It is VERY important to understand what it is you are measuring and why.
For example, there are two ways to crank the engine..........by using the key, or by jumpering the starter relay. These two small differences create BIG differences at the coil during cranking
1....When cranking the engine "normally" using the key, the ignition switch sends FULL BATTERY voltage direct to the coil. This is the brown bypass circuit, "IGN2" which comes from the switch, to the coil+ side of the ballast. During cranking, you should have "whatever battery" voltage is, and certainly more than 10V
2....If, instead, you jumper the starter relay, the bypass contacts in the ignition switch are NOT active, and NOW you have the ..........reduced battery voltage due to the starter.........minus the additional drop through the ballast resistor to the coil. The coil will have MUCH less voltage
++++++++++++++++++++++++++++++++
SO!!!
A....Turn key to run, measure coil POS and NEG. Compare to actual battery voltage. This tells you a couple of things.......whether the coil is getting "normal" expected voltage from the battery, which varies due to the ballast, and ........also whether the coil is drawing current through the ECU. This could give you a clue as to whether there's a bad connection at a connector or whether the box is poorly grounded. Coil NEG should be quite low, down around 1 volt while getting say, 8-10V at the coil+
B.....Clip voltmeter to coil POS and crank the engine USING THE KEY. Now you are checking for the bypass circuit being active, and good connections in the harness. Here the battery should be above 10V during cranking, as well as the coil POS connection. If this is low, I would either fix this first, or work around it by........
C.....Clip an alligator clip lead to the coil + and run over to battery source. NOW you have full battery to the coil. Now see if you have spark. Don't leave this connected any longer than necessary to test.
+++++++++++++++++++++++++++++++++++++++
When testing for spark, FIRST eliminate ALL high tension wiring, including the COIL wire. Get yourself a spark tester from the parts store, and "rig" it from the coil tower to ground, where you can see it under the hood gap. I use a Lisle, less than 10 bucks
A good working system should produce blue snappy sparks at least 3/8" and more typically 1/2" long
ONCE YOU get this far, THEN worry about the coil wire, the cap and rotor, and the plug wires and plugs
+++++++++++++++++++++++++++++++++++++
Along with everything else, I wrote up a pictorial test some time ago
===============================================
Maybe this helps, and maybe not
Here's a post I made about the simplest way I know to get across testing the ECU/ ignition parts
74 duster electrical problems
You need a coil, the ECU and the distributor
Lay it out on the bench. Follow the diagram. Find the two distributor pickup terminals on the ECU. Hook them to the distributor
Hook the ECU case to battery NEG
Coil does NOT need grounded
Distributor does NOT need grounded.
Hook coil + to the power lead terminal on the ECU. Get a clip lead hooked there and let dangle. This is your battery "hot" when you are ready
Hook something from coil "case" to a probe for testing spark.
Hook up your power clip lead. Twist the distributor shaft while holding the test probe near the coil tower. The thing should make sparks
If not, unhook distributor. Take first one, then the other pickup clip leads, and "tap tap" ground them at the battery connection. Coil should make 1 spark each time you do so.
If not, try another coil. If that does not fix it replace the ECU
IF you hook it all up and it WORKS, then there is something AFU in the car harness. SUSPECT a bad ECU connector OR a bad DISTRIBUTOR connector
=============================================
This is all you need to test the basics of the ignition. You can easily test the ballast separate. A battery, the ECU, distributor and a coil, and of course some test leads
Below, the basic diagram for a 4 pin ECU
Below, the wire for testing spark. I use my 12V test light. No, LOL the spark won't blow up the bulb
Below, the ground connection. ALL you need is one wire from batt NEG to the ECU case
Below, the two distributor connections. In the car these are polarity sensitive, but for testing does not matter
Below, the coil NEG connection
Below, battery PLUS connection, one wire to this terminal of ECU and jumpered over to + side of coil
Below, all hooked up and ready to test (except for battery ground). Should produce sparks at least 3/8" and typically 1/2" long
Below, distributor "one wire" test. I have removed the other distributor wire for simplicity. Take the bare connector end or this clip lead (the yellow) and with everything hooked up, ground it repeatedly. Each grounding should result in a spark (In this photo you need to hook up the ECU ground wire, I left it off for the photo)
This new improved software sure makes it damn hard to do anything......
