Automobile Maintenance: Testing of Electrical Relays

This write-up concerns the general-purpose electrical relays. The specialized ones like timer relays etc are not discussed here.


For testing of electrical relays, one must first understand their function and working principle. Fucntion of a relay is to allow the use of a low-amperes circuit to control a high-amperes circuit. By control is implied switching on and switching off operation. The low-Amps circuit is the control side and high-Amps circuit is the feed side of the relay.


Many online videos suggest that if a relay clicks, then it is working. Clicks are an audio test at best and does not confirm if a relay is actually working. A click confirms that the control side of the relay is good but nothing can be said definitely about the feed side based on a clicking sound.



The control-side circuit consists of a solenoid and feed-side consists of a switch that is turned on or off by the solenoid's magnetic field. Sometimes this switch of the feed-side would melt from heating and would make a very weak or no contact at all and there would still be a clicking sound. A melted relay terminal is shown below.

Before you conduct the tests, it is imperative to know the control and feed side terminals. Onto the French/Italian relays, terminals 1-2 belong to the control-side and terminals 3-5 belong to the feed-side. On the German/American relays, terminals 85-86 belong to the control-side and terminals 30-87 belong to the feed-side. Now is the time to put the theory to the test.

(1) Attach a digital multimeter in Ohms mode to the control-side terminals and read the value in ohms. It should not exceed 60-70 ohms.

(2) Next attach multimeter in Ohms mode to the feed-side terminals of the relay using clips on the meter leads. The meter should read infinity or no continuity.

(3) Now as shown in the figure, attach a 12 Volts or 9 Volts battery to the control-side terminals of the relay using two electrical wires. There should be a click and the multimeter should read one to two ohms of resistance. This would constitute a complete set of basic tests. Advanced tests would surely include amperes test for given battery voltage and voltage drop for a given resistance of the control-side solenoid.

If a relay fails these tests, it is defective. There is a usage of the defective relays too. They can be transformed into relay shunts as shown in the picture below.