How are Horn Relay issues diagnosed?
Diagnosing a relay is simple with testing. On the exterior of most relays there is a diagram of the switches inside the relay. In the case of the horn relay, there normally be only one switch. Applying power and ground to the small circuit with proper polarity should make the relay click. This, however, is only the first half of the test. The secondary test is to measure the resistance across the relay, and compare that to the data on the relay or from the service manual. If the resistance is too high across a relay, the powered component will not receive the high amperage current it needs, and therefore, the relay must be replaced. If the relay does not click, the same applies, but the second part of the test will yield infinite resistance.
How are Horn Relays replaced?
Replacing a relay is only a matter of identifying the relay according to diagrams in the owner's manual, under the lid of the power distribution or fuse block, or in the service manual for the vehicle. Once testing of the relay is complete, the failed relay is thrown away only after ensuring the new relay is exactly identical in function and size, and the new relay is firmly pressed in, pulled out, and once again pressed into the power distribution or fuse block. This will ensure that complete contact is made between the relay and the terminals on the vehicle.
RepairPal Recommendations for Horn Relay issues
Never attempt to repair a relay. There is generally nothing serviceable in a relay, and erroneous reconstruction will only lead to humidity entering the relay, a relay that does not function properly, or even melting of the entire relay. Relays are inexpensive sealed units that should be replaced, not tampered with.
What to look out for when dealing with Horn Relay issues
If a relay is proven good, but still does not function, the terminals can be tested in the junction block with a test light or multimeter. A multimeter is prefered because of the accuracy of readings, but a test light will inform the technician if power is present or not. Also, the horn switch may provide a switched ground to the horn relay, or it may provide a switched hot (battery voltage). This should be known when testing the relay and the power distribution block. In the case of a switched ground, there will only be continuity between the battery negative cable and the relay when the horn switch is pressed.