Question about the coils how to test?

[JAX S50]

So the service manual says on page 6e3-c4-14 that in step 3 to check the resistance of the coils both primary and secondary windings, so where exactly would i measure across?

[JAX S50]

I think I just found what I needed

[Hammer]

Post up so we who don't know can add to our knowledge.
Thanks

[JAX S50]

John, it looks like you just take your ohm meter and place the leads across the two secondary posts on the coil (the one the spark plug wire goes on) and then you need to take a reading across the primary so make something to stick under the coil where the primary slots are, I just found some thin strips of copper and cut them to fit. hope this helps. Here are my readings any thoughts ? they still look good to me
PRIMARY SECONDARY
1 & 6 01.9 5.76
3 & 2 01.3 5.83
4 & 7 01.0 5.87
5 & 8 00.8 5.80

[flyin ryan]

Can test the 'Low' side as well as the 'High' side. If one 'Side' is bad don't matter...coil is dunn, get it 'DUNN'...spent to long in the sun today .

[Hammer]

Originally Posted by flyin ryan

Can test the 'Low' side as well as the 'High' side. If one 'Side' is bad don't matter...coil is dunn, get it 'DUNN'...spent to long in the sun today .

That's no ****.

[ZR1Vette]

I read the numbers but what is spec?

[Paul Workman]

For those having worked with high voltage, this is common knowledge. But for those not familiar w/ high voltage, what we're actually doing when checking coils is testing to see if the insulation has broken down between windings, or breaks down under "normal" operating conditions. We're checking to see if secondary winding insulation is breaking down, resulting in individual windings shorting out and lowering the resistance reading, checking to see if primary windings are shorting out, or in some cases checking to see if the windings are grounding out to the coil's ferrous core. (The coils in the LT5 (according to the FSM) are stand-alone devices where the primary and secondaries are isolated (read: windings are not physically connected see below).



Ordinary ohm meters will detect insulation that has broken down only to the extent the conductor of the coils or windings are in physical contact with one another, but they do not produce enough voltage to test insulation breaking down under load conditions, i.e., a standard ohmmeter may indicate a "normal" reading when in fact pinholes in the insulation allow arching between the windings that go undetected by the ordinary VOM.

Not that they are relatively available, we really need a megger; a special high-voltage ohmmeter designed to test insulation breakdown to evaluate the coil's health. Without such a test instrument, it is possible to get "normal" readings and yet the coil is "kaput"!

OK, so a megger is not something we all have in our tool box. My point being this: A good reading taken with a VOM (DVM, if you must), my not be a true indication of the coil's health. However, a relative simple technique can shed more light on the situation:

After the engine is good an warm, have a helper shut it off and be ready to start it up again. Now, pull a plug wire and insert a good spark plug and find a GOOD ground point (like the cam cover bolts) and have that helper fire the engine up while you observe the relative spark intensity*. Repeat for each of the 4 plugs on one side of the engine**.

*(NOTE: Per Marc H, it is a very bad idea to pull a plug wire off a plug while the engine is running. Extremely high voltages result from the open end; enough to punch through a damaged plug wire. And, if that damage is close to the low voltage wire harness adjacent to the coils, some bad juju could result to the ECM, should an ECM wire be hit with a few thousand volts!)

**

(Note: the plugs on the test side are fouling as the engine runs - fuel, but no spark applied - so that if you were to test again from the opposite side, your results could be affected by plug fouling resulting from the first 4 tests. Plug condition (and more to the point) fouling can play a role in the subjective analysis during this test...Nothing is totally simple, huh?)

Some small differences in spark are common. But, if the relative spark on one or perhaps more is substantially weaker, it could be the plug on the opposite side of the coil is bad, or a bad plug wire - either on your side or on that of the opposite plug.

If changing the opposite plug (from the respective coil) results in no improvement in spark intensity, then it narrows down to either the coil or the plug wires - one or the other in most cases - or connections between the coil and the ECM, or the ECM itself, in rare cases.

Considering the hassle in pulling the plenum to get at the coils and wires, many opt to replace the under plenum hardware at that point and be done with it. But, if your luck is like mine often is lately, a new part can be just as likely bad out of the box as not (these days). Point being, it is always better to troubleshoot and isolate the bad component and replace it than it is to "shotgun" the problem - replace parts until the problem goes away, IMO.

Now, all that said, looking at your VOM results makes me think your 5/8 coil is for sure suspect, relative to the 1.9 ohms of the 1/6 coil. I would make sure your probes had a good connection and re-measure ... with the coil HOT if possible! But, definitely a reading that is less than 50% of the the highest reading sample (5-8 coil), screams BAD to me! Not so much at all on the secondary sides. They all seem to group relatively equal.

Hope you find something useful here.

P.

[9T1 Red ZR1]

Hello,
Page 6E3-C4-13, chart shows primary resistance to be 0.35-1.45 ohms,
secondary resistance to be 5000-6500 ohms.

Thanks.

[9T1 Red ZR1]

Sorry, didn't realize this was so old!!