How to Test HT Leads.

Adam Mechanic

How to test engine HT leads with a multimeter. How to check them incase of misfire. Please follow and like for more content!

Transcript

00:00Hi everyone just a quick video on HT leads what they are and what they do and how to test them

00:06to see if it's the reason for either a rough idle on a car or possibly a misfire on a cylinder. So

00:12HT leads literally deliver high voltage and high current from either your coil pack or your

00:17distributor on towards your spark plug so the spark plug can fire under extreme pressure and

00:23burn that fuel layer mixture in your cylinder. Obviously I've got these off a four cylinder

00:27engine so I've got one HT lead per cylinder normally they are marked up when they're new

00:32for which cylinder they're going when we talk about cylinders one to four one is normally the one that's

00:37at the timing belt end and you work from there so I've got four HT leads here all slightly different

00:43lengths because of obviously the length of the engine so number one is the longest in this case

00:47and number four is the shortest so when I do my test in a minute I'm going to be expecting the

00:51longest one to have the highest resistance and a gradual step down in resistance as the length

00:56decreases within one of each each of these HT leads you've got a copper wire and it's thickly insulated

01:03to make sure that the current cannot escape and it does go to its intended path which is

01:07onto the spark plug so the outside is normally rubber or silicon or PVC or something similar

01:13and inside is just copper and in this section here you'll have a ceramic insulator to make sure

01:19again that that high current gets to its and high voltage gets to its destination

01:24this would this end normally plugs into the coil pack or the HT lead sorry into the coil pack or the

01:30distributor and this one goes shrouds around the spark plug and make sure it goes again onto its

01:35destination so um multimeter wise I've got my multimeter set to 20k which is 20k ohms

01:41uh it's one kilo ohm per thousand ohms so we're expecting high resistance in these leads

01:45and obviously I've got my black uh probe into the comb for common earth and my red uh probe into the

01:52vr or ohms because I'm using the ohm setting so I'm going to start from the top the longest one I

01:57expect the highest resistance in this one I'm going to work my way down expecting the resistance to drop

02:02each time uh staggered quite evenly um just mentioned if one of these does have high resistance it's going

02:09to cause a spark to fire for less time so you're going to end up having rough idle and lumpy uh

02:17running conditions if it's broken all together or there's a crack in the wire or a bad connection and

02:22it's it's open circuit you'll have a misfire on that particular cylinder so let's start with the

02:26longest one first um I've labeled these up so I know exactly which end each of these cables is going into

02:33for myself on this distributor it's actually marked with each uh cylinder that it's going to a joint

02:38so it's quite easy but it's always worth marking them up if not so I'm just going to put my negative

02:43on one end it doesn't matter which way around you do these and the positive on the other end

02:48and again at 1000 at killer ohms the longest wire has 14

02:55around about 14 uh ohms killer ohms i'm gonna put 14 there next one down then I'm going to expect less

03:06in this one as I say so again it doesn't matter which way around again but we'll go the same as

03:11before so one into that one deep into there make sure you find the metal contact otherwise you won't

03:16get reading so that particular one the resistance drops a little bit as the current finds it easier

03:24and easier to pass through the wire so we'll go for 8.38 there we go to the third one

03:37again make sure you find the metal contact and then on to the negative on the other end here

03:43we're definitely on the metal contact in there and i'm getting absolutely no reading so one shows open

03:53circuit sometimes it'll show ol for open loop so actually the little voltage coming out of the

03:59battery in the multimeter passing through this cable is not getting to the other end of the cable

04:03so straight away i can now say that the misfire on this particular engine

04:08is definitely at least number three cylinder so i'll put ol for open loop there is no continuity

04:18in that particular lead and then the last one let's have a look so i've got one in there

04:26one in there i'm getting the reading of on the shortest lead

04:3110.35 we'll go with 10.35 so actually the resistance in the shortest lead is greater than

04:43the one in the second longest so for me this is on its way to failing so there's bad either a bad

04:49connection on one of the clips or the wire itself got a kink or some damage in it so that one would

04:55ideally be replaced because that was potentially going to cause a misfire sooner or later but number three

05:01this one has failed so somewhere within here there is a full break in the ht lead and that is causing

05:07misfire on number three cylinder on this engine um so i will be ordering a new set of ht leads and

05:12distributor cap sometimes it's worth checking checking the distributor cap as well because these contacts

05:17as the rotary arm spins around hits these four contacts for each cylinder these contacts do wear

05:22down over time and eventually will lose connection and it won't be able to send the high voltage across to

05:27the corresponding port for the ht lead in this case they're not too bad a little bit corrosion on that

05:34one but as part of this packet will be coming with a new distributor cap and a new set of ht leads for

05:40this particular car hope that makes sense see you on the next one

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