Current Flowing Through Demister/Defogger Switch

[Tillsy]

Looks to me like the demister/defogger relay is nothing more than a timer, it does not offload current from the switch but rather the element, relay itself, and the bulb/LED draw straight through it.

To that end does anyone know off-hand how much current is being pulled through that circuit?

[David T]

Looks to me like the demister/defogger relay is nothing more than a timer, it does not offload current from the switch but rather the element, relay itself, and the bulb/LED draw straight through it.

To that end does anyone know off-hand how much current is being pulled through that circuit?

The relay has no timer function in the Delorean. If you turn on that defroster it stays on until you notice you turned it on with your elbow. It lights up a light in the console (if the bulb isn't burnt out).
David Teitelbaum

[DrJeff]

The relay has no timer function in the Delorean. ...

I thought the relay is a timer of sorts, a cycle timer. Doesn't the relay 'switch on the current for a period, switch it off for a period, then repeat' and like you said it keeps doing this until you flip the defrost switch to off. It isn't a timer in the sense that it switches off the current and it remains off.

I think this means everyone is correct :D

[Bitsyncmaster]

I thought the relay is a timer of sorts, a cycle timer. Doesn't the relay 'switch on the current for a period, switch it off for a period, then repeat' and like you said it keeps doing this until you flip the defrost switch to off. It isn't a timer in the sense that it switches off the current and it remains off.

I think this means everyone is correct :D

I did test it some time ago and I think you are correct. It turns on the heat full power for 30 seconds and off for 30 seconds. Those number are not correct but it's about that timing.

[Tillsy]

Correct, I didn't measure the actual length of time - merely that, while tracing that it passes power to the demister directly from the switch, it kept turning off and on again at regular intervals. Hence my realisation that it is nothing more than a timer rather than the normal role of a relay to offload power from the switching source.

[David T]

Correct, I didn't measure the actual length of time - merely that, while tracing that it passes power to the demister directly from the switch, it kept turning off and on again at regular intervals. Hence my realisation that it is nothing more than a timer rather than the normal role of a relay to offload power from the switching source.

I stand corrected, the defoger timer does (or at least is supposed to) cycle the power to the rear window grid. The light will NOT cycle on and off, only the power to the window. As for practical value, I find I never had need for it but have turned it on inadvertently with my elbow often. Someone way back sold a plastic U channel you laid over the buttons so you wouldn't do that. You are also correct, it is NOT a relay, it is only a timer serving to stop the flow of power from the defroster switch to the rear window. It is one of the reasons you must be careful which relay you stick into each position. If you switched the timer and another relay you could make a mess of your wiring harness! They are NOT all the same!
David Teitelbaum

[Tillsy]

Yes, as David T points out, the light/LED is hardwired directly to the switch hence it stays on even though it turns out the defogger actually cycles on and off.

I have now rewired my LED to feed off the relay rather than the switch - thus when the defogger is turned on the light cycles on and off with the defogger (another project I am working on with solve the problem of not knowing the defogger has accidentally been left switched on even if it is in the off cycle).

My results were a little unexpected:

Turning on the defogger results in the light illuminating (and thus the defogger running) for 4 minutes 8 seconds
It then turned off for 5 minutes 27 seconds
It then turned on for 3 minutes 5 seconds
It then turned on again and I ran out of time to test further (need to head out)

Given these varying results so far I wonder if the timer is based on 1) feedback from the defogger - say as it reaches temperature resistance changes and the timer circuit cycles, 2) the timer decreases during each cycle or perhaps just after the first time (since the first cycle would get rid of initial mist buildup, subsequent cycles are just a quick refresher), 3) something else.

Might observe further another time.

[Tillsy]

Alright some more results:

ON - 4:57
OFF - 5:29
ON - 3:09
OFF - 5:29
ON - 3:09
OFF - 5:29
ON - 3:09
OFF - 5:29
ON - 3:09

I have done several tests now and each time the first "on" sequence seems to vary, but the timings thereafter seem consistent. Perhaps nothing more than a bit of static when it first powers up that scrambles the initial timing.

[DrJeff]

I rebuilt my defroster circuit when I installed the window mirror heaters. The new relay-timer switches the heat on for 10 minutes and then switches off permanently. The switch is a momentary switch that can toggle the timer on and off. The light is only on when the defroster is actually heating.

- Click the defroster button to start the timer, light comes on, heating starts
- If left alone, after 10 minutes the defroster switches off, light goes out
- If you click the defroster button while the light is on, the defroster switches off, light goes out

Adding the mirror heaters doesn't complicate the wiring at the relay-timer (but it does mean changes in the doors and behind the rear panels). I also reworked my button panel so that the defroster is a momentary switch.

