Ignition issues with Rover V8
Discussion
Hi all. I'm struggling a bit on a problem that I've got with my kit car and was hoping that someone on here might have an idea of what my problem might be.
First a quick list of the current set up. I've got a P6 Rover V8 in my Rush with a Mallory electronic pointless distributor, Mallory ballast resistor and 9V Mallory coil. I'm using an Odyssey 925 sealed racing battery and have what I think is a Land Rover alternator that I have refurbed.
What's happening currently is that I can start the car when cold and run it for approximately 5 minutes, after which time the engine suddenly dies. Any attempt to restart the car fails. If left overnight then I can then restart it and run it for 5 or so minutes. I have borrowed a 12V coil from a XJ6 Jaguar and it will restart straight away and run indefinately (I've run it for about half an hour so far, starting and stopping occasionally, with no issues).
I have ended up with the Mallory coil and ballast resistor due to a similar fault I have had. Originally I had an Accel coil (the type Real Steel supply as standard for use with the Mallory distributor) and the engine appeared to run fine. I drove 25 miles to the local VOSA station for my SVA test and got half way through and the car then died (I should add I drove to the SVA with the the headlights on). We couldn't restart it and traced the fault to the coil. The test station was only down the road from Real Steel, so we got in the support car and got a replacement coil (a Mallory 9V). The first of these was faulty (what are the chances!) and so we returned it and got a second Mallory coil. This allowed the car to run again and we thought we might finish the test, only for the car to fail after 5 minutes of running.
I've done all the tests I can possibly think of to diagnose the fault and I think I may have narrowed it down, but was hoping for some feedback on my reasoning.
When running with the Jag 12V coil I noticed on the +ve side of the ballast resitor reads about 13.7V and the coil side reads about 12.2V. If this is the case then is it possible that the coil has been supplied too high a voltage for the 9V coil to handle (I know this is related in part to the internal resistance of the 9V coil). If this is the case, could my problem be related to having too large an alternator supplying too small a battery?
I tried to test the theory this morning. I disconnected the alternator and fitted a short belt between just the crank and the water pump so that I was running just on the battery. The engine ran for approximately 15 minutes before cutting out. I think the cutting out may have been related to the radiator fan kicking in because it was running when the engine died (it draws 10A and the voltage at the coil a few minutes before it died had already dropped to just under 8V).
I've borrowed a larger capacity battery and will do the test again to see if it'll last longer than 15 minutes. The car's quite tricky to restart when hot and I don't have enough hands so I'll have to leave it to cool down a little first.
To summarise, could having an alternator that is too big cause this kind of problem? Does anyone have any suggestions of things for me to look at or other things that may be at fault?
Sorry for the long post and thanks in advance!
First a quick list of the current set up. I've got a P6 Rover V8 in my Rush with a Mallory electronic pointless distributor, Mallory ballast resistor and 9V Mallory coil. I'm using an Odyssey 925 sealed racing battery and have what I think is a Land Rover alternator that I have refurbed.
What's happening currently is that I can start the car when cold and run it for approximately 5 minutes, after which time the engine suddenly dies. Any attempt to restart the car fails. If left overnight then I can then restart it and run it for 5 or so minutes. I have borrowed a 12V coil from a XJ6 Jaguar and it will restart straight away and run indefinately (I've run it for about half an hour so far, starting and stopping occasionally, with no issues).
I have ended up with the Mallory coil and ballast resistor due to a similar fault I have had. Originally I had an Accel coil (the type Real Steel supply as standard for use with the Mallory distributor) and the engine appeared to run fine. I drove 25 miles to the local VOSA station for my SVA test and got half way through and the car then died (I should add I drove to the SVA with the the headlights on). We couldn't restart it and traced the fault to the coil. The test station was only down the road from Real Steel, so we got in the support car and got a replacement coil (a Mallory 9V). The first of these was faulty (what are the chances!) and so we returned it and got a second Mallory coil. This allowed the car to run again and we thought we might finish the test, only for the car to fail after 5 minutes of running.
