Weak spark on the RV8... any idea's?
Discussion
Besides driving TVR, i also have a Marcos Mantara which originally had the 3.9 EFI. Engine was swapped with a 4.6 V8 developments. All the 3.9 parts were put on the 4.6. For quite some i had a rough running engine which also misfired. Turned out that I had mixed up 2 HT leads. I had Magnecor wires on, but some of the leads did not click very well on the spark plug. At first i suspected this was the problem. Changed to anothet set of plug leads and checked the ignition system as far as I could. In the mean time I changed my lucas coil for a new Bosch. Fitted a new 3 pin remote ignition amplifier. A new lucas distributor cap and rotor. After the 2 plug leads were in the right position the engine ran nicely, but...… I still have a rather weak yellow spark. The coil gets the needed 12V. When I test it directly at the coil, it still is a weak yellow spark. The Marcos uses a standard rover loom, just like many TVR's. It has the Lucas 35D distributor with the remote amplifier. I know RPI engineering has a amplifier which should create bigger sparks, but surely the rover set up must be amble to generate a bright blue spark also…. I am now pretyy sure the weak spark goes to all the 8 cilinders at the right time, but what can be looked at further?
Forget the Rpi box of tricks, concentrate on what you have.
It’s well known replacement modern components for the ignition system are poor quality, your Bosch coil should be good but where did you buy the cap/rotor and module from?
Some useful test stuff here.
http://g33.co.uk/pages/technical-ignition-system.h...
It’s well known replacement modern components for the ignition system are poor quality, your Bosch coil should be good but where did you buy the cap/rotor and module from?
Some useful test stuff here.
http://g33.co.uk/pages/technical-ignition-system.h...
Edited by Belle427 on Friday 17th July 17:11
Ok then
The coil supply voltage can only be measured correctly by having the coil load on/in the circuit
A bad supply or a supply through a ballast resistor will still measure as 12 volt when the coil load isn't on/in the circuit
You need to remove all cables from the ignition coil negative terminal
Use a jump lead to connect the coil negative to battery negative or a known good earth
Now switch the ignition on and measure the voltage at the coil positive
Don't leave the ignition on for too long as the coil could overheat, 10 to 15 seconds isn't a problem
You will have then measured the supply voltage to the coil while it is under coil load (coil is drawing current)
The coil supply voltage can only be measured correctly by having the coil load on/in the circuit
A bad supply or a supply through a ballast resistor will still measure as 12 volt when the coil load isn't on/in the circuit
You need to remove all cables from the ignition coil negative terminal
Use a jump lead to connect the coil negative to battery negative or a known good earth
Now switch the ignition on and measure the voltage at the coil positive
Don't leave the ignition on for too long as the coil could overheat, 10 to 15 seconds isn't a problem
You will have then measured the supply voltage to the coil while it is under coil load (coil is drawing current)
You wont get 12v at the coil like that once the circuit is engaged the coil is transforming the supply into electro magnetism so it seems to disappear as the field builds up and the coil readys itself to deliver 30kv and yes dont constant earth it for too long or else this process will overheat the coil which should only be triggered for milliseconds in normal operation.
flyingdutchie said:
We.measured the 12volt at the coil.with a multimeter. It dropped a bit into the 11 volts when starting. The cap, rotor an amplifier are from rimmerbros. The cap is original land rover. The other parts are all lucas.
Ok, Lucas no longer exist so the components now are mainly Chinese tat in fancy Lucas boxes.Here in this topic https://www.pistonheads.com/gassing/topic.asp?h=0&...
You will notice that somebody with much knowledge confirms that the voltage at the coil positive should be close to battery voltage when earthing the coil negative
Don't be mislead by comments stating anything else
You will notice that somebody with much knowledge confirms that the voltage at the coil positive should be close to battery voltage when earthing the coil negative
Don't be mislead by comments stating anything else
GreenV8S said:
Sounds like you have shown it is an ignition fault then. That's good news because it means you can eliminate anything to do with the ECU, sensors, fuel supply etc. The ignition is very straightforward.
The first test is that you have 12v on the +ve side of the coil - I understand you've confirmed this.
The second test is to momentarily earth the -ve side of the coil and show that you get a spark on the king lead. If you don't get a spark then you probably have a duff coil, but just double check that the +ve side is still showing around 12v while the -ve side is earthed (don't hold it earthed for more than about a second while you do this or you risk overheating the coil).
If you do get a spark then you need to do a similar test for the ignition amp and if that checks out OK you may have a duff ignition pickup.
Bear in mind that it's easy to damage HT components while you're plugging and unplugging things so even if you find and fix the fault, don't assume that everything else is still good.
And since it's an ignition fault you've almost certainly flooded the plugs by now so once you've sorted the ignition, fit a new set of plugs before you make a serious attempt to start it. And make sure the battery is kept charged up. It's far harder to start an engine if the battery is starting to run down and you need everything on your side until the problem is sorted.
The first test is that you have 12v on the +ve side of the coil - I understand you've confirmed this.
The second test is to momentarily earth the -ve side of the coil and show that you get a spark on the king lead. If you don't get a spark then you probably have a duff coil, but just double check that the +ve side is still showing around 12v while the -ve side is earthed (don't hold it earthed for more than about a second while you do this or you risk overheating the coil).
If you do get a spark then you need to do a similar test for the ignition amp and if that checks out OK you may have a duff ignition pickup.
