can a coil have a short and still work?

With an aftermarket ignition system, you'll probably have to consider the entire package.

On a standard breaker point ignition, you'd be running plain voltage (usually through a ballast resistor) to the primary coil circuit, and interrupting the ground path, to force the magnetic field to collapse on the secondary windings, creating the voltage spike to fire the plugs.

On a CDI system, though, (especially an early design) the coil wiring is often more complex, involving dumping capacitor energy into the coil.

The Crane XR700 has been around roughly forever --- how old is the one in your machine?

For a capacitive discharge system, I would expect the capacitors to start going bad by the time it's 20 years old.

 

you can disconnect the coil and test it easily enough, use a multi-meter and check the resistance (ohm), depends on the rating of that coil, but typically is 10K to 11K, any more or less then it's time to change it

 

This sounds quasi-normal to me -- i.e., if the optical wheel in the dist is blocking the light path, the Crane box should be trying to put current thru the coil primary winding and a rough static I= V/R for a coil is in the 10 amp ballpark. Is this a new (unexpected) behavior of the ammeter that you've noted, or has it always been this way?

 

Quick test, with the ignition switched on pull the molex connector from the Crane XR700/3000 ignition to the distributor pickup wire, your ammeter should now go back to normal.

• If it does, the Crane box should be ok but your coil might have several shorts in the primary winding, measure the resistance between the two screw terminals (remove one of the wires first), you should see a value somewhere around 1.6-3 Ohm.

• If the ammeter does still indicate a 10 ampere load, the Crane/Allison box might be defective. What happens if you remove the red wire coming from the Crane/Allison box, do you still see the 10 ampere load?

 

Don't forget to do the simplest test of all with car ignition problems. Have a look at what's happening under the hood when it's dark. Get someone to start the car too and see of there are sparks anywhere there shouldn't be.

 

The resistance goes down, the primary winding does not touch the case but the insulation between the primary windings itself breaks down.

It's quite possible to retrieve the wire with the molex connector from above without having to remove the wheel and liner, give it a try.

Good luck,
Adrian

 

Whether your ignition coil or a transformer in your washing machine, a common failure mode is insulation breakdown causing adjacent turns shorting yet it functions adequately. A shorted turn causes heat, then more insulation breakdown, then another shorted turn, more heat, more shorted turns until the smoke escapes. A resistance measurement is meaningless now when looking for a 20% change as you don't know what resistance it started at.

The other obvious possibility is the amplifier which simply supplies a closed position simulating breaker points but has advantages over them. They are supposed to present an open with engine not running not all early ones did. My bet is here.

The XR700 uses a standard (non internal ballast resistor) coil, which are about $20. Why not start minor money throwing and remove any doubt about the coil? I pulled the 246 wiring diagram and find a spare ignition coil marked 'emergency'. Worse case you will be back to original with a spare coil. But it will end your doubts.

 

That is interesting, good chance that the box is shot. While you did that test the coil was still connected to the red +12V wire from the car wiring?
Background, the red wire to the Crane box only supplies the electronics, which switches ground to the coil, e.g. there should be virtually no load (<0.5 A) issued by the box itself.

That sounds normal for an inductive coil, although the Crane XR700 usually wants to see a ballast resistor in front of the coil. The coil current (~7 ampere) with your current setup might have been a bit too much for the ignition box.

Looks like it, i don't think that the coil is responsible for the issues you see. Would be great if you could have a try at the Molex removal test, though.
BTW, could you check if your XR600 box is potted (filled with epoxy)? If not, it should be quite straightforward to fix it.

 

As I wrote above, I think the XR700 is bad but recommended buying another coil as a cheap way to sort things out.

The function of an amplifier is to provide a short to ground for the coil when, and only when, the distributor rotates to that precise magic point dictated by points or sensor. Having a short at all times (10A) is unusual but you claim the engine runs, so it's hard to beat reality. Somehow the full short might somehow clear and return to the normal few milliseconds short to achieve spark. The coil must then be returned to open-no current condition to ready itself for the next spark.

Ballast resistor - Ignition coils are wound for a specific input voltage. Early cars simply applied the full battery voltage to coils designed to run at battery voltage. The ballast resistor was an innovation around the 60's. It's purpose is to help engines start easier by providing a high spark plug voltage during cranking then dropping back to an adequate voltage for normal running.

The 246 doesn't have provision to switch the ballast resistor out during start, therefore requires a coil designed for no ballast resistor, i.e. the full nominal 12v of the battery. And your original resistance of 3.1 ohms sounds right for a 12v coil. I was in O'Relieys yesterday for wiper blades and they said such a coil is about $20.

Yes, when it's working properly regardless of manufacturer or type.
True, the only ground for the coil is through the boxes output transistor.
Yes

I read otherwise. The box will ultimately fail if the coil requires a ballast resistor and it's not used...but not all coils require a ballast resistor, again, depends on it's design.

 

Ignore the coil for a moment, the question is why there is ground switched through the coil when the engine is not running. Either you always park the Dino with the rotor blocking the optical switch (not very likely) or there might be a defect with the optical switch/trigger input stage of the XR700 leading to such a symptom. Your last test proved that the output stage of the XR700 is ok, so we need to focus on the input stage/optical trigger.

With the ignition switched on and the ammeter showing -10 ampere, try to either temporarily remove the molex connector going through the distributor pickup, or put the Dino in fourth gear and push it an inch or two so the trigger wheel in the distributor passes the optical switch.
Check if the ammeter still shows -10 ampere or goes (temporarily) back to normal.
With the molex connector pulled, there should be no current flowing through the coil at all.

 

Um, I'm not all that familiar with the Crane*, but in a normal breaker points system, the connection to ground *opens* when you want the coil to fire.

The ignition voltage creates a current through the primary windings, establishing a magnetic field on the core. The current through a coil resists changing instantly. So when you open the breaker points, interrupting the current, the magnetic field then creates a very large voltage spike on the secondary windings, causing a spark at the plug (dumping the magnetic field energy to the plug).

(This is also why you sometimes see burnt contacts on 308 ignition key switches: the current in the starter solenoid will resist a sudden disconnect, and you can get a negative voltage spike which can arc across the key switch contacts.)

So it's when you want the coil to fire that you *open* the ground connection.
(The "condenser" in a points system is to prevent arcing across the breaker points.)
So you'd expect the current to build through the coil (primary) to a steady state value, except when the coil is firing (the dwell period).

Some early electronic ignition systems ignored the field collapse effect, and used a coil as a simple transformer, dumping a spike of energy into the primary (usually from a capacitor). But if the Crane isn't a CDI system, I'd expect that it'd be opening the ground contact to fire the coil.

(* I looked into after-market ignition systems for the Alfa, back around 1980, but my mechanic put it in perspective: you can buy many sets of points for the cost of an electronic system (back then). )

 

Fully correct In an 'inductive' electronic ignition such as Jim's Crane XR700 the transistor does the job of the points switch, as transistors can handle a much higher current (points can go up to 4-5 ampere only) and you can do nice things on top such as electronic control of dwell and maximum coil current.

BTW, the Dinoplex ignition which was factory installed in the Dino is a 'CDI' capacitive ignition.

 

No

 

Yes, absolutely! Please remove the green wire and check your ammeter again.