retiredinoregon

My '98 4.6l is only showing me a dull yellow spark in my tester... not the bright blue spark I should be seeing.

What are the parts that can cause this?

Coil packs... right?

Is there another component? A capacitor I should be checking that boosts the voltage or something, or do the coil packs take the 12 volt signal and step it up all on their own?

Are they solely responsible for that nice blue spark?

jstockert

I had a weak coil pack that would slowly cause a p0303 code. I would replace the plug and be ok for about 200 miles and then p0303. It would just miss slightly, just enough to know something wasn't right. Replaced coil pack on that side and no problems since.

RCButler

Well from the looks of it you have 1 coil pack servicing 2 spark plugs, yes?

I'm finding several different types from a dual to a single to a 4 pack by MSD and Accel (https://www.summitracing.com/search/...tyle/coil-pack)

Do you have a weak spark on both plugs or just one on 1 coil?

Have you checked all the coils? ( )

If you are getting weak spark out of only 1 side of the coil then it's most likely a bad plug wire. If both are weak it's the coil.

In any case examine the wires on all coils for wear/age/damage when you replace the coil pack(s). Old wires can expedite coil failure as the coil attempts to push the same voltage through a poor conduit.

mbb

If its all coils

Low voltage on primary side
Bad ground on pcm Ignition circuit
Bad pcm

The pcm/ecu grounds and interrupts the primary side to produce the discharge in the secondary. A poor circuit on primary side will result in weak discharge.

All coils dont fail at once. No start is usually a single failure point.

A shop with ignition analyzer could check this out In blink of eye.

retiredinoregon

Okay,thanks.... so this morning,

1) i will check for 12 volts at the center terminal on both coils, to make sure I'm getting power.

2) i will check spark directly at coil towers with my HEI tester

3) if the spark is good, i will assume wires

4) If spark is not good, I will check resistance across the secondary. Info I have says it should be 6.5-11.5 kohm, so I will check across 4/7, 8/2, 1/6, and 3/5

5) If this looks "weak", I will look to the ground connections for the PCM


So, it IS possible to have a solid 12 volts to the coil from the PCM, but still have a problem, because it is not 'interrupting' the ground side properly for the signal... is that right?

This explains a lot to me... I kept wondering how the coil could have voltage, but it was just weak.... I think I know where my "Gremlin" is hiding.... and I think I'm closing in on him.

Thanks very much mbb... I'm going to run this down, take the measurements and post them all here so we can follow the logic of this diagnosis.

white89gt

How are you checking the spark? Just know that you're dealing with a waste spark system. This means you have two plugs firing simultaneously off each coil (Each coil pack only has 2 coils inside). One plug is on the compression stroke, and the other on exhaust stroke. The exhaust stroke has a weaker spark, so that may be what you are seeing.

mbb

yes, bad ground would show 12V but no current would flow and minimal discharge when circuit broken, because the change in the primary magnetic field is minimal.

Its the change in the current flowing, that results in change in magnetic field, that induces current to flow in the secondary windings. Without changing magnetic field , a transformer (coil) does nothing.



This is where scope or graphing analyzer with capacitive pickup is worth its money.

retiredinoregon

I completed my measurements of the coil packs and cleaning and tightening ground connections.

I put the fully charged battery back in and cranked it... no start, but it stumbled, and more frequently than before, but would not start.

My coil pack readings are as follows:

Driver's side (replaced 2 months ago)

Primary for tower 4/7: .6 ohms
Primary for tower 8/2: .5 ohms

Secondary tower 4/7: 12,100
Secondary tower 8/2: 12,100



Passenger Side (Original equipment)

Primary for tower 5/3: .5 ohms
Primary for tower 1/6: .6 ohms

Secondary tower 5/3: 12,600
Secondary tower 1/6: 12,500

I checked the output on the towers by sticking an old plug gapped to .050" in my spark tester, and sticking it directly in the tower in place of a plug wire. I grounded the spark plug with a jumper cable. I did it this way so I would have both the LED in my tester and the spark plug to give me indications of spark.

I did this for all towers on both coils, and the results were consistent: Weak yellow spark. In looking at the LED in my tester, I can tell they are firing consistently, regularly, but by looking at the spark plug while cranking, not nearly hot enough. It doesn't "crackle" when it jumps.

I suspect that the secondary side of both coil packs have partially failed.

The primaries look fine but their secondary is supposed to ohm between 6500 and 11,500... and these are a good deal beyond that.

Though it seems very odd to me that both of these coil packs, one 17 years old, and one 2 months old, would fail almost simultaneously in this way.

But.... I am told that given the state of my old spark plugs, and the fact they were the wrong ones, these coils were BOTH having a tremendous amount of current drawn through them via the ever-increasing gap on my crap plugs.

In other words, bad plugs, if it's pretty much all of them, CAN ruin your coil packs, and this is how they would do it... on the secondary side, while the primary side would be generally unaffected.

Now I will freely admit that I am getting all of this information by reading other parts of this forum and others, but being as I did a sizable amount of electrical work before I retired, it does make sense to me. Pull too much amperage, of any voltage, through any device, and eventually it will fail for various reasons.

Still, before I rush out and spend $63 apiece on new coil packs... I would like to hear it from you guys, lol.

So please... tell me what you think.

retiredinoregon

Well, I have the original equipment coil pack off, and the battery inside with me for the night... and it just occurred to me;

Since I read 12v coming to each coil pack in my efforts to make sure they were getting voltage, can I just take the place of the PCM for a minute, and feed this coil pack 12 volts from the battery, and intermittently ground the other side of the primary, and see if it produces a nice blue spark from the secondary in my spark tester?

This would also tell me if the coils are indeed bad... or whether I have a PCM ground problem, right?

guyina4x4

So the truck won't start?
and you do have spark?

did you recently change the plugs?
was it starting/running before you changed the plugs?