1320

iTrader: (6)

on...

fans
hi and low

and the fuel pump?

For some reason....I want to say they switch ground on the fans and B+ on the fp.....set me straight please so I dont have to probe wires.

01WS6/tamu

iTrader: (37)

ground on all of the above

Old Geezer

iTrader: (3)

Huh??

flame

iTrader: (13)

You are correct

lizeec

iTrader: (57)

^^^^^^
WRONG WRONG

You are straight and correct

98 Beast

Prints show that the PCM only switches the ground side to permit voltage flow.

Gary Z

Lower current devices such as fuel injectors are switched directly by the PCM. A relay is used to switch higher current devices. For high-current devices the relay coil, which itself requires low current, is directly connected to the PCM and high current flows to the device through the relay's contacts. In ALL CASES the PCM switches the ground leg of the circuit. If the circuit includes a relay, the relay contacts switch the "B+" side of the circuit. The fuel pump and both fan motors are controlled through relays. I hope I haven't increased the confusion.

2xLS1

iTrader: (25)

You are wrong on the in ALL CASES. The PCM provides +12v to the fuel pump relay windings to energize it. AFAIK, that is the only relay circuit energized by +12 and not a ground.

Gary Z

The schematic shows 2xLS1 is correct - I was wrong. My only excuse is that I'm an electrical engineer who specialized in microprocessor control systems.

01WS6/tamu

iTrader: (37)

Should have specified exactly what year and platform you need information for to get exactly what you need. FYI 04 silverado fuel pump relay uses a ground command from the pcm in order to close the contacts.

2xLS1

iTrader: (25)

Looks like 12v control to me. At least on the primary. I guess you could say with dual tanks the secondary relay is controled by a ground, but only after the primary is energized by 12v.

What specific 04 Silverado are you refering to and I'll look it up because I've not seen one controled by a ground. I thought it was GM engineering's way of preventing a run a way pump if the circuit gets shorted to ground.

Here is a 2004 Chevy Truck Silverado 1500 2WD V8-5.3L VIN T schematic.

mantycarguy

Up until aprox.03 GM used almost exclusively ground side switching on their fuel pump circuits.I dont know why they switched to + switching but the fact clearly indicates this change. Long story short you must consult the proper wiring schematics for your project. It is simple enough to check this circuit with a multimeter by probing the wire in question with one lead and by probing either the positive or negative with the opposite lead while an assistant operates the key. So in conclusion both are correct.

flame

iTrader: (13)

The fuel pump circuit has always been B+ trigger on all GM vehicles. I believe it is so that in a crash if the control circuit gets grounded out your pump doesn't stay running creating a fire hazard.

mantycarguy

Flame,If you check some older wiring diagrams you will find that all of these systems used a computer controlled ground to activate the relay for the fuel pump. Look at a 2002 Silverado vinT for a good example of this. I work as a GM technician specializing in electrical and computer diagnostics.I have been doing this for 20+ years and have seen many changes come along. I still say consult the proper diagram to confirm circuit operation.

2xLS1

iTrader: (25)

Well here is a 2002 Chevrolet C1500 VIN T schematic. The FP relay winding is hard wired to the ground distribution and the PCM supplies B+ to turn the relay on. No different than the 04 I posted. I have randomly looked at GM vehicles, both cars and trucks, back to 1994 and have yet to see any that the PCM supplies a ground to turm on the FP relay. The first schematic I posted is from a 99 F-body. Your statement about "Up until aprox.03 GM used almost exclusively ground side switching on their fuel pump circuits" is wrong.

2002 Chevrolet C1500 VIN T

flame

iTrader: (13)

You can go back to the 80's when they started fuel injection and they always used power to trigger the fuel pump relay. Been doing this for 15 years now.

mantycarguy

I apologize. My mistake, Flame is correct.

1320

iTrader: (6)

so in conclusion........I think I was correct, at least some of the time, when the sun is partly clouded by the winds from the north.