jjbunn

I pulled this relay while checking for a solution to my problem with the heated rear window. It's a fairly big "relay", and unlike the other relays doesn't have a circuit diagram printed on it.

So I decided to open it up. In fact, it's a three-transistor circuit board:



There are two relays. One of them actuates immediately the power is applied to the correct spades. My theory is that the second actuates after a time delay, which is what the transistors are for - the capacitor gets slowly charged (at a rate set by the potentiometer), and at a certain voltage, triggers a monostable that then causes the second relay to actuate ... perhaps boosting the voltage to the rear window heating element?

This is all supposition: if somebody can point me to any documentation on this circuit, I'd be grateful.

I checked the trannies for shorts, and the diode, and they all look good. The thing may be working just fine, but how best to check it? Any ideas welcome.

dfinnegan

Not sure if this helps . . . I've got an ME not EE background :-)

Let me know if you need more input from the wiring diagrams.

jjbunn

I think that's the switch on the dash, rather than the relay?

I'd be interested in the part of the circuit diagram that shows the connection to the rear window heater element, and the relay, if that's easy for you to find?

Thanks!

dfinnegan

Sorry, I was heading out. I think this is more what you're after.

jjbunn

Excellent! Thanks ...

Any idea where T36 is physically located in the car?

mojorizing

From the circuit, it looks like the middle heating element is energized, then a the top and bottom elements energize after a time delay. Maybe to heat the window from the middle to the edges to avoid thermal stress? These germans think of everything!

jjbunn

Yes ... I came to the same conclusion.

I'm going to measure the resistance between pins 3 and 2 on the relay connector and ground: that will tell me if the circuit through both heating elements is complete ... I'm fearing that it wont be and that I may have a broken element (or two!).

jjbunn

Well, I will have to mark this down as an Unsolved Mystery. I checked the resistance through the socket via the heating elements to ground, and got a few Ohms as I should - which indicates that the wiring is OK. I checked that the first relay actuates when I plug in the relay with the demister switch on the dash engaged, so even if the timed second relay doesn't work, I should at least get some demisting.

Perhaps the rear window defogger is just not very effective in these cars?

I think instead of bothering with this any more I will just drive the car!

dfinnegan

Sorry to hear that you didn't work it out. For what it's worth, I use my rear defogger from time to time and it seems work quite well.

jjbunn

Thanks, and thanks for your help withe the schematics, which was invaluable ... it's possible that all my fiddling around has resolved a bad connection: it's hard to be sure if it's really not working without a fogged window

dfinnegan

Toss a bag of frozen peas on the back window and see what you get! :-)