Testing HT circuit continuity and resistance - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 127 (permalink) Old 12-30-2011, 09:43 AM Thread Starter
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Testing HT circuit continuity and resistance

The last few days have seen a number of posts where the Ignition coil needed testing, at least for resistance.

On the 360º engines (Bonneville and T100 models) the ignition coil secondary or HT winding is double-ended and isolated from ground. This means that without dismantling anything important we can measure the resistance and continuity of the 3 main items in the HT circuit:

1.- The HT (High Tension) side of the coil or secondary winding. This being the thinnest wire within the coil, most likely to break or get damaged, punctured or burned insulation as it's the most highly stressed part due to the high voltages involved, (>20Kilo volts).

2.-The plug caps which contain 5000 Ohm resistors that can fail sometimes. These resistors are fitted for radio interference suppresion.

3.-The HT cables that on our bikes are made with copper conductor cores and not likely to break, but we can check them just the same as the connections at both ends can come adrift from the coil terminals or the plug cap terminals.

Once again we need a digital multimeter obtainable for a few dollars in lots of places. Set the range switch to a suitable resistance (Ohms), 50K, 100k, 200k, etc and insert the probes into the disconnected plug caps. Ensure the metal part of the probe can reach the internal contact. Luckily the waterproofing rubber fitted to the plug cap often serves to hold the probe in position leaving both hands free. If this is not so you'll have to hold the probes in there while performing the test.

Here's a diagram showing how the 360º twins HT circuit is arranged, and showing the connections of the Ohmmeter:





The reading on the meter should come to a total of around 18 to 20 Kilo Ohms for carburetted models and some 30-32 Kilo Ohms for EFI models.

If an open circuit is detected (meter reading infinite resistance), sometimes symbolised on the display with a "1", first of all check the plug caps are attached properly to their cables.

The connection consists of a kind of self-tapping screw fixed where the cable enters the cap and sometimes the cable can become "unscrewed" from it. Hold the cable firmly and twist the cap to "screw" it back into the HT cable.

If OK go on to the coil posts where the HT cables are attached. This connection consists of a sort of spike inside shrouded coil terminals. The cable is fitted with a pressure gland and plastic nut that forces the cable onto the spike.

If the previous items are OK then it's safe to assume the coil's secondary or HT winding is broken internally.

If you get a suspiciously low resistance reading the chances are that the internal insulation of the coil winding has broken down leading to an internal short circuit.

The resistance of the coil secondary only, should be in the region of 8-9K for the carburetted coils and around 20-22k for the EFI ones.



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Last edited by Forchetto; 12-31-2011 at 02:58 PM.
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post #2 of 127 (permalink) Old 12-30-2011, 11:56 AM
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Forchetto,
Thanks for the info.
See my post on "Ignition No Spark Need Advise" and let me know what you think. I value your opinion when it comes to "electrickery" stuff.
I much prefer mechanical problems as they are absolute and not always intermitent within a sealed circuit board.

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post #3 of 127 (permalink) Old 12-30-2011, 10:19 PM
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Good info. Since my HT coil was reading open, I replaced it, but still have other issues that are causing my woes. Once I get them sorted out, I'm going to try putting my old coil back in just to satisfy curiosity if a coil will still function despite the bad reading. I'll be sure to post my results.
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post #4 of 127 (permalink) Old 12-30-2011, 11:23 PM
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As a caveat when testing the leads.....you can only test solid core leads this way. 90% of the other leads out there used in chevy, fords and aftermarket leads have a crushed carbon and fibereglass stranded center which has intential resistance to lower rfi. Your ohm meter will jump all over.
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post #5 of 127 (permalink) Old 04-12-2012, 02:51 PM
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Does anyone have any ideas of what causes them to go bad?

Thanks
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post #6 of 127 (permalink) Old 04-12-2012, 03:32 PM Thread Starter
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Do you mean the HT leads?. In the case of our bikes not much as they're copper-cored like normal electrical cables with a reinforced insulation to withstand the high voltages (>20kv).

They'll last the life of the bike. However they can become unscrewed from the plug caps or pull away from the coil connection posts. In this case and given the high voltages, the system could still work in a fashion. This is because the very high voltage can jump a fair distance to make up for a defective connection. In this case you'll often hear a "click, click" sound as the spark jumps the gap.

The coil secondary winding can be easily damaged by disconnecting part of the HT circuit and operating the system. If the high voltage has nowhere to go then the voltage in the coil will keep on rising until something breaksdown, usually the enamelled wire insulation that the winding is made from.

Take care when spark testing to ensure that the plug is firmly in contact with the metal of the cylinder head to avoid this. If not sure, use a spring clamp to hold the plug body in firm contact, like this:



There are several types of HT testers on the market. These fit between the plug cap and the top of the plug and allow the engine to work as normal. The indication is given by an internal lamp that works off the HT voltage in the case of testers 1 and 3, and by a visible and adjustable spark gap in the case of tester number 2:

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post #7 of 127 (permalink) Old 04-12-2012, 03:56 PM
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Thanks. I was primarily meaning the winding in the coil itself. Since I'm suspecting the one I got a few months ago is heading south, I was concerned as to why before putting a replacement on order. I did a fair bit of spark testing so perhaps I didn't have enough of a contact to the block a couple of times. Education is expensive....

Thanks for the info!
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post #8 of 127 (permalink) Old 04-26-2012, 10:44 PM
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what makes one side of the coil + or - ?

theoreticly, could the coil be switched 180?

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post #9 of 127 (permalink) Old 04-30-2012, 03:24 AM
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Quote:
Originally Posted by HacksawsGarage View Post
what makes one side of the coil + or - ?

theoreticly, could the coil be switched 180?
That's correct Hacksaw. Typically the coil is an isolated component and thus has no reference.

If the coil, for some reason, had some polarity discrete components contained within, then polarity could become an issue. But this is usually not the case.

One test is worth a thousand "expert" opinions!
Curiosity was framed,ignorance killed the cat
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post #10 of 127 (permalink) Old 04-30-2012, 04:13 AM Thread Starter
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Coils where the ground side of the HT winding goes to chassis through the casing had a polarity marked on them. See photo below. The idea was that the -VE side went to the contact breaker and the +VE to the ignition switch. This ensured the spark polarity was correct, i.e. the +VE to the centre electrode of the spark plug and the negative to the ground electrode.

Apparently this required 30% less voltage to spark properly. You could connect the coil the wrong way around and things would still work though.

Note the + and - signs on the LT terminals:



On our 360º engines with a wasted spark system and non-grounded double-ended coils, it means that one of the spark plugs is theoretically sparking with the wrong polarity. It doesn't seem to make much difference though...

Last edited by Forchetto; 04-30-2012 at 04:16 AM.
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