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I was attempting to install a lighted push button switch for my elec fan and I was surprised to see it light up when the ignition was turned off! Why is my pcm fan gnd putting out 12V when off? Ive attached a diagram of how I have it wired. Its working fine. I simply have the fan hi switched to gnd.
The switch has 5 pins, its a push button latch. I could isolate it with a relay, but I'm really curious as to why its getting power in the first place? Don't know if its damaging the pcm or not..
The PCM is not powering them with +12V.
This type of outputs are grounded by the PCM to activate them, when deactivated they go to a "floating state" and the voltage you see at the pins is a residual or leaked voltage that passes thru the relay coil. If you want the bulb to remain off then you should use a IGN wire to power the relay 86 terminal from your drawing instead of having it permanent from battery.
The PCM is not powering them with +12V.
This type of outputs are grounded by the PCM to activate them, when deactivated they go to a "floating state" and the voltage you see at the pins is a residual or leaked voltage that passes thru the relay coil. If you want the bulb to remain off then you should use a IGN wire to power the relay 86 terminal from your drawing instead of having it permanent from battery.
Ok, now this makes total sense. I suspected it was caused by the relay. I have it wired to 12v constant just so i can run the fan while ign is off. Guess i will have to run relay just for switch bulb if i want it to shut off.
Is there a relay I could use that does not leak voltage?
Relays are just switches, depending on how you wire them, they are a open / close circuit. Open contacts do not bleed voltage, unless their shorted (welded together) or broken. The coils need power in and a source on ground. The only way a relay can maintain power when voltage is remove, it will be known as a timer relay which has a capacitor (that hold current) in it to maintain voltage at a timed / determined period.
Ok, now this makes total sense. I suspected it was caused by the relay. I have it wired to 12v constant just so i can run the fan while ign is off. Guess i will have to run relay just for switch bulb if i want it to shut off.
You can try a solid state relay, but the solution is there, use a IGN feed instead of a direct connection from battery which you should not be doing anyways.
Relays are just switches, depending on how you wire them, they are a open / close circuit. Open contacts do not bleed voltage, unless their shorted (welded together) or broken. The coils need power in and a source on ground. The only way a relay can maintain power when voltage is remove, it will be known as a timer relay which has a capacitor (that hold current) in it to maintain voltage at a timed / determined period.
Well, as soon as I unplug the relay, with IGN OFF (with pcm) the switch light goes out. Ive tried another relay and it did the same. I will have to bench test these to confirm..
Originally Posted by MontecarloDrag
You can try a solid state relay, but the solution is there, use a IGN feed instead of a direct connection from battery which you should not be doing anyways.
I guess you are right, I should just switch them to IGN.
[QUOTE=JensenB;20086962]Well, as soon as I unplug the relay, with IGN OFF (with pcm) the switch light goes out. Ive tried another relay and it did the same. I will have to bench test these to confirm..
I need the make & model # of the switch. to see how it should be wired.
04-22-2019, 01:24 AM
PCM Cooling Fan Wire Hot When Off 
