Component Testing

Since it is not possible to test a computer in the car, most servicing approaches are designed to eliminate all other causes of trouble. If everything else tests out okay, then the computer is the last possible cause. In order to do this, some components must be tested first.

Sensors can be checked to see if they open or close, or if their resistance values are correct at specified temperatures. Solenoids can be checked by applying voltage and observing their operation. Solenoids should also have a measurable resistance across the windings. They should not be shorted to ground or open.

If sensors and solenoids are okay, the system circuits should be checked. Disconnect wiring at both ends of a circuit and measure resistance. If more than about 5 ohms is measured, then the connections need to be cleaned or wires repaired. Next, connect the ohmmeter between one end of the wire and ground. There should be at least several hundred thousand ohms resistance. If not, 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 check out okay, then the computer may be the problem. If possible, substitute a known good computer. If performance is restored, install a new unit. Once again, since control units are very expensive and RARELY fail, be sure ALL other causes are eliminated before replacing.