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How
to test the igniter (ICM) off-the-car
back to Start Problems
(NOTE:
If your car is new enough to
not have a
distributor, forget this page entirely. You can't do any of
the tests contained herein.)
A limited on-car test for igniter
input can be
found here (a 336 kB printable
PDF).
Another on-car
test was suggested to me recently (April 21, 2008).
There are
two more on-car
tests that can be performed. One with a dwell meter, and one without
(this last from Remco):
"...you could put a small low current 12V bulb (so we're not talking
about a headlight or tail light - just a Radio Shack 12V bulb) between
the +12V and the output of the igniter. (envision hooking it up
across the coil but taking the coil out of the circuit).
If the bulb flashes when cranking, your igniter is ok. If not, it will
take a little detective work to make sure that you indeed are getting a
periodic input on the igniter."
Yet
another on-car test from another contributor, this one taking current
AFTER the coil (the negative side): "Pull the two
connectors to the coil and the wire that
goes to pin 2 on the ICM. Then hook up a 12V test
light between the 12V negative coil wire and pin #2. Have someone
else try
to start the car, and see if the ICM still flashes the test
light.
You could also test the ECU by putting another test light between pin 4
and ground and see if the ECU is doing its job by switching pin 4's
voltage from 10V to zero." Warning:
If you do the ECU test just mentioned, disconnect the coil from its
power feed! Allowing the coil to charge up during the test will
probably damage it.
As a wire
identification
for Remco's test above:
The coil and igniter have two wires in common: one is 12-volt (goes to
both coil and igniter) and the igniter output (which goes between
igniter and coil but nowhere else). This last wire we'll call wire "X".
Disconnect this wire X from the coil and hook the light bulb across the
igniter's 12V wire and wire X. You basically disconnect the coil
but hook the bulb in its place.
NOTE: Never allow the coil to receive
power from any source without
grounding its high-tension lead!, or removing the ignition fuse,
which is usually a low-numbered one.
A proper, full test
requires that the igniter be removed from the
vehicle. Off-car testing is as
follows, with details and photos below:
NOTE: Not shown here is the
tachometer connection to the igniter. Do not connect anything to the
tach connector during this test!
 |
Logic
table for this graphic:
|
GOOD IGNITER
| Terminal
4 |
Bulb |
| V+ |
0 |
| V0 |
1 |
| Vfloat |
0 |
|
BAD
IGNITER
| Terminal 4 |
Bulb |
| V+ |
0 |
V0
|
1 |
| Vfloat |
1 |
|
|
An igniter
is just a
switch, just like an old-fashioned Kettering
contact point set, (only
a little more sophisticated in that it also handles dwell). Being a switch, its job is
to, um,
switch.
Switch between one state and another that is, with those states being on and off. Switching, and the effect it has on
the
ignition coil,
is
what creates the massive voltage and current spike that fires the spark
plugs. No switching, no spike.
One
particular, very
knowledgeable, contributor has made much of this page possible. In my
contributor's words:
"Basically, left floating, the bad igniter locks hard ON rather than
OFF, so the bulb is not
switched by a negative-going ECU pulse
which is what I would expect in high electrical noise
environment. Regarding our past conversation on whether failed
igniters destroy coils, if the igniter fails ON, the coil will get
pretty toasty pretty quick! I used a small 8W bulb for testing."
It appears
that if the
igniter fails hard ON, the coil will continue to be charged up as
long as the ignition key remains in the ON position, which may overheat
the insulation, damaging the coil.
It is
possible for the
igniter to fail OFF as well. The end result is the same though: No
switching, no spike, and thus no spark, except that the coil will not
be damaged.
However, being
a semiconductor, and in that regard being very much unlike the old Kettering mechanical
switch, the igniter's transistor is capable of sort of failing. Sort of failing means current never
quite cuts off. A certain amount of "bleed" occurs, continuing to
charge the coil when current is supposed to be stopped. This can -- and
does -- burn out the coil just like hard "on".
Another
knowledgeable contributor tells me this: "If you substitute a voltmeter
for the 12V bulb in your diagram you'll have a more accurate picture of
what's happening. My bad igniter shows 2 volts when pin 4 is floating
(bulb doesn't light, however) and ~11V when pin 4 is grounded (bulb
lights as normal). Brand new igniter: ~0V when pin 4 is floating and
12V when pin 4 is grounded." That 2V is the "bleed" which should not
happen.
The
images below
show my
first contributor's equipment setup, using an 8-watt bulb instead of a
multimeter. The bulb should switch on or off as it
is switched by the lead on Terminal 4. The igniter in this test had
failed ON, and will only switch off when grounded, but not when
Terminal 4 is floating. A good igniter would switch when floating.
 |
The test
cabling
setup, using an 8-watt bulb.
|
|
Setup indicating no switching
(igniter has
failed hard "on").
If igniter had failed hard "off", bulb would not light up at all.
If the igniter had sort of
failed as detailed earlier, the bulb would have lit up as it
should have had the igniter been switching, but you would not detect
the electrical "bleed" that would have exposed a bad transistor.
Substitution of a multimeter for the bulb in this setup would show that
"bleed".
|
Last update April 01/08
