Ignition coil tests in manual M:01:03

[Andy_]

Im following the workshop manuals tests (m:01:03) and im not getting the expected results.
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Image quality is poor so copied text here;
CHECKING THE TRANSISTORIZED IGNITION SYSTEM
Checking the Ignition Coil:
L.T circuit connect ohm meter between Battery + terminal and distributor terminals on the ignition coil, meter should indicate approximately 1 ohm.
H.T. circuit connect the ohm meter between the Battery + terminal and H.T. output socket on the ignition coil, meter should indicate approximately 7000 ohm.
WARNING: THESE RESISTANCES ARE HIGH


I have done the above with ignition off, on, and while cranking and am not getting any ohm readings. Ive got 12.4v at the white ballast feed, but cant even get a reading from white to jump start post.
What am i doing wrong ?
Thanks for any help

[DMC-Ron]

Andy,
May just be a misunderstanding. The B+ post being referred to in the manual is the B+ post on the coil. This is the post on the coil that receives the battery power coming from the resistors (on my car it is the post on the right as you face the coil from the rear). The easiest way to check the ohm readings of the coil is to first disconnect all the wires. Then take a resistance reading with one probe on the right connector and one probe on the left. You are getting a reading through the primary coil (L.T. circuit):
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Manual indicates proper reading between 0.95 and 1.4 ohm:
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Next, connect your meter between the high voltage output (top connector that goes to the distributor) and either of the other posts:
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Reading should be 5.5 to 8.5 kohms (5500 to 8500).

As you can see, I'm going through the same issue and it looks like my coil's primary resistance is too low (almost a direct short). I'm wondering if I can pick up a coil at my local auto parts?

Ron

[Bitsyncmaster]

Andy,
May just be a misunderstanding. The B+ post being referred to in the manual is the B+ post on the coil. This is the post on the coil that receives the battery power coming from the resistors (on my car it is the post on the right as you face the coil from the rear). The easiest way to check the ohm readings of the coil is to first disconnect all the wires. Then take a resistance reading with one probe on the right connector and one probe on the left. You are getting a reading through the primary coil (L.T. circuit):
71626
Manual indicates proper reading between 0.95 and 1.4 ohm:
71627

Ron

I tested the stock coil primary resistance to be 0.4 ohms. I use a bench power supply driving 1.00 amps of current and measure the voltage drop to get accurate low resistance measurements. If your just using an ohm meter, short your test leads and note the reading. Then you subtract that value from the value you get when you measure the coil resistance.

[DMC-Ron]

I tested the stock coil primary resistance to be 0.4 ohms. I use a bench power supply driving 1.00 amps of current and measure the voltage drop to get accurate low resistance measurements. If your just using an ohm meter, short your test leads and note the reading. Then you subtract that value from the value you get when you measure the coil resistance.

Dave,
That's encouraging since my coil is reading only 0.2 lower. However, your 0.4 reading is a lot lower than the low end tolerance given in the workshop manual. On my car I'm getting 0.2 ohm on the primary and 8.6 kohm on the secondary. I have a relative button on the Fluke that allows me to zero out the probe leads. The low resistance on the primary has me concerned. I am wondering if my testing messed up my coil. I disconnected the wire going to the ICU (coil [-] terminal) and connected an open jumper wire to it. I connected a test spark plug to the upper terminal high voltage connector and grounded the plug. I turned on the ignition switch. I touched the jumper wire momentarily to ground; looking for a plug spark when ground was opened. At first it seemed I got a small spark. Then no spark. I would like to recheck my test, but I am worried that I maybe I should not put full ground potential on the negative side of the coil? This test is static. The engine is not cranking, Both ballast resistors are in the circuit. It is a simple coil test I have used on my other cars to verify a working coil.

Would you know a model number for a substitute coil with a primary resistance of ~1 ohm that would be available at a auto parts store?

Ron

[Bitsyncmaster]

Dave,
That's encouraging since my coil is reading only 0.2 lower. However, your 0.4 reading is a lot lower than the low end tolerance given in the workshop manual. On my car I'm getting 0.2 ohm on the primary and 8.6 kohm on the secondary. I have a relative button on the Fluke that allows me to zero out the probe leads. The low resistance on the primary has me concerned. I am wondering if my testing messed up my coil. I disconnected the wire going to the ICU (coil [-] terminal) and connected an open jumper wire to it. I connected a test spark plug to the upper terminal high voltage connector and grounded the plug. I turned on the ignition switch. I touched the jumper wire momentarily to ground; looking for a plug spark when ground was opened. At first it seemed I got a small spark. Then no spark. I would like to recheck my test, but I am worried that I maybe I should not put full ground potential on the negative side of the coil? This test is static. The engine is not cranking, Both ballast resistors are in the circuit. It is a simple coil test I have used on my other cars to verify a working coil.

Would you know a model number for a substitute coil with a primary resistance of ~1 ohm that would be available at a auto parts store?

Ron

I will do another test on the stock coil I have. Maybe it was a little lower, I posted that 0.4 ohms from memory. I don't think you could have damaged anything in the coil. It's just copper wire wrapped around an iron core and submerged in oil.

[Bitsyncmaster]

I measured the OEM primary coil at 0.3951 ohms.

I bet your ohm meter may not be able to give you accurate low resistance readings. It's really hard to get consistent readings below one ohm. Your leads and the contact resistance make the reading change.

Now after seeing the manual state "caution, high resistance", do you really think the author knew what he was doing?

The Bosch part number on the bottom of the coil is 0 221 122 001

[David T]

You cannot do accurate resistance tests on an induction coil. It is only good for checking for shorts to ground or opens. The proper way to test a coil is with an impedance meter or by substitution with a known, good one. The OEM coil is very uncommon to fail but if you do NOT have an OEM coil it is a VERY common failure point. Over the years many owners have tried to 'improve" the Delorean's performance by getting a "performance" ignition coil only to have it fail soon after. Common failure points in the ignition system include the connections on the ballast resistor and the pick-up coil in the distributor. Verify spark at a spark plug or at least the coil wire before considering a failure in the ignition system. If no spark, the next check is +12 Volts at the ignition coil.

[Bitsyncmaster]

You cannot do accurate resistance tests on an induction coil.

Your wrong. Testing resistance with DC voltage does give you accurate resistance. Your ohm meter uses DC voltage also.

[DMC-Ron]

I measured the OEM primary coil at 0.3951 ohms.

I bet your ohm meter may not be able to give you accurate low resistance readings. It's really hard to get consistent readings below one ohm. Your leads and the contact resistance make the reading change.


Dave, thanks for the additional resistance reading. It is helpful and has convinced me to do more mockup diagnosis with a test spark plug. I have great confidence in my Fluke 787 process meter, but you are absolutely correct that resistance readings this low are very tricky.


Now after seeing the manual state "caution, high resistance", do you really think the author knew what he was doing?
Well, I took this as some "inside humor" interjected by the writer just because it'll be funny to those that catch it. But it could be a less than knowledgeable technical editor that knew enough about coils to understand they can be dangerous, saw the 7000 value and added the warning without understanding. I prefer to go with the inside humor option.

Ron

[DMC-Ron]

You cannot do accurate resistance tests on an induction coil. It is only good for checking for shorts to ground or opens. The proper way to test a coil is with an impedance meter

David,
I am with Dave on this one. This is not an AC coil. I am skeptical the inductance meter would work because there is no AC frequency that could be set for the test. The primary coil is pulsed DC. Perhaps adding bridge rectification to simulate the pulsed DC would give you some sort of result on the primary coil, but unlikely it would tell you anything about the secondary. If there is a test set up that provides useful results, I'm all for learning something new and understanding how it works. And I do appreciate your helpful comments on the forum.

