So the customer drove to your workshop with the complaint that something does not work. It could be the horn, a light, the wipers, an electric door window, anything really.

In most cases, we start with the basics: whatever does not work, does it have power and ground? It the subject was the horn, then check the power and ground for the horn. If there’s power and ground, then the horn must be faulty.

While this is correct, we need to focus a little more on power and ground. A horn pulls a few amps when honked. So your test light should reflect that. Also, if you remove the horn and connect the power and ground to your testlight, you’ll still not know what is at fault if it doesn’t light up. So instead of the connector, we aim to use a known good plus and a known good minus.

The horn:

To check for power, we hook up the test light minus to a known good minus, preferably the battery. Then we check the horn’s power lead for power.

If this is ok, the test light lit up, then we need to check the horn minus or ground lead on the connector. So we hook up the minus of our test light to the battery plus and check the horn connector for minus. If the minus is good, the light should light up.

If the test light lit up in both tests, the power and ground are good, so it must be the horn that is faulty.

If the ground was good, but the power was not, then we have to look further. It can still be the horn, but less likely so – it is not often that two parts of a system break at the same time.

In the old days, you’d check the horn lead from the horn switch on the dashboard, and check the fuse. Today, it’s quite different. The horn is probably switched by the BCM, the Body Control Module. The switch on the steering wheel goes to the BCM, which either provides power or ground to the horn. If we had a correct ground, but no power, it could now be one of the following (in no particular order):

• Steering wheel switch defect
• Fuse defect
• BCM defect
• A wiring defect (an open connection)
• …

The cooling radiator fan (engine runs hot):

This is quite similar to the horn, but there’s no dashboard switch. In old cars, there was a thermo contact in the radiator that switched the fan on at a certain temperature. Today however, most likely the radiator fan is controlled by the ECM that has all the data required to make it run. The fan can be ground-switched or power-switched. In a ground-switched fan, the fan motor always has power. When the BCM grounds it, it starts to run. With a power-switched daf, the power is switched on or off by the BCM, while the fan always has ground.

In order to fix this problem, you need to know how the system works, what the fan circuit looks like. In short, we need a wiring diagram.

…

Paul
2025-09
Sweden

This page is part of a series. You can find the main page for the series here.