Low coil voltage on cranking
Discussion
1973 Rover P5B Coupe. It's had a new coil and fitted with electronic ignition. After its winter lay up it is proving a little difficult to start at times. I thought initially the original mechanical fuel pump may need overhauling or replacing with an electric pump. It turns out now that I've only got just over 6V at the coil when cranking, over 9V when it's running.
I assume I should try to get a switched 12V feed to the coil when I'm turning the key.
Is this difficult? Is it something a competent auto electrician should have no problem with?
SD.
I assume I should try to get a switched 12V feed to the coil when I'm turning the key.
Is this difficult? Is it something a competent auto electrician should have no problem with?
SD.
It sounds more like you have a fundamental electrical issue.
You should have 12.6v with a fully charged battery & 13.5v minimum with the engine running (at the battery) if you are getting such low voltages it seems like you have a knackered battery or poor connections at the major terminals for the battery/alternator/starter/engine/chassis.
You should have 12.6v with a fully charged battery & 13.5v minimum with the engine running (at the battery) if you are getting such low voltages it seems like you have a knackered battery or poor connections at the major terminals for the battery/alternator/starter/engine/chassis.
Can you find out whether that vehicle has a ballasted coil? If it has, that may explain the problem.
In a ballasted system the normal supply from the ignition switch to the coil goes via the ballast resistor that drops the voltage by several volts. These often run at 6V. The coil has a secondary supply from the starter which brings the coil up to full battery voltage when the starter is engaged. This might be as low as 9V on some vehicles.
If the supply from the starter has failed, it would be running through the ballast resistor while cranking, which would result in a very low supply voltage.
You may be able to work that out from the voltage printed on the coil, but don't assume that's the correct coil for the car unless you already know it is; if you fit a non-ballasted coil to a car designed for a ballasted coil it'll be a pig to start unless you also remove the ballast from the circuit.
If it's definitely not a ballasted car then you need to trace through the circuit to find where that voltage drop occurs. On a non-ballasted car, the coil should see full battery voltage, less a couple of tenths for voltage drops along the loom.
In a ballasted system the normal supply from the ignition switch to the coil goes via the ballast resistor that drops the voltage by several volts. These often run at 6V. The coil has a secondary supply from the starter which brings the coil up to full battery voltage when the starter is engaged. This might be as low as 9V on some vehicles.
If the supply from the starter has failed, it would be running through the ballast resistor while cranking, which would result in a very low supply voltage.
You may be able to work that out from the voltage printed on the coil, but don't assume that's the correct coil for the car unless you already know it is; if you fit a non-ballasted coil to a car designed for a ballasted coil it'll be a pig to start unless you also remove the ballast from the circuit.
If it's definitely not a ballasted car then you need to trace through the circuit to find where that voltage drop occurs. On a non-ballasted car, the coil should see full battery voltage, less a couple of tenths for voltage drops along the loom.
Thank you, very informative. It's a late model so "shouldn't" have a ballast, but so many previous owners have been enthusiastic with maintenance the wiring diagram is a prize winning work of fiction.
I replaced the coil with one that should have fitted. I may have to check everything in the entire circuit although with all the changes made over the years it may be time to recruit someone who knows what they are doing!
SD.
I replaced the coil with one that should have fitted. I may have to check everything in the entire circuit although with all the changes made over the years it may be time to recruit someone who knows what they are doing!
SD.
GreenV8S said:
Can you find out whether that vehicle has a ballasted coil? If it has, that may explain the problem.
In a ballasted system the normal supply from the ignition switch to the coil goes via the ballast resistor that drops the voltage by several volts. These often run at 6V. The coil has a secondary supply from the starter which brings the coil up to full battery voltage when the starter is engaged. This might be as low as 9V on some vehicles.
If the supply from the starter has failed, it would be running through the ballast resistor while cranking, which would result in a very low supply voltage.
You may be able to work that out from the voltage printed on the coil, but don't assume that's the correct coil for the car unless you already know it is; if you fit a non-ballasted coil to a car designed for a ballasted coil it'll be a pig to start unless you also remove the ballast from the circuit.
If it's definitely not a ballasted car then you need to trace through the circuit to find where that voltage drop occurs. On a non-ballasted car, the coil should see full battery voltage, less a couple of tenths for voltage drops along the loom.
this very much im old enough to remember these, and been caught out by it when fitting coils.In a ballasted system the normal supply from the ignition switch to the coil goes via the ballast resistor that drops the voltage by several volts. These often run at 6V. The coil has a secondary supply from the starter which brings the coil up to full battery voltage when the starter is engaged. This might be as low as 9V on some vehicles.
If the supply from the starter has failed, it would be running through the ballast resistor while cranking, which would result in a very low supply voltage.
You may be able to work that out from the voltage printed on the coil, but don't assume that's the correct coil for the car unless you already know it is; if you fit a non-ballasted coil to a car designed for a ballasted coil it'll be a pig to start unless you also remove the ballast from the circuit.
If it's definitely not a ballasted car then you need to trace through the circuit to find where that voltage drop occurs. On a non-ballasted car, the coil should see full battery voltage, less a couple of tenths for voltage drops along the loom.
also alot of the old ballast resisters were just fitted in the wiring, looked alot like a normal plug, and they would corrode.
They were fitted to prevent points wear, so if the dizzy is electronic, it may not need a ballast,
If its had a electronic dizzy upgrade, some still use a ballast coil others dont, so worth finding out what is fitted and going from there.
Easy answer is fit a 12v coil and wire direct to ignition live. Just check that the dizzy can cope with this.
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