Spark Plug Lead Resistances

[lubeit]

Well I had just got 50 kays up on my newly acquired AU1 futura and today the engine suddenly starts running real rough, shaking and delivering less power under acceleration.

I’ve checked for spark and alls good except number 5 has no spark. I removed the lead (it’s a top gun 8.0 mm, don’t know how old) and it measures approx 7.48 kohms. The Gregorys manual says the number 5 lead should be 9 to 21 kohms. Could the low resistance measurement on the lead cause a complete loss of spark? (I’m a bit suss on this because the manual specifies the number 6 lead to be 7 to 19 kohms, so I thought even though number 5 measures a bit low it should still supply some spark…)

With regard to the coil pack, the gregorys manual circuit diagram shows number 2 and number 5 are supplied high tension by the same winding. Is it correct that the coil pack supplies plug pairs? If so, is it possible that number 5 could not be getting high tension whilst number 2 is sparking ok? Due to the fact that my number 2 is sparking ok, I don’t know if I should suspect the coil pack…

Any help would be appreciated.

[howesy]

Just swap the number 5 with the number 6 lead and see if number five still has no spark. if not more than likely the coil. The lower the resistance the better the transfer of spark. your measuring resistance not conductivity although they do relate to each other.

[lubeit]

Just carried out some further troubleshooting and now I’m more confused!

Swapped lead 5 to coil pack position 6 and lead 6 to coil pack position 5. Lead 5 now delivers spark to test plug and lead 6 (from coil pack position 5) now delivers no spark. So I checked the secondary coil resistances between position 2 and 5 on the coil pack and it measures ok per the gregorys manual specs : approx 9.26 kohms.

Whats goin on? The coil pack primarys measure ok at around 1 ohm each, the coil pack secondary between 2 and 5 measures ok at 9.26 kohm, but both number 5 and number 6 lead wont deliver high tension to the test plug from position 5 on the coil pack but they both deliver high tension to the test plug from position 6 on the coil pack!

Lead 5 resistances was rechecked : 7.45 kohms
Lead 6 resistance checked : 8.1 kohms

Its got me stuffed!

[MNM96]

I always set a limit of 12 ohms resistance. The reason they vary is due to the lead length, the longer they are, the more resistance.

Sounds like the coil. If you are stuck, let me know. I got one here.

[lubeit]

Thanks for the advice all.

Im starting to suspect the coil pack too. I guess even though the secondary coil for 2 and 5 measures ok, maybe its breaking down under operation, although I still dont understand why number 2 would still be firing ok when number 5 doesn’t…

Im going to have to change the leads now anyway. I got a bit careless removing number 4 lead and left the lead clip behind on the plug! I’ll get a new set of leads and plugs tomorrow and try that out. If still no good I might have to talk to you, MNM96 (Mike), about a quote on a coil pack!

Thanks again for the assistance.

[MNM96]

I am happy to pass it on for free, on the proviso that, if you don't need it, you do the same for someone else in need! If you would like it, drop me an addy for the mailman.

[Mechan1k]

I had this happen in my ute ... was doing it occasionally though ... ended up being one of the terminals on the coilpack was breaking down ... replaced it and all was good after that.

[lubeit]

A bit more info for the forum…

I replaced my leads and plugs this morning (good thing I did this anyway because the old plugs were in a pretty sad state) but number 5 still had no spark.

So with a bit of fiddling around I got the coil pack out and did some checks with my multimeter: all three primarys measured ok around 1 ohm and all three secondarys measured ok around 9 kohms. But then I got my megger and did some insulation resistance checks, and when I applied 1000V between the primary and secondary of number 2 and 5 the insulation broke down! Meggar checks on the other 2 sets of windings tested ok. So the coil pack is cactus…

Moral of the story: meggers are handy!

A couple of things I found when changing the leads and removing the coil pack:

I found it essential to remove the airbox and ducting all the way to the throttle body for access to the coil pack.

A quarter inch drive socket set with long extensions (like 12 inches worth) makes it easy to get the two bolts out on the inboard side of the coil pack. Go down from the top, past the throttle body. You can see the bolts with a torch. I loosened them off completely and then retrieved them with a $2 telescopic magnet (a very handy tool at times). The two outboard side bolts were accessed with just the ratchet and a deep socket. I then manipulated the coil pack rearwards, then outboard around some hoses and forward and up out of the engine bay. All work was done from above the engine bay.

I hope this is helpful to others as the Gregorys manual doesn’t have any details on removing the coil pack.

[MNM96]

I will post out my coil pack Monday Morning. Got your Email.
Thanks,
Mike

[lubeit]

Thanks Mike, much appreciated.