ignition coils problem?

ghostdancing

ghostdancing

True Classic
i recently upgraded my car fitting a brand new marelli SE100CX breakerless dizzy in place of the old tired s135 points unit; i did not changed the i.coil: it's the time honoured OEM 46 years old marelli canister one.
the car works well since then, but yesterday i had some misfiring when driving on an hill so i'm wondering if the OEM coil can be the issue: can be uncorrect and lead to power module failure?
i have some basic electricity knowledge (very basic tough..) so i can figure a low impedance coil can let flow too high current in the electronic mudule?
 
ghostdancing said:
i recently upgraded my car fitting a brand new marelli SE100CX breakerless dizzy in place of the old tired s135 points unit; i did not changed the i.coil: it's the time honoured OEM 46 years old marelli canister one.
the car works well since then, but yesterday i had some misfiring when driving on an hill so i'm wondering if the OEM coil can be the issue: can be uncorrect and lead to power module failure?
i have some basic electricity knowledge (very basic tough..) so i can figure a low impedance coil can let flow too high current in the electronic mudule?
Click to expand...
Some coils have a built in ballast resistor (like the early cars), and some require an external ballast resistor (like the 79-> cars with no points). If you use a coil designed for use with a ballast resistor and don't use it in the primary circuit, it will draw too much current (probably about 2X what it should). That will overheat the coil, and likely the driver circuit inside the electronic distributor.

The primary winding resistance of the 79 coil is spec'd at 1.1 to 1.7 ohms. The required ballast is spec'd at .85 to .95 ohms so you can see that if the ballast is left out, you could be running almost 2X the current. Since power = current squared x resistance, you can see that current going up and resistance going down is not a good thing for power dissipation.

I recommend you check the coil primary resistance and see if you need a ballast resistor or not. The older coils with a built in resistor are spec'd at 3 to 3.4 ohms. If your coil measures in that range, you likely don't need an external ballast. If it is in the 1.1 to 1.7 ohm range, you definitely need the external ballast resistor
 
ghostdancing said:
thanks for your inputs; the coil doesnt have any ballat resistor fitted externally..so i must assume it is built in inside (it's the classic cylinder bodied coil);

you say resistance should be 3.4O ohms..but the coil intended fot the breakerless dizzy (modern style body) has a resistance way higher (here a sample: says 3500 ohms..

https://www.ebay.it/itm/New-Genuine...ity/192437641299?_trksid=p2485497.m4902.l9144
Click to expand...
The 3500 ohm number in the link is for the secondary winding. It also states that the primary winding is 0.75 ohms. I would guess that it will require a ballast resistor. You might want to check the distributor specs to see what the minimum primary resistance it will tolerate, but my guess is that it would like to see something higher than 0.75 ohms. Adding a ballast resistor in series should be an easy solution.
 
please, explain me how to measure the coil resistance: coils have 3 terminals: positive\negative\center high volts.. which leads are the 2 wiring primary\secondary)

btw a searched the web for more distributor tech details but with no luck..

this is the shematic of the ignition
nlublh.jpg
 
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