Ron

[Bitsyncmaster]

David,
I am with Dave on this one. This is not an AC coil. I am skeptical the inductance meter would work because there is no AC frequency that could be set for the test. The primary coil is pulsed DC. Perhaps adding bridge rectification to simulate the pulsed DC would give you some sort of result on the primary coil, but unlikely it would tell you anything about the secondary. If there is a test set up that provides useful results, I'm all for learning something new and understanding how it works. And I do appreciate your helpful comments on the forum.

Ron

Resistance is the basic DC value that impedes current flow. When you get into alternating current then you have a value of impedance which is usually measured with a sine wave at a specific frequency. They are the same when there is no inductance (but technically everything has some inductance). We are not looking for impedance values we are looking for resistance values as stated in the manual.

[David T]

Resistance is the basic DC value that impedes current flow. When you get into alternating current then you have a value of impedance which is usually measured with a sine wave at a specific frequency. They are the same when there is no inductance (but technically everything has some inductance). We are not looking for impedance values we are looking for resistance values as stated in the manual.

Pulsed DC is really just another way of saying AC. We are pulsing DC into the coil. For testing purposes, an impedance test would be a more accurate test. IMHO the main reason there is no spec for impedance is that an impedance tester is expensive and it is just simpler to try a known, good coil. In any case, if you are getting + 12 volts DC to the B+ side of the coil and the other side is intermittently grounding, if you don't get a spark (a nice strong, blue one) then you can figure the coil is bad. Using an ohmmeter cannot tell you if you have shorted windings. That test is not sensitive enough unless you have a LOT of shorted windings. Think of it this way, how does an ignition coil (a transformer) work in a DC circuit. It can't. That is why you are pulsing the DC, to create an alternating field (AC) so you can transform the voltage from 12 volts to 5,000 inside the ignition coil. It is in effect, a mini Tesla coil. Look up Kettering, he was the one way back who was clever enough to design this. What we have in the Delorean is just a solid-state version of points and condensor.

[Bitsyncmaster]

Pulsed DC is really just another way of saying AC. We are pulsing DC into the coil. For testing purposes, an impedance test would be a more accurate test. IMHO the main reason there is no spec for impedance is that an impedance tester is expensive and it is just simpler to try a known, good coil. In any case, if you are getting + 12 volts DC to the B+ side of the coil and the other side is intermittently grounding, if you don't get a spark (a nice strong, blue one) then you can figure the coil is bad. Using an ohmmeter cannot tell you if you have shorted windings. That test is not sensitive enough unless you have a LOT of shorted windings. Think of it this way, how does an ignition coil (a transformer) work in a DC circuit. It can't. That is why you are pulsing the DC, to create an alternating field (AC) so you can transform the voltage from 12 volts to 5,000 inside the ignition coil. It is in effect, a mini Tesla coil. Look up Kettering, he was the one way back who was clever enough to design this. What we have in the Delorean is just a solid-state version of points and condensor.

I don't agree that an impedance test is more accurate than a resistance test for finding shorted windings. They would both show the same percentage of change. The primary resistance is a critical value for controlling the current flow of the ignition system. The coil current is mostly DC current when the ignition ECU is grounding the coil to saturate the iron core with magnetism. (at idle that is about 90% of the time). You don't care what the current is when the ECU opens the circuit.

[Bitsyncmaster]

I don't agree that an impedance test is more accurate than a resistance test for finding shorted windings. They would both show the same percentage of change. The primary resistance is a critical value for controlling the current flow of the ignition system. The coil current is mostly DC current when the ignition ECU is grounding the coil to saturate the iron core with magnetism. (at idle that is about 90% of the time). You don't care what the current is when the ECU opens the circuit.

At idle with a modern ECU, the on current has a shorter time % (known as dwell) at low RPMs. The stock ECU has very little dwell adjustment and therefor wastes a lot of current at lower RPMs.