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Mk3 Cortina Ignition Coil/Ballast Resistor Questions


Lumby

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Morning all,

After a bit of a search and a google, I can't find exactly what I am looking for so hoping someone will be able to shed some light for me on replacement options/issues/implications;  

It seems the GT40R ignition coil blew in the cortina yesterday - having tested and checked other parts of the system, it seems to be the only piece that is faulting. 

The replacement options I have found state that a ballast resistor should be used - having checked through and chased the wiring as much as I can, I can't see any sign of a ballast resistor being present.  Some of the online discussions suggest that Ford used a resistance wire to drop voltage at the coil instead, but the measured voltage across the coil is 12, suggesting this isn't the case / enough changes have been made over the years that it is long gone if it was ever there. 

Am I best to simply replace the GT40R on account of it was running prior to the fault, should I switch to something like the GT40 which *suggests* 12v direct is fine through the coil or should I be looking to wire a ballast resistor inline?

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GT40stands for Resistor (not Racing), so it probably blew because it had too much voltage going into it if you dont have a resistor installed

(and your measure of the power at the coil was = to  battery voltage, right - ie not with engine/alternator running whcih would be 14V+ so 12v when running would imply some resistance)

The R is added to older cars because of voltage drop when cranking with older battery tech, so the coil still works, tricky bit is that it is UN-resisted when cranking (getting all the volts avalaible) but needs to be resisted when its not cranking/alternator is spinning  else it blows up

If you going to get another coil go with an unresisted set up (ie GT40, no R)

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4 minutes ago, azzurro said:

(and your measure of the power at the coil was = to  battery voltage, right - ie not with engine/alternator running whcih would be 14V+ so 12v when running would imply some resistance)

The R is added to older cars because of voltage drop when cranking with older battery tech, so the coil still works, tricky bit is that it is UN-resisted when cranking (getting all the volts avalaible) but needs to be resisted when its not cranking/alternator is spinning  else it blows up

If you going to get another coil go with an unresisted set up (ie GT40, no R)

Yes the voltage measure is to battery voltage only.  I was under the impression that if a resistance wire or ballast was present, it should show ~9v at the coil connection with ignition switch open, but not cranking or running.  

Are there any issues with running a straight non-resisted set up? 

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1 hour ago, Lumby said:

Yes the voltage measure is to battery voltage only.  I was under the impression that if a resistance wire or ballast was present, it should show ~9v at the coil connection with ignition switch open, but not cranking or running.  

Correct. - the resistor will be bypassed when cranking, so depending on your starters draw, the coil might see >9V temporarily (bigger spark when turning over), the issue is that it cant handle it for ages (ie normal running)

1 hour ago, Lumby said:

Are there any issues with running a straight non-resisted set up? 

I dont think so, unless your starter draws so much battery current that the voltage drops at the coil sufficiently that it no longer fires or has a weak spark (ie the problem the resistor was designed to fix, still exists)

If it does, then wire up a resistor (and a coil to match) as per the factory, otherwise enjoy having the same voltage at the  coil as all the other electrical components in your vehicle?

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2 minutes ago, azzurro said:

I dont think so, unless your starter draws so much battery current that the voltage drops at the coil sufficiently that it no longer fires or has a weak spark (ie the problem the resistor was designed to fix, still exists)

 

Ahhh, and the penny drops! Thanks

 

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The terminology has always been confusing. A 'ballasted coil' is one that needs to be run with an external ballast where as a 'non ballasted coil' is one that has an internal ballast.

As a quick rule of thumb the primary winding on an average 'non-ballasted' coil is around 3 ohms while a ballasted one should be approx. 1.5 ohms.

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And to add to the above wise words; a coil run without ballast (internal or external) will fry your points in short order.

I've stuffed points in less than 15 mins by bypassing the ballast resistor (but a quick sand gets you operational again to limp to the nearest points stockists). Others may have gotten months out of points like that though.

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12 hours ago, Lumby said:

Are there any issues with running a straight non-resisted set up? 

Absolutely. See above. You will not get far before your points arc, pit, and stop working.

P.S: a coil will 'burn out' if the ignition is left on inadvertently when the points are closed for a period of time. Apart from that they were at least moderately reliable as they're no moving parts and they're basically an oil filled canister that has some wire in it.