I've done all the tests I can possibly think of to diagnose the fault and I think I may have narrowed it down, but was hoping for some feedback on my reasoning.
When running with the Jag 12V coil I noticed on the +ve side of the ballast resitor reads about 13.7V and the coil side reads about 12.2V. If this is the case then is it possible that the coil has been supplied too high a voltage for the 9V coil to handle (I know this is related in part to the internal resistance of the 9V coil). If this is the case, could my problem be related to having too large an alternator supplying too small a battery?
I tried to test the theory this morning. I disconnected the alternator and fitted a short belt between just the crank and the water pump so that I was running just on the battery. The engine ran for approximately 15 minutes before cutting out. I think the cutting out may have been related to the radiator fan kicking in because it was running when the engine died (it draws 10A and the voltage at the coil a few minutes before it died had already dropped to just under 8V).
I've borrowed a larger capacity battery and will do the test again to see if it'll last longer than 15 minutes. The car's quite tricky to restart when hot and I don't have enough hands so I'll have to leave it to cool down a little first.
To summarise, could having an alternator that is too big cause this kind of problem? Does anyone have any suggestions of things for me to look at or other things that may be at fault?
Sorry for the long post and thanks in advance!
When it dies and won't restart, what's wrong? I think you need to track down the source of the failure and the solution will then be obvious. It sounds as if you're concentrating on an ignition related fault, and that's probably what it is, but don't assume it is without confirming it. I recommend putting a passive strobe in line with the king lead. You can get dedicated HT testers for the job, or just use an ordinary cheap passive timing light. (The more expensive amplified timing lights are no good for this job.)
If you have a 9V coil then I'd expect to see 9-10V across it when the points were "closed". Obviously you need to ensure you don't fry the coil when you do this, but shorting the -ve side to earth for a fraction of a second would be enough to let you measure the voltage across the coil. If you're really getting 12v then either your ballast resister isn't a high enough resistance or (more likely) you've wired the starting circuit up wrongly and the ballast resister is actually being bypassed while the engine is running. It should only be bypassed while you're cranking.
Hope this helps.
If you have a 9V coil then I'd expect to see 9-10V across it when the points were "closed". Obviously you need to ensure you don't fry the coil when you do this, but shorting the -ve side to earth for a fraction of a second would be enough to let you measure the voltage across the coil. If you're really getting 12v then either your ballast resister isn't a high enough resistance or (more likely) you've wired the starting circuit up wrongly and the ballast resister is actually being bypassed while the engine is running. It should only be bypassed while you're cranking.
Hope this helps.
Hi Peter. Thanks for the reply. 
I don't have the starter bypass at all. The instructions with the distributor say specifically not to use one. I don't even have the wiring there to do it even if I wanted to so this definately isn't the problem.
I don't suppose you have any other things that I might want to try to do? I'm a bit
at the moment!
GreenV8S said:
When it dies and won't restart, what's wrong? I think you need to track down the source of the failure and the solution will then be obvious.
I have confirmed that the two coils I have returned have been faulty. They were both tested on Real Steel's test rig. We tried various things but what confirmed it were the following things. We put a spark plug in the HT lead between the coil and the distributor and held the plug to ground. With the ignition on we then shorted the coils -ve terminal to ground repeatedly (to simulate the distributor), but there were no sparks, despite there being over 12V measured between the +ve terminal and ground. We then tried it with another lead to make sure that wasn't faulty and we had the same response (or lack there of).GreenV8S said:
If you have a 9V coil then I'd expect to see 9-10V across it when the points were "closed". Obviously you need to ensure you don't fry the coil when you do this, but shorting the -ve side to earth for a fraction of a second would be enough to let you measure the voltage across the coil.
