why did my coil melt?
Discussion
Mk1 Golf gti with electronic ignition.
Car wouldn't turn over after months of not being used. After much effort with various chargers, boosters and jumps starting I replaced the starter motor.
Engine span really fast but would not fire. I left it for a while and put some extra fuel in even though it had some and it started straight away. Too quickly to have sucked the fuel through, I would have thought.
Take it for test drive. All good for 20 minutes then cuts out instantly. Coil is so hot it has melted the plastic cover and dropped molten plastic all over the heater pipes.
What could have caused it? Do I need to check anything before fitting new coil?
Thanks
Car wouldn't turn over after months of not being used. After much effort with various chargers, boosters and jumps starting I replaced the starter motor.
Engine span really fast but would not fire. I left it for a while and put some extra fuel in even though it had some and it started straight away. Too quickly to have sucked the fuel through, I would have thought.
Take it for test drive. All good for 20 minutes then cuts out instantly. Coil is so hot it has melted the plastic cover and dropped molten plastic all over the heater pipes.
What could have caused it? Do I need to check anything before fitting new coil?
Thanks
oakdale said:
Coil should be fed through a ballast resistor when running but fed by direct battery 12v from a terminal on the starter motor during cranking.
Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
Indeed.Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
Old rover v8's are the same.
Norfolkandchance said:
What would you do?
Thanks
Sorry, my ost should have had a queston mark st the end Thanks
If you've repllaced rhe srarter motor, then check that it's wired up correctly. Also ensure that youndidn't dislodge anything else when doing the work.
Electrical problems are all about methodical and tedious diagnosis, in my experience...
oakdale said:
Coil should be fed through a ballast resistor when running but fed by direct battery 12v from a terminal on the starter motor during cranking.
Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
Very plausible. Will check tom. Assume engine neededent be running to look for feed as long as the ignition is on. Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
Thanks
Norfolkandchance said:
oakdale said:
Coil should be fed through a ballast resistor when running but fed by direct battery 12v from a terminal on the starter motor during cranking.
Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
Very plausible. Will check tom. Assume engine neededent be running to look for feed as long as the ignition is on. Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
Thanks
oakdale said:
Coil should be fed through a ballast resistor when running but fed by direct battery 12v from a terminal on the starter motor during cranking.
Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
There are virtually no cars made with electronic ignition AND a ballast resistor (feel free to post up any ones that do but I'm pretty sure a MkI Golf isn't amongst them). The ballast system was designed to increase spark power during cranking whilst extending the life of the points under normal running conditions. Electronic ignition does not use points.Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
Doofus said:
Bad earth
The standard (usually incorrect) internet answer to every electrical query.For a coil to melt it will generally be pulling too much current through it, a bad earth would reduce the current flow.
OP, I've had a quick google and would expect your electronic ignition would look a lot like this -
It would be worth checking out the wiring colours etc before trusting this diagram!
If it is the same then the ignition module is what switches the earth on and off to the coil (in the same way the points used to). If you were to disconnect your melted coil and fit a new one you should find that with the ignition on you can measure 12v on both sides of the coil. If the -ve side (green wire) is at zero volts or earth then disconnect the terminal from the coil quickly as that's what melted your old one; alternatively you can link a low wattage bulb (side lamp or curtesy light) between the coil terminals instead of the coil, when the ignition is switched on the bulb should not light up.
If the -ve side is at earth (bulb lights up) then disconnect the ignition module, if the wire goes open circuit (bulb goes out) then you've got a faulty ignition module and the constant flow of current through the coil is what melted it.
If you do have 12v on both sides of the coil with the ignition on (or bulb not lit) then chances are you had an internal fault in the original coil although you could have had an intermittent fault in the module, if it were to cut out again turn the ignition off quickly to try to save melting the new coil.
HTH
Steve H
Steve H said:
The standard (usually incorrect) internet answer to every electrical query.
For a coil to melt it will generally be pulling too much current through it, a bad earth would reduce the current flow.
OP, I've had a quick google and would expect your electronic ignition would look a lot like this -
It would be worth checking out the wiring colours etc before trusting this diagram!
If it is the same then the ignition module is what switches the earth on and off to the coil (in the same way the points used to). If you were to disconnect your melted coil and fit a new one you should find that with the ignition on you can measure 12v on both sides of the coil. If the -ve side (green wire) is at zero volts or earth then disconnect the terminal from the coil quickly as that's what melted your old one; alternatively you can link a low wattage bulb (side lamp or curtesy light) between the coil terminals instead of the coil, when the ignition is switched on the bulb should not light up.