Bear in mind that it's easy to damage HT components while you're plugging and unplugging things so even if you find and fix the fault, don't assume that everything else is still good.
And since it's an ignition fault you've almost certainly flooded the plugs by now so once you've sorted the ignition, fit a new set of plugs before you make a serious attempt to start it. And make sure the battery is kept charged up. It's far harder to start an engine if the battery is starting to run down and you need everything on your side until the problem is sorted.
O mage said:
You wont get 12v at the coil like that once the circuit is engaged the coil is transforming the supply into electro magnetism so it seems to disappear as the field builds up and the coil readys itself to deliver 30kv and yes dont constant earth it for too long or else this process will overheat the coil which should only be triggered for milliseconds in normal operation.
NonesenseGents, don't go chasing coil voltage as the cause. I've two Wedges and both measure 9 volts with power on. Both coils state they are 12 volt units and both engines run great. The issue will be something else. I've had three ignition modules fail on me over the last few years. I also had a new coil, distributor and HT leads fail within 7 weeks of purchase all bought from a well known ignition specialist near Worcester, so new equipment doesn't mean it's not the fault. The faults with the new coil and HT leads were diagnosed by running the engine at night in the dark. The top of the coil was covered in a spiders web of HT discharge and the king lead glowed blue where the insulation had broken down and was leaking HT to the rocker cover. It was surprising and impressive to see.
Edited by KKson on Saturday 18th July 09:32
^^^^^^
Yes we have been through this in another thread and confusion reigned and we wasted multiple pages on it, all because of failure to understand how to test an ignition coil and what readings to expect. That person will claim a ballast resistor fault explain's the reading seen at the coil even when the car runs and drives. If You have 12v feed and you load it up with a headlamp bulb and it lights fully then this proves you have 12v and that the feed can pull 5 amps. Job done feed checked if you then attach the feed to the coil and start checking the coil in operation then you will see a drop.
Yes we have been through this in another thread and confusion reigned and we wasted multiple pages on it, all because of failure to understand how to test an ignition coil and what readings to expect. That person will claim a ballast resistor fault explain's the reading seen at the coil even when the car runs and drives. If You have 12v feed and you load it up with a headlamp bulb and it lights fully then this proves you have 12v and that the feed can pull 5 amps. Job done feed checked if you then attach the feed to the coil and start checking the coil in operation then you will see a drop.
KKson said:
Gents, don't go chasing coil voltage as the cause. I've two Wedges and both measure 9 volts with power on. Both coils state they are 12 volt units and both engines run great. The issue will be something else. I've had three ignition modules fail on me over the last few years. I also had a new coil, distributor and HT leads fail within 7 weeks of purchase all bought from a well known ignition specialist near Worcester, so new equipment doesn't mean it's not the fault. The faults with the new coil and HT leads were diagnosed by running the engine at night in the dark. The top of the coil was covered in a spiders web of HT discharge and the king lead glowed blue where the insulation had broken down and was leaking HT to the rocker cover. It was surprising and impressive to see.
NonesenseEdited by KKson on Saturday 18th July 09:32
Coil voltages are and always have been the first things to get right before looking elsewhere
9 volts will not drive a 12 volt ignition coil correctly, this is nothing new
Primary circuit current drops when the primary voltage drops
Secondary voltage is governed by the strength of the magnetic field that is created by the primary winding
Lowering the voltage supply to the coil lowers the secondary voltage at the spark plugs
The OP hasn't got a good spark, hence the first thing to check is the primary voltage
The above has always been the case, it's nothing new, it's basics
The voltages around your vehicles ignition system are not going to help fix the OP's problem
Another very knowledgeable poster when it comes to ignition systems posts the facts here https://www.pistonheads.com/gassing/topic.asp?h=0&...
Apart from the information being readily found at reputable websites, the very same information can be found at the PH forums
Apart from the information being readily found at reputable websites, the very same information can be found at the PH forums
blitzracing said:
Im not convinced a rusty reluctor could have caused the origional fault (that came on suddenly- unlike rust). The rotor simply acts as a piece of iron that closes a magnetic circuit to a coil pickup, with the actual magnet being in the side of the distributior, its nothing fancy or technical. The coil on my 3.9 is a ballasted type so its running voltage is about 10 volts, not 12. If you fit a 12 volt coil, and then feed it 10 volts, its output is going to be much lower than it should be, albeit probibly enough to fire the plugs some of the time. If you want to check if the circuit you have is ballasted, ground the -ve side of the coil (to by pass the amps switching) and then measure the voltage on the +ve side of the coil with the ignition on. Only if you get the full 12 volts its a non ballasted supply to the coil.
O mage said:
You wont get 12v at the coil like that once the circuit is engaged the coil is transforming the supply into electro magnetism so it seems to disappear as the field builds up and the coil readys itself to deliver 30kv and yes dont constant earth it for too long or else this process will overheat the coil which should only be triggered for milliseconds in normal operation.
A gentlemans bet wouldn't go amissWhile we are on the subject again regarding ballast resistors. A vehicles system is 12v and ignition systems were 12v. they were only used on 9 volt coils the idea being that this 9 volt coil would put less stress on the points and condenser or switching amp. During cranking this 9v coil gets a full 12v to aid starting as the ballast is bypassed then when you turn the key back it gets fed 9v. If you have a 12v coil like most should be then you have no need for a ballast and if you did the 12v coil wont work.on 9 v. And you would be able to see it drop before the coil that everyone see's 12v at. the feed.
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