You can do quite a few ignition system diagnostics with a timing light, even if the engine isn't running

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12 hours ago, sr2 said:

The terminology has always been confusing. A 'ballasted coil' is one that needs to be run with an external ballast where as a 'non ballasted coil' is one that has an internal ballast.

As a quick rule of thumb the primary winding on an average 'non-ballasted' coil is around 3 ohms while a ballasted one should be approx. 1.5 ohms.

10 hours ago, Unclejake said:

And to add to the above wise words; a coil run without ballast (internal or external) will fry your points in short order.

I've stuffed points in less than 15 mins by bypassing the ballast resistor (but a quick sand gets you operational again to limp to the nearest points stockists). Others may have gotten months out of points like that though.

Thanks, I think I understand.. so given that there was a ballasted coil installed (but no sign of ballast), the risk of damaging the points would be lessened by installing a non-ballasted coil rather than going the other direction? The risk would be more if there is resistance somewhere in the system already that I can't see (i.e a hidden ballast or resistance wire), it will run badly on account of weak spark? It may mean that the points are already damaged - I haven't been running the car much as I work on panels, and i don't recall seeing damage but I will check these again to be sure.

Similarly, to fry the points the only way to achieve that would be to bypass a ballast resistor in the system? A non-ballasted coil wouldn't carry that risk on account of the internal ballast?

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I assume UJ means running a ballast coil with no ballast resistor, as this will be the case where a coil expecting 6-9v is running off permanent 12-14v, otherwise the output of the two systems on the output or HV side (which is in the 20-40kV range) should be pretty similar.

The advantage of the ballasted set up, is on cranking as they get MORE than their normal or rated voltage when the starter is cranking (with the resistor by passed, but the starter cranking, the coil might see say 11-12V), which is when you need the strongest spark, but the coil (and seemingly the points which should only be handling the 12v ground side switching) cant handle this for long.

I dont imagine a non-ballasted coil on 12v will hurt the points, but running a ballast coil with no ballast installed could, i guess

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The terminology sure does get confusing, but a GT40R is a 'high performance' coil that is designed to work with an external ballast resistor. That means the coil gets all of the available voltage when the starter is being cranked, but once the starter stops being spun the voltage to the coil returns to whatever the external ballast resistor delivers the coil (I thought it was 6v but I've never looked it up so your 9v will probably be correct). 

I also thought (as SR2 wrote) that the GT40 had an internal ballast resistance, but I have no idea how that works.

A high performance coil like a GT40n isn't any advantage to a normal engine, but it won't hurt much either. The worst case is that it'll feed 'too much' high current to the ignition system and wear out ignition system components (points, capacitor, spark plugs) a little faster than a 'standard' coil.

I ran a GT40, a GT40T, and a couple of other coils on my Mk1 Cortina race car ... and never noticed a lick of difference TBH.

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Yea the details I could find got a bit confusing at the outset but I think i have a clearer picture now on how it should work!

I'll have a hunt around and see what the prices /options are for a replacement non-ballast; it isn't a race car so just needs to be reliable for a Sunday driver when I get it back on the road. 

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mmkay pops in -as ya do says hi to sr2 [Simon you have email re inverter] **Waves at unca Jake** Okily Dokily on the old Mark 3 Go Tina...the ballast wire was about a foot or so long with the standard old round bullet connectors and was a thick GREY wire. Recall has it possibly under the  bonnet catch radiator support upper panel area... pic of wire (with ends reterminated here)

https://thumbsnap.com/1q1cINlX

End of the day running a ballast resistor with a bypass is actually a tad better for starting. However if you decide to go that way a standard issue common garden 3k/4k toyota earlier mitsis etc  as here

https://www.repco.co.nz/en/brands/oex/oex-oex-ballast-resistor-acx1955/p/A1195390

Can simply be mounted and wired direct from the ignition feed to the ballast coil-as to the ballast bypass giving more available power on cranking/starting 

 well as simple as a diode fitted in to a wire connected to the starter solenoid feed and connected direct to the coil...

 

Hope This Helps

 

Edits to add **newbie**.....Yeah Right.............(Passes round Tuis......-As Ya Do...)

 

 

Reason for edit

 

 

Older Experienced Time Served Qualified Mechanic **Newbie**

 

Edited by snoop
Older Experienced Time Served Qualified Mechanic **Newbie**
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