If the -ve side is at earth (bulb lights up) then disconnect the ignition module, if the wire goes open circuit (bulb goes out) then you've got a faulty ignition module and the constant flow of current through the coil is what melted it.
If you do have 12v on both sides of the coil with the ignition on (or bulb not lit) then chances are you had an internal fault in the original coil although you could have had an intermittent fault in the module, if it were to cut out again turn the ignition off quickly to try to save melting the new coil.
HTH
Steve H
Pretty sure a Metro turbo ran a ballast resistor with a full 12v bypass for cranking. Technically it wasnt a resistor as such, but the wire from source to coil was a resistive wire and marked as such within the loom ) to reduce normal running voltage at the coil.For a coil to melt it will generally be pulling too much current through it, a bad earth would reduce the current flow.
OP, I've had a quick google and would expect your electronic ignition would look a lot like this -
It would be worth checking out the wiring colours etc before trusting this diagram!
If it is the same then the ignition module is what switches the earth on and off to the coil (in the same way the points used to). If you were to disconnect your melted coil and fit a new one you should find that with the ignition on you can measure 12v on both sides of the coil. If the -ve side (green wire) is at zero volts or earth then disconnect the terminal from the coil quickly as that's what melted your old one; alternatively you can link a low wattage bulb (side lamp or curtesy light) between the coil terminals instead of the coil, when the ignition is switched on the bulb should not light up.
If the -ve side is at earth (bulb lights up) then disconnect the ignition module, if the wire goes open circuit (bulb goes out) then you've got a faulty ignition module and the constant flow of current through the coil is what melted it.
If you do have 12v on both sides of the coil with the ignition on (or bulb not lit) then chances are you had an internal fault in the original coil although you could have had an intermittent fault in the module, if it were to cut out again turn the ignition off quickly to try to save melting the new coil.
HTH
Steve H
As others have said, pretty sure some old Rover V8's did the same
Steve H said:
There are virtually no cars made with electronic ignition AND a ballast resistor (feel free to post up any ones that do but I'm pretty sure a MkI Golf isn't amongst them).
stevieturbo said:
Pretty sure a Metro turbo ran a ballast resistor with a full 12v bypass for cranking. Technically it wasnt a resistor as such, but the wire from source to coil was a resistive wire and marked as such within the loom ) to reduce normal running voltage at the coil.
As others have said, pretty sure some old Rover V8's did the same
I think the Metro ballast was just for the points versions, same for the V8s but I could be wrong, it's been a while since I've seen one!As others have said, pretty sure some old Rover V8's did the same
Either way, these are the exceptions not the rule, and not anything other than a distraction to the OP.
I'd just like to go over what an ignition ballast resistor was for, it was not to increase the spark during cranking but to just maintain the spark to normal values during cranking.
Anyone who remembers starting a car on a cold morning in the 70s will know that it was a hit and miss affair with the engine turning over very slowly due the batteries and starters of the day not being as efficient as they are now and the thicker oil that was used then.
Because of this there was a massive voltage drop to the ignition system which resulted in it being supplied with a voltage much lower than it was designed to run on.
To overcome this a lower voltage (lower resistance) coil was used in series with a ballast resistor during normal running but the coil was supplied with battery voltage (which would be similar to the voltage supplied through the resistor during normal running) when starting.
Although lower resistance coils can be used with electronic ignition the same principle still applied and in the early day of electronic ignition many vehicles still used ballast resistors, although this was not always obvious as most used a special resistance wire between the switch and the coil.
If my memory serves me correctly these Golfs did have a resister wire but I accept that I could be wrong.
Having said that I wouldn't accept some doodle that someone has put on the internet as proof that it doesn't,
Anyone who remembers starting a car on a cold morning in the 70s will know that it was a hit and miss affair with the engine turning over very slowly due the batteries and starters of the day not being as efficient as they are now and the thicker oil that was used then.
Because of this there was a massive voltage drop to the ignition system which resulted in it being supplied with a voltage much lower than it was designed to run on.
To overcome this a lower voltage (lower resistance) coil was used in series with a ballast resistor during normal running but the coil was supplied with battery voltage (which would be similar to the voltage supplied through the resistor during normal running) when starting.
