Ignition coil question.

[GK]

Today while driving to work my car's engine suddenly went (as though a switch was flicked) from purring like a kitten to running as rough as guts. Rang work to inform and book the day off against leave and did a U-turn for the transmission shop. It was overdue for a transmission service and they aren't open Saturdays. The transmission had nothing to do with the problem, but since I was now having a non-public holiday weekday off I figured I'd get that out of the way.

After the trans service I barely made it home though, engine's ill running progressing towards being terminal and finally stalling when pulling into my driveway. It was quite a while since I changed the ignition leads, dizzy cap and rotor  (actually overdue for renewal), so I bought what I needed while the car was in the transmission shop and renewed all of that after limping home. Engine still idled like cwap, barely starting. By this time other checks ruled out any cause other than ignition, so I swapped out the next obvious thing (the ignition coil) with another known to be good that I had handy.

There was no change, ruling out the coil, and all I am currently getting whilst cranking is a wimpy orange spark barely strong enough to bridge the gap of a spark plug.
It was then time to break out the Rigol. My engine is equipped with a Bosch HEI electronic distributor and matching HEC716 ignition coil. The potted "HEI" module (mounted withing the distributor housing) has failed on me before and it looks like it has gone again. The first time it just died outright while I was driving in the right most lane of four in freeway-like traffic, in low 40 degrees Celsius heat. Handy that; and so far the only time in the 14 years I've been driving the car that I need to call a tow truck. That was an easy diagnosis, because the coil (-) just remained at the same potential as the (+) terminal. The switching transistor inside the potted module had died open circuit.

Attached are scope traces of the potential at the (-) terminal while cranking the engine. I've got ~30mS of dwell which looks OK for the first 5mS only, followed by ~3mS of wimpy burn. Bosch rate the HEC716 coil primary current at 6A peak and the DCR at 0.41 ohms. I'm pretty sure the inductance is in the vicinity of 8 to 10 mH. 

The first 5mS of dwell looks OK. With 8-10mH of inductance it will take approx that long for the coil current to reach the specified 6A, at which point the HEI module should start current limiting. Current limiting post ~5mS is where things appear to be going pear-shaped. 6A at 0.41 ohms DCR = ~2.5V, so post 5mS the (-) terminal potential should rise to the battery voltage minus 2.5V and remain there until the end of the dwell period, and this is what happens initially, but then things begin to look seriously abnormal.

The current limit is oscillating all over the place, with a mean potential across the coil primary of approximately half the battery voltage, which computes to an average approximate primary coil current (given the 0.41 ohm DCR) approaching 15A.

So here is my question. Are these high-energy-ignition-compatible coils particularly susceptible to saturation if driven beyond their peak primary current rating? The loss of inductance is the only mechanism I can think of ATM that is causing that massive (abnormal) coil primary current to give me the woefully inadequate spark that I am getting.

[akis]

I do not understand much however I am very curious how this works

 If you have a very old system with a distributor with a rotor arm and a cap, that must be serviced, and an external coil that generates the 50KV or whatever it is, the question to my untrained eye is what is the purpose of the "electronic distributor" since there is already a mechanical one? I sort of suspect that the electronic system maybe feeds the low tension in the coil in a more refined way that the rotor arm can achieve so you can that way control the advance ? Is there a vacuum diaphragm attached to the distributor?

Edit: I forgot there must be (is there?) a set of contacts which are also serviceable and you did not mention them, as well as a cylindrical capacitor? Or maybe the electronic distributor does away with the contacts?

[GK]

question to my untrained eye is what is the purpose of the "electronic distributor" since there is already a mechanical one?

The coil primary current is not switched by mechanical "points". In place of the points is a multi-spoked reluctor wheel and pick-up. An electronic circuit switches the coil. Google the MC3334 IC datasheet.     

[Seekonk]

I had a Toyota with a bad ignition module. Without even looking the dealer told me it was $220, back when that was real money.  I dug through this nasty slime and soldered three wires to the output transistor on the thick film assembly and made a 5 transistor darlington.  It worked for at least 60K miles before I sold it.

That module used PWM to limit coil current. A shorted turn would definitely increase current.

