Replacing Problematic Relays

Coursing through the copper, inside a myriad maze of wires stuffed into the nooks and crannies of the modern automobile is the electricity required to run primary systems such as the starter motor, and secondary gadgets, like heated rear view mirrors. Since different levels of electrical power are required for running some devices over others, the automobile incorporates a relatively old-school electromechanical device to activate more modern innovations.

Isolating and Replacing Problematic Relays

Passing on the Power
This device, known as a relay, is essentially an electromagnetic switch that acts the same as one runner passing a torch on to the next in a "relay" race. When activated, the relay passes the signal on to send electricity flowing either this way or that—and the race begins. Insofar as automotive applications go, the relay usually passes the torch onto a bigger, faster, more powerful runner.

The good majority of relays in automobiles are used to channel a small amount of power in one circuit in order to trigger another, requiring a large amount of power, such as the tiny little electronic switch on a cabin climate control computer that activates the big old electromagnetic clutch on an air conditioning compressor. That being said, even fancy computers in modern cars still rely on the old-school electromechanical relay in order to make things work.

Recalcitrant Relays
For various reasons, not the least of which includes turning on and off thousands of times, and spending life in an environment that is alternately baking hot and freezing cold, relays may eventually stop working. When this happens, the switch to activate the heated seat may work fine, but the switch signal will stop at the relay and the big electrical power needed to heat up the coils under your hiney to keep your keister cozy will never be reached.

Worse, the engine heat can occasionally cause a relay to act on its own with complete disregard to switch commands, sending electrical power pell-mell. A malfunctioning relay can also cause the most difficult of all electrical problems: one that occurs intermittently, and at random.

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.


Step 1 - Disconnect the negative post of the battery before working on the electrical system.


Step 2 - Isolate the problem circuit and find the relay in question. This cluster is home to six relays that control everything from the AC condenser fan operation to the compressor magnetic clutch activation.


Step 3 - Remove the screw or fastener holding down the relay. Release connector tabs. Disconnect the relay from the connector by grasping the relay and connector itself. Never pull on wires to remove a connector.