Engine/Transmission (1998.5 - 2002) Lift pump electric questions

[mgoncalves]

Does the computer monitor lift pump pressure and then vary voltage to maitain pressure? I guess the term is modulation??



If not, is the pump supplied 12 volts all the time?





Thanks,

Matt

 

[Ncostello]

Its my understanding that during cranking, the lift pump is "duty cycled" by the computer... meaning its voltage is lower than normal to provide around 4-5psi for starting. Then its full speed ahead with 12 Volts to provide the 15psi design pressure for running. High pressure at start up can sometimes cause hard starting. And its generally the feeling that going beyond 20psi can be harmful to the VP44's seals. But there are cases where high pressure during startup and running cause no problems for owners.



The only thing I wonder is when I key on without starting, the pressure will go to 15psi then fall off back to 0psi if you wait a few seconds. I can hear the pump start then stop after a few seconds. Does the pump immediately start back up again during cranking at the duty cycle voltage level? I cant tell cause I cant hear it when cranking... or when the engine is running for that matter.



Hope this helps a little.

 

[kaffine krazed]

No the computer does not monitor fuel pressure. During cranking the computer uses pulse width modulation (PWM) to lower the pressure output of the pump but the computer doesn't actually know what the pressure is the on time and off time is preprogramed into the ECM. Pulse width modulation means that the computer is pulsing 12volts to the pump. Changing the on time vs the off time gives you diffrent pressures. The lift pump normally runs for 2 seconds after key on and 30 seconds after a failed start attempt like when you bump the starter to fill the fuel filter after changing it, some flashes seem to change the times the lift pump runs for key on and after bumping the starter. If the engine is running the lift pump is supplied with a constant 12 volts. If the engine stalls the lift pump shuts off.

 

[rbattelle]

No the computer does not monitor fuel pressure. During cranking the computer uses pulse width modulation (PWM) to lower the pressure output of the pump but the computer doesn't actually know what the pressure is the on time and off time is preprogramed into the ECM. Pulse width modulation means that the computer is pulsing 12volts to the pump. Changing the on time vs the off time gives you diffrent pressures. The lift pump normally runs for 2 seconds after key on and 30 seconds after a failed start attempt like when you bump the starter to fill the fuel filter after changing it, some flashes seem to change the times the lift pump runs for key on and after bumping the starter. If the engine is running the lift pump is supplied with a constant 12 volts. If the engine stalls the lift pump shuts off.

Is this true? If so, what about people who use the ECM signal to trigger a relay to an aftermarket pump (like FASS)... wouldn't the PWM make the relay chatter like crazy?



-Ryan

 

[mgoncalves]

Wow here's an old thread.



PWM is so fast that it is just like having a lower voltage. So to answer your question, As long as the relay can be pulled in with the lower voltage, no it will not chatter.



Matt

 

[rbattelle]

Wow here's an old thread.



PWM is so fast that it is just like having a lower voltage. So to answer your question, As long as the relay can be pulled in with the lower voltage, no it will not chatter.



Matt

Makes sense.



-Ryan