Test and measure

Understanding function and measurement can help achieve a quicker diagnosis

Published:  04 August, 2014

By James Dillon

When considering the physical measurement aspect of circuits, a full understanding of the circuit being tested will give the technician a distinct advantage when it comes to testing, measurement and diagnosis of a fault condition. We have a golden rule which we refer to during our electrical fault finding technical training course; if you don't know what value you expect to read with a multimeter within a circuit when testing, then you are not ready to make that test. The logic follows that if you don't have a measurement expectation or criterion, how will you qualify the reading you've just made?

Figure 3

Relay 1 'control side' is supplied a voltage from a fuse. They do not share a supply but they do share a ground! Relay 3 is providing a ground path for 'control sides' of relays 1 and 2.

Having an understanding of the function and the expected voltage levels under differing conditions (key on, key off cranking, etc.), will allow you to develop a test plan. There are subtleties in the wiring diagram, which unless you spend time understanding, will set you on the wrong path. It would be easy to make assumptions in the case of our Honda, which could lead you to the wrong conclusion.

I truly believe that time invested in analysing the circuit and developing a test plan with expected values will save a multiple of five times (so 10 minutes research and planning turns into a 50 minutes saving) when compared to getting on the car to 'do some testing', gung-ho style, meter swinging. Using your plan to guide and record your measurement steps and test values is also a great help when writing up the invoice. The plan also helps prevent multi-measurement (where you go back to measure the same thing several times as you can't remember what was measured before as you progress further into the fault).

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