ECM and fuel pump relay control

**belaw** · 07-30-2012

Okay, one more trip to the well.... (this is what you guys get for being so helpful).

I finished stripping my harness and my (hopefully) last issue of confusion is with the 2 fuel pump relay connections at the ecm.

Pin A1 is listed on the 7747 wiring diagram as the "Fuel Pump Relay Control," which based on the diagram on top of the relay does exactly what the diagram says.

What has me stumped is the "fuel pump input" at B2; on my harness the wire taps into the wire from the switched output of the relay to the fuel pump, and as near as I can tell this wouldn't do anything other than tell the ecm that the fuel pump has power. Is that right? If so, why? More to the point, do I need or want the B2 fuel pump input circuit on a carb to TBI swap?

Also, when the relay is not energized the fuel pump input circuit is switched to another relay output (it would be pin 87a on a Hella/Bosch relay) that is shown as "fuel pump test" on the wiring diagram. What the heck is that?

**dyeager535** · 07-30-2012

Ooo let me try to answer something for once!

The ECM watches fuel pump voltage and I believe uses that to "correct" for the amount of fuel injected at that voltage (which I assume theoretically corresponds to pressure).

**EagleMark** · 07-30-2012

The fuel pump test wire is for just that. Hook power to it and run pump to test or prime.

**RobertISaar** · 07-30-2012

i find no flaws here. :)

generally, if you're watching both battery voltage and fuel pump voltage via the datastream, fuel pump voltage will almost always appear smoother. could be due to capacitor filtering inside the ECM for each channel's A/D circuit itself being different or just that the fuel pump tends to get a cleaner source of power, but i have yet to see a vehicle where fuel pump voltage looked noisier than battery voltage. GM may or may not have realized this as well and chose to use fuel pump voltage rather than battery voltage for injector corrections.

**dyeager535** · 07-30-2012

Heading off on a tangent...is it normal to see a ~-1V difference at the fuel pump vs. battery voltage in the datastream?

Years back when I met with Mark Mansur and he was checking the data on my truck, he made mention that the difference in voltage (between batt and fuel pump) needed to be corrected. But what you say makes sense, perhaps I need to see what is actually getting to the pump voltage-wise?

**RobertISaar** · 07-30-2012

mine are generally within about .3 volts of each other.... with the fuel pump voltage almost always being higher. there are exceptions, like when a large electrical draw is kicked on (fans, lots of brake lights(8 on a 88-96 grand prix), A/C etc). then all bets are off, since battery voltage will always dip, yet fuel pump voltage may stay close to where it's at, sometimes it will also drop. may depend on which load causes it.

**EagleMark** · 07-30-2012

Originally Posted by RobertISaar

i find no flaws here. :)

That's not as funny as "I approve this message!" or was it "I approve this purchase!"

Your lucky to have one voltage in data on1227747, but that incoming line is important.

As for the voltage drop, yeah on a conversion but it shouldn't be. Injector voltage compensation tables will adjust for some. Probably wire gauge to pump from relay, or just lower alternator charging of the conversion vehicle, or in older truck conversions the ground back off pump to body or frame. In conversions I run the ground back to block.

**sciroc** · 04-21-2013

I am currently at a wall with my setup. While data logging my bat volts are 13-14v but my fuel pump voltage is 5-6v. The car runs like crap when this happens. I traced the tan/white wire from the relay to the ECM and it is getting 13-14v but it appears it is loosing or miscommunicating the voltage in the ECM. I think it is time to try another ECM to rule out an internal clitch.