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Resistance spec for Primary Side of Coil

7869 Views 4 Replies 2 Participants Last post by  tvpierce
My bike is a '79 CX500 Deluxe with the non-CDI ignition.

Is there a resistance spec for primary? I looked in the GL 500/650 pdf service manual, and it says 2-3 ohms, but that's for a CDI equipped bike.

The test I've found for the non-CDI CX involves using a special tool for a 3 point spark test.

Thanks in advance.

(EDIT: No, apparently I do have a CDI. I don't understand this ignition system. Can somebody explain it to me. Thanks in advance.)
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A 79 would be a CDI unless someone put a later motor in it

( like me with my 82 Eurosport motor

To check the CDI coils primary resistance you need to first check the resistance of the

meter leads

To do this set the meter to the lowest resistance readings

short the leads together and note the reading

You may find its 1 ohm or less.

Now attach the probes to the yellow or pink LT wires and get a good contact

the centre slug

You may need to scratch down to bright metal to do this

the usual reading fo the LT coils is 0.8 of an ohm

an ohm or two should not matter.

Test the HT side by removing the cap and use range to read say 10K

usually its about 8K ohms between the centre slug and conductor.

While your at it test the plug caps

Original ones had resistors many of which have burnt out

You would expect to see about 5K ohms between the HT lead connector

and the plug top contacts.

If you get a dead short look up the hole where the plug goes

If you DO NOT see a slot for a screwdriver thats fine

Its a later non resistor type cap


It may be an original cap that someone has had the resistor burn out

and replaced it with an alloy or brass rod

You can dismantle these to check if you wish

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Hi Reg.

That's extraordinarily helpful... thank you!

I was getting 1.1 ohm testing between the primary lead and the rear mount/ground.

To be perfectly honest, I don't quite understand how this ignition system works. I expected there to be two leads on the primary side: a switched 12v lead, and a "signal" lead that makes and breaks to create the spark... but there is only one lead. Am I correct to assume that the CDI is making and breaking full voltage to the coil? That seems like it would put a lot of stress on the CDI.

Thanks in advance for sharing your knowledge.
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What you describe fits the TI system but the CDI works a bit


The stator has two coils which charge 'dump caps' in the CDI box

The pulsers inside the rear cover send trigger pulses to this CDI box

and SCR's then dump the charge in the caps which collapses the magnetic

field in the primary windings and in turn induces a much higher voltage on the HT

side and produces the spark.

the voltage to the coils on a CDI system is much higher than on a TI which

as you say is 12v or battery voltage

This means they are not interchangeable with TI coils.

Using my rough and ready test,I expect to see an open circuit voltage from the CDI LT output of about 150-200 VDC

CDI boxes can and do break down, usually suddenly with no warning

The main weakness is the dreaded 'stator failure'

The part of the stator that generates electrical power is often fine

and often its just the 2 CDI power windings that have failed.

Also the advance windings on the rear of the case can break down and

the bike still runs in this case but loses its get up and go

the Ignitech is a solution to these all too common failings.
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Very thorough and informative Reg... thank you very much.

Let me digest that info... I'm sure I'll have follow-up questions.

Again, thanks!

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