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Testing Spark Plug Leads

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#1 KernowCooper



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  • Name: Dave
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Posted 06 April 2013 - 11:21 AM

Due to the Minis design putting the electrical system at the front of the engine, Its important to check the ignition leads to ensure the ignition system is in top order to avoid bad starting and misfiring.

You see lots of posts here in TMF where members are asking for help on misfiring, bad starting and engines running on 3 cylinders, they have replaced the Plugs and fitted a new Condenser and a set of Plugs but they can't cure the problem? I don't see much mentioned they have also fitted a new set of Ignition Leads

I have written this as a test of the Ignition Leads which really should be part of servicing your car.

Engine misfire often is caused by faulty spark plug leads, so visual inspection could reveal what's wrong, the outer insulation may be damaged from the outside by abrasion caused by engine vibration, and the ignition lead rubbing on another component. Take the leads off for inspection and pay attention to the outer sheath, checking for cracks,burn marks and splits. If you see any of these signs replace the leads

Look inside the large rubber boot on the plug end and see if the metal connection has turned back and dirty, then check the metal connection is intact, your looking for anything which could stop the lead making a good connection in the distributor cap and onto the spark plug.

Look and listen with the engine running If you see little electrical arcs, or hear a crack, there is high-voltage electrical leakage on the lead, you can even see lots of leakage in the dark far better so a test them might reveal the misfire you've been chasing for weeks

Ok so assuming your leads have no cracks, splits and the boots and connections are looking good the next test is a resistance check.

1. Set your Multimeter to a scale of 20k ohms reading, touch the Red and Black Probes together and the meter reading should be "0" which indicates a perfect circuit.

2. Check for any printed writing on the Ignition leads which for example could be 7.5k ohms /ft - 10k ohms /ft or - 15k/m, this will give the manufactures readings on the cable when new.

3. Take a reading on your Ignition Leads with the Red probe one end and the Black probe the other end, it doesn't matter which way you put the probes.

4. If the reading is within a acceptable range as given by the manufacturer (old leads show higher ohms as they age the resistance goes up) but if any reading which is more than 30% higher indicates the resistance is increased to a point where it could break down and cause a misfire then replace the leads.

5. If you get no reading on the Multimeter or its flashing "- - - -" this indicates the plug lead is either broken internally or the resistance is more than 20k ohms, replace the lead.

I have tested a Brand New set of Ignition Leads for a Mini as a indication of what reading to expect,

Longest to Shortest Lead

Cylinder No 1 = 6.04k ohms
Cylinder No 2 = 6.33k ohms
Cylinder No 3 = 6.40k ohms
Cylinder No 4 = 4.70k ohms

King Lead from Distributor = 5.68k ohms
And these were the set I tested http://www.simonbbc....-leads-in-black

How long do Ignition leads last?

Depending on the quality of the lead some where between 2-5years you could see a increase in its resistance, a lot will depend on things like heat from the engine and proper maintenance of the leads.

So if the leads look old and tired and you've just fitted new plugs and points and done the timing then it may be time for new leads.

Edited by KernowCooper, 06 April 2013 - 11:38 AM.

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