For example, there are two ways to crank the engine..........by using the key, or by jumpering the starter relay. These two small differences create BIG differences at the coil during cranking
1....When cranking the engine "normally" using the key, the ignition switch sends FULL BATTERY voltage direct to the coil. This is the brown bypass circuit, "IGN2" which comes from the switch, to the coil+ side of the ballast. During cranking, you should have "whatever battery" voltage is, and certainly more than 10V
2....If, instead, you jumper the starter relay, the bypass contacts in the ignition switch are NOT active, and NOW you have the ..........reduced battery voltage due to the starter.........minus the additional drop through the ballast resistor to the coil. The coil will have MUCH less voltage
++++++++++++++++++++++++++++++++
SO!!!
A....Turn key to run, measure coil POS and NEG. Compare to actual battery voltage. This tells you a couple of things.......whether the coil is getting "normal" expected voltage from the battery, which varies due to the ballast, and ........also whether the coil is drawing current through the ECU. This could give you a clue as to whether there's a bad connection at a connector or whether the box is poorly grounded. Coil NEG should be quite low, down around 1 volt while getting say, 8-10V at the coil+
B.....Clip voltmeter to coil POS and crank the engine USING THE KEY. Now you are checking for the bypass circuit being active, and good connections in the harness. Here the battery should be above 10V during cranking, as well as the coil POS connection. If this is low, I would either fix this first, or work around it by........
C.....Clip an alligator clip lead to the coil + and run over to battery source. NOW you have full battery to the coil. Now see if you have spark. Don't leave this connected any longer than necessary to test.
+++++++++++++++++++++++++++++++++++++++
When testing for spark, FIRST eliminate ALL high tension wiring, including the COIL wire. Get yourself a spark tester from the parts store, and "rig" it from the coil tower to ground, where you can see it under the hood gap. I use a Lisle, less than 10 bucks
A good working system should produce blue snappy sparks at least 3/8" and more typically 1/2" long
ONCE YOU get this far, THEN worry about the coil wire, the cap and rotor, and the plug wires and plugs
+++++++++++++++++++++++++++++++++++++
Along with everything else, I wrote up a pictorial test some time ago
===============================================
Maybe this helps, and maybe not
Here's a post I made about the simplest way I know to get across testing the ECU/ ignition parts
74 duster electrical problems
You need a coil, the ECU and the distributor
Lay it out on the bench. Follow the diagram. Find the two distributor pickup terminals on the ECU. Hook them to the distributor
Hook the ECU case to battery NEG
Coil does NOT need grounded
Distributor does NOT need grounded.
Hook coil + to the power lead terminal on the ECU. Get a clip lead hooked there and let dangle. This is your battery "hot" when you are ready
Hook something from coil "case" to a probe for testing spark.
Hook up your power clip lead. Twist the distributor shaft while holding the test probe near the coil tower. The thing should make sparks
If not, unhook distributor. Take first one, then the other pickup clip leads, and "tap tap" ground them at the battery connection. Coil should make 1 spark each time you do so.
If not, try another coil. If that does not fix it replace the ECU
IF you hook it all up and it WORKS, then there is something AFU in the car harness. SUSPECT a bad ECU connector OR a bad DISTRIBUTOR connector
=============================================
This is all you need to test the basics of the ignition. You can easily test the ballast separate. A battery, the ECU, distributor and a coil, and of course some test leads
Below, the basic diagram for a 4 pin ECU
Below, the wire for testing spark. I use my 12V test light. No, LOL the spark won't blow up the bulb
Below, the ground connection. ALL you need is one wire from batt NEG to the ECU case
Below, the two distributor connections. In the car these are polarity sensitive, but for testing does not matter
Below, the coil NEG connection
Below, battery PLUS connection, one wire to this terminal of ECU and jumpered over to + side of coil
Below, all hooked up and ready to test (except for battery ground). Should produce sparks at least 3/8" and typically 1/2" long
Below, distributor "one wire" test. I have removed the other distributor wire for simplicity. Take the bare connector end or this clip lead (the yellow) and with everything hooked up, ground it repeatedly. Each grounding should result in a spark (In this photo you need to hook up the ECU ground wire, I left it off for the photo)
This new improved software sure makes it damn hard to do anything......
