Odd electric relay/current draw problem.

[CarreraS2]

I have an electric fan that is triggered by a standard square 4 post Bosch relay.

The relay is doing something odd. Here's the scene:

1. Ammeter shows something is causing a minor draw on the battery when the car is off.

2. Turns out it's related to the relay. When I unplug the 12V constant power supply wire from the relay, the current drain goes away. (Of course, the testing is done with the car stone cold, so the fan isn't running).

3. If I leave the relay alone, but unplug the fan, the current drain is still there! So that seems to conclusively eliminate the fan itself as the problem (or at least the direct problem).

4. I'm thinking it's a bad relay because this exact same problem happened about a couple of months ago. I replaced the relay, retested with the ammeter, and zero current draw.

Assuming that when I replace this relay again for the second time in 2 months the problem is solved (will have to get another relay tomorrow), here's my questions:

1. What could be causing the relays to fail? Just bad luck? (that seems unlikely).

2. What is happening inside the relay to cause it to draw power even when the device it is triggering (the fan) is disconnected?? That almost seems impossible, but my tests seem to show that it is happening.

[Jim Sims]

Does the relay have a diode across the coil (between terminals 86 and 85)? If so, terminal 85 should be connected to ground and terminal 86 should be getting the signal power to activate the relay. If you have these two terminals reversed there will be current flow across the diode. The diode could also be failing and causing the current leakage even when properly hooked up. Is the surge starting current into the fan motor above the relay's current rating?

[CarreraS2]

Thanks Jim, that helps me a lot in understanding what's going on.

I don't know if the failed relay has a diode or not, there's no diagram on it.

But the replacement that I got this morning does not have a diode, so that issue will be gone.

I don't know what the surge starting current into the fan motor is, or even what kind of current the fan is drawing. It is a big cooling fan, around 10 years old. My understanding is that as they get older, they can start drawing more.

Seems mosts relays are 30 amps, I got a 40 amp replacement, so hopefully that solves the problem.

Thanks again for your help.

[donporfi]

To protect from voltage spikes when disconnecting an inductive load like an electric motor, some relays have a diode between terminal 85 and 86. That is why there is a difference between round black relays and red round relays in Porsches (I learned it here).

[Mysterytrain]

I'm missing something here..what fan? what car?