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How to test the igniter (ICM) off-the-car
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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!

Off-car test schematic
Logic table for this graphic:
Terminal 4 Bulb
V+ 0
V0 1
Vfloat 0

Terminal 4 Bulb
V+ 0
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.

Cabling setup for test

The test cabling setup, using an 8-watt bulb.

Igniter fail hard "on" - bulb on permanently
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