Continuity check on plug leads

Thu Jul 10, 2008 5:03 pm

1 for you electrickery experts!

When I was at the MOT station the other day the tester commented on the engine missfiring. I couldn't really hear it, but then what do I know,eh!
Any way he suggested that a plug lead may be breaking down and that I should check the lead continuity. I think I can just about manage the test, but I aint sure about the results.

Can anyone help interperate??

Results as follows;
1. 5.57
2. 5.89
3. 5.67
4. 5.66
5. 5.63
6. 5.92

Meter was set to 20k in the Ohms section of the meter.

Thu Jul 10, 2008 6:41 pm

The tolerance is between 4-6k, so they are fine. They are also within the 10% value fluctuation of each other.

You could check the plugs themselves and the distributor cap/rotor.

Fri Jul 11, 2008 12:15 pm

Thanks m8!

Sat Jul 12, 2008 1:45 pm

rj wrote:The tolerance is between 4-6k

does this apply to any engine, like my m10?

Sun Jul 13, 2008 11:18 am

Plug leads 'breaking down' could be the insulation rather than continuity.....i.e the spark is 'escaping' before it reaches the plug. Lift the bonnet when it's dark and have a good look along the run of the leads to see if you can see any arcing to ground.

Sun Jul 13, 2008 3:18 pm

Lacy wrote:Plug leads 'breaking down' could be the insulation rather than continuity.....i.e the spark is 'escaping' before it reaches the plug. Lift the bonnet when it's dark and have a good look along the run of the leads to see if you can see any arcing to ground.

On any other make of car, maybe, but BMW design is such that the leads aren't subjected to that kind of electrical stress.

Sun Jul 13, 2008 4:17 pm

Brianmoooore wrote:On any other make of car, maybe, but BMW design is such that the leads aren't subjected to that kind of electrical stress.

Interesting....I'd like to know more. I don't question what you say one bit....I've learnt from my lurking here for so long that you are a great font of knowledge but presumably the HT voltage will be similar to, or higher than, any other ignition system; so is it the design of the lead that makes them less susceptible to breaking down and arcing out? if not, what in the design is so different?

It has been (many!) years since I did my apprenticeship at Lucas, (prince of darkness) so I'm well rusty on current ignition system designs. ( I left Lucas in 1982!)

Sun Jul 13, 2008 8:41 pm

Lacy wrote:
Brianmoooore wrote:On any other make of car, maybe, but BMW design is such that the leads aren't subjected to that kind of electrical stress.
Interesting....I'd like to know more. I don't question what you say one bit....I've learnt from my lurking here for so long that you are a great font of knowledge but presumably the HT voltage will be similar to, or higher than, any other ignition system; so is it the design of the lead that makes them less susceptible to breaking down and arcing out? if not, what in the design is so different?

It has been (many!) years since I did my apprenticeship at Lucas, (prince of darkness) so I'm well rusty on current ignition system designs. ( I left Lucas in 1982!)

Because the leads are held well away from any earthed metal by elaborate plastic guides.

Mon Jul 14, 2008 2:40 pm

Plug leads all ok. I have just checked dizzy cap & rota. Think they are a bit worn. Slight pitting in the contact area, these have cleaned up with a bit of fine emery, but think they could do with a change in the very near future.

Gennuine BMW or ECP??

Also can some one confirm spark plug gap? I have just looked at the manual and it says a gap of 0.8mm specifically for champion plugs. Does it matter about the make or are they all the same?? I think I have NGK,s fitted.

Thanx!