# Continuity of spark plug wires

If I take a simple continuity tester (penlite type with alligator clip) and test a spark plug wire(touching the metal parts inside the wireplug ends. It should light correct?

True. But be aware that the resistance of automotive spark plug wires can vary from about 40 ohms/ft to 10,000 ohms/ft depending on application. If you have the high-resistance wires, there is a chance your simple tester might not register continuity.

I’ve not tried that, but I don’t believe that is a good test - The plug wires are very high voltage and having a resistance in series helps to reduce EMI (and other stuff). It doesn’t take a lot of resistance to keep the test lamp off (maybe 50 ohms or so)

Try measuring the resistance with a VOM. Would be interested to see what you read - I’m going to wild ass guess somewhere between 1,000 Ohms (1K) and 50,000 Ohms (50K)

There we go - even 40 Ohms will kill a test lamp

There we go - even 40 Ohms will kill a test lamp

The penlight test lamp uses a 1.5 volt battery. The lamp in it needs about 50 ma of current (.05 Amps)

Ohms law says Voltage = current X resistance

So 1.5V = current X 40 Ohms

1.5/40 = current = .0375 amps (37.5 ma) which is not enough to light the lamp!

No. The spark plug wire resistance is too high. It’s supposed to be several thousand ohms, isn’t it? The battery powered tester is useful for very small ohm values, not large ohm values.
Walmart has a decent, cheap, “voltmeter” (multimeter) for under \$25. If you are going to do any electrical testing, or troubleshooting, you need a good digital, high impedance, multimeter. It’ll pay for itself the first time you use it and you DON’T change that part.

You can not test the ignition wires this way.

Check the resistance of the wires with an ohmmeter. The specs for a ignition wire should be about, minimum - 250 ohms per inch, maximum - 1000 ohms per inch. If you listed the year, make model, and engine I could be more specific.

By the way - using your penlight to check battery cables won’t work either! The lamp will light if there is a few ohms of resistance - but under starting load of about 100 Amps - or even an operating load of 10-20 Amps- a few ohms will drop the voltage so low the starter won’t turn or other stuff won’t work properly!