The distributor is a Mallory pointless electronic distributor that uses a hall effect sensor as I understand it. Is there a way I could replicate this test without the use of a distributor? Perhaps leave the -ve lead shorted to ground and turn the ignition on and quickly measure the voltage across it?GreenV8S said:
If you're really getting 12v then either your ballast resister isn't a high enough resistance or (more likely) you've wired the starting circuit up wrongly and the ballast resister is actually being bypassed while the engine is running.
The ballast resistor is the one supplied for use with the coil so I think under normal circumstances this should be the right match. With the alternator disconnected the voltage appears to be about right between the +ve terminal of the coil and ground.I don't have the starter bypass at all. The instructions with the distributor say specifically not to use one. I don't even have the wiring there to do it even if I wanted to so this definately isn't the problem.
I don't suppose you have any other things that I might want to try to do? I'm a bit
at the moment! Edited by gingerpaul on Thursday 1st November 14:59
A 9V coil is used on a points system so that normal running passes 12V through the ballast (be that a resister or resister lead) to produce 9V at the coil.
During start the coil is supplied direct from the starter motor. This is done because when starting high currents are drawn on the battery and the voltage drops (surprise, surprise) to about 9V.
If you are running electronic ignition I would expect you to be running a 12V performance coil and no ballast as the voltage to the coil from the electronic ignition will be very high. It will also be immune to some of the effects of low voltage during start.
I would not touch that 9V coil after the engine has stopped as I suspect it is close to melt-down.
Steve
During start the coil is supplied direct from the starter motor. This is done because when starting high currents are drawn on the battery and the voltage drops (surprise, surprise) to about 9V.
If you are running electronic ignition I would expect you to be running a 12V performance coil and no ballast as the voltage to the coil from the electronic ignition will be very high. It will also be immune to some of the effects of low voltage during start.
I would not touch that 9V coil after the engine has stopped as I suspect it is close to melt-down.
Steve
Hi Steve. Thanks for responding.
Once again, thanks for your help!
Edited to clarify an ambiguous point.
Steve_D said:
A 9V coil is used on a points system so that normal running passes 12V through the ballast (be that a resister or resister lead) to produce 9V at the coil.
During start the coil is supplied direct from the starter motor. This is done because when starting high currents are drawn on the battery and the voltage drops (surprise, surprise) to about 9V.
When I got the engine running initially I was running this set up because of the voltage drop and I was using the original points distributor. Ironically, the main reason for getting this set up was getting better reliability as the original set up had messed me about a bit too (unrelated to the current issue mind you). One of the reasons I got the Odyssey battery is that it doesn't suffer from this effect as much as an ordinary battery. With the current set up I agree that it shouldn't be necessary. The reason I've got it is because Real Steel had ran out of the normal coil they supply (Accel) and couldn't get hold of any and hence got the Mallory one in as a temporary replacement.During start the coil is supplied direct from the starter motor. This is done because when starting high currents are drawn on the battery and the voltage drops (surprise, surprise) to about 9V.
Steve_D said:
If you are running electronic ignition I would expect you to be running a 12V performance coil and no ballast as the voltage to the coil from the electronic ignition will be very high. It will also be immune to some of the effects of low voltage during start.
What kind of coil would you recommend for a system such as mine? Are there particular brands / models that are good for Mallory electronic distributors? Real Steel were keen to point out that I need to be careful with what kind of coil I use with it as the electronics inside are delicate. I don't know enough about it to make a decision either way.Steve_D said:
I would not touch that 9V coil after the engine has stopped as I suspect it is close to melt-down.
Indeed! I'm doing a final test with it tomorrow morning and seeing how it performs. I'm almost certainly not going to put it back on the car though.Once again, thanks for your help!
Edited to clarify an ambiguous point.
Edited by gingerpaul on Thursday 1st November 20:34
What voltage is the coil designed to have across it? I would expect the coil supplier to tell you, and it'd probably have it written on the coil, packaging etc.
If it's a 9V coil then it should have around 9V across it. However, this is rather tricky to measure due to the way the coil is used.
