ZX Coil Testing
Moderator: RichardW
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furyspyder
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ZX Coil Testing
Has anybody got experience of testing ignition coils with a normal multimeter? I have a ZX 2.0 16v which has four coils. Haynes manual reckons that the resistance across the primary windings should be 0.65 ohms. One of my coils has been causing a misfire, intermitantly, and I have pinned this down to the No.2 coil (by pulling each spark plug cap whilst the misfire was occuring). However, No.1, 2, and 4 coils give a reading of approx 1 ohms and No.3 coil gives a reading of 2.6 ohms. What does it all mean? I have a new coil on order. Perhaps I should do all four? !![?]
Try swap the suspect coil that makes that one cylinder a no-change with plug lead on/off. You should ONLY swap the coil - do NOT make ANY changes to the wiring [:)]
If the problem moves with the coil - you know what to do.
Your meter can not measure that accurate in the 1-10 ohm range - the contact resistance alone on the probes gives you a varying 0.2 - 0.6 ohm offset on your readings.
It's VERY important that contact points are clean metal when taking measurements with an ohm meter - rub down coil contacts and probe tips.
If you then still have that clear out-of-spec indication on one coil - it may be on it's way out.
But usually it's the secondary windings of ignition coils that fails - by temperature - may even still work (but dodgy) with an interrupted winding. These have large number of thin gauge windings - while the primary has low count heavy gauge windings.
Usually the secondary windings have resistance in the 5K-10K range - should be fairly even on all 4 coils - which are measured directly from the HT connector to the LV connector. Of course you'll measure the primary in series with this - but the primary have less than 1 ohm compared to the secondary of 5K (5000) ohm - no influence on the result.
If the problem moves with the coil - you know what to do.
Your meter can not measure that accurate in the 1-10 ohm range - the contact resistance alone on the probes gives you a varying 0.2 - 0.6 ohm offset on your readings.
It's VERY important that contact points are clean metal when taking measurements with an ohm meter - rub down coil contacts and probe tips.
If you then still have that clear out-of-spec indication on one coil - it may be on it's way out.
But usually it's the secondary windings of ignition coils that fails - by temperature - may even still work (but dodgy) with an interrupted winding. These have large number of thin gauge windings - while the primary has low count heavy gauge windings.
Usually the secondary windings have resistance in the 5K-10K range - should be fairly even on all 4 coils - which are measured directly from the HT connector to the LV connector. Of course you'll measure the primary in series with this - but the primary have less than 1 ohm compared to the secondary of 5K (5000) ohm - no influence on the result.
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furyspyder
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The only reading (and only continuity) I can get is in the order of 1 ohm (and the one coil which gives me 2.6 ohms)which is across the LT pins on the connector. The (haynes) manual does not quote a figure for the secondary windings, just the 0.65 for the primary. Is this figure a maximum or is there likely to be a tolerance on it?
The probes and connectors are all clean(it is a nearly new multimeter and quite an expensive one too. The accuracy in the measurement of resistance is +/- 1.5% in the 0-320ohm range alledgedly) and I get a very stable and repeatable 1.0/1.1 ohms across the pins of the LT connector each time (except the 2.6 ohms one). Unfortunately the misfire does not occur all of the time, so I have been unable so far to swap coils to ensure that it is ONLY the one on No.2 cylinder!! I still don't understand why the No.3 coil is the only one to give the slightly high ohm reading, whereas the No.2 coil has so far proved to be the faulty one? It will be interesting to measure the same resistance on the new unit. At about £45 each, I don't really want to have to replace all four!!
The probes and connectors are all clean(it is a nearly new multimeter and quite an expensive one too. The accuracy in the measurement of resistance is +/- 1.5% in the 0-320ohm range alledgedly) and I get a very stable and repeatable 1.0/1.1 ohms across the pins of the LT connector each time (except the 2.6 ohms one). Unfortunately the misfire does not occur all of the time, so I have been unable so far to swap coils to ensure that it is ONLY the one on No.2 cylinder!! I still don't understand why the No.3 coil is the only one to give the slightly high ohm reading, whereas the No.2 coil has so far proved to be the faulty one? It will be interesting to measure the same resistance on the new unit. At about £45 each, I don't really want to have to replace all four!!
