Slow-action Relay - rear window defrost problem
 

Hi. I have seen a similar problem described in this forum, but on an older model with a different relay.

Mine is a 1989 GT, and the rear window defroster is not switching off automatically as it should. The switch for the defroster is a turn/push knob, with continuous on, 15 minutes on high then continuous low, or 15 minute defrost then off. With switch depressed, and in the 0 position it should run full defrost for 15 minutes and then turn off automatically. Well, it doesn't. Not even after the ignition is switch off, so it drains the battery. If I restart the engine, it switches off as part of cutting all power drains while cranking the engine.

I believe this action is controlled by the relay in the I & II position, a so called slow-action relay, part number 928 615 115 00.

Is there any known way of testing this unit? I have some lab equipment with 12V power source, and instruments. As this is an expensive part, it's worth putting some time into fixing it.

Here is what i looks like (picture borrowed from Alan on Rennlist):

http://forums.pelicanparts.com/uploa...1356822662.jpg

Mine does the same thing. Does it work when you only turn the knob to defogger position and do not push in for defroster? Mine works fine as long as you do not push the knob in. Of course that only gives you half the heating.
The relay is bad. Essentially you have tested it with the symptoms you describe. You may want to open it up and see if there are any solder connections that have cracked and repair them. But the relay has a timer in it and it may be bad which means replacing the relay. As you have discovered if you turn the ignition key to start it resets the relay and turns off the defroster. If I use the defroster, I make sure the light in the defroster knob is out and if not, restart the car to turn it and the defroster off. $200 for a relay is ridiculous but that's what happens with and old Porsche.

It has a transistor emblem on the case you posted, so that might be the timer circuit.
You might try cleaning the internal contacts if you can open it up. If you know a little about electronics you might be able to connect DC to it and see if the relays operate.
Naturally check the simple things first, switches, etc.

I agree - you have already tested it - it fails. It could be replaved or you could make an atemp at fixing it.

Unless you can reverse engineer the circuit - I'd limit your work to checking & cleaning the contacts - there are 2 seperate SPDT relay contacts - see if they are sticky - if so you may be able to clean. You could also try refreshing the solder contacts - could be a simple fix...

Otherwise get another one, in this case a used one may be OK - many cars aren't driven in winter conditions - so many of these don't get used that much.

Alan

Thanks. As soon as the weather gets a little warmer, I will open it up and look for obvious faults.