How to Test Spark Plugs.

Adam Mechanic

How to test the internal condition of your spark plugs with a multimeter. Please like and follow for more content.

Transcript

00:00Hi everyone just a quick video on your spark plugs and how to check them for the resistance

00:05so why is it important to know the spark plug resistance well if the spark plug resistance

00:10gets too high then the voltage won't be allowed through and you won't get the spark across the

00:15electrode gap and therefore you'll develop a misfire so it's very important to check these

00:19and make sure that the resistance isn't too high it's mostly going to show up under high

00:23load when you're accelerating when the demand is at its highest so it's worth testing them

00:29i've got my multimeter set up onto omega there and i've got my earth in the comm and the black lead

00:36and the red lead if you can see that is in the ohms setting as well so they're both ready to go

00:43i'm expecting between sort of one and five thousand ohms and depending on the type and the design of the

00:49spark plug bear in mind that some have multiple contacts some don't the resistors that live inside

00:56here will be different so they'll allow more or less um current through so do bear in mind that you

01:02want to be checking light for light spark plugs if you're ever replacing a set of spark plugs make

01:06sure they're exactly the same ones for all four cylinders to make sure the car runs properly

01:11um so yeah what we've got here is four brand new spark plugs and they're all identical so they should

01:16be giving similar readings i'm not expecting big big differences but because you've got four as a

01:20reference if i had a misfire and i checked the other things on the car and i wanted to check the

01:24spark plugs this would be a really good way of finding higher resistance because i've got some

01:29references here so i've said it's 20 000 because it's not going to be any higher than that and i've

01:34got my two test leads here it's just worth mentioning the construction of the spark plug

01:38the high voltage comes down here it goes to the little tip here and then it jumps across so you think

01:44of that the positive side if you like jumps across to the electrode tip here and then down the body

01:50and this is screwed into the engine block or the cylinder head sorry so this is earthed to the engine

01:55so you've got your positive side if you like and then your earth return is the actual body itself

02:00so if you're testing for resistance there should be no resistance between here and here because

02:05they shouldn't be linked there should be two sides of the circuit so if i want to actually test

02:09the insulator and the condition of the spark plug inside i'll put my earth on that side

02:14it doesn't matter which way around you do the leads technically and then on the contact tip here

02:20if you can see that there so i've got one on the back end there and one on the contact tip there

02:26i'll have to slide my multimeter down it's 4.57 so slide that down so you can see

02:344.57 ohms so i'm going to put down for this one 4.57 ohms at

02:4420k so that's my reference now for the next three so next one along i'm going to check

02:50yet again i'm going to put my earth

02:54side onto here and then this one onto that little contact there

02:584.94 ohms at 20k so i know what setting i've got my multimeter on next one along again

03:16earth onto the contact tip

03:245.4

03:25and then the last one

03:38is

03:414.85

03:42so even though these are all absolutely brand new they've never been in a car

03:50they've come straight out of the factory no two spark plugs are exactly identical so again even

03:56though they come straight off the production line this particular spark plug is has higher

04:00resistance the other three so that's actually internally in worse condition than the other

04:06ones as you can see they're quite similar in terms of the readings um but yeah clearly that uh second

04:12one down has higher resistance so therefore the structure inside isn't quite as good one of the tests

04:18worth doing if you are testing spark plugs as i say if we just set it to continuity which will just

04:24give me a buzz if there's a contact so if i go between here and here where i have before

04:32it shows it's complete but what as i said before this is the return side of the circuit so there should

04:37be no if i hold it still here on earth put it on the live on the other end of the circuit there should

04:42be as a zero or a one uh show me that there's an open circuit it should be the same all the way along

04:48there should be no contact because it's two sides of a circuit so that one's okay so yeah so from

04:57that if you've got one with an extremely high resistance compared to the other three that could

05:02very well be the cause of your misfire and it'd be worth replacing the spark plugs for the cost of

05:06them uh to see if that uh diagnoses and removes your misfire hope that makes sense and see you next time

05:14cheers

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