I thought I would treat my car to a new set of HT leads, it probably does not need them, 23,000 miles but nearly eight years old. Bought a set, (Blueprint £54) from my local motor factor. I tested the resistances before fitting. Working on around 16k.ohms/metre. Two leads around 120k.ohms one 19 the other 14k.ohms. Now this seemed ridiculous to me so I returned them. Factor then obtained another set manufactured by a company by the name of Cambaire. I have never heard of them has anyone else? These were much cheaper which made me suspicious so I declined them. Genuine Mazda leads are £79! Eurocarparts £34 but no manufacturer stated, I prefer a known scource. I have read a good deal on the forum on this topic and it would appear that the recommendations of some members are then denounced by others based on their experience, a great deal of controversy over those supplied by MX5 parts. I would prefer to buy locally in case they prove unsatisfactory as were the Blueprint ones. The problem did not arise previously as I did not keep cars for very long before the MX5. so new leads were never needed. Reliability is my top priority so I am inclined toward the Mazda leads but as a last resort due to the price. Finally I notice that some leads are 8mm, standard are 7mm. will the guides on the cam cover accept 8mm, (tight squeeze), or do the need to be enlarged? Comments advice etc. anyone who thinks I do not need to replace the leads? I think I am right with 16k.ohms/metre, unless someone can correct me!
Andrew at Autolink does some cheapie grey ones which are actually Lucas and great value
Moss Europe were doing a promotion recently with Jackson Racing 8mm leads @ £20 - not sure if the deal is still on, I have the JR red ones on my supercharged and they seem fine and more importantly are a perfect fit on the coilpack and into the plug holes in the head.
I have the JR ones too and have had no problems with them (I’m usually an advocate of OE leads). As Taff says, they are a good fit which is an important aspect of HT leads in my opinion.
I was reading Geoff’s comments on testing the leads resistance. I had a play with my multimeter and not sure if its me but measuring resistance does not seem to be that easy. Got over a dozen sets of leads around, mainly OE 7mm leads but also Magnecore 8mm, Sportline 8mm, etc.
I sell lots of coilpacks and see the condition of the replaced parts and other issues with the MX5. Water ingress, particularly where the leads enter the coil packs is a very common issue. Water ingress in the spark plug holes is almost non existent but worn cam cover gaskets allow oil to pool in that area which can’t help. I believe the main problem is the coil HT port area with water somehow getting in. I have tried to work out how this could happen as the ports are covered by the rubber shroud on the leads - guess it must be some expansion/contraction causing a vacuum or summit.
People don’t usually worry about these things until there is a problem and then it’s replace everything. My advice is to carefully fit a good set of leads, 7mm OE is fine but make sure that there is no oil pooled in the plug holes and more importantly that the coilpack HT ports are clean and corrosion free.
It is very noticable when the engine is misfiring. If your engine is not misfiring, you have got it right.
Should be easy enough, just put it in the correct range and look for about 5 or so kOhm from end to end. (actually 16kOhm per metre if you want to get your tape measure and calculator out).
But measuring the exact resistance isn’t important. You just need to make sure it is neither very low, nor very high.
That was the kind of reading I was getting but with Geoff talking about readings of 120, 19, etc, I thought my use of a multimeter was again suspect.
My own theory is that the lead breakdown is more likely to occur at the coilpack HT end rather than in the lead itself. Expensive 8mm leads fitted to a coil pack with corroded HT ports is a total waste of time and money.
I think the biggest problem by far is moisture ingress through perished insulation. The joints at either end are the weakest point. Once dampness gets in it’s only a matter of time.
I’m not sure of the lengths, but I’d guess about 0.3m down to about 0.1m? So the longest should be 16/3 ~5kOhm and the shortest about 2kOhm as an estimate…according to the book.
So it would seem 1 & 2 should be chucked, and 3 & 4 are about ok.
If resistance rather than largely untestable insulation is that important, I would agree.
That being the case the advice has to be to buy a good secondhand set of 7mm OE leads rather than supposedly higher quality 8mm leads. The worst readings by far come from the Sportline leads which are as new and would have cost over £50.
It’s quite possible they have altered the factory spec resistance for the sake of ‘performance’. I’m not really well versed on high tension electrics…I’d assuse lower resistance would mean a bigger spark, but I don’t know if that’s necessarily going to be good for the coils or the plugs.
But yeah, in the end it’s insulation that’s the main problem and without a megaohm tester, it’s impossible to test.
Thankyou for your replies everyone. The snag with leads which have a high resistance is that the plug will take as much current as it needs to spark, the coilpack is only a glorified transfomer after all, albeit a very expensive one. High resistance could overload the coilpack, not a good idea on the MX5 where it seems to be something of an Achilles heel. Merely for academic interest I measure my leads, they are the originals, every year or so and the readings have been quite consistent since new. No1. 6.87 No.2. 5.31 No3. 3.95 No.4 3.32 (all kiloohms) The JR leads evidently, fit the Mk.2 but not the 2.5, Why I do not know. The two sets I tried apart from the resistances being very high were all around 4cm. too long. Not normally a problem, but difficult to ‘lose’ that amount due to the support brackets etc. So, the quest continues. I have been told by NGK that their leads are not available in this country yet Moss Europe sells them!
There are a couple of simple tests to check on how good a state the leads etc are in.
Insulation. Open the bonnet in a dark place and run the engine, also blip it a couple of times as this changes the effective resistance across the plug gap. If you can see any blue sparks or discharge around the leads, then the spark is escaping, this could be because the plugs are going high resistance or the leads are failing. The outside of the oil pack can also track the spark away, again visible in the dark. Late one winter afternoon I took back a brand new set of expensive “Sparkrite” leads simply because they leaked spark like a sieve, and kept with the old set. I was able to demonstrate to the disbelieving Halfords manager just how carp the leads were with the open bonnet test; he thanked me and pulled the whole range.
Resistance 1. Measure the leads conductor end to end with a test meter, expect no more resistance than the figures mentioned above. However in the past I went for copper core leads and the resistors built into the plugs, this way having a consistent match between cylinders, big snag - the resistor plugs were less reliable so I needed to carry an old one as a spare. Some resistance is required, firstly for interference suppression, and secondly it extends the spark duration just long enough to ensure the fuel charge in the cylinder has fired (electronics people think about lowering the system’s resonant Q).
Resistance 2. Measure the plugs too, this is important. A couple of times on various cars over the years I’ve experienced plugs going open circuit, and this can cause the spark to punch through the lead insulation or cook the coilpack so that these also needed to be replaced.
I’m out of date on ‘offers’. These days I tend to stick with guaranteed items from big name suppliers or the car manufacturer because there is too much cheap rubbish flooding the after market.
Interestingly, in the US, Mazda does not supply the original black plug leads from Yazaki. Instead they migrate owners to NGK Blues.
Its a lottery. There is no point to Magnacors (and they’re a bit short on 1.8s), and I have a set. I like Racing Beat leads, but only because I’ve met the 80 year old they employ to hand assemble and test each set. But they use Ultra8 connectors, which can scorch a coil pack if fitted incorectly. I have No-brand yellow Autolink cheapies on one car, and they’ve never given cause for concern.
