I am not talking only about fuel pressure, I am talking condition of the fuel pump brushes and armature.
This test gives you and idea of how good or bad the brushes and armature contacts are within your fuel pump. This is valuable in diagnosing a failing pump way before it quits on you, and a pump that may be down on RPM and not supplying enough fuel to keep the pressure where it should be.
I have used this test on my Saab 95 Aeros to see how good the pump is, as well as my old V8 Chevy pickup. My 911 is MFI so these is an electic pump to feed the MFI pump, but the final injector pressure is done in the pump.
You actually are reading volts on the scope. An inductive current probe is actually reading millivolts. Most current probes have two settings, 10mV/A and 100mV/A. So when set to 10mV/A, each 10mV displayed, equals 1 amp. And when set to 100mV/A, then each 100mV displayed equals 1 amp.
By checking an amperage waveform, you can not only see how many amps the pump is drawing, the electrical integrity of the pump itself. This can be extremely useful especially in rough running, low fuel pressure circumstances.
When we are looking at a waveform you are actually looking at the relationship between the brushes and the commutator.
When looking at the waveform you will want to see the rise and fall in amperage at a consistent rate.
the humps are relatively identical. They represent each time the armature moves across the brushes. Here is a good wave form.
Here is what a bad form would look like:
