Component Testing

Since it is not possible for a computer to be tested in the field, most servicing approaches are designed to eliminate all other causes of trouble, and then recommend replacing the computer. In order to do this, some components must be tested.

Sensors can be checked by seeing if they open or close at the right time, or if their resistance values are correct. Solenoids can be checked by applying voltage with a jumper wire and watching to see if they operate. Solenoids should have a measurable resistance - that is the windings should not be shorted to ground or open.

If sensors and solenoids are okay, circuit wiring should be checked. Disconnect wiring at both ends of a circuit and measure resistance of the circuit. If more than about 5 ohms, connections need to be cleaned or wires repaired. Then connect the ohmmeter between one end of the circuit and ground. If resistance is less than several hundred thousand ohms, the wire is shorted to ground and must be repaired. Wiring diagrams are available for all systems and should be used to check connections and resistance values.

If all sensors, control devices, and wiring are good, the control unit or computer is suspect. If possible, substitute a known good unit. If vehicle performance improves, install a new unit. Since control units are expensive, be sure all other possible causes of failure have been checked before replacing one.