Houston, we have a problem

Next Article

Fluid-injection for NOx reduction

Electric fuel pump diagnosis – Part II

Houston, we have a problem

By Mike Mavrigian

Last issue in Part I of this article, we discussed several tests that should be performed to ascertain whether or not a fuel pump is at fault for a vehicle’s no-start or hard-start condition.

This time, we’ll continue our diagnosis.

Closed circuit tests

Although open circuit testing is a valuable tool for checking and locating opens, it is not effective for locating a poor electrical connection that has not gone “open” yet. Poor electrical connections add unwanted electrical resistances to a circuit and can cause a fuel pump to operate at lower than normal pressures and f ows.

In order to locate these types of failures, current will have to be f owing in the suspect circuit so the effects of the poor electrical connections can be detected in the form of a voltage drop. This type of testing is referred to as closed circuit because a “closed” circuit is necessary in order to have current f owing to measure voltage drops.

Voltage drop testing

The equipment you will need to perform

this test includes:

• A DMM set to the DC volt scale. • Any necessary test leads or electrical terminals to properly probe an electrical connector. • A substitute load tool. • A wiring diagram of the fuel delivery circuit.

To perform this test: 1. Disconnect the vehicle wiring harness from the fuel pump or fuel module. 2. Carefully connect the substitute load tool

to the power and ground terminals of the in the fuel pump circuit. The amount of vehicle wiring using the appropriate test voltage that can be lost in a fuel pump leads or terminals, see Figure 4. circuit will vary, based upon the amount 3. Connect the positive lead of the DMM to of current f owing in the circuit when the power terminal in the vehicle wiring the voltages are being measured. Check harness, see Figure 4. OEM or equivalent service information for 4. Connect the negative lead of the DMM to specif cations on the maximum voltage the ground terminal in the vehicle wiring drop in the circuit. harness, see Figure 4. 5. Set the DMM so it reads voltage values If no specif cation is available, a guidedown to 0.001. line is the voltage drop should not exceed 6. Energize the fuel pump circuit and record 0.5 volt when normal fuel pump current is the DMM voltage once it stabilizes. f owing. Voltage drops greater than this will 7. Remove the DMM from the vehicle elecaffect pump performance. trical connector and connect it to the If an excessive voltage drop is found vehicle battery. further testing can be done to locate which 8. Energize the fuel pump circuit and record portion of the circuit has the excessive drop the DMM voltage once it stabilizes. so repairs can be made. This can be done by 9. Subtract the voltage recorded at the following the procedure in the box below. vehicle electrical connector from the voltage recorded at Voltage drop #2 (Battery voltage) 12.44 volts the battery to determine the Voltage drop #1 (Fuel pump voltage drop) -12.04 volts amount of voltage that is being Fuel system voltage drop 00.40 volts lost while current is f owing

Fuel pump power circuit testing

1. With the load tool connected to the power and ground terminals of the vehicle wiring harness, connect the DMM positive lead to the positive post of the battery and the negative lead to the power terminal in the vehicle wiring harness, see Figure 5. 2. Set the DMM so it reads voltage values down to 0.001. 3. Energize the fuel pump circuit and record the DMM voltage once it stabilizes. 4. If the value noted on the DMM is 0.3 volts or higher there is an excessive drop in the circuit and repairs in this circuit will be required.

An excessive voltage drop in the fuel pump power circuit can be caused by these problems:

• A corroded or damaged electrical connector or junction in the power circuit causing excessive resistance.

• A failing fuel pump relay with burned or corroded contacts. • Damaged wiring due to failing wiring insulation.

Fuel pump ground circuit testing

1. With the load tool connected to the power and ground terminals of the vehicle wiring, connect the DMM positive lead to the ground terminal in the vehicle wiring harness and the negative lead to the negative post of the battery, see Figure 6. 2. Set the DMM so it reads values down to 0.001 volts. 3. Energize the fuel pump circuit and record the DMM voltage once it stabilizes. 4. If the value noted on the DMM is 0.2 volts or higher there is an excessive drop.

An excessive voltage drop in the fuel pump ground circuit can be caused by:

• A damaged ground wire connection at

Figure 6: Shown here is the load tool and DMM connected between battery ground and pump relay ground to measure the voltage drop on the fuel pump ground circuit.

Among the potential causes of zero fuel pressure include a broken/poor fuel pump ground, or this could be the result of an impact sensor that was activated during a collision, shutting down the pump circuit. Attempt a reset before delving further.

the vehicle chassis due to rust/corrosion. • A missing ground wire connection due to work that was done previously on the vehicle. • Damaged wiring due to failing wiring insulation.

If an excessive voltage drop is detected in the power or ground circuit further testing to locate the drop can be done by using the DMM, the load tool and the vehicle wiring diagram to systematically test the suspect portion of the circuit. To test the power circuit connect the positive lead of the DMM at the positive battery post and the negative lead to any junction points along the power circuit starting at the power terminal of the vehicle electrical connector and working through the circuit back toward the positive post of the battery.

Figure 7: With DMM connected to measure the current f ow in the fuel pump circuit.

The ground circuit can be checked by connecting the DMM negative lead at the negative battery post and the positive lead to any junction points along the ground circuit starting at the ground terminal of the vehicle electrical connector and working through the circuit back toward the negative post of the battery.

The location of the excessive drop can be isolated between the last normal voltage drop value and the excessive drop. The repair work can then be focused in this particular area of the circuit.

Testing fuel system amperage

The amount of current f owing through the fuel pump circuit can be measured and may provide valuable information when diagnosing the condition of a fuel pump. Although when performing this test care should be taken to make certain factors such as the voltage being applied to the pump and the mechanical load on the pump are taken into consideration.

A pump operating with low voltage due to wiring or battery issues or under a heavy load due to a restricted fuel f lter may cause a suspect current reading that would mistakenly condemn a correctly operating fuel pump.

The equipment you will need to perform this test includes:

• A DMM with a 20 amp DC fused in-line ammeter function. • Any necessary test leads or electrical terminals to properly probe the fuel pump fuse of the vehicle. • A wiring diagram of the fuel delivery circuit.

To perform this test:

1. Remove the fuel pump fuse from the vehicle.

2. Set the DMM to the 20 amp DC in-line ammeter position. 3. Using the test leads, connect the positive lead of the DMM to the power side of the fuse holder in the vehicle and the negative lead of the DMM to the load side of the fuse holder, see Figure 7. 4. Energize the fuel pump circuit and record the DMM amperage once it stabilizes. 5. Compare the current reading to the service specif cations to determine if there is a problem.

Be sure to reinstall the fuel pump fuse when all testing has been completed.

An excessively high current reading can be caused by:

• A binding pump due to internal pump damage or contamination. • A shorted or grounded fuel pump. • An unwanted short or ground in the electrical wiring to the pump.

An excessively low current reading can be caused by:

• Excessive electrical resistance in the fuel pump circuit wiring due to corrosion, rust or poor connection. • A failing or failed fuel pump relay. ●

Aside from a worn-out pump, a faulty or stuck fuel pressure regulator can be the root cause of low fuel pressure. Check the regulator before dropping the tank to access the pump.