A circuit is provided for detecting abnormal conduction of current conducted through a solenoid in response to the pulse width modulation signal. The circuit includes a measuring means, for detecting conduction current conducted through the solenoid in response to the pulse width modulation signal and measuring continuous conduction time of the conduction current, and determining means, for determining a conduction to be an abnormal conduction when the continuous conduction time is longer than a predetermined time. Thus, the circuit can be miniaturized and the accuracy of the circuit is not affected by the accuracy of its components. And, the circuit is capable of detecting the half short state of the transistor driving the solenoid and not malfunctioning even if tile power is turned off.

A tester for individually testing diode packages, double pole relays and single pole relays includes a circuit contained within a housing, which circuit connects the diode package or relay being tested to a source of 12-volt direct current and transmits the direct current through the diode package, or relay, to an array of indicator lamps under the control of one normally open and two normally closed switches. A forward and reverse bias switch controls the direction of current through the diodes of the diode package and the internal diode of the relays to determine if the diodes and internal diode are oriented properly and conduct current properly.

A switch circuit for activating and deactivating a motor vehicle cruise control includes a 555 model timer integrated circuit which has a trigger terminal, a reset terminal, a threshold terminal, a control voltage terminal and an output terminal. A substantially constant voltage is applied to the threshold terminal. A first normally-open switch connected between the vehicle ground and the trigger terminal, and a second normally-open switch connected between vehicle ground and the reset terminal. Separate pull-up resistors connect the trigger and reset terminals of the timer to a positive voltage source and a capacitor couples the control voltage terminal to vehicle ground. A relay has a coil connected between the output terminal and vehicle ground, and has a switch that is connected between the common terminal and the ON control terminal of the cruise control. The 555 timer is configured as a latching circuit having an output which toggles in response to operation of the two normally-open switches.

A device for monitoring the switch position of a switch including an auxiliary voltage source, which is connected to the switch, a device for converting the switch position into an electrical signal, a device for transmitting the electrical signal and a device for evaluating the transmitted information. The converting device uses a pulse-duration modulation, the duty cycle of which becomes higher as the voltage U.sub.0 of the auxiliary voltage source becomes smaller. This enables the monitoring device to be used for different auxiliary voltages, without any need for adaptation.

An on-vehicle electronic controller which controls the operation of a device of an automobile or an internal combustion engine is disclosed. A relay intermittently supplies power to a controlled apparatus. The controller first causes the apparatus to be operated for a predetermined period of time while the relay is off, thus ensuring that any residual voltage at the relay terminal is discharged. After the predetermined time, the relay terminal voltage is compared to a predetermined voltage. If the voltage is greater than the predetermined voltage, it is determined that the relay is welded shut.