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    HT Leads

    I wrote about my recent misfiring episode on another thread, but I thought I would pass on what I discovered today.

    My Stag has been running well up until recently, with the HT leads I fitted 7 years, 26,000 miles ago. These were new items bought from a well-respected supplier of Stag parts. Following the work to fit a reconditioned distributor and new electronic ignition, it began suffering from a misfire under load. This disappeared when I swapped the full set of HT leads with the old ones I took off 7 years ago and threw into a box of spares. I have since ordered another new set which should be with me in a couple of days.

    Today, curiosity got the better of me, so I did some testing of the "faulty" lead set using a multimeter. Of the 9 leads (including the king lead) 5 were showing open circuit with my meter set on 20kOhm. The other three were around 12kOhm. I then checked the 9 old leads which I fitted on Sunday. One of these was also showing open circuit, with the remaining 8 showing between 7 and 12kOhm, depending on length. I have now replaced the open circuit lead with one of the good ones from the "faulty" set.

    I am no electrical expert, but how the engine ran at all with 5 apparently faulty leads I don't know. I guess the high voltage was enough to overcome the broken leads up to a point, but disturbing them during the dizzy replacement was enough to tip them over the edge.

    If my findings are relevant, you may want to check the leads on your car as all may not be as good as it seems. I don't know if the figures I've obtained are healthy, or how they compare with so-called high performance leads. Perhaps someone could enlighten me. I do know that I shall be keen to test the new leads when they arrive and before I fit them, for comparison.
    Dave
    1974 Mk2, ZF Auto, 3.45 Diff, Datsun Driveshafts. Stag owner/maintainer since 1989.

    #2
    Hello Dave,

    I've just had a new set of Magnecor leads delivered, which I've not yet fitted. Here's the readings that I measured with 20k ohm out of the box:
    Coil lead- 3.48;
    1- 11.81;
    2- 7.72;
    3- 9.08;
    4- 7.03;
    5- 8.07;
    6- 5.44;
    7- 7.23;
    8- 5.40.

    Hope that helps.

    Other Dave

    Comment


      #3
      Thanks OD. I'll compare with the LDP ones when they arrive.
      Dave
      1974 Mk2, ZF Auto, 3.45 Diff, Datsun Driveshafts. Stag owner/maintainer since 1989.

      Comment


        #4
        Originally posted by DJT View Post
        Thanks OD. I'll compare with the LDP ones when they arrive.
        When I fitted my current set of Magnecor leads, which was about 15 years ago, Tony Hart suggested I do an exhaust gas test before & after. I have an ex-garage Snap On gas tester, which was calibrated when I bought it, so was reasonably sure of its accuracy. Sure enough, as Tony suggested, the emissions dropped considerably with the new leads. Whether this was due to the leads being new, or due to the brand, I can't say. I plan to do the same again when I get round to fitting my new set.

        Jam First, Not Cream, Dave

        Comment


          #5
          On another car, I have had brand spanking new Bosch HT leads which were faulty.

          Comment


            #6
            LDPart HT Leads are 7mm and fit into the standard ht lead clips without pinching the leads. Wonder if it is the pinch that causes the failure?

            Or the excessive heat transfer from the exhaust manifolds which are mm away from the leads
            Stags and Range Rover Classics - I must be a loony

            Comment


              #7
              my running exhaust temp is around the220 degree mark with coated headers not sure how that compares with cast iron

              Comment


                #8
                Are your HT Leads silcone,and do you know if they have a wire centre core or carbon core ?
                73 mk 1/2 now gone to the dark side BLUE

                Comment


                  #9
                  Originally posted by new to this View Post
                  Are your HT Leads silcone,and do you know if they have a wire centre core or carbon core ?
                  7mm Silicone with a carbon core. No sign of any visible heat damage.
                  Dave
                  1974 Mk2, ZF Auto, 3.45 Diff, Datsun Driveshafts. Stag owner/maintainer since 1989.

