Rear screen heater relay causing battery light on problem?

[BrillPhil]

I have a really wierd issue and would appreciate any views! About the end of the initial lockdown my 6J Ibiza, which hadn't moved much for a while, started showing intermittent battery light on. I was unable to find any problem with the alternator or battery, voltages seemed fine. Eventually I decided to replace the battery which was the original, so getting old anyway. Sadly the issue returned. Very recently I realised that the battery light problem was somehow linked to the rear screen heater, the momentary switch controlling it was apparently inoperative and unlit whenever the battery light was on, and the red light went out about the time I would expect the timed heater to switch off (10 minutesish). Restart and switch press works normally for a short time but then the issue recurs, and the switch light goes out as the battery light comes on. Sometimes it has been days without any problem, and "working" the relay sometimes seems to help, when it is clicking normally.
I've ordered a replacement relay but wonder if I am barking up the wrong tree. it seems to me that the relay must be somehow leaking current but I don't know how that could happen without blowing the fuse (checked of course and looks perfect). Any ideas?? Thanks in anticipation.

 

[RUM4MO]

Have you load tested this alternator, easy way to do this without using the heated rear screen could be to switch the main beam on and check the alternator voltage at idle and fast tick over to make sure that its output is still within expected range, ie somewhere over 13 volts?

Edit:- maybe even get the cabin fan running at a high speed and the AC as that will bring the "radiator" cooling fan on.

 

[BrillPhil]

Have you load tested this alternator, easy way to do this without using the heated rear screen could be to switch the main beam on and check the alternator voltage at idle and fast tick over to make sure that its output is still within expected range, ie somewhere over 13 volts?

Edit:- maybe even get the cabin fan running at a high speed and the AC as that will bring the "radiator" cooling fan on.

Hi RUM4MO
I really appreciate your help. This is the strange thing alternator has never dropped below 14.4v under any load conditions including the fault state. Battery voltage was reasonably healthy even before I replaced it. This morning it was playing up again with the previous apparently heated rear screen related issues worse than ever. I almost convinced myself that the relay had failed completely. This afternoon I pulled out all the relays that I can see (don't know for sure which one it is) all superficially looked OK i.e terminals all bright and clean. Guess what all seemed normal afterwards and screen heater working! I'll probably just replace the relay next time it happens. I think (but don't know) that it's a multi purpose relay under electronic timer control rather than an old fashioned timer relay?


Sent from my SM-A105FN using Tapatalk

 

[BrillPhil]

Update yes I am barking up the wrong tree! As far as I can established this function is under the control of the onboard control unit, rather than a separate relay. As this clearly hasn't affected any other function controlled by this unit yet I'm just going to hope I'm suffering from nothing more than an intermittent poor contact. It seems the problem is worse in cold and damp weather which is maybe another clue. Alternator and battery voltages are fine so I'm just going to leave it for now. I guess the battery light is also controlled by this unit.

 

[RUM4MO]

I think that some one somewhere on these VAG forums, has opened up a BCM and replaced a relay using one sourced via ebay, but that would be a long drawn out repair waiting for a new relay to turn up.
I'm not sure what sort of money would win you a used BCM via ebay, ideally you would have access to VCDS or similar and so use that to find out the part number and coding of the one currently fitted and source as close as possible and change the coding using VCDS to get it to match your original one.

 

[BrillPhil]

Thanks again RUM4MO. Prices for second hand on ebay are not prohibitive, so replacement is a possibility if it comes to it. I wouldn't want to tackle trying to replace a relay in the BCM, and appreciate the complication of getting it coded. No sign of the issue today so I'll see how it goes for a bit. Maybe just having my hands on the unit has helped if my poor contact theory is correct. I'm tooo old for this, when I was young ok things went wrong more often, but you could fix just about anything yourself! Nowadays I usually limit myself to routine service and easy jobs on my cars. I've just got the Ibiza Toca and a Renault cabbie now. Said goodbye to my much loved 52 plate Leon Cupra last year.

 

[BrillPhil]

UPDATE - SOLVED? I thought I'd write again in case anybody else experiences this odd issue. Thinking about the clues and bearing in mind the fact that there didn't seem to be any real charging issue, my suspicions fell on the two wires running from the alternater connector - one goes to ECU and one to BCM. As for the link to the heated rear screen, I noted the manual comment that this was disabled under certain (charging) conditions. I came up with the theory that condensation was causing poor contact to one of these wires. The imaginary charge problem reported could cause the BCM to cut off the rear screen heater as well as bringing on the charge light. I got hold of some silicon WD spray, which I'd seen on a Youtube clip being used to waterproof electrics, gave the outside of the connector a thorough soaking, and once the stuff had dried off a bit the problem ceased.

 

[RUM4MO]

That will do nicely!