Help with testing a coil w/a multi-meter
 

(1987 600F Hurricane w/24,000 mi)

Symptoms are runs, suddenly dies. runs, suddenly dies. All the fuel and compression issues are fine and my previous thread pointed me to shoot the wiring system. Starting at the battery, I'm good through the primary side of the coils where I read DCV in and out and 3.1 ohms (within spec) on the low/primary side of the coil.

My question is in measuring the high/secondary side of the coil. I can measure no resistance what so ever across the spark plug towers. Measures like it is open which would indicate its broke and of course will not have an output when triggered.

Now measuring ohms resistance isn't hard IF you are making good contact and I don't know if I am. I rum my thin probes deep into the plug wire sockets, wiggle them around, do everything I can and STILL don't get any reading at all. OK, if my coil is really broken/shorted, I'll just get some new ones but what if I'm just not measuring correctly?

Is there anything down inside the socket that is hard to contact with a probe? The plug wire has to connect to something? I'm guessing a metal spike that goes into the center of the plug wire and it can't be THAT hard to contact with a probe. Hell, my coil test so far sure indicates a shorted HT side of the coil so replacement would normally be stupid obvious... if I am actually reading the high/secondary side contacts. Is the coil bad or is there a trick to the coils' high side terminals? :icon_gunhead:

My experience is coils fail with age and start intermittanly. An '87 with 24k on it with an ever increasing failure rate sure smells like a coil to me. Anyone agree or have any other ideas?

check the starter/ kill switch, inside scontact points.

Always a great place to start as this switch is prone to exposure and corrosion.

However, I have power to my coils. The circuit runs as:
Batt>
Ign Switch>
Off/Run Switch>
Spark Unit>
Coils>

With power to my coils and no spark, I see three possibilities:
> bad coil on the high voltage side (trying to test now)
> bad pulse/trigger to fire the coil
> bad spark unit between the trigger and coil

Anyone wanna guess on where to go from here?

Check to see if there is pulsing signal at the coil, using a 12v test light.

Check the coil trigger with a test light to see if it's the coil or something else. Test light connected to power and to the negative coil terminal should flash while cranking. If you're getting a good trigger and power at the coil while cranking it's a coil/wires/plug issue.

edit: I type too slow lol!

Slow, but accurate and usually...correct! ;)

Measuring the resistance between the (+) and (-) terminals you should get 2.5-3.2Ω

To answer your question of plug cap to plug cap, 21-27Ω

There are resisters in the plug caps. Remove the wires from the coil and you should have 11-17Ω

If you don't get these readings, the link that Hueristic provided is a cheap mod to make it run like it has never run before. Trust me, I made that mod myself.

Also, join us in the hurricane section. Problem solving in that section may be easier for you.

Thanks Tim & Tahoe!

I like the stick coil mod but that may come after I verify what I have.

OK, to test the pulse/trigger, it looks like I am using a simple 12v test light. So far, my coils have something like 12v in and out and measure 3.1ohms across the low side windings. I like that.

As I understand coil firings, the low side saturates it's coil (coil rise time) when the power supply is interrupted, the stored power in the coil jumps/shorts to the high side (step up to high voltage) windings and seeks the natural ground at the plug gap.

If so, my pulse/trigger will be breaking/interupting my 12v supply to my coil so that the fields may collape. If so, my 12v buld test light probe could go on the bare spade on neg side of coil with the alligator clip on the neg batt term.

The light will indicate the 12v supply and when cranking, the light should flash as the trigger interupts the supply from the spark unit.

That it... or do I have my head up my ass. :p

Great place and I will migrate to the 600F forum for more model specific questions later.

I can't say how much I appreciate all your help. Sometimes my garage gets lonely out there by myself.

45 minutes later:

Test light clipped to the neg batt term and the test light tip on the left #1/#3 coil's bare neg terminal. Light on with ignition on. So far so good.

Upon cranking, the light appears to stay on with no noticable real on/off flashing. Some dimming from the cranking but no 'flash'.

Pulse unit then?


Now 3 hrs later, after doing points, plugs and timing on a '70 BMW 75/5, I tested the right side #2/4 coil and get the same thing, solid light with no flash or even a flicker.

Can both go bad... or would that be more of an indication of a faulty spark box? :sad: