Voltage drop, is this normal or excessive?
Discussion
My classic Sunbeam Alpine is a pain to start when hot. I don't think it is fuel vaporisation. The engine temperature shows normal on the gauge but the rocker cover does seem very hot and the coil gets very hot (too hot to touch). I have checked the timing, points gap etc and am starting to suspect the coil may be a problem (Petronix Flamethrower). It is about 6 years old.
The battery is fully charged and showing around 12.7V. If I turn the ignition on with the coil disconnected it remains the same on the voltmeter. If I connect the coil it drops to around 9.7V.
I expected to see a voltage drop as soon as the ignition circuit was activated but this seems excessive. I thought that I had found the problem but when I swapped it for another old coil the figures were more or less the same. Is this correct? Surely it needs to be at least 10-5V to start the car reliably.
Does anyone have any other ideas as to what could be causing this?
Puddles.
The battery is fully charged and showing around 12.7V. If I turn the ignition on with the coil disconnected it remains the same on the voltmeter. If I connect the coil it drops to around 9.7V.
I expected to see a voltage drop as soon as the ignition circuit was activated but this seems excessive. I thought that I had found the problem but when I swapped it for another old coil the figures were more or less the same. Is this correct? Surely it needs to be at least 10-5V to start the car reliably.
Does anyone have any other ideas as to what could be causing this?
Puddles.
What is the battery voltage at the battery when you connect the coil? As some cars have a balast resistor to drop down the voltage to the coil, and then a second 12v feed for when it is cranking.. This maybe totally incorrect for yours, but it rings a bell.
Ive just done a quick googling to check my reply, some info here.. https://www.askthemechanic.co.uk/classic-cars/sunb...
Ive just done a quick googling to check my reply, some info here.. https://www.askthemechanic.co.uk/classic-cars/sunb...
Edited by xtruss on Friday 2nd August 15:46
Hi, This system doesn't have a ballet resistor. I just went and put a meter across the battery and it stays at 12.6-12.7 regardless of whether the ignition is on or off. The volt meter is showing 9.8 when I turn the ignition on with the coil connected if I disconnect the coil it shows 12.7. Does this mean the coils (both the one fitted and the old one) are at fault or could it be something else in the ignition circuit?
Puddles
Puddles
It does sound suspiciously like it’s linked to the resistance wire though as that is all the coil will get unless the engine is cranking, - as then it gets 12v feed to it via the starter solenoid.
(That’s how it works on my 1970’s escort anyway, as I had to put in a separate 12v feed for:the firebox to a Lumenition module, as the coil feed voltage was too low ).
TEST. If you connected a separate positive feed directly from the battery to the coil and the gauge then read the full 12+volts, it would confirm that the volt meter connection is reading from the ignition coil feed circuit and not one of the ignition supply circuits,.
(That’s how it works on my 1970’s escort anyway, as I had to put in a separate 12v feed for:the firebox to a Lumenition module, as the coil feed voltage was too low ).
TEST. If you connected a separate positive feed directly from the battery to the coil and the gauge then read the full 12+volts, it would confirm that the volt meter connection is reading from the ignition coil feed circuit and not one of the ignition supply circuits,.
Edited by Hol on Friday 2nd August 16:19
There is 12.7V at all of the fuses on the permanently live fusebox when the ignition is off. If I connect the +ve terminal of the coil directly to this supply (bypassing the ignition switch and other wiring) it drops to 10.98V across all of the fuses in this box and 10.78V at the coil.
It also feels as if the voltage reading at the coil is creeping UP as the temperature of the coil comes DOWN but that could be my imagination. I'm sure it was constantly reading in the 9s then in the very low 10s and now is mid10s.
Puddles.
It also feels as if the voltage reading at the coil is creeping UP as the temperature of the coil comes DOWN but that could be my imagination. I'm sure it was constantly reading in the 9s then in the very low 10s and now is mid10s.
Puddles.
If it is build up of resistance under load / when hot then a “hot start relay” ‘will likely help. It is very much a workaround but is a decent ‘get you out of jail’ solution until you found and address the underlying cause. And for very little expense is actually a useful part of the diagnostic process. I use them on my old vw campers which suffer from long wiring runs and degraded wiring but the principle should work on your car
I have looked into the Hot Start Relay idea and I like it. I will do that tomorrow but I am not sure that this is what is causing the problem, the starter spins very quickly as it is now. It is the voltage at the coil that is dropping too low.
As the car has continued to cool down, the voltage readings have continued to rise. The meter is now showing 11.4V at the coil when I turn the ignition on.
Interestingly it is the same with both the usual coil and my old spare (which is stone cold) so perhaps something else is cooling down and becoming less resistive. Ignition switch maybe?
As the car has continued to cool down, the voltage readings have continued to rise. The meter is now showing 11.4V at the coil when I turn the ignition on.
Interestingly it is the same with both the usual coil and my old spare (which is stone cold) so perhaps something else is cooling down and becoming less resistive. Ignition switch maybe?
Puddles of Oil said:
My classic Sunbeam Alpine is a pain to start when hot. I don't think it is fuel vaporisation. The engine temperature shows normal on the gauge but the rocker cover does seem very hot and the coil gets very hot (too hot to touch). I have checked the timing, points gap etc and am starting to suspect the coil may be a problem (Petronix Flamethrower). It is about 6 years old.
The battery is fully charged and showing around 12.7V. If I turn the ignition on with the coil disconnected it remains the same on the voltmeter. If I connect the coil it drops to around 9.7V.
I expected to see a voltage drop as soon as the ignition circuit was activated but this seems excessive. I thought that I had found the problem but when I swapped it for another old coil the figures were more or less the same. Is this correct? Surely it needs to be at least 10-5V to start the car reliably.
Does anyone have any other ideas as to what could be causing this?
Puddles.
12.7V resting suggests a fully charged battery.The battery is fully charged and showing around 12.7V. If I turn the ignition on with the coil disconnected it remains the same on the voltmeter. If I connect the coil it drops to around 9.7V.
I expected to see a voltage drop as soon as the ignition circuit was activated but this seems excessive. I thought that I had found the problem but when I swapped it for another old coil the figures were more or less the same. Is this correct? Surely it needs to be at least 10-5V to start the car reliably.
Does anyone have any other ideas as to what could be causing this?
Puddles.
But check it is fully charged.
Pulling your battery down to 9.7V suggests either a very big current or a pretty sick battery.
But it could be a wiring fault, depending on where you're measuring the voltages?
Personally, I'd measure the current.
Or test the battery with a known load.
What does the battery drop to if you put the headlights on with the engine stopped?
Does the voltage hold up running them for ten minutes or so?
It's always posssible to be very, very wrong when diagnosing stuff over the interweb.
But personally I can recommend Tayna for the battery I think you need.
All the best!
Sounds as if there is a steady current across the LV side of the coil which, unsurprisingly, is causing it to heat up as it is only intended to have an intermittent current which activates the HV side. In days of yore leaving the ignition on when the points happened to be closed was a cause of coil failure hence it was drummed into drivers not to, just in case.
Maybe a problem with the points, or equivalent, rather than the coil?
Maybe a problem with the points, or equivalent, rather than the coil?
I have now fitted a “Hot/Hard Start Relay”, the car is starting and running but it is not especially hot today. I am going to try and get things as hot as I can and see what happens.
Dogwatch, I agree that would explain things but I have had hot start issues both with the previous electronic ignition and the currently installed points set up. I will keep investigating.
Dogwatch, I agree that would explain things but I have had hot start issues both with the previous electronic ignition and the currently installed points set up. I will keep investigating.
In case anyone has been following this thread and is interested.
I have now fitted the Hot Start Relay. Today has been hot and the car would usually not want to start but whatever I do I can't make the car play up. It is starting correctly even if I deliberately try to make it extra hot and provoke her to misbehave. The Hot Start Relay is a great tip, I wish I had done this sooner.
I have a brand new coil but I haven't fitted it.
I took the following meter readings today, all are 3ohm coils:
Fitted to the car currently, Flamethrower. 3ohm Primary circuit, 10,260 Secondary circuit.
New Accuspark coil 2.7ohm Primary circuit, 8,300 Secondary circuit.
Old Lucas coil 3.0 ohm Primary circuit, 5,600 secondary circuit
I am guessing that it is the bigger secondary field figure that gives the performance coils the extra power.
Voltage measured across the battery terminals is 12.52
Voltage measured at the Primary fusebox 12.50
As soon as the ignition is switched on whichever coil is fitted the voltage on the voltmeter drops to 10.3/10.4V.
The Flamethrower coil does still get hot (too hot to touch) after a drive on the motorway but it is definitely all working correctly and the car is starting and driving well.I can't see any reason why the coil gets so hot, everything seems to be within specification. Perhaps they all get this hot.
Thank you all for your help and assistance.
Puddles
I have now fitted the Hot Start Relay. Today has been hot and the car would usually not want to start but whatever I do I can't make the car play up. It is starting correctly even if I deliberately try to make it extra hot and provoke her to misbehave. The Hot Start Relay is a great tip, I wish I had done this sooner.
I have a brand new coil but I haven't fitted it.
I took the following meter readings today, all are 3ohm coils:
Fitted to the car currently, Flamethrower. 3ohm Primary circuit, 10,260 Secondary circuit.
New Accuspark coil 2.7ohm Primary circuit, 8,300 Secondary circuit.
Old Lucas coil 3.0 ohm Primary circuit, 5,600 secondary circuit
I am guessing that it is the bigger secondary field figure that gives the performance coils the extra power.
Voltage measured across the battery terminals is 12.52
Voltage measured at the Primary fusebox 12.50
As soon as the ignition is switched on whichever coil is fitted the voltage on the voltmeter drops to 10.3/10.4V.
The Flamethrower coil does still get hot (too hot to touch) after a drive on the motorway but it is definitely all working correctly and the car is starting and driving well.I can't see any reason why the coil gets so hot, everything seems to be within specification. Perhaps they all get this hot.
Thank you all for your help and assistance.
Puddles
I have had a couple of "hot" coils in the past, both of which were caused by the coil's internal insulation breaking down. Both of these coils gave running problems, though they were intermittent and occurred after the coil had become hot through use.
Coils do run a little hot in normal operation, as far as I know, but I'd keep an eye on one that routinely gets too hot to touch. Someone with more experience in this area may be able to comment on the expected temperature of a coil in good condition.
You could also consider having a spare coil mounted where you could just swap over the high and low tension connections if the original gave trouble. This was common practice as I understand it in rally cars in period.
Coils do run a little hot in normal operation, as far as I know, but I'd keep an eye on one that routinely gets too hot to touch. Someone with more experience in this area may be able to comment on the expected temperature of a coil in good condition.
You could also consider having a spare coil mounted where you could just swap over the high and low tension connections if the original gave trouble. This was common practice as I understand it in rally cars in period.
Gassing Station | Classic Cars and Yesterday's Heroes | Top of Page | What's New | My Stuff