Can you test ignition leads with a multimeter?

[Scoboblio]

Bit of a dumb question, but here goes.

 

Can you test ignition leads with a common-or-garden multimeter? I’ve only ever used mine for testing alternators. .

 

I’m trying to track down a mystery starting problem on a Vauxhall

[400BHP]

yes you can test them, a haynes manual will be able to give you the resistance reading they should give you, its in the k ohm range if i remember from college (12 years ago!) but if you take a reading from all 4 and one is significantly different then it may indicate a faulty lead.

[Scoboblio]

Excellent, thank you!

[Soop Dogg]

Any reading you get from a multi meter won't tell you much, if anything, worthwhile about your leads.

 

The main failure with ignition leads isn't the lead going 'open circuit', it's the insulation. You might get a decent reading resistance wise when you pass a few volts from a meter down it, but when you pass thousands of volts down it from the ignition coil, and if the insulation is knackered, it will arc all over the place. (You'll see this happen if you look at the running engine after dark.) That's where your spark will usually be lost.

[Scoboblio]

Any reading you get from a multi meter won't tell you much, if anything, worthwhile about your leads.

 

The main failure with ignition leads isn't the lead going 'open circuit', it's the insulation. You might get a decent reading resistance wise when you pass a few volts from a meter down it, but when you pass thousands of volts down it from the ignition coil, and if the insulation is knackered, it will arc all over the place. (You'll see this happen if you look at the running engine after dark.) That's where your spark will usually be lost.

 

thanks Brian, I'll give replacement plugs a go first before thinking about how to try out the leads.

[400BHP]

Any reading you get from a multi meter won't tell you much, if anything, worthwhile about your leads.

 

The main failure with ignition leads isn't the lead going 'open circuit', it's the insulation. You might get a decent reading resistance wise when you pass a few volts from a meter down it, but when you pass thousands of volts down it from the ignition coil, and if the insulation is knackered, it will arc all over the place. (You'll see this happen if you look at the running engine after dark.) That's where your spark will usually be lost.

 

 

That's not strictly true, the resistance in the lead can go sky high and stop the current from flowing to the end at all.

Most leads are now made with a carbon conductor which can break down over time and stop the passage of electricity, even if the insulation were to break down its unlikely that you'd see it arcing all over the place" as the insulation would have to almost disappear entirely for the spark to pass through it.

 

Most faults occur near either end of the lead where the connectors are, try pulling the lead off of each plug with the engine running, if when you pull of a lead there is no difference in the speed/note of the engine then you have found the faulty cylinder. (use insulated pliers).

 

Also check the plugs, see if they have a brown ring around the porcelain where it meets the metal body, this would be a build of of metallic oil which can lead to something called the corona effect where the spark travels along the outside of the plug and down to the block, this can be almost impossible to see actually happening.

 

As for looking at the engine at night and seeing some sort of light show that will give away the faulty part, its very very unlikely.