[H3LZSN1P3R]

Power comes straight off the ECM. While the engine is cranking, the lift pump runs at a 50% duty cycle to keep fuel psi to 7 or so if you Bump the starter you will get normal 100% duty cycle for 15 to 30 second run.

No the lift pump does not run when cranking the ECM only has On or off for lift pump power thats why you get the 1/2 second lift pump prime when you turn the key to the on position(if ecm is up to date it will be 1/2 second if not it may be 2 seconds)

 

[Norton]

There is a fuel pump relay in power distribution box, one of the wires goes to the ECM for relay control.

 

[jross036]

I believe that relay is for the VP.

 

[Diesel Pl]

No the lift pump does not run when cranking the ECM only has On or off for lift pump power thats why you get the 1/2 second lift pump prime when you turn the key to the on position(if ecm is up to date it will be 1/2 second if not it may be 2 seconds)

Okay you are correct on when the pump is actually turning on and off but trust me despite what everybody thinks when you see the wait to start light (2 sec + -) the pump is running in 50% duty cycle PWM.

If you don't believe it grab a oscilloscope set it to single shot and let me know what you see.

 

[H3LZSN1P3R]

There is a fuel pump relay in power distribution box, one of the wires goes to the ECM for relay control.

Mine never had a relay for the lift pump there it was separate because of the in tank pump like the early 3rd gens but the VP relay is the one you pointed out i also bought my truck new off the lot and had the in tank conversion done as part of the sales deal

Okay you are correct on when the pump is actually turning on and off but trust me despite what everybody thinks when you see the wait to start light (2 sec + -) the pump is running in 50% duty cycle PWM.

If you don't believe it grab a oscilloscope set it to single shot and let me know what you see.

really would only be if you are running the factory carter anyway as every other pump has a separate relay harness or at least should since these ECMs are really not great with the factory pump load let alone an aftermarket pump

 

[Diesel Pl]

Opss it is at 25% percent duty-cycle.

 

[Local2Ed]

Just to try to update this thread I'll try to explain what I've done since last post.
I switched my original ECM with a used unknown condition ECM.
The used one performed with voltage all over the place as monitored directly going into the fuel pump.
I put back in the original ECM and Cleaned the mounting points that touch the block which had excessive corrosion as I had done with the used ECM I had just removed.
That seemed to fix the severe voltage swings.

So after watching the voltage for several months now at the lift pump this is what is happening:
From about 13.5 volts to 13.9 volts the pump puts out lower pressure then the maximum pressure it should.
From 13.4 to the closer the voltage gets to 13.0 the pump puts out the maximum pressure bolt at Idle and driving.
To be clear, I like the pressures when the voltage is 13.4 and closer to 13.0 volts measured at the lift pump.

I was wondering if the higher voltages were causing the lift pump to run to fast possibly cavitating the fuel?
The truck runs and starts fine I'm just not sure why the higher the voltage at the lift pump causes lower fuel pressures.

I'm going to call Delphi, the maker of the lift pump and see if they have any answers.

 

[jross036]

Keep in mind, if you have the in tank mod done, then there's a relay for it somewhere. Or there should be. Usually on the firewall drivers side. Not in the PDC. ECM triggers that relay. I would be looking for/at that relay and the grounds.

 

[Local2Ed]

Lift pump is on the side of the engine as delivered from factory.
The ECM regulates the voltage from what I understand.
The lift pump doesn't like voltage over 13.5 volts on what is a 12 volt pump.
Alternator tested good at reputable starter/alternator shop.
Charging system shows good on Auto Zone scanner with live data .
Turn the key but not start the lift pump prime pressure hits maximum pressure every time.
What I understand is 13.9-14.0 volts running is normal, but those voltages cause the lower pressures from the lift pump.
I'm starting to wonder if the ECM regulates the voltage down for the lift pump because the closer to 13.0 volts the pump puts out maximum pressures.

 

[Andyba20]

Have you checked to make sure your fuel tank vent isn’t clogged?

 

[Local2Ed]

Have you checked to make sure your fuel tank vent isn’t clogged?

No I haven't but would like to hear the reason why the lift pump behaves the way it does at the stated voltages because of a clogged tank vent.

Can I access the vent to clean it without removing tank?
It's worth a try.
Thanks.

 

[Diesel Pl]

The ECM use PWM (Pulse Width Modulation) to control electric power inside the motor coil. The output power is controlled by repeatedly turning the output ON and OFF.
Does PWM drop voltage?
PWM does not lower the (peak) voltage. PWM reduces the average current (and in consequence the average power).

 

[bigfish95971]

What are the actual fuel pressures you are talking about?

 

[Local2Ed]

What are the actual fuel pressures you are talking about?

Maximum pressure is 14psi. coming off the VP at idle.
Driving pressure is 10 psi.
Hard acceleration drops to 8psi.
These numbers are when I'm reading the voltages closer to 13-13.4 volts. I'm happy with those pressures.

At the high 13.5 to 13.9volts the pressures drop to 8-10 psi. at idle.
Driving pressure 5-6psi.
All pressures are one psi. higher when measured at the fuel filter.

 

[Local2Ed]

The ECM use PWM (Pulse Width Modulation) to control electric power inside the motor coil. The output power is controlled by repeatedly turning the output ON and OFF.
Does PWM drop voltage?
PWM does not lower the (peak) voltage. PWM reduces the average current (and in consequence the average power).

This does make sense from what I read.

I am taking my voltage readings tapped at the lift pump with a pigtail I made with a VOM.
Could whatever components control the PWM in the ECM be going bad causing what actions I'm seeing in pump output pressures?

Thanks.

 

[Local2Ed]

Have you checked to make sure your fuel tank vent isn’t clogged?

I did drive with a loose filler cap for several weeks over the summer now that I remember but It made no difference.

 

[bigfish95971]

Those pressures are fine and no problem. Stop overthinking it. Forget about voltages. If you have a stock pump it is fine. If you have an aftermarket pump like Fass it should be wired so the the ECM wire triggers a relay for key or battery power to run the pump.
Direct ECM power is insufficient amperage to properly run many aftermarket pumps.