[Bitsyncmaster]

I had posted the timing on an old thread that I got with mine. But the circuit is a very crude capacitor timing circuit so the timings may vary greatly. I never figured why they would cycle the power unless it was to prevent overheating the elements in the window.

I like your 10 minute idea. It's more like a modern car.

[DMC5180]

UH OH, I see there gears turning for another DM-eng product. A simple time-out circuit the protects the whole circuit if the switch gets inadvertently turned ON (which happens a lot) and works like modern day cars.

[Bitsyncmaster]

UH OH, I see there gears turning for another DM-eng product. A simple time-out circuit the protects the whole circuit if the switch gets inadvertently turned ON (which happens a lot) and works like modern day cars.

Not this year. I'm struggling keeping up with what I sell now. My AC panel had to take a back seat to building inventory.

[DMC5180]

not this year. I'm struggling keeping up with what i sell now. My ac panel had to take a back seat to building inventory.

lol j/k

[John Rogers]

I rebuilt my defroster circuit when I installed the window mirror heaters. The new relay-timer switches the heat on for 10 minutes and then switches off permanently. The switch is a momentary switch that can toggle the timer on and off. The light is only on when the defroster is actually heating.

- Click the defroster button to start the timer, light comes on, heating starts
- If left alone, after 10 minutes the defroster switches off, light goes out
- If you click the defroster button while the light is on, the defroster switches off, light goes out

Adding the mirror heaters doesn't complicate the wiring at the relay-timer (but it does mean changes in the doors and behind the rear panels). I also reworked my button panel so that the defroster is a momentary switch.


On the off chance that Dr Jeff still monitors this forum. What did you replace your rear defroster timer relay with? Perhaps a defroster timer relay from a different car. I have a broken window switch that can operate as a momentary switch and a broken defroster switch to donate the rocker with appropriate decal.

[John Rogers]

On the off chance that Dr Jeff still monitors this forum. What did you replace your rear defroster timer relay with? Perhaps a defroster timer relay from a different car. I have a broken window switch that can operate as a momentary switch and a broken defroster switch to donate the rocker with appropriate decal.

I have found a relay that I believe I can wire up to turn off the rear screen defogger after ten minutes. The problem is working out how the old relay is wired so as to be able to rewire it for the new 10 minute timer relay.

There are two wires going to the relay as well as the ground wire. One wire is unpowered and goes to the rear screen defogger. The other wire should be coming from the rear screen switch except that the switch does not interrupt the power as it is continually powered when the ignition key is in the first or second positions. However, if the old green timer relay is in position then the switch does interrupt the power. The wiring diagram shows ground to pin 85, the wire from the switch to pins 86 and 87 and pin 30 powering the defogger.

Does anyone know why the wire from the switch only interrupts the power when a relay is in position?

My alternative solution is to simply run a new wire from the switch to pin 86, move the defogger from pin 30 to pin 87 and plug the continually powered wire into pin 30.

[DMC-Ron]

I have found a relay that I believe I can wire up to turn off the rear screen defogger after ten minutes. The problem is working out how the old relay is wired so as to be able to rewire it for the new 10 minute timer relay.

There are two wires going to the relay as well as the ground wire. One wire is unpowered and goes to the rear screen defogger. The other wire should be coming from the rear screen switch except that the switch does not interrupt the power as it is continually powered when the ignition key is in the first or second positions. However, if the old green timer relay is in position then the switch does interrupt the power. The wiring diagram shows ground to pin 85, the wire from the switch to pins 86 and 87 and pin 30 powering the defogger.

Does anyone know why the wire from the switch only interrupts the power when a relay is in position?

My alternative solution is to simply run a new wire from the switch to pin 86, move the defogger from pin 30 to pin 87 and plug the continually powered wire into pin 30.

John,
First of all I don't haven't worked on the defogger circuit because I've never had a need for the rear window to be defogged. But your description got me curious. I've attached the defogger timer circuit schematic. I've cleaned up the schematic a bit and added a standard auto relay pinout. Assume this were just a standard form C auto relay, When power comes from the defogger switch, it energizes the relay coil (pin 86), relay pulls in and pin 87 is connected to pin 30 and power goes to the rear defogger. I'm speculating here, but I would expect the intended relay to be a special timeout relay, such that if the coil remains powered for a given period of time, the relay contact opens and remains open. When power is removed, the timer resets waiting for coil power to again begin the timeout sequence. If this were a standard form C auto relay the defogger would turn off and on with the switch but it would not automatically turn off after a given period of time. It would remain on until the defogger switch was turned off. If your timeout relay will open the contact after a 10 minutes, then it may work depending on its operational sequence. Would you have a technical sheet on the relay?

Ron71509