fuel pump controller module voltage drop wire or signal feed

[ozzinc]

on the 90 to 93 q45 which wire on the fuel pump module controller harness actuallydrops the voltage in stages, or does it work on resistent on the grounding side.

[trobbinsfromoz]

I've just repaired my controller (1989 Q45a) 1700/60U00, which had melted one of the two FET switches and heat stress on the other FET and pin 4 of the socket, and the 12V input filter electrolytics also looked to have had a hard life. An easy job if you're into electronics, as they just use simple power FETs to duty-cycle modulate the average current level (I would guesstimate, as there is no current sensing or ballast resistors in the circuit).

The pin 4 input (white) from the negative side of the pump is switched to controller chassis (ground) by either/both of the two FET switches (2SK519, but I used simple IRF540 with an insulator pad wrapped around the tab). As suggested by others, inside the controller it's best to bolster pin 4 to the pcb land using tinned copper wire to spread the heat, and also the drain/sources on the FETs, and I added an external 'white' wire connection (via a spade lug connector) to parallel the pin 4 connection as that is a weak link. Hopefully the 15A fuse will go first next time!

[Skibane]

they just use simple power FETs to duty-cycle modulate the average current level (I would guesstimate, as there is no current sensing or ballast resistors in the circuit).

That's a good guess, since you can sometimes hear a whine from the fuel pump motor that sounds like PWM ("pulse width modulation").

(PWM works by rapidly turning the fuel pump motor on and off. Depending on how long the "on" length is in comparison to the "off" length, you get different pump motor speeds - and thus different fuel output).

[trobbinsfromoz]

Initial test of my repaired controller showed low pump voltage at idle. Oscilloscope showed a poor jittery 20kHz PWM waveform applied to the pump. Replaced C8, C9 (1nF) to fix - they get any FET drain spikes so will be under stress.

For my own amusement I internally bridged pin 7 to pin 5, and added a 1nF SMD across R20 to assist cleaning up the battery feedback signal, and used 100pF 1kV ceramic SMDs for C8, C9 to offset the larger FETs I'd used (C8, C9 are FET drain-gate added capacitance to slow down the switching transitions).

The control circuit uses PWM at 20kHz to regulate the pump voltage, based on the two control inputs from the ECU. Pin 1 held low (0V) will force idle pump voltage. Pin 1 floating and Pin 2 held low will force medium pump voltage. Pins 1 and 2 floating will cause full battery voltage across pump.

Ciao, Tim

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