1988/09/01 Caterpillar

This article provides a general test procedure with the relay installed on the product. The procedure is for both 24 and 12 volt systems. The only test equipment needed is the 6V7070 Digital Multimeter, or equivalent such as the 6V7800 or 6V3030 Multimeter.

How A Relay Functions

Basically, a relay is a remote-controlled electrical switch that is turned on and off by a small amount of electrical current to let a large amount of current pass through its contacts to power a load (such as horns and starters). Low current flows through the relay coil and forms an electromagnet, which opens or closes the relay contacts (switch). See Illustration 1. Note the locations of the coil terminals and the contact terminals. Unless otherwise shown, contact designations of "normally open" (N.O.) and "normally closed" (N.C.) relate to the switch contact state without power applied to the relay coil.

Illustration 1. Schematics of two types of relays.

When the relay contacts are closed, a large amount of current can now flow through the two contact terminals to power the desired load.

Testing The Relay

This test determines if there is an open in the relay coil AND if the voltage drop across the contact terminal is within specification. For 24V systems, this voltage drop should typically be less than 0.5V, with a maximum allowable drop of one volt. For 12V systems, the drop should be less than 0.25V and the maximum allowable is 0.5V.

NOTE: The test should determine faulty relays. However, there are many other variables that can cause electrical problems.

1. Provide power to the relay, and if possible, turn on all equipment that is powered by the relay contacts. The engine can either be off or running.

2. Connect the multimeter across the terminals as shown in Illustration 2 to check for system voltage at the relay coil terminals. For 24V systems, this voltage should be more than 22V. For 12V systems, it should be more than 11V. If voltage is less than specified or zero, proceed with the remainder of Step 2, otherwise go to Step 3.

... Connect the multimeter between the battery side of the relay coil and machine ground. The multimeter should read system voltage.

... If not, there is a faulty connection between the source of battery positive and the positive connection of the coil. Correct this fault before proceeding with the test.

... If system voltage is present between the positive side of the coil and ground, but not to the ground side of the coil, then there is a faulty connection between the ground side of the coil and frame ground. Correct fault before proceeding.

Illustration 2. Checking to see if system voltage is available across the relay coil terminals.

3. Connect the multimeter as shown in Illustration 3 to check for system voltage at the relay contact battery terminal. If no voltage is present, correct this condition before proceeding.

Illustration 3. Checking to see if system voltage is available at the relay contact battery terminal.

4. After you are sure that system voltage is present across the relay coil terminals and at the contact battery terminal, you are ready to check the relay contacts (switch). Connect the multimeter across the contact terminals as shown in Illustration 4. Use the 200 DCV scale.

Illustration 4. Checking the relay contacts for correct operation.

5. If the relay contacts are closed during these conditions, the meter should read less than one volt maximum. (For 12V systems, 0.5V is the maximum.) For relay contacts that are open under these conditions, the meter should read system voltage.

6. Disconnect positive Wire (A), shown in Illustration 4, from the coil terminal. This should cause the contacts to change positions (closed contacts to open, and open contacts to close). As you touch and remove Wire (A) to the coil terminal, you should hear a "clicking"noise caused by the contacts opening and closing. This indicates a good relay coil, meaning it is not "open."

7. With Wire (A) disconnected, measure voltage drop across the contact terminals. Leave the meter connected as shown in Illustration 4. The meter should show system voltage for relay contacts which were formerly closed, but are now open. For relay contacts which were formerly open, but are now closed, the meter should show the voltage drop to be less than one volt maximum for 24V systems (0.5V for 12V systems). Reconnect Wire (A) and repeat this test several times to verify the results.

8. When Wire (A) is disconnected and reconnected, the voltage across the contact terminals MUST alternate between system voltage and the allowable voltage (one volt max. for 24V, 0.5V max. for 12V) across the contact terminals. If this does not occur, replace the relay.

... If the voltage across the relay contacts remains constant during this step, either the relay is defective or the circuit to the load is open or turned off.

... If the circuit to the load is not open, replace the relay as stated.

... If the circuit to the load is open, correct the problem and repeat the entire contact operation test beginning with Step 5.