                  Comment


                    #10
                    Hi Chaps
                    Back in the seventies i didn't have a Stag but did run fords and BMC stuff that seemed to run ok on copper leads and from what i can remember were pretty much bomb proof. do we need these fancy kinds of leads for the Stag.
                    Not trying to be funny just curious and always willing to learn. Thank's Chris.
                    Chris TV8 O/D Carmine, Scirocco TDI 180 R Line

                    Comment


                      #11
                      I had a misfire recently and searching for it, I could hear it arcing but couldn't see anything. Turned out to be one of the leads on the distributor had pulled away from the plug crimp. I put a new set on from LD Parts, the 7mm version but i've kept the old set as they were copper core and would have just needed the plug crimping again. I only discovered this when fitting the new leads.

                      Comment


                        #12
                        Originally posted by DJT View Post
                        Today, curiosity got the better of me, so I did some testing of the "faulty" lead set using a multimeter. Of the 9 leads (including the king lead) 5 were showing open circuit with my meter set on 20kOhm. The other three were around 12kOhm. I then checked the 9 old leads which I fitted on Sunday. One of these was also showing open circuit, with the remaining 8 showing between 7 and 12kOhm, depending on length. I have now replaced the open circuit lead with one of the good ones from the "faulty" set.

                        I am no electrical expert, but how the engine ran at all with 5 apparently faulty leads I don't know. I guess the high voltage was enough to overcome the broken leads up to a point, but disturbing them during the dizzy replacement was enough to tip them over the edge.
                        Dave, chances are that the 5 leads that apparently showed open circuit were the longer leads in the set (I am guessing here) and the fact that you checked the resistance on the 20kOhm range on your meter means they would have shown as open circuit if their resistance was more than 20kOhms (actually 19.99kOhms). I have seen references to HT lead resistances as being a maximum of around 12kOhm per foot, so shorter leads would measure OK, but the longer ones may not. (NOTE: the 12kOhm per foot figure is just a generalisation and may not apply to your specific make of lead). This only applies to resistance type leads. You can get copper cored leads where the resistance will be very low (effectively 0). Resistance leads were introduced to reduce the amount of interference on radios and TV's. (remember the days of crackling sound on the radio or lines on the TV as a badly maintained motorbike went down the road outside your house?)

                        A small break in the conductor in the lead would should not be an issue in any case. In theory the high voltage would most likely jump the gap - in a conventional distributor type ignition system there are 2 gaps anyway - one between the rotor arm and distributor cap contact and the other is the spark plug gap itself. The issue with failed leads is usually where the insulation has broken down or the conductor resistance has risen to such a high level that the insulation breaks down somewhere from the coil to the lead meaning the voltage leaks away before reaching the spark plug. The high voltage will look for the easiest path to earth (battery -ve), be it along the cable and via the spark gap at the end of the spark plug, or through a weak spot in the insulation on the lead, or tracking down the rotor arm in the distributor through a dodgy rivet and slightly conductive black plastic - you get the picture. The leads will deteriorate with age. The heat from the engine, chafing and rubbing, oil, etc all conspire to degrade the insulation. Also the carbon/graphite conductors suffer from migration of the carbon away from the central conductor leading to the increased resistance. (you may have seen "black wire corrosion" [other names are available] on some cables in the wiring loom - you strip them back and instead of seeing shiny copper you see blackend copper where carbon has migrated out of the plastic which is a similar process).

                        When testing the leads with a meter bear in mind they are different lengths so will have different resistance readings. You need to measure the length as well and do a calculation to get a figure of Ohms per foot or Ohms per meter. You can see this in Dave022's readings. The longer leads to plugs 1 and 2 have higher readings than the shorter leads to 7 & 8 (odd leads are longer than the even leads)

                        One final comment - changing the coil and or ignition system often results in a higher available high voltage. In your case fitting the new Pertronix module may have upped the HT voltage which increases the electrical stress all along the ignition system. This will, can and does seek out any weaknesses possibly causing electrical breakdown which results in misfiring etc.

