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Spark Plug/Wire testing

the_hairy_baboon

Active member
Joined
Dec 1, 2021
I have a 1990 Haynes manual for 240 model Volvos and there's a very odd piece of troubleshooting they have in the ignition section

3 If the engine still fails to start, check that the current is reaching the plugs, by disconnecting each plug lead in turn at the spark plug end, and hold the end of the cable about 3/16th inch (5mm) away from the cylinder block. Spin the engine on the starter motor

All spark plug wires (or COPs) have a large boot to prevent arcing to anything that's not the spark plug. Therefore the end of the spark plug lead shouldn't be able to arc to the engine body.

The online link for checking spark plugs at haynes.com says to either use a tester OR pull the lead off, remove the spark plug from the engine body, reconnect to the lead, and then hold close to the engine body to check for spark.

Why would they initially suggest a method that wouldn't work? Was it because the original spark plug wires just used vast distances to prevent arcing and therefore didn't need a boot? Everything I can see shows stuff with boots AND even with a metal body outside the rubber interior boot there's still not arcing.
 
Gently push a screwdriver into the boot and hold it 3/16" off the head or valve cover.

So basically continuity from Coil-->Dist-->SparkPlugLead-->Screwdriver which can then arc from part of the screwdriver length to the engine block. Makes sense.

Still doesn't explain the history of why Haynes would suggest something that doesn't work (and for a very common test) unless the Lead boots were different back in the day. Any Historians here?
 
I have a 1990 Haynes manual for 240 model Volvos and there's a very odd piece of troubleshooting they have in the ignition section



All spark plug wires (or COPs) have a large boot to prevent arcing to anything that's not the spark plug. Therefore the end of the spark plug lead shouldn't be able to arc to the engine body.

The online link for checking spark plugs at haynes.com says to either use a tester OR pull the lead off, remove the spark plug from the engine body, reconnect to the lead, and then hold close to the engine body to check for spark.

Why would they initially suggest a method that wouldn't work? Was it because the original spark plug wires just used vast distances to prevent arcing and therefore didn't need a boot? Everything I can see shows stuff with boots AND even with a metal body outside the rubber interior boot there's still not arcing.

You need a spare spark plug to do that old school test.
 
If you have Boricord wires and they are more than a couple years old, they are likely junk.

The described test is a good way to electrocute yourself.

The proper way to check plug wires is with an ohm meter.

1k ohm per inch I think is a hairball spec. So you don't want to see any wires over ~30k.
I tug on them too while testing to verify that they don't go open when you pull.

If you don't have an ohm meter, use your tongue.
 
The spark plug wire spec is 500 ohms per foot for the bougicord wires. Also you want to take a look at the engine at night and make sure there isn't any wires that broke down the insulation and are shorting out. You can also use a water filled squirt bottle and spray the wires to make sure they are good.
 
If you have Boricord wires and they are more than a couple years old, they are likely junk.

The described test is a good way to electrocute yourself.

The proper way to check plug wires is with an ohm meter.

1k ohm per inch I think is a hairball spec. So you don't want to see any wires over ~30k.
I tug on them too while testing to verify that they don't go open when you pull.

If you don't have an ohm meter, use your tongue.

Of course all that stuff before the tongue. :shh:
 
If you have Boricord wires and they are more than a couple years old, they are likely junk.

OEM wires have traditionally degraded with time, but for consumer usage, this is not an issue until many years pass.

In point based distributor days, these carbon conductors would have to be replaced every ten thousand miles or so for performance based engines.
 
I've stuck a big cotter pin in the wire end before to check spark to ground. It's easier to start with the coil wire though and check that first, it's exposed enough to use by itself.
 
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