Where's the defroster delay?
 

Frosty morning the other day so reminded me to try the rear window defroster/defogger for first time. Worked fine then I got on with some work on another part of the car (heater control valve).

When I came to idle the engine to check the valve actuator battery was flat, which puzzled me as I'd turned off the interior lights.

Explanation turns out that the defroster is not turning off after time delay. Spotted this only yesterday driving back in the dark from the illumination staying on in the knob! Yanked out the fuse and this resets it, but I'd like to make a permanent fix.

I see there are two relays (one a 'double' size) controlling this function. One is a standard sort (Relay VIII) so I could swap it with another (horn say) on the board. But on the wiring diagrams I can't find the 'delay' for the rear window heating power-off. Is it inside the double relay or where? Is the problem more likely to be relays sticking on or a fault in the (hidden!) delay module?

Have done a search and only one reference to similar problem but this only suggested swapping the small relay, and made no mention of the double relay (Relay II from memory).

Thanks in advance,

Adrian

The big double relay 928 615 115 00 is the rear defrost and it has the time delay feature built inside and has probably failed. Cost is about $88 for new $40 used with one year warranty.

Or if your car is like mine, you can rotate the defroster knob/button clockwise to turn on and rotate back counter clockwise to turn off. If you depress the knob/button, you require the relay time delay feature to be functioning to auto-off.

Thanks Jim - I've added it to my wish list and will order with a few other items.

Adrian

Thanks for useful workround till I fix permanently Cameron, though I'd be nervous of accidentally pressing in the knob and coming back to a flat battery! Also I think that on my 88 S4 you have to push in to get full de-icing, otherwise it's just a demist setting but I may be wrong.

Either way you've reminded me that the timer is only on push, so I just have to remember not to push the darn thing!

Adrian

For some reason, operating the starter turns off the stuck defrost relay...

Brilliant tip thanks Wally!

Saves me having to yank out fuse if necessary.

Adrian