How to test ignition cable?

I would not remove the HT leads from the coil. The easiest method of testing the leads and the plug caps is to measure resistively from the inside of one plug cap to the inside of the complimentary plug cap. If you look at the diagram in heavy maintenance manual you will see how they are paired. 

Effectively you are just measuring through the resistance of the secondary of the coil which is quite low. The plug caps are about 5000 ohms and the HT leads are pure copper and so measure less than 1 ohm. I cannot remember the coil resistance but all this will be in the manual. 

If necessary, you can unscrew the plug caps off the HT leads. You may need to shorten the lead slightly before reattaching it. 

Hi All

The spark plug boot resistance must be checked with it removed from the high tension wire.  They unscrew.  The boot has a threaded prong inside.  There is a threaded prong inside the termination point on the coils also, the wires are replaced at TBO.  To check the spark plug ends see the MMH section 74 for details.  

To check the high tension wires the best way is to remove the boots from the common coil.  Each coil has 2 wires, the resistance is the high tension wire from one end to the other as it passes through the coil.  Normally we never remove the wire from the coil unless the resistance is too high.  You can not check the wires with the spark plug boots installed as each boot has some 4 to 6 K ohm resistance on their own.  The values shown are for without the boots installed.  

Cheers

MISTAKE...the arrow for the resistance should point to the secondary, not the blue wire.  it will read 5K to 7.6K ohms wire end to end as it passes through the coil.  

Cheers

Thank you for the advice, Kevin.  Got a couple follow up questions:  

1) I tried to check the resistance across the complimentary plug caps as you said, but got no reading (zero load). In this case, I measured across the 3T and 4T caps since both connect to Coil 4 per the MMH.   So instead I unscrewed the plug cap in question (3T) and tested it directly and it measured 5k ohms just as it should, so it's good.  But I'd like to understand why the cross-check method didn't work.  Did I miss something?  I should have gotten a reading of 10k ohms, right?

2) The plug cap seemed to push and screw back on just fine.  You mentioned possibly needing to shorten the leads slightly first, but that's only if it the threads don't engage properly and won't tighten, right?

Appreciate your help!       

Thank you for the advice and tips, Rotax Wizard.  The threaded prong description and diagrams were especially helpful.  Got a couple questions for you:

1) As you may have seen in my previous reply, I could not get a reading across the complimentary plug caps through the common coil.  Which matches what you said.  But why is that?  A combined resistance of 10K ohms seems to be within the measuring range of a multimeter.

2) The page you provided with the resistance values was very helpful but does not appear in the latest MMH.  It's labeled page 7 of the section  74-00-00, yet the manual I have only has 4 pages in that section.  Do you know why?  Are there different versions of the MMH? 

Appreciate your help!