Isolating and replacing problematic relays

Errant electrical power can, at the very least, cause things not to work and can, at worst, be downright dangerous. An electrical circuit switching on all by itself can overload and create an electrical fire. Intermittent electrical problems are extremely difficult to trace but can often be narrowed down to a recalcitrant relay. Other examples of failed or failing relays can be fuel control micro switches, horns not working despite valiant added hand signals, or a radiator fan not switching on when the coolant reaches a certain temperature.

Remove and Replace
While pulling out the multimeter and testing every relay under the hood, dash or carpets is one way to commence attack on mysterious electrical problems, a better way is to obtain a service manual and isolate the circuit in question by consulting a wiring diagram. First check and see if the connector has become corroded or clogged with road grime. A quick cleanup of the contacts may be the lucky fix. One of the easiest ways to test a relay, once the circuit and relay are isolated, is to swap it out with a duplicate that is known to be good and see if the circuit and relay work together once again.


Relays of the same voltage and amperage are often used throughout the vehicle, so just find one that has matching numbers and pin connectors, swap it out, and test away. When swapping out relays, make sure to only swap with replacements of the same voltage and amperage rating, and keep in mind that some relays may require a diode to complete the circuit.


A service manual can help determine the correct replacement relay, as can a qualified parts counterperson. An electric radiator fan that doesn't switch on when an engine gets hot can lead to overheating, which could, in turn, lead to a blown head gasket or cracked cylinder head—an expensive mess caused by the failure of an inexpensive part. The failure of a five-dollar part can cause the five hundred dollar problem, so swapping out the relay in question with a new one may be the best plan of action.