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.

New pumps include a new pigtail, to eliminate known or potential connection problems.

New pumps include a new pigtail, to eliminate known or potential connection problems.

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 flows.

In order to locate these types of failures, current will have to be flowing 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 flowing 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.

[PAGEBREAK]

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 vehicle wiring using the appropriate test leads or terminals, see Figure 4.

3. Connect the positive lead of the DMM to the power terminal in the vehicle wiring harness, see Figure 4.

4. Connect the negative lead of the DMM to the ground terminal in the vehicle wiring harness, see Figure 4.

5. Set the DMM so it reads voltage values down to 0.001.

6. Energize the fuel pump circuit and record the DMM voltage once it stabilizes.

7. Remove the DMM from the vehicle electrical connector and connect it to the vehicle battery.

8. Energize the fuel pump circuit and record the DMM voltage once it stabilizes.

9. Subtract the voltage recorded at the vehicle electrical connector from the voltage recorded at the battery to determine the amount of voltage that is being lost while current is flowing in the fuel pump circuit. The amount of voltage that can be lost in a fuel pump circuit will vary, based upon the amount of current flowing in the circuit when the voltages are being measured. Check OEM or equivalent service information for specifications on the maximum voltage drop in the circuit.

If no specification is available, a guideline is the voltage drop should not exceed 0.5 volt when normal fuel pump current is flowing. Voltage drops greater than this will affect pump performance.

If an excessive voltage drop is found further testing can be done to locate which portion of the circuit has the excessive drop so repairs can be made. This can be done by following the procedure below.

 

Voltage drop #2 (Battery voltage)                           12.44 volts

Voltage drop #1 (Fuel pump voltage drop)          -12.04 volts

______________________________________________

Fuel system voltage drop                                         00.40 volts

Figure 4: With load tool and DMM connected to measure the voltage drop on the fuel delivery circuit.

Figure 4: With load tool and DMM connected to measure the voltage drop on the fuel delivery circuit.

[PAGEBREAK]

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.

Check the fuel pump relay with key on. An audible click should be evident (placing your finger on the relay and feeling for the click is useful in a noisy shop).

Check the fuel pump relay with key on. An audible click should be evident (placing your finger on the relay and feeling for the click is useful in a noisy shop).

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.

Figure 5: Shown here is the load tool and DMM connected between battery ground and pump relay ground to measure the voltage drop of the fuel pump power circuit.

Figure 5: Shown here is the load tool and DMM connected between battery ground and pump relay ground to measure the voltage drop of the fuel pump power circuit.

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 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.

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.

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.

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.

[PAGEBREAK]

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.

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.

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.

Testing fuel system amperage

The amount of current flowing 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 filter 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 specifications to determine if there is a problem.

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

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

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

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.

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.

0 Comments