Newbiie - How to test resistance on a Bosch Ignition Coil

[Malik]

Hello,

Does anyone have a "lamens" explanation handy for a newbie to electric
and VW's altogether? We suspect the ignition coil but not sure how to
test the resistance.

TIA

[Speedy Jim]

For 12V coils, the primary resistance between the push-on terminals
should be roughly 3-5 ohms.
The secondary resistance from one push-on terminal to the
hi-tension terminal is about 10,000 ohms.

Better is to test the coil under actual load.
Disconnect the distributor wire.
Turn the key On or connect + 12V to the + terminal.
Momentarily ground the other terminal repeatedly.
If you hold the hi-tension lead near ground, you should
see a small spark.
The spark won't be nearly as big as it would be with the
condensor connected, but this will at least tell you
if the coil is working.

If that checks, connect the distributor up again.
Connect your voltmeter from ground to the (-) terminal
(where the distributor wire goes).
With key On, turn the engine pulley.
The voltage should go from zero to + 12V twice
each revolution.
If it stays at zero volts, the points or condensor are shorted.
If it stays at 12V, the points are not closing or there
may be a problem with the points wire or even the grounding
pigtail on the advance plate.

Speedy Jim
http://www.nls.net/mp/volks/

[Mike Fritz]

Lamens? I don't know, I'll tell you like it is, and maybe you can figure
it out. It would be a good idea to pick up a few books, or take a course
or two in basic electronic theory.

Amps * Volts = Watts
Amps * Ohms = Volts

The coil has a certain amount of resistance. Why? To reduce current. If
the coil didn't have resistance then when the points close, you will have
12 volts, / 0 Ohms, which is not possible. This would cause infinite
amount of current, up to the batteries limit to go through the coil, and
points. This would quickly burn up your points. To avoid this, the coil
has a built in ballast resistance.

I don't remember the resistance, maybe it's 1,000 ohms? Let's assume
1kohm. 12 volts / 1000 ohm = 0.012 amps, a lot less. This lowers the
effenciency of the coil, but it's a lot less wear on your points.

No I haven't taken into account the charge time of the coil, discharge
time, and the resistance natural to coils as inductors when current is
first applied, but that could/would get complicated.
Mike

Malik wrote:

> Hello,
>
> Does anyone have a "lamens" explanation handy for a newbie to electric
> and VW's altogether? We suspect the ignition coil but not sure how to
> test the resistance.
>
> TIA

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