<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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