[james_s]

What car is this? I have a similar system in my Volvo. The older one used a hall sensor in the distributor and the later one uses a crank angle sensor that reads divots in the flywheel. On the '87 I had a problem like this that turned out to be deteriorating insulation on the engine wiring harness. I cut it open and the section of harness with the main bundle of fuel injection and ignition wires was just a bundle of bare copper wires in a pile of rubber crumbs. It's remarkable that it ran as long as it did like that.

[mmagin]

Interesting.  Some electronic ignition systems seem to be much better than others.  I really liked the GM DIS system that my 1990 Chevy Corsica (inline 4) had.  It was a system that had one coil per two cylinders (waste spark) and it seemed like it never needed new spark plug wires.

Overall though I have found that drivability problems on modern cars seem to usually be a failed sensor.  My 1998 Honda Accord had a coolant temperature sensor intermittently opening up and making the engine think the coolant was at -40 C. 

[james_s]

Bad sensors and damaged wiring have been the two most common issues I've seen by far. I also had an ignition power stage fail once and have kept a spare in the glove box ever since. Other problems I've had were fuel pumps, had the main pump suddenly fail in one car when the drive pegs sheared off the motor shaft and I've had the pre-pump in the tank fail. These were 20+ year old cars with >200k on them so I can excuse such things. I've seen bad electrolytic capacitors in ECUs too, and bad caps and solder joints in Bosch relays.

[GK]

That module used PWM to limit coil current. A shorted turn would definitely increase current.

After some more testing this evening it appears that mine does as well - that waveform I posted where the dwell appears to go all whoopie after the first ~5mS is the ignition module regulating/limiting the coil current via PWM.

This evening I plugged a 0.05 ohm resistor in series with the coil primary to serve as a current sense. Despite the potential across the primary winding looking to be all over the place the current waveform is a textbook-clean linear ramp from zero to 7.2A during the first 5mS of the dwell period, which then holds steady for the remaining. I also probed and loaded with a dummy load the + voltage feed to the coil. With 10A of load the ignition power feed doesn't sag below 11V, so that rules out anything wrong there like dodgy connections.   

And despite what I reported at first, it turns out my spark isn't wimpy at all. Last night I had a grounded spark plug plugged into the coil HT via a short test lead. That lead is old and as it turns out dud. Viewing that perfectly fine coil primary current waveform this evening and knowing the coil to be good I figured the spark can't possibly be defective, so I fished out another lead for testing, but this time no plug, just arcing to the engine - bingo, 3/4" white-blue arcs no problem.

All of his still doesn't rule out the HEI module; it could be triggering erratically off the reluctor while the engine is running, but there doesn't appear to be any suspect behavior while cranking. Starting to rethink my initial conclusion  that the ignition is the fault now, not being able to find anything wrong with it. A grossly incorrect and progressively worsening mixture metering could explain the behaviour, so maybe I have a blocked jet in one of the carburetors or something.   

Bloody cars.

[tautech]

Check grounding leads and straps. Seen some funny stuff with them. 

Does it run a ballast on the coil supply Glen ?
Seen them give trouble.

On the same subject seen the coil supply from the switch as a lossy resistive lead that go too lossy and not noticed at cranking when ballasts and lossy leads are bypassed to give full battery voltage when cranking.

[drussell]

Yeah, if there is a ballast resistor in the circuit, check that...

They are usually wired so as to be bypassed during cranking then in circuit when in "run".

[Molenaar]

With my '67 Mercedes I've had the problem that the distributor cables would sometimes arc over. When I sprayed water vapor on them at night, the cables lit up like a christmas tree.

[GK]

No, no ballast resistor in series with the coil; the current limiting function of the HEI control module makes that superfluous. Power for the HEI module comes straight from the + terminal of the coil, like I said I loaded this (the ignition power feed) at the coil to 10A with a dummy load and it didn't drop below 11V.

But so far I've only 'scoped things while cranking. I can't fark around with running this thing at night after I get home in the dark so it's just going to have to wait for the weekend.

[P90]

I had an issue with my Volvo, it turned out it was a defective spark plug with no visible signs of damage. New set of plugs, and she ran like new at 200k