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An Introduction to
FUEL PUMP TESTING
PROBE HOOKUP:
Most late model fuel injected vehicles have some
sort of fuel pump control circuit that is
fairly easy to access and test. The value of the inductive current
probe is that by clamping
around a single fuel pump feed wire and operating the pump we may
determine the actual
loaded "health" or condition of the pump under operating
conditions.
Fuel pump amp draw, operating rpm, brush
contact and even wear is easily determined.
SCOPE SET-UP:
Today, lab scopes are like scan tools with many
brands and many features. Discussing
the good and bad points of different lab scopes has become a
discussion that equals the
Ford versus Chevrolet debates and will go on forever. My opinion
(called Linderism in some
parts) is that ANY lab scope is better than not having one at all !!
The majority of our fuel pump testing uses a
voltage setting of 100mv per division
and a time setting of 2ms.
In some cases we expanded the time
base to 5ms for a better look at the waveform
produced by the pump.
KNOWN TEST PARAMETERS:
CURRENT DRAW:
Current draw will increase with load and decrease
with lack of load. Also poor brush contact
will lower current draw. Brush spring tension is critical to the
motor operation! For some
reason, the negative (ground) brush always is the first to fail.
This spring will be the one to
show heat, burnt brush and loss of spring tension of every failure
we have seen and
inspected to date.
MOTOR COMMUTATOR BARS AND RPM:
We have concluded that the majority of automotive
pumps (motors) we have inspected have
a total of 8 commutator bars.
Using that number and the time base of the scope
we may now figure the actual operational
rpm of the pump motor.
In case study one, 8 bars contact in 12.7 ms or
one revolution every 12.7 ms and
1 minute = 60,000 ms
60,000 divided by 12.7 = 4724 rpm. (this is
normal for most TBI units)
PREVENTATIVE MAINTENANCE:
Using the printout we can now advise the customer
as to the condition of the fuel pump
and the actions that should be taken as a maintenance measure.
Think this when explaining the waveform:
" If this pump was in a airplane would you wish to fly with
it ?"
CASE STUDY #1
GOOD FUEL PUMP WAVEFORM:
What does the fuel pump waveform above
indicate?
- The pump is defective and needs to be replaced.
- Current draw is very high at 14 amps.
- Pump has defective armature and is going to
fail soon.
- This is a known good pump with proper
waveform.
Using some very basic DC motor fundamentals (from
my radio controlled car motor days)
we can break the pattern above into sections of discussion and make
many diagnostic
decisions in regards to the in-tank fuel pump operation.
- We know by testing that the amp draw is correct
and within specifications for the
vehicle tested. Higher fuel pressure = higher current draw or
motor load.
- Brush contact is very regular and consistent.
CASE STUDY #2
BEGINNING STAGE OF FAILURE
What does the fuel pump waveform above
indicate?
- The pump is defective and must be replaced.
- Current draw is excessive at 14 amps.
- Pump shows damaged com section every
eighth bar.
- Vehicle has excessive AC ripple and needs a
alternator.
This is a great example of a known good (?) fuel
pump that is showing some damage on one
single section of the pump. Every eighth brush cross-over to the
next commutator bar
shows some variation in current. If this was your customer you might
not wish to replace the
pump, but would want to monitor its wear as time goes on and the
vehicle returns for service.
CASE STUDY #3
DUAL TANKS ONE GOOD, ONE BAD.
The pump waveforms shown above are from a dual
tank GM truck, what do these
waveforms show?
- Both pumps test ok with one under load.
- Both pumps are noisy.
- One pump is ok (lower), and the other
pump is defective (upper)
- One pump is OEM, the other is aftermarket.
This is a good example of a quick fuel pump test
on a dual tank equipped truck.
The TBI units on these vehicles are somewhat difficult to test pump
pressure and volume due
to the absence of a fuel test point. This vehicle had a performance
related problem only
when on the rear pump. A new fuel pump repaired the condition.
CASE STUDY # 4
DEFECTIVE PUMP AND DEFECTIVE ALTERNATOR
What does a quick test of fuel pump waveforms
on this vehicle indicate?
- The pumps are in good condition.
- The vehicle not only has one defective
fuel pump, but the alternator should be
checked for AC leakage.
- One tank is empty and the other is full.
- Both pumps should be replaced.
Ford truck with dual tanks, history of poor
performance and 02 sensor codes (AC ripple
was very present on 02 waveforms also). A single test at the fuel
pump relay shows one
pump with very high current draw, low rpm and poor brush contact. AC
ripple was checked
and alternator was also defective! Patterns smoothed out with
alternator unhooked.
Another case for preventative maintenance
CASE STUDY #5
RETURNLESS FUEL SYSTEM
What does this pump on a 1994 Dodge (returnless
fuel system) show?
- Returnless systems have a different pattern or
waveform.
- Pump is defective and will have a short
life cycle!
- Pattern is normal for a returnless system
- Pump has a defective ground circuit and low amp
draw.
The advent of returnless fuel systems will
challenge the OEM and the aftermarket as present
designs must be altered.
Most OEM's suggest removal of the entire
fuel tank and a total flush of the tank unit
prior to pump replacement.
Repair facilities have shown some
reluctance to do this service as suggested! This will
compound the damage done to returnless systems as multiple pump
replacements are
common on these vehicles.
Take a Quiz
and see how you score on Current Ramping Fuel Pumps.
E-mail your questions and comments about this
Fuel Pump Testing Procedure to jimlinder@lindertech.com |
