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Old 05-07-2016   #1
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Coil consumption

Hello!
Tried searching the forum, but couldn't really find any answers..

My dear little 500 with 126P 652 engine eats approx. 1 coil per 60 km's.
It gets really hot and stops working after a while.

I have recently replaced all ignition service parts. Checked plug gap and looked over the ground cables. It looks fine.

Do you have any idea what could be the problem?
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Old 05-07-2016   #2
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Re: Coil consumption

That sound serious and potentially dangerous, not to mention....EXPENSIVE!
Could it be related to the fact that some of these systems use a coil that is rated for 9 volts and which use a ballast resistor to moderate the coil voltage under normal running?
I'm no expert on that but I remember my 126 BIS had one in the front wheel well, of all places.
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Old 05-07-2016   #3
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Re: Coil consumption

Hi Beaver,

As above plus ...

Please bear with me.

Coils are transformers which convert a chopped 12VDC signal to around 50kV to fire the plugs. The current flowing through the primary winding creates heat which is usually quite efficiently dissipated through the coil casing without much increase in temperature.

If there is a higher current flowing for a longer time, the coil will get hot.

A higher current can result from either a short circuit in the coil primary or too low a primary resistance. The primary circuit coil resistance should be not less than 3.2 ohms. Modern coils fitted to cars with electronic ignition have primary resistances as low as 0.8 ohms. Lower voltage coils (6V or 9V as suggested above) also will have low primary resistance. Below 3.2 ohms is too low for our cars and these coils will get very hot if fitted (without an appropriate ballast resistor).

Current can flow for excessive times if the dwell angle (the time during the ignition cycle when the points are closed) is too large. This happens when the points gap is too narrow though does not usually result in an excessively hot coil, just pitted points and an engine that runs poorly.

I guess I'd start by checking the primary resistance of your coil and searching for that resistor..

Lecture over

Chris
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Last edited by Bambino; 05-07-2016 at 11:00.
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Old 05-07-2016   #4
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Re: Coil consumption

Thanks guys!

Yes, it is indeed an expensive habit.. She 'only' ate two of them so far (The first one was switched without knowing if that was tha problem) though.

I'll try fitting one with a ballast resistor next. I'll get back to you with result. Maybe using smoke signals.
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Old 05-07-2016   #5
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Re: Coil consumption

Quote Originally Posted by fiat500 View Post
That sound serious and potentially dangerous, not to mention....EXPENSIVE!
.
Not as expensive as finding out that the powder coater had left had left sand blast grit hidden behind the struts inside the rocker cover then you fit said cover to a rebuilt tuned engine with 1,200 of new parts fitted but that is another story.
I fitted the very basic twin spark ignition on the rebuilt engine so points , condenser but no rotor arm or distributor cap just the twin spark coil. What can go wrong as everything was brand new? Everything was fine driving around town and on short runs. Then went off to a car show about 15 miles away and had to queue for about an hour in very hot weather for about an hour. Just got into the site and engine died. While testing for a spark the coil burnt out and produced a massive amount of smoke. Thinking that it was because I had forgotten to tell my wife to turn off the ignition I then fitted another new coil. Everything was fine then the next long run the 2nd new coil burnt out. So tatty old coil fitted now but with a ballast resistor. Just this week I was checking out a new coil on the Axel Gerstal site and they say that it is important to fit the ballast resistor.
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Old 05-07-2016   #6
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Re: Coil consumption

I thought that the purpose of having a coil that normally operates at less than 12 volts is so that when the full voltage is applied, normally when starting the engine, the coil can take that load for a short time and produces a very strong spark.
So I presume there needs to be an additional step in the starting circuit which bypasses the ballast resistor on startup. That would imply the need for a different keyswitch for the starter. Possibly that's not the case on this problem car.
Running with a sub-12 volt coil without having this clever advantage would seem pointless to me, although I believe that these coils tend to run cooler in normal circumstances.
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Old 05-07-2016   #7
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Re: Coil consumption

The ballast resistor is normally bypassed when the starter is engaged. This ensures full spark even though the battery voltage drops under the load of the starter. On later cars this is part of the ignition (key) switch. If you have a separate starter switch or mechanical starter control you have to add a diode or relay to provide the bypass. Simplest to understand is a relay with the normally open contacts connected across the ballast resistor and the coil to the starter motor. A diode would go between the starter (anode) and the coil side of the ballast resistor (cathode) You would need a 3A 100V or higher rated diode. Don't forget to put a fuse in any wire connected to the high current terminal of the starter.


