Testing for Available Voltage

I have uploaded and modified a Cavalier horn circuit diagram from Alldata.  Alldata is a paid professional program used by technicians to get detailed knowledge about anything on a particular car, basically a sophisticated service manual.  Alldata does offer some home user plans as well.

In this circuit, I will describe how to test for available voltage, why to test there, what results to expect, and an explanation of where to go from there.  This part of my article may be a bit complicated for some, but it is much easier to describe where and how to test when using a real life circuit with a make believe problem. In most circumstances, the first testing step is to verify that the component has power and ground. In this example, I am only showing you how to test the circuit.

  1. The first step in ANY diagnosis is to verify the customers complaint.  If the customer says "My horn does not work" then you should try the horn first.  This often prevents customer miscommunication. 
  2. Note how the positive lead is placed onto the fuse terminal and the negative lead is placed on a common ground or the negative terminal on the battery. All voltage available tests are done in this manner.  Keep the tester grounded and only move the positive lead from point to point. The Power Probe 3 allows you to hook a positive and negative to the battery to test that on the fly.Voltage Available Test - Circuit Diagram for Horn
  3. If the horn does not work, you should then test the easiest component next.. the fuse. The diagram provides the example of how to test available voltage after the fuse on this circuit (point 1).  During this test, you would expect to get a reading of 12V on a good circuit. If you do not have 12V, you should check the fuse itself and the power before the fuse.
  4. Next is to test voltage at point 2 and 3 at the relay terminals. The relay diagram is usually on the back of the relay. Those pins should match those of your diagram.  You first have to remove the relay to test at the terminals or install a Relay Tester Kit. You should have 12V in a good circuit. If not, you have a problem in the wiring between the fuse and the relay.
  5. You will only have a ground at point 4 if you activate the horn switch, otherwise your reading will be nothing with the relay removed. Point 5 should always be ground until the circuit activates the relay. Once the relay is activated (by grounding point 4 with the relay plugged in or by using the horn switch) it will complete the circuit to the horn.  You should have voltage on points 5 and 6 once it is active.  If you do not, check the relay for a problem.
  6. Point 7 will be a ground regardless if circuit is open or closed.  If you have 12V here, check your ground for a loose or bad connection.
  7. What if you have power at point 6, ground at point 7, and your horn still doesn't work? Sounds like you need a new horn.
  8. A good way to check ground is to test from point 7 to chassis ground. Should read zero.
  9. Be Careful to not to accidentally apply power to the control side of the circuit. The control side is the part of the relay on the bottom left hand side of the diagram. Applying power here could damage the connected turn signal switch assembly or even the BCM.

Now lets review what we have done. This procedure of testing works on all relays. Every voltage test you performed is a Voltage Available test.  Available voltage tests can tell you exactly where power is located at, home much power is available at any given time in the circuit, and if you have a good ground circuit. Just remember that anytime you are testing a circuit for voltage available you must always set up the tool in parallel to the current circuit, otherwise you defeat the purpose of the test.

Don't forget to check out my other articles.  The next article after this one should be about how to test Voltage Drop.