Snap-On EETA308D, Reference Manual

Page: 14 / 25

7
Tech Note
If the fuel pump is difficult to access, it may be possible to
check the fuel pump current at the fuse box (if
applicable).
To check the fuel pump current at the fuse box:
1. Locate fuse box and remove the fuel pump fuse.
2. Install an inline fuse harness in place of the
fuse.
3. Clamp the Low Amp Current Probe around the
inline fuse harness.
Fuel Pump RPM Calculations
Most fuel pumps run at about 5000 to 6000 RPM and
draw about 6 to 8 Amps. Pumps may cause driveability
symptoms below 3000 RPM. Most fuel pump
commutators have 8 segments. Using an 8-segment
commutator as an example, RPM may be estimated by
measuring the delta time of the 8 humps present in the
waveform. One of the formulas to find RPM is 60,000 (the
number of ms in a minute) divided by the time
measurement of the 8 humps of the waveform. Let’s say
if in our example that the time measurement of the 8
humps is 12 ms, we would divide 60,000 by 12 to get
5000 RPM.
To measure amperage, measure the average voltage of
the waveform and use 100 mV = 1A for calculation.
3.6.3 Ignition Coil Current Measurement
Recommended Probe Setting: 20A
Note: Before taking measurements or clamping the probe
jaws around any wire, complete the Setup steps below.
Setup
1. Use the On/Off and Range Selector switch to select
the 20A setting.
2. Perform the Low Amp Current Probe Zero
Adjustment procedure (refer to section 3.3).
Test
To measure ignition coil current:
1. Clamp the Low Amp Current probe around the B+
side or the control side wire of the ignition coil. Do
not clamp the probe around both wires.
2. Verify that the probe jaws are fully closed.
The ignition coil current readings display.
3. If a negative waveform appears, open the probe jaws
and clamp the probe in opposite direction. The ignition
coil waveform should have a gradual rise or ramp as
current begins to flow (refer to Figure 4).