                        Bit of a long winded way of saying a) not all of your leads may have been faulty and b) it may not have been the fact you disturbed the leads but that they had deteriorated and they could not withstand the higher HT generated by the new ignition module. Though in practice the actual limit of the HT voltage should be governed by the plug gap.

                        Roger

                        So many cars, so little time!

                        Comment


                          #13
                          Thanks Roger.

                          I’ve been out and checked the leads using the higher ranges on my multimeter (200kO and 2MO) and got the following results:


                          Coil. 10.5” - 0.43 - 5.0
                          1. 33” - OC - 14.04
                          2. 28” - OC - 9.8
                          3. 33” - 1.36 - 12.57
                          4. 26” - 1.06 - 13.33
                          5. 28” - 1.17 - OC
                          6. 20” - OC - 7.4
                          7. 25” - OC - 10.16
                          8. 19” - OC - 7.27

                          Lengths in inches. First column of results are the 7-year old silicone leads. Second column are the leads I took off 7 years ago. Of the good leads in each set that I could compare, I was surprised to see one set has resistance 10x that of the other. Dave in post #2 measured another set with readings in between.
                          Last edited by DJT; 12th June 2019, 09:04.
                          Dave
                          1974 Mk2, ZF Auto, 3.45 Diff, Datsun Driveshafts. Stag owner/maintainer since 1989.

                          Comment


                            #14
                            A very interesting discussion! I've been bugged for a couple of years with what seems like a spark breakdown in one cylinder. When I'm driving there seems to be a very subtle 7 cylinder feel to the engine. Hard to describe. I had something on another V8 and it was more noticeable and quickly diagnosed. On the Stag it's so subtle so doesn't seem to be a complete failure of a particular cylinder. I've changed out points, swapped to electronic ignition, second new distributor cap, third new rotor arm, third type of spark plug, second new coil, (all being from different manufacturers). After each of the above "disturbances" to the system, the "7 cylinderness" went away, but crept back over the following days and weeks.

                            I'd originally fitted those green leads with the correct angled plug connectors which did the rounds a few years back. I now changed to the thicker blue ones which a chap on eBay makes up to order. Again, the 7-cylinderness disappeared but returned.

                            As it doesn't seem like it is a case of one cylinder "missing" but rather one cylinder not getting a healthy spark, I can only think that this is HT lead related.

                            Looking forward to your LDP lead report Dave!

                            Incidentally, Klaus put together the different sizes from the Beru plug lead standard programme - perhaps it's time for me to do the same - especially if you find out that the leads are the problem.

                            Drew
                            If you can't say something nice, don't say it !

                            Comment


                              #15
                              Originally posted by DJT View Post
                              Thanks Roger.

                              I’ve been out and checked the leads using the higher ranges on my multimeter (200kO and 2MO) and got the following results:


                              Coil. 10.5” - 0.43 - 5.0
                              1. 33” - OC - 14.04
                              2. 28” - OC - 9.8
                              3. 33” - 1.36 - 12.57
                              4. 26” - 1.06 - 13.33
                              5. 28” - 1.17 - OC
                              6. 20” - OC - 7.4
                              7. 25” - OC - 10.16
                              8. 19” - OC - 7.27

                              Lengths in inches. First column of results are the 7-year old silicone leads. Second column are the leads I took off 7 years ago. Of the good leads in each set that I could compare, I was surprised to see one set has resistance 10x that of the other. Dave in post #2 measured another set with readings in between.
                              That's interesting! During the course of my saga, I'd identified that a suspicious area where the spark plugs don't seem to be doing their work fully all the time are cylinders 3 and to some slight extent 1. These are, as you list, the longest HT leads.

                              Drew
                              If you can't say something nice, don't say it !

                              Comment

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