Broken heated rear window SOLUTION!

[RIDDLER]

Just talked to a friend of mine who knows a little about electrics and he says the following:

My guess( without looking at a wiring diagram) is that it is a faulty relay. The original window heater would have drawn a significant current and to avoid having a very "heavy" switch on the dash, the current to the heater would have been switched on and off by a relay, which in turn was operated by the dashboard switch. It's possible that this relay also acts like a trip switch, such as you may have on your domestic fuseboard, and the fault is that it's tripping out on too low a current.

[malcolmpaul]

Originally Posted by RIDDLER

Did you try it in the cigarette lighter first? Sounds like it is somehow overloading the heated rear window circuit, but if that was the case it would blow the fuse which it clearly isn't doing. I trust you havechecked the car's heated rear window fuse AND the fuse that is on the wire for the fan heater? Had no problems with mine so it SHOULD work!

Connected it direct to the battery and it works fine. There are no fuses blown. Can't imagine this little unit draws more power than the proper HRW, especially with the heat turned off and just trying to run the fan only, and then a test light across the HRW terminals stays on. You're absolutely right Riddler, it should work, but it doesn't, and I can't think of a reason why not

[malcolmpaul]

Originally Posted by RIDDLER

Just talked to a friend of mine who knows a little about electrics and he says the following:

My guess( without looking at a wiring diagram) is that it is a faulty relay. The original window heater would have drawn a significant current and to avoid having a very "heavy" switch on the dash, the current to the heater would have been switched on and off by a relay, which in turn was operated by the dashboard switch. It's possible that this relay also acts like a trip switch, such as you may have on your domestic fuseboard, and the fault is that it's tripping out on too low a current.

Thanks Riddler, only just noticed your second post, will check the wiring diagram. Only thing is if it trips out because of a low current why does a test light stay on?

[RIDDLER]

Maybe the answer is to take it to any good auto electrician who will probably work it out - though it will of course add to the cost! But it is worth it, because mine has been much better than before when I could never see a thing out of the back window! Good luck.

[RIDDLER]

One other thought - I take it you bought 'female' spade connectors to put on the wires from the fan heater to then connect to the 'male' connections on the HRW terminals. Are the terminals all nice and clean so they are making a good connection?

[morbias]

I would also look at the relay. The light in the switch is on the low current side of the relay so it shouldn't be affected if the heater is drawing too much current. The only way i can see it causing the light to go out is if the internals of the relay are making slight contact with eachother. A test light might not have enough current draw to show it up, but the draw from the fan might cause the circuit to ground out through the relay making the potential difference across the light in the switch zero, causing it to go out.

It makes sense in my head, but that's not a guarantee of anything

[EDIT] when I say current draw, I actually mean impedance, sorry -- man flu is messing with my head. In other words, the fan has higher impedance than the faulty relay and the high side of the circuit will take the easiest path to ground.

[malcolmpaul]

Many thanks for the advice fellas, where is the relay situated on a 72 240Z?

[malcolmpaul]

After about 6 months finally got round to it! Couldn't understand why it lost power when the heater was connected, it turns out it was dirty contacts in the HRW switch, took it apart, cleaned everything up and, hey presto, now works fine

[RIDDLER]

Fantastic news! Mine does the job - not as efficient as a normal HRW, but it certainly helps and you'll appreciate it in the winter months when you steam up!