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 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.