I'm curious to know how the primary winding resistance could rise by 1.6 ohms. The winding is a simple thickish wire as Anders states and no doubt its connections are soldered or welded. If the winding itself burned then it would get thin but with your meter operating at say 9 volts this wouldn't show until the wire itself was very nearly burned through, in which case it would have probably failed altogether by now or the failure would be imminent.
Much the same would happen with the connections if they were to fail, especially as at 1 ohm the primary will carry 12 amps - a substantial current! - I appreciate inductance and all sorts of complicated magnetic effects will cause this to drop when the engine is running)
Like Anders I think your problem is likely to be in the secondary. Isn't the circuit from the plug lead to earth? With the secondary the likely problem is failed insulation and an internal short which may not show up with a low voltage meter anyway.
jeremy
Much the same would happen with the connections if they were to fail, especially as at 1 ohm the primary will carry 12 amps - a substantial current! - I appreciate inductance and all sorts of complicated magnetic effects will cause this to drop when the engine is running)
Like Anders I think your problem is likely to be in the secondary. Isn't the circuit from the plug lead to earth? With the secondary the likely problem is failed insulation and an internal short which may not show up with a low voltage meter anyway.
jeremy
The accuracy of multimeters is always given at maximum deflection (or max range with a DVM). So your +/- 1.5% is at the maximum reading for the range you are useing. And 1 ohm is a long way from 320 ohms (max reading in range).
I would take my multimeter to a breakers and see what sort of results I got on a few other cars.
I also agree withj the others - the most likely cause of your problem is on the secondary side of the coil.
//NiSk
I would take my multimeter to a breakers and see what sort of results I got on a few other cars.
I also agree withj the others - the most likely cause of your problem is on the secondary side of the coil.
//NiSk
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David Goddard
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I wouldn't worry about such small differences in the DC resistance on the primary side. In coils, it's more to do with the number of turns, the impedance (AC "resistance"), the inductive coupling to the secondary coil, the no. of turns of the secondary coil which determines the output voltage.
Anyway, if the primary and secondaries have continuity, I would always condider an insulation breakdown between the secondary and earth. If the primary and secondaries are totally isolated, it may be possible to look for a leakage on the Megohm range.
Have you tried to use a strobe light. The inductive pick-up type are very good at spotting a high voltage drop-out due to internal shorts or open circuits in coils. Should flash at a regular time. Missed flash = a problem. Look at each plug lead in turn.
Anyway, if the primary and secondaries have continuity, I would always condider an insulation breakdown between the secondary and earth. If the primary and secondaries are totally isolated, it may be possible to look for a leakage on the Megohm range.
Have you tried to use a strobe light. The inductive pick-up type are very good at spotting a high voltage drop-out due to internal shorts or open circuits in coils. Should flash at a regular time. Missed flash = a problem. Look at each plug lead in turn.
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furyspyder
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Well, the multimeter I have does actually hav an inductive pick-up attachment, for measuring RPM etc, but as my car has the coils on top of the spark plugs, I can't get access to a high voltage signal over which to clamp the pick-up, as I would if I had conventional HT leads. I can get a signal on the LT wires, but I have to turn the sensitivity right up and I think I am then picking up a signal from the other wires in the vicinity as it is all over the place!!
I will have to see what I get out of the new coil, and see if that cures the problem. In the meantime I'll just have to keep trying to induce (or catch) the misfire when it is actually occuring and take measurements then. Sods law says that it never misses when you want it to!! As I mentioned in my original post, it was only chance that I managed to catch it the first time I started investigating the problem, and found that by pulling the No.2 coil/spark plug cap the misfire did not change.
