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Mon Sep 25, 2017 8:56 pm

I'd been wondering if my original black Bosch coil on my 71 had become marginal and had been meaning to measure the coil.

Yesterday, I accidently left my ignition on for a couple of hours and realized it only when I went to measure the coil and found it hot. Initially, I tried to cool it down, but then decided to take some measurements at various temperatures. This is what I found:

@ 67C/153F - 5.3ohms/11/5Kohms (primary/secondary)
@38C/100F - 4.3ohms/9.5Kohms
@25C/77F - 4.1ohms/8.5Kohms

I did some searching last night and found nothing on whether there should be any or this much change in resistance when hot. Is my ambient temp reading ok? or is that 2 high on primary? And, are the higher temp readings ok?

I found this Bosch chart on an another forum - showing the differences between the E/K/KW coils. The VW stock coils are of the E variety (stamped E12 under the coil). I've read that there is a lot of variation in the newer Blue coils. Are these new coils also stamped K12? Or can I distinguish a good blue coil by looking for the K12 marking? The red coils are marked KW12, but Bosch also made some non-resistor Black KW coils (BMW 2002s had them).

Byas

Mon Sep 25, 2017 9:20 pm

The Temperature Coefficient of Resistance for copper is +0.393%/°C.

Mon Sep 25, 2017 11:38 pm

Thanks! So a 4ohm @ 25C should become ~ 4.6ohm at 65C. I'm measuring nearly twice the increase at 5.2ohm. But, the 65C I measured was at the surface of the coil canister. Perhaps, the actual coil temp is itself much higher.

Maybe, the coil connectors would have been better places to measure temp instead of the surface?

Thanks!
Byas

Tue Sep 26, 2017 10:27 am

The actual wire temp of the primary windings would have been much higher than the outer case temps. The canister type coils are either oil or epoxy filled to act as a coolant for the windings.

When you leave your ignition switch ON for more than a few minutes you risk the coil overheating, which yours apparently did. It all depends on whether the engine stopped with the point contacts OPEN or CLOSED. There is about a 50/50 chance of either (45deg dwell on a 4-cyl engine means the points are closed for 45 degrees and open for 45 degrees for each cylinder). If the points are CLOSED and your ignition is ON current will flow into the #15 (+) terminal; thru the ignition coil; out the #1 (-) terminal and thru the points to ground. The primary coil windings are just a wire with a 3-4ohm resistor. A 3.0ohm resistance in a 12.6v circuit means 4.2A will flow thru the coil. This amount of current flow creates heat in the coil and in the points over time, resulting in premature failure.

Did this damage your coil? Maybe.
Will it have an impact on the lifespan of your coil? Probably.
Can you still use your coil? The resistance tests above indicate your coil is still within spec. As long as it is still able to create a spark that will fire your spark plugs you can continue to use it. Test the spark at your plugs and confirm it is a strong blue-white spark and not a weak orange-red spark. Make sure there is 12.0v or higher reaching the ignition coil.