<VV> Flamethrower, HEI Diagnosis
Larry Forman
Larry at forman.net
Thu Mar 24 00:58:52 EST 2005
At 09:39 PM 3/23/2005 -0600, Roger Gault wrote:
>With the key on you'll get one of two situations, depending on whether the
>points are open or closed.
>If the engine stops with them open, you'll get 12V at both terminals.
>If it stops with them closed, you'll get 12V at one and ground at the other.
>UNLESS, of course, the wire between the coil and the points is broken or not
>making contact at one end or the other.
>
>Place the distributor end of the coil output wire so that it's close to
>ground (less than 1/4") (or connect it to a spark plug and ground the plug
>housing). Do NOT hold the wire with your hand unless you like jumping
>really high and cussing. Turn the engine until thepoints are open and short
>across the points with a screwdriver, etc. (yes, I am crude) You should get
>a spark out of the coil. If not, touch the "-" terminal of the coil with a
>wire connected to ground. Touch and remove several times, simulating the
>points. You should get a spark. If you can't get a spark connecting to the
>"-" terminal, the coil's dead. If you get one there, but not from shorting
>the points, the wire to the "-" terminal is bad. If you get a spark both
>ways, but not by turning the engine then the points are somehow messed up.
>
>Hope that helps,
> Roger Gault
Roger has a good approach here. And it is simple and quick. Some
additional ideas:
1. Measure the POSITIVE terminal of the ignition coil when you short the
negative lead to ground. You will likely have a voltage drop condition
caused by the ballast wire. It will likely drop the positive terminal on
the ignition coil to between 6 and 9 volts, or so. If it is substantially
less than that, you likely have some higher resistance in the ballast
wiring circuit or connections. If it is less, do more snooping to get the
voltage up to where it needs to be. Remember that when the "points" are
open, even with a high resistance you will have 12 volts on both the
negative and positive coil terminals. That is because Ohm's law says you
will not have any voltage drop if you have no current flow. Points open
means no current flow, and thus same 12 volts.
2. If it is less voltage, you can TEMPORARILY run a 12 volt clip lead from
the battery positive to the coil positive and force the voltage to 12
volts. You should have a VERY strong spark when you short the coil
negative lead to ground and then release it. The spark is caused by the
collapsing magnetic field in the ignition coil wanting to continue the
current flow. Then the ignition voltage rises enough to cause the current
flow to continue for a short duration. Lenz's law (might be incorrect
spelling).
3. What might cause the negative terminal not being well connected to
ground is a relatively poor ground contact on earlier Pertronix
modules. The later models have an additional grounding wire to connect
from the "points plate" to the grounding screw on the distributor
body. Just run an additional grounding contact or a short length of wire
braid and some insulation to prevent shorting to the wrong things.
-- Larry
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