Tempting Fate? or Hot Fire?

[Cookie]

Good day all. Bit of an odd question for you electrical geniuses out there. It's come to my attention that to get my Tempter running (sorry for not updating, time to post pics is scarce these days), I'm going to need some new coils. Now, I've been fixing cars for about 10 years and I don't think I recall ever having to match Primary Coil Resistance to my vehicle so I'm a bit lost.The GR650 Serice Manual says my coils need to have a Primary Resistance of 3-5 Ohms. Why is that? I assume it's to act as a protection for the ignition control? Not sure here...
The reason I ask is this: Over the years, I've come into a small collection of "known good" ignition coils from various Mazda and, more recently, Jaguar cars. Being a cheap S.O.B, I'd much prefer to use a coil that i can get (or already have) for free, rather than drop hundreds on new coils. However, the Primary Ressitance on the coils for a 2000 S-Type Jaguar is spec'd at 0.5-1 Ohm (The ones I have are measuring 0.8 Ohm). Can I put a 2-or-3-Ohm resistor in line with my Jag coil to bring Primary Resistance up, or is this a lost cause? Will I nuke the ignition controller on a 28 year-old bike that was only made for 2 years in N.America or will I end up with a useable (possibly hotter) system?

 

[mydlyfkryzis]

0 ohms is a dead short. Current is Volts/Resistance. A 5 ohm coil draws: 12V/5 = 2.4 amps. A 3 ohm coil, using the same formula, draws 4 amps. Since a point type ignition system has no other current limiting feature, you need to limit the current or you will burn out the points. the points take a beating with 3 ohm coils too. A 1 ohn coil draws 12Amps....too much....

A CDI ignition is different, with the current being limited by the CDI circuitry, so those types usully have 0.1 ohm resistance (nearly none).

Automotive coils from a point type system generally work. Ones from electronic ignition generally don't as they are similar to CDI ignitions, in that the current is limited somewhere in the circuitry.

Generic coils, for a motorcycle (smaller, so they fit) are in the $24 each range. Not too steep...But measure the automotive coils and if they are at least 3 ohms, you can use them. Make sure the condensers are good too, as they absorb high voltage transients and keep the points from arcing and becoming non-conductive. No conduction, no spark.

 

[Cookie]

Thanks mydlyf, I'll check that out.

Just for the sake of clarity. The tempter uses CDI (or at least a 1980's approximation of it) to power its coils. The coils are similar in design to the coils on my buddy's CL350. Suzuki's spec states 3-5 Ohm resistance. My Jaguar coils are 0.8 Ohms each so, going by electrical theory, the'll be drawing 12v / 0.8 Ohm = 15amps. That would certainly be a "no".

I pose this: could I put a 3 Ohm resistor in series with the coil to help drop my current draw. 12 Volt / (0.8 Ohm +3 Ohm) or
12/3.8 = 3.15 amps, give or take...

Would this work or am I wasting my time? I'm not financially well-endowed and I'm also a cheap prick. Spending $50 bucks on new coils versus free coils plus up to 80 cents for resistors sounds really good in my head. I'll spend the money if I need to, I'd just much rather not

 

[mydlyfkryzis]

If the FSM says 3-5 ohms, then stick to it...

The coil all look alike, the resistance has nothing to do with appearance.

 

[Cookie]

alright. Thanks!

 

[harrydahl]

Hi! Be careful swapping coils around different ignition systems. A CDI system work quite differently to a points or transistor system. A CDI does not work with 12V to the coil, instead they use something like 400 V, which of course calls for a totally different type of coil.