Ignition resistors
03-01-2012, 12:14 AM
#1
Thread Starter
Ignition resistors
OK, I tried to search, but it seems dead right now... Can someone explain what the 2 large (0.4 ohm and 0.6 ohm) resistors on the LH wall of the engine bay do? The 0.4ohm is inline on a wire going to the starter, and I'm trying to find out if I need to keep the resistor installed for my MegaSquirt conversion.
Thanks,
Thanks,
03-01-2012, 01:21 AM
#2
Rennlist Member
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The car uses an eight or nine volt coil. During normal running, coil power goes thru both the 0.4 Ohm and the 0.6 Ohm resistors in series. During cranking, the wire from the starter bypasses the 0.4 Ohm resistor to increase cranking voltage to the coil slightly.
03-01-2012, 02:14 AM
#3
Nordschleife Master
What Wally said, they are called Ballast resistors and my guess is that any system would use them, but it depends on the details of how your ignition system works if its changed. Pretty much all cars have used them for many many years, maybe ending with the coil on plug stuff.
03-01-2012, 03:20 AM
#4
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Wally:
I've never been able to measure more than about 4 volts at the coil on an early 928...usually lower than that. I've had several cars in that have "American" coils, designed to run on 8-9 volts and every single one of those cars wouldn't run correctly, because of the coil. I install the "correct" Bosch coil and everything is fixed.
I suspect that 12 volts running through 6 ohm and then 4 ohm resistors might not provide 8-9 volts and the Bosch coil is designed to run on lower voltage?
I've never been able to measure more than about 4 volts at the coil on an early 928...usually lower than that. I've had several cars in that have "American" coils, designed to run on 8-9 volts and every single one of those cars wouldn't run correctly, because of the coil. I install the "correct" Bosch coil and everything is fixed.
I suspect that 12 volts running through 6 ohm and then 4 ohm resistors might not provide 8-9 volts and the Bosch coil is designed to run on lower voltage?
__________________
greg brown
714 879 9072
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Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
03-01-2012, 03:57 AM
#5
Addict
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Wally:
I've never been able to measure more than about 4 volts at the coil on an early 928...usually lower than that. I've had several cars in that have "American" coils, designed to run on 8-9 volts and every single one of those cars wouldn't run correctly, because of the coil. I install the "correct" Bosch coil and everything is fixed.
I suspect that 12 volts running through 6 ohm and then 4 ohm resistors might not provide 8-9 volts and the Bosch coil is designed to run on lower voltage?
I've never been able to measure more than about 4 volts at the coil on an early 928...usually lower than that. I've had several cars in that have "American" coils, designed to run on 8-9 volts and every single one of those cars wouldn't run correctly, because of the coil. I install the "correct" Bosch coil and everything is fixed.
I suspect that 12 volts running through 6 ohm and then 4 ohm resistors might not provide 8-9 volts and the Bosch coil is designed to run on lower voltage?
Engine Running you are measuring a one sided square wave (or thereabouts) so with a meter that averages (or RMS's) AC voltage you are going to see something around 4 volts (being the average of 8 and 0V). EDIT that's not quite right as the coil fields are made to collapse by removing the ground which will have all sorts of strange effects due to back emf etc. but you get the picture.
What you see with the meter on DC will depend on the meter.
Engine off you should see the true peak voltage of the square wave, but then again you may see nothing if the coil is not energised until the engine is turning.
03-01-2012, 04:17 AM
#6
Rennlist
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All depends upon how and when you measure it
Engine Running you are measuring a one sided square wave (or thereabouts) so with a meter that averages (or RMS's) AC voltage you are going to see something around 4 volts (being the average of 8 and 0V). EDIT that's not quite right as the coil fields are made to collapse by removing the ground which will have all sorts of strange effects due to back emf etc. but you get the picture.
What you see with the meter on DC will depend on the meter.
Engine off you should see the true peak voltage of the square wave, but then again you may see nothing if the coil is not energised until the engine is turning.
Engine Running you are measuring a one sided square wave (or thereabouts) so with a meter that averages (or RMS's) AC voltage you are going to see something around 4 volts (being the average of 8 and 0V). EDIT that's not quite right as the coil fields are made to collapse by removing the ground which will have all sorts of strange effects due to back emf etc. but you get the picture.
What you see with the meter on DC will depend on the meter.
Engine off you should see the true peak voltage of the square wave, but then again you may see nothing if the coil is not energised until the engine is turning.
03-01-2012, 11:00 AM
#7
Thread Starter
Thanks All,
With the LSx coils I'm using needing 12V feed, I will eliminate these ballast resistors. I think I can also eliminate the BK/YW wire from starter post #16 also. My coils are powered from a relay on the CE pulling power from #30 circuit, then fused RH/LH for each bank.
With the LSx coils I'm using needing 12V feed, I will eliminate these ballast resistors. I think I can also eliminate the BK/YW wire from starter post #16 also. My coils are powered from a relay on the CE pulling power from #30 circuit, then fused RH/LH for each bank.