I still don't seem to be able to measure any continuity/resistance between the coil output and any other point though, although like you say (Jeremy) the breakdown in the insulation could only manifest itself at high voltages, which obviously a digital multimeter can't replicate.
I will have to see what I get out of the new coil, and see if that cures the problem. In the meantime I'll just have to keep trying to induce (or catch) the misfire when it is actually occuring and take measurements then. Sods law says that it never misses when you want it to!! As I mentioned in my original post, it was only chance that I managed to catch it the first time I started investigating the problem, and found that by pulling the No.2 coil/spark plug cap the misfire did not change.
I still don't seem to be able to measure any continuity/resistance between the coil output and any other point though, although like you say (Jeremy) the breakdown in the insulation could only manifest itself at high voltages, which obviously a digital multimeter can't replicate.
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furyspyder
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Okay, now I'm confused !!
I have just been taking some more measurements and the following summarises what I have found. Before, I couldn't get any reading of the secondary winding resistances (partly because the haynes manual didn't list anything) but I have found that by putting the +ve probe of the multimeter on the coil output, and the -ve on the middle of the three connector pins I can get a reading but not with the probes swapped. However, on the No.1 coil alone I can get a reading both ways! Why is this?
No.1 Coil No.2 Coil No.3 Coil No.4 Coil
Primary Secondary Primary Secondary Primary Secondary Primary Secondry
1 ohm 4Mohm 1 ohm 10Mohm 1 ohm 10Mohm 1 ohm 10Mohm
(24Mohm)
The above figures are approximates. The 2.6 ohm reading on the No.3 coil primaries that I mentioned before seems to have dissapeared but the 24Mohms in brackets above is the figure I get by connecting the multimeter -ve probe to the coil output as described above.
Do I need one replacement coil, are all of them suspect? I thought that perhaps it was the No.2 coil that was faulty (see original post) but now I am wondering if it is the No.1 Coil?
I know that at low voltages you can't really check these things properly, so maybe I am barking up the wrong tree !!!??
I have just been taking some more measurements and the following summarises what I have found. Before, I couldn't get any reading of the secondary winding resistances (partly because the haynes manual didn't list anything) but I have found that by putting the +ve probe of the multimeter on the coil output, and the -ve on the middle of the three connector pins I can get a reading but not with the probes swapped. However, on the No.1 coil alone I can get a reading both ways! Why is this?
No.1 Coil No.2 Coil No.3 Coil No.4 Coil
Primary Secondary Primary Secondary Primary Secondary Primary Secondry
1 ohm 4Mohm 1 ohm 10Mohm 1 ohm 10Mohm 1 ohm 10Mohm
(24Mohm)
The above figures are approximates. The 2.6 ohm reading on the No.3 coil primaries that I mentioned before seems to have dissapeared but the 24Mohms in brackets above is the figure I get by connecting the multimeter -ve probe to the coil output as described above.
Do I need one replacement coil, are all of them suspect? I thought that perhaps it was the No.2 coil that was faulty (see original post) but now I am wondering if it is the No.1 Coil?
I know that at low voltages you can't really check these things properly, so maybe I am barking up the wrong tree !!!??
To me it seems obvious that the coils have build-in frewheeling (or blocking) diodes - complicating matters on trying to do simple resistance measurements.
You MAY instead try do the diode test measurements on the coil contacts - which would give you voltage figures on the diodes conducting - if the coil winding resistance is not too high for a valid result.
You MAY instead try do the diode test measurements on the coil contacts - which would give you voltage figures on the diodes conducting - if the coil winding resistance is not too high for a valid result.
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furyspyder
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Yes, have just realised that myself. My multimeter ha a diode checker aswell which has confirmed this. The 24Mohms I recorded must have been the "leakage" past the diode in the reverse direction. Still have a lower resistance in the correct direction across the secondaries of the No.1 coil though.
Still no further in deciding which, if any (or all!!), coil is faulty after a weekend of fiddling !!
Still no further in deciding which, if any (or all!!), coil is faulty after a weekend of fiddling !!