[T3] Coil Resistance and Heat

[Jim Adney]

On 2 Jun 2017 at 4:55, Jaime Gurrola wrote:

> I ran a new SVDA Pertronix distributor with Flamethrower I yesterday 
> with the FT III coil.  The coil got ridiculously hot then the car just 
> stopped running after about 10 minutes of running.  The coil reached 
> +250 F degrees.  The rest of the engine was at normal temps. I replaced 
> the coil with the Bosch Blue coil but haven't been able to re-start it.

Put a dwell meter on the coil and get someone to run the starter while you 
watch the meter. If the dwell sticks at 100%, the Pertronix is fried. They are 
known to do this occasionally.

Coils DO get hot in operation, just from the intermittent current that runs thru 
them. This is actually worst at idle, because the combination of coil 
inductance and resistance lets the current rise to higher levels when the 
dwell time (different from dwell angle) is longer.

> I measured the Bosch Blue coil resitance between the + & - terminals and 
> got 4.5.  I measured the the resitance in my Pertronix Flamethorwer III 
> coil and it was 1.4 ohms and the box it came in says  .32 ohms.

Lower resistance lets the coil and ignition run to higher RPMs, but it also 
draws more current, meaning more stress on whatever is controlling the 
current (points or Pertronix.)

Most high performance ignition "upgrades" don't really offer any advantages 
to our VWs, because the standard ignition runs perfectly well past our 
engine's red line. The downside is that you get reduced reliability from all the 
aftermarket parts.

> Have I damaged either the coil or the ignition module or both? Why did 
> the coil get so hot?  How does a change in resistance affect ignition 
> system?

I'm not a fan of most high performance ignition "upgrades" because most of 
them offer us no advantage, along with reduced reliability. The one upgrade 
I DO like are the Delta Mark 10 series of CDI (Capacitive Discharge Ignition) 
units. These won't give any performance improvement over a well working 
standard ignition, but they do make the plugs and points last a lot longer. 
They will work with normal points or with the Pertronix. The Pertronix should 
also be more reliable because of the reduced stress on it.

The Pertronix DOES fix the need to do occasional dwell/timing checks and 
one problem it will fix is a worn distributor cam, that results from long term 
running with a grit filled rubbing block, which wears out new rubbing blocks 
quickly. I polish those cams when I rebuild a distributor, but sometimes the 
wear groove is just too deep.


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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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