Although lower resistance coils can be used with electronic ignition the same principle still applied and in the early day of electronic ignition many vehicles still used ballast resistors, although this was not always obvious as most used a special resistance wire between the switch and the coil.
If my memory serves me correctly these Golfs did have a resister wire but I accept that I could be wrong.
Having said that I wouldn't accept some doodle that someone has put on the internet as proof that it doesn't,
The doodle was picked just because it actually gave a pretty clear layout for the OP's assistance. I'll have a look later but TBH I'm not sure I'll still have any "proper" wiring diagrams from back then!
Resistive wires in the looms did exist on some BL cars IIRC (the Metros that StevieTurbo suggested rings a bell, white with a pink trace??) but I don't recall them being in much else - pretty poor system as they will inherently get warm and have potential to fail.
As to why cranking bypass was used, I think we are saying similar things in different ways. It needed a bigger spark when cranking than it would otherwise have had, whether that was down to cranking voltages, poor cold fuelling and combustion or just the basic weakness of the points system is arguable but 10v cranking was considered a sensible threshold then as it is today so I'd have said it was more of the latter two.
The reason they were able to do away with ballasts with electronic ignition despite using even lower resistance coils than on the ballasted system was that from a very early stage, EI was capable of running variable dwell meaning that the coil was given only the amount of power required to saturate the primary windings. With points, a high dwell angle was necessary to ensure full saturation at high rpm but at lower speeds this fixed dwell meant the coil received more on time than it required which without ballasts and higher resistance coils resulted in excessive heat and failures.
As I said before, it's been a while since I worked regularly on these systems but if the OP follows the route I've posted above I think he'll be OK.
Resistive wires in the looms did exist on some BL cars IIRC (the Metros that StevieTurbo suggested rings a bell, white with a pink trace??) but I don't recall them being in much else - pretty poor system as they will inherently get warm and have potential to fail.
As to why cranking bypass was used, I think we are saying similar things in different ways. It needed a bigger spark when cranking than it would otherwise have had, whether that was down to cranking voltages, poor cold fuelling and combustion or just the basic weakness of the points system is arguable but 10v cranking was considered a sensible threshold then as it is today so I'd have said it was more of the latter two.
The reason they were able to do away with ballasts with electronic ignition despite using even lower resistance coils than on the ballasted system was that from a very early stage, EI was capable of running variable dwell meaning that the coil was given only the amount of power required to saturate the primary windings. With points, a high dwell angle was necessary to ensure full saturation at high rpm but at lower speeds this fixed dwell meant the coil received more on time than it required which without ballasts and higher resistance coils resulted in excessive heat and failures.
As I said before, it's been a while since I worked regularly on these systems but if the OP follows the route I've posted above I think he'll be OK.
Steve H said:
oakdale said:
Coil should be fed through a ballast resistor when running but fed by direct battery 12v from a terminal on the starter motor during cranking.
Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
There are virtually no cars made with electronic ignition AND a ballast resistor (feel free to post up any ones that do but I'm pretty sure a MkI Golf isn't amongst them). The ballast system was designed to increase spark power during cranking whilst extending the life of the points under normal running conditions. Electronic ignition does not use points.Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.
Doofus said:
Bad earth
The standard (usually incorrect) internet answer to every electrical query.For a coil to melt it will generally be pulling too much current through it, a bad earth would reduce the current flow.
OP, I've had a quick google and would expect your electronic ignition would look a lot like this -
It would be worth checking out the wiring colours etc before trusting this diagram!
If it is the same then the ignition module is what switches the earth on and off to the coil (in the same way the points used to). If you were to disconnect your melted coil and fit a new one you should find that with the ignition on you can measure 12v on both sides of the coil. If the -ve side (green wire) is at zero volts or earth then disconnect the terminal from the coil quickly as that's what melted your old one; alternatively you can link a low wattage bulb (side lamp or curtesy light) between the coil terminals instead of the coil, when the ignition is switched on the bulb should not light up.
If the -ve side is at earth (bulb lights up) then disconnect the ignition module, if the wire goes open circuit (bulb goes out) then you've got a faulty ignition module and the constant flow of current through the coil is what melted it.
If you do have 12v on both sides of the coil with the ignition on (or bulb not lit) then chances are you had an internal fault in the original coil although you could have had an intermittent fault in the module, if it were to cut out again turn the ignition off quickly to try to save melting the new coil.
HTH
Steve H
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