HTH,
Robert G8RPI.
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Old 05-07-2016   #8
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Re: Coil consumption

Quote Originally Posted by g8rpi View Post
This ensures full spark even though the battery voltage drops under the load of the starter.
Thanks for filling in the gaps in my half-understanding of this.
I bet it's the lack of a switch that's the problem since the car has been converted to run on a later engine.
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Old 05-07-2016   #9
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Re: Coil consumption

This site holds a great deal of information about coils, points, ballast resistors etc. Also sells the pointless ignition system.
http://www.accuspark.co.uk/index.html
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Old 05-07-2016   #10
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Re: Coil consumption

I have fitted an 'Accuspark' ignition system, along with their coil recommendation (red coil). It has performed perfectly. Before I fitted the system, I statically set the timing to 10 degrees BTDC (as per the 'book of words'), tipexed a marker on the distributor base and crank-case and noted where the rotor arm was facing. When I put the distributor back in, I retarded the ignition point slightly. After starting the engine (almost first time), I checked the timing and adjusted accordingly. You HAVE to use a timing-light to set the timing with electronic ignition--you CANNOT statically time it.
AccuSpark do a deal that enables you to buy a perfectly good Timing light at a special price when you buy the ignition kit. Total time to fit the kit?---less than an hour from starting (with a COLD engine) to crank-up.
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Old 05-07-2016   #11
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Re: Coil consumption

[quote=Franko500;4183934 Also sells the pointless ignition system.
[/quote]

I don't see the point to that.
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Old 05-07-2016   #12
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Re: Coil consumption

This might appear a silly question but is a ballast resistor needed when running a 123 electronic ignition?
I have a Bosch blue coil and don't want to fry it!
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Old 05-07-2016   #13
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Re: Coil consumption

Quote Originally Posted by AndrewHarvey View Post
This might appear a silly question but is a ballast resistor needed when running a 123 electronic ignition?
I have a Bosch blue coil and don't want to fry it!
It depends on the coil rather than the 123. If the coil say 12v on the side I think you're OK.
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Old 05-07-2016   #14
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Re: Coil consumption

Quote Originally Posted by AndrewHarvey View Post
This might appear a silly question but is a ballast resistor needed when running a 123 electronic ignition?
I have a Bosch blue coil and don't want to fry it!
The Bosch blue coil is a normal 12V coil for a points (or points replacement electronic ignition) , the Bosch ballast resistor compatible version is red IIRC.

But as said earlier in this thread, some electronic ignition systems (the so-called constant energy type) work best with a coil with really low primary resistance.
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Old 05-07-2016   #15
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Re: Coil consumption

A Bosch blue coil has a primary resistance in the region of 3.4 ohms so effectively, as the resistance is so high it has its own ballast resistor built into the primary coil. The model number you should have for the 500 is 221 197 027. This model number is specifically for cars that do not have an increased switched voltage for starting.

As Chris says any resistance below 3.4 ohms will fry on a 650 engine without a ballast resistor. The twin dry wasted spark coils as fitted to the 126 definitely needs a ballast resistor, so when converting a 500 to 650 with this setup you need a ballast resistor. Hence why as Peter said the 126 was fitted with one.

123 specifically recommend Bosch Blue for use with the 123 distributor. Therefore in theory a Bosch Blue should work fine on a 650 conversion so long as you don't have a twin dry coil.

Tony
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