency: 1
Reset counter: 187
Time Indication: 0

Putting a meter across the two ends of the feed to the element shows 5.7v, whether the engine is running or not. This voltage stays until system shutdown following locking.
I don't get it

 

[mmi]

Are we talking about these below? On mine they both have continuity to ground. If there's a short across the element wouldn't the BCM sense that and generate an error?

I've had to leave it for now, I have a couple of runs to the tip and then some plumbing repairs to do.
View attachment 38294

Yes, those exactly. The left is +12V feed when engine running and heater switched on. The right is permanently connected to chassis ground. The resistance of heating element is just approx. 0.5 ohms (the current draw approx. 22 Amps). The left/right in direction of travel just to avoid any confusion.

I think there is no short - just very low resistance between terminals makes it look that both are connected to ground.

In the VCDS by flipping the heater switch the following measurement values change their state:
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Address 08: Auto HVAC (7E0 907 047 AC)

17:31:12
IDE00993-MAS01277 Status of input signals 11-Rear window heater actual value OFF
IDE00993-MAS01278 Status of input signals 11-Rear window heater target value OFF

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

Address 08: Auto HVAC (7E0 907 047 AC)

17:32:13
IDE00993-MAS01277 Status of input signals 11-Rear window heater actual value On
IDE00993-MAS01278 Status of input signals 11-Rear window heater target value On

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

 

[mmi]

Ok, unplugging both sides of the heating element produces the expected error

00975 - Heated Rear Window (Z1)
011 - Open Circuit
Freeze Frame:
Fault Status: 01101011
Fault Priority: 3
Fault Frequency: 1
Reset counter: 187
Time Indication: 0

Which address 08:HVAC (J301) or 09:Centr.electr (J519)?

 

[andys]

Which address 08:HVAC (J301) or 09:Centr.electr (J519)?

09 Cent elec

I'll go back out and check the voltage between each side and ground

 

[Dellmassive]

Putting a meter across the two ends of the feed to the element shows 5.7v, whether the engine is running or not. This voltage stays until system shutdown following locking.
I don't get it

The 5v you are reading is the BCM low current/voltage that is being used to monitor the circuit. (thats how it detected you disconnected it)

do a ohms test to see which side cable is the perm ground, the other side will be your +12v switched.

do your voltage tests reference to a chassis ground like the latch catch or other.

 

[andys]

Ok the voltage between the contacts and ground are +5.7v on the left and 0v on the right. Pressing the HRW button doesn't change this

 

[Dellmassive]

ok so you have found the supply and ground, thats good,

and the BCM is monitoring the circuit - thats good too.

but its not working or flagging a fault code - which would relate back to a coding issue?

have you tried looking at @mmi measure blocks above to see if the BCM is seeing the button request?

also a bit out there, but did you try the hard reset? - disconnect battery for 30mins

 

[andys]

ok so you have found the supply and ground, thats good,

and the BCM is monitoring the circuit - thats good too.

but its not working or flagging a fault code - which would relate back to a coding issue?

have you tried looking at @mmi measure blocks above to see if the BCM is seeing the button request?

also a bit out there, but did you try the hard reset? - disconnect battery for 30mins

The fault code is only produced by disconnecting the element though, with connections back on the fault code goes away.

How do I monitor the button press in VCDS? I'll disconnect the battery now while I have my tea

 

[Dellmassive]

@mmi

quote: How do I monitor the button press in VCDS? I'll disconnect the batter now while I have my tea

i havent got HRW on mine can you help @andys with displaying the Live measure blocks for the switch you detailed above (show him how to do it)?

like your readout below?

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------


Address 08: Auto HVAC (7E0 907 047 AC)

17:31:12
IDE00993-MAS01277 Status of input signals 11-Rear window heater actual value OFF
IDE00993-MAS01278 Status of input signals 11-Rear window heater target value OFF

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

Address 08: Auto HVAC (7E0 907 047 AC)

17:32:13
IDE00993-MAS01277 Status of input signals 11-Rear window heater actual value On
IDE00993-MAS01278 Status of input signals 11-Rear window heater target value On

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

 

[Dellmassive]

basically you go into the module and choose "measuring blocks" and "advanced measuring blocks" then start moving up through the groups till you find what data you are looking for.

looks like @mmi was in Address 08: Auto HVAC (7E0 907 047 AC)
17:31:12
IDE00993-MAS01277 Status of input signals 11-Rear window heater actual value OFF
IDE00993-MAS01278 Status of input signals 11-Rear window heater target value OFF

 

[andys]

basically you go into the module and choose "measuring blocks" and "advanced measuring blocks" then start moving up through the groups till you find what data you are looking for.

looks like @mmi was in Address 08: Auto HVAC (7E0 907 047 AC)

Thanks I'll try that next if the hard reset doesn't work

 

[mmi]

Thanks I'll try that next if the hard reset doesn't work

Yes, module 08: HVAC, and then "Adv. Meas. Values"

 

[mmi]

That should open two subwindows, the selection list is similar to this one... (this is from another module but gives the idea)


then just need to find and tick these
IDE00993-MAS01277 Status of input signals 11-Rear window heater actual value
IDE00993-MAS01278 Status of input signals 11-Rear window heater target value

 

[Dellmassive]

Thanks @mmi =]

lets see what @andys comes back with

 

[andys]

Thanks guys, ok this line does not change when the button is pressed - IDE00993-MAS01277 Status of input signals 11-Rear window heater actual value - OFF

This one changes to ON briefly when the button is pressed (the same length of time the light is on, about a second) before going to OFF again - IDE00993-MAS01278 Status of input signals 11-Rear window heater target value - OFF

 

[Pauly]

Interesting, you can do output tests with VCDS to activate all functions in the T6
You could try doing an output test on the button light to confirm it activates and stays on then you could also do an output test on the screen itself and see if it warms up (dont think it will light the button for this though ?)

 

[andys]

Interesting, you can do output tests with VCDS to activate all functions in the T6
You could try doing an output test on the button light to confirm it activates and stays on then you could also do an output test on the screen itself and see if it warms up (dont think it will light the button for this though ?)

That sounds really useful, I'll have a go at then