fuel pump voltage

[Dave J. Andruczyk]

> found a few odd things.  The first finding was the obvious, a swinging
> DC voltage between 9-12 volts with occasional 14volt readings (can't
> figure why though, doesn't moven between 12-14 volts).  Then I switched
> to AC readings, I noticed 16volts at a frequency between 60-70khz.  The
> AC reading would sometimes fluctuate, but the more gas the car was given
> the lower the AC voltage and higher the DC voltage.
> Does the ECU convert the DC to AC to drive the pump and then convert it
> back finally into DC again?  And what is the deal with the frequency?
> Sometimes it jumps around, but I could not see any correlation to
> throttle position and frequency.

Answer:  PWM  Pulse Width Modulation.  The computer is using PWM to slow
down the pump.  The effects are "varying DC voltage" especial when
measured with a DMM,  USe a scope and it;ll make much more sense. The
computer changes slows down the pump when less fuel is required,  which
reduces the load on the pump, and on the pressure regulator.

DAve