The coil has 12V supplied to the +ve terminal (via a resister in your case). The -ve terminal is isolated most of the time but pulled down to earth briefly each time a spark is required. While the -ve side is isolated you'll find both sides of the coil at the battery voltage of around 12V (engine stopped) or 14V(engine running) i.e. no Voltage across the coil. It is only when the -ve terminal is pulled down to earth that current will flow through the coil, the -ve side of the coil will be pulled down to around 0V, the positive side will be pulled down to around 9V and the remaining 4-5V will be dropped across the ballast resister.
The best way to measure what's happening to the coil is:
Switch the ignition on. Measure the voltage between the coil +ve and ground, it should be around 12V.
Connect the volt meter across the coil. It should read 0V.
Take a wire and briefly short the coil -ve terminal (perhaps half a second, no longer than a second).
Bear in mind that when you take this short off the coil will spark.
While the short is in place the voltage across the coil will go from 0V to about 9V.
When you disconnect, it will go back to 0V.
The voltage across the coil while the ignition is on and the -ve side is earthed is the important thing. If it's significantly higher than the voltage that the coil is rated for then that would explain why the coils keep blowing.
By the way it is not normal to user ballasted coils with electronic ignition. Consider going to a standard 'electronic' 12V coil and get rid of the ballast resister; I suspect this will solve all your problems.
If it's a 9V coil then it should have around 9V across it. However, this is rather tricky to measure due to the way the coil is used.
The coil has 12V supplied to the +ve terminal (via a resister in your case). The -ve terminal is isolated most of the time but pulled down to earth briefly each time a spark is required. While the -ve side is isolated you'll find both sides of the coil at the battery voltage of around 12V (engine stopped) or 14V(engine running) i.e. no Voltage across the coil. It is only when the -ve terminal is pulled down to earth that current will flow through the coil, the -ve side of the coil will be pulled down to around 0V, the positive side will be pulled down to around 9V and the remaining 4-5V will be dropped across the ballast resister.
The best way to measure what's happening to the coil is:
Switch the ignition on. Measure the voltage between the coil +ve and ground, it should be around 12V.
Connect the volt meter across the coil. It should read 0V.
Take a wire and briefly short the coil -ve terminal (perhaps half a second, no longer than a second).
Bear in mind that when you take this short off the coil will spark.
While the short is in place the voltage across the coil will go from 0V to about 9V.
When you disconnect, it will go back to 0V.
The voltage across the coil while the ignition is on and the -ve side is earthed is the important thing. If it's significantly higher than the voltage that the coil is rated for then that would explain why the coils keep blowing.
By the way it is not normal to user ballasted coils with electronic ignition. Consider going to a standard 'electronic' 12V coil and get rid of the ballast resister; I suspect this will solve all your problems.
Hello again Peter. Thanks for the advice. I'll do what you have suggested tomorrow and see what results I get. The coil I was supplied was a 9V unit by the way. This was because they were out of stock of the Accel coil that they normally supply (which has a ballast resistor built in, so I'm told).
Do you have any recommendations for brands / models if I were to replace the coil and get an electronic 12V version? Presumably some are better than others. I've also been told I need to be careful not to get something that will damage the distributor. Do you know what I need to look out for to ensure compatibility?
Do you have any recommendations for brands / models if I were to replace the coil and get an electronic 12V version? Presumably some are better than others. I've also been told I need to be careful not to get something that will damage the distributor. Do you know what I need to look out for to ensure compatibility?
Tried to run on the Mallory 9V coil today and it wouldn't start at all. Swapped to the Jag 12V one and it ran fine for an hour without stalling. Looks like I need to get myself a decent 12V one then and see how I get on. Incidentally, I did the tests that you suggested Peter and got the results you predicted on the 12V coil (numbers adjusted for 12V of course).
Thanks for your help. I'll keep running the engine and see how I get on.
Thanks for your help. I'll keep running the engine and see how I get on.
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