8/3/2019 Prueba de Consumo Electrico de La Bomba de Combustible

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66 March 2003

Last month, we explainedhow to use voltage droptesting to determinewhether an electric fuelpump is receiving ade-quate supply voltage for

proper operation. This month, well ex-plain how to use an inductive low-ampprobe and a digital storage oscilloscope(DSO) to look inside an electric fuelpump and assess its health.

Inductive amp probes come in allshapes and sizes, and they can handlethe full range of currentsboth AC andDC. A low-amp inductive probe can beused to measure the relatively low cur-rent flow (usually below 10 amps) in afuel pump circuit.

The inductive probe converts the sig-nal into one that can be understood bythe voltage-reading DSO. The DSOwill still be thinking its seeing volts, butwell know theyre amps.

Most low-amp probes have two set-tings10mV/A and 100mV/Aandthey affect the way the fuel pump wave-form is displayed on the DSO screen. Ifthe probe is set on the 10mV/A scale,for each 10mV displayed on the DSO,the probe is measuring 1 amp of cur-rent. If the probe is set on the 100mV/Ascale, for each 100mV displayed on theDSO, the probe is measuring 1 amp.

When viewing fuel pump current waveforms on your DSO, start with atime base of either 1 or 2mS per divi-sion, and a voltage scale of 100mV perdivision. The waveform can be analyzedin greater detail at 1mS per division, while the rotational speed of the fuelpump can be more easily calculated at2mS per division.

Adjust the scope and probe to thesettings above, then use the inductiveprobes ZERO dial to move the wave-form to the 0 volt (ground) positionon your DSO before clamping the

probe around the fuel pump wiring.The amp probe clamp can be placed

at any point in the fuel pump circuit, aslong as its in series between the voltagesupply and the ground for the load.Clamp the probe to one of the pumpwires, if its accessible. Or clamp itaround the pumps power wire at thefuse box, at a relay or inertia switch or atan underhood test connector.

While voltage may be different at various points in a circuit, current in aseries circuit is the same at all points. Somake sure there are no additional com-

ponents, like a transfer pump, wired inparallel to the main pump, as these willaffect the test results.

The engine and other electrical com-ponents must not be running while thefuel pump is being tested, as they mayaffect the available voltage to the pump,which will also affect its amperage read-ings. Some manufacturers provide aseparate underhood pigtail test connec-tor to power the fuel pump. Othersplace a fuel pump test pin in the diag-nostic connector. To operate the fuelpump with the test connector and theengine OFF, connect a poweredjumper lead to the connector.

Check the wiring diagram before us-ing this method. You may be bypassingthe fuse, relay or other switching com-

ponent when power is provided to thepump via an external source. If so, thepump current draw may be within spec-ifications during the pigtail test. But avoltage drop in the bypassed circuitcould cause low current at the pumpduring normal vehicle operation. Ideal-ly, the fuel pump should be poweredON using the same wiring thats usedduring normal operation.

A good pump will produce a patternsimilar to the one shown here. Thescope is set to 100mV per division andthe probe is on the 100mV/A scale.Each vertical division is equal to 1 amp,so the pump is drawing between 5 and6 amps.

If the pump is good, the waveformhumps should be uniform. Poor contactbetween the brushes and one or moreof the commutator segments will pro-duce jagged or low-amplitude humps.

Mechanical resistance inside thepump can result in higher-than-normalaverage pump current. A clogged fuelfilter or fuel line restriction will also in-crease pump current. Electrical resis-tance, either in the pump or elsewherein its circuit, lowers available pump volt-age and reduces pump current.

High-pressure pumps, like those inport fuel injection (PFI) systems (35 to45 psi), require more current than low-pressure pumps in throttle body injec-tion (TBI) applications (9 to 13 psi). ATBI pump may have a normal currentdraw as low as 3 to 5 amps, with PFIpumps pulling 4 to 6 amps on average. AGM central point injection (CPI) pump(55 to 64 psi) needs 8 to 10 amps.

Visitwww.motor.comtodownload a free copy of this

article. Copies are also availableby sending $3 for each copy to:

Fulfillment Dept., MOTOR Magazine,5600 Crooks Rd., Troy, MI 48098.

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:KarlSeyfert

FuelPumpDiagnosis:

Amp TestingBY KARL SEYFERT