[dan624]

Hello
i wanted to drain the stale petrol out of the tank of my Nissan Micra k11 1999, which had stood for a long time, so I shorted out the 85 86 contacts at the base of the fuel pump relay.
Now the fuel pump does not run.
I am an idiot. Anyone any idea how I might get the fuel pump running again? Thanks. Terry

[NMNeil]

@dan624 Maybe trying to scan it with a cheap OBD2 tool. If the tool and PCM won't talk to each other it's probably fried.
A quick check would be to connect a multimeter positive probe to the socket in the fuse box which corresponds to pin 85 of the relay and the other probe to ground, turn on the ignition and see if you have battery voltage, if not repeat but using pin 86.
If there is no battery voltage on either start looking for blown fuses. If one shows battery voltage connect the positive lead of the voltmeter to the opposite non battery pin and the other probe to ground, set the meter to read resistance and before you turn on the ignition check the resistance. If it's very high or infinite turn on the ignition and it should drop substantially as the PCM tries to activate the relay to prime the pump.
If the resistance with the ignition off is low of if there is no change when you turn on the ignition the PCM is probably fried.
But all may not be lost if you have a soldering iron.
The innards of the PCM work on 3.3 or 5 volts but the battery is at 12V or more, as the 'brains' can't turn on the relay directly at that voltage, it sends 3.3 or 5 volts to the gate of a mosfet which then grounds the relay coil to activate it.
With me so far?
The maximum current that the mosfet can handle would only be about 500mA because that's more than enough to activate the relay coil.
You gave it a few amps when you shorted it so it got fried.
Have a look at the innards of a mosfet and the gate would be connected to the brains, the drain to the relay coil and the source to ground.
The good news is that they are under $2 for 10.
https://www.mouser.com/datasheet/3/1...BCC3C032D2.pdf

[NMNeil]

Terminal 86 should go to the battery and have 12v more or less, this is one side of the coil inside the relay. The PCM will ground terminal 85 to activate it. The relay coil has roughly 100 ohms of resistance which gives a current draw of about 120 milliamps at 12 volts. This will change depending on battery voltage, but not by that much.
By shorting the two contacts together you sent the maximum current the wiring could carry into the PCM, which may have fried it.
It would not have affected the fuel pump as contacts 30, 87 and 87a which go to the pump were on a totally separate circuit inside the relay.
This may help you understand the inner workings and it looks like this one has a freewheel diode to snub the inductive spike when the coil is deactivated.
https://www.farnell.com/datasheets/103809.pdf

[dan624]

@NMNeil
Thanks for the reply. is there a way to confirm the pcm (aka the ecu) is fried, and is it doable to replace it? Terry

[Rolebama]

FWIW: There are quite a few ecu repairers around. A quick Google found 3 local to me, so I would think you should be able to find a suitable one.

[dan624]

@NMNeil this is the most complete, forensic explanation I could have wished for.
Now I just want to get stuck in , and try and fix the car. many thanks. regards Terry

[dan624]

@Rolebama
Hello that's a possibility, thanks for the suggestion