Roger 04 RT
Registered user
The early twin-spark R1150R/GS/RTs connect all three ignition coils (2 stick coils for the primary plugs and one conventional coil for both secondary plugs) to the battery, directly through the contacts of the key switch. Later R1150RTs added a "2nd Load Relay", which connects the stick coils directly to the battery through a relay. Why did they do this?
From reports, the measured resistance of the secondary coil is less than 2 ohms. My measurement of the stick coils shows a primary resistance of less than 1 ohm. That means that the saturation current (maximum current flow) through each stick coil is 12 amps (24 amps total) and for the lower-plug coil is 6 amps--those current levels are only achieved momentarily, for a few thousandths of a second. At saturation current, the total momentary current is 30 amps. The parallel resistance of all three coils is about 0.4 ohms.
I measured the contact resistance of the key switch on my twin-spark at about 200 milli-ohms, or 0.2 ohms. Do the electrical engineering math on this and knowing that it takes time for an inductive coil to charge and reach saturation current, if the battery voltage was 12V and when all three coils reach saturation, the voltage at the top of the coils will be 8V and it will be 8 volts for anything connected to the key.
What is connected to the key circuit:
--two stick coils
--the lower plug coil
--Fuse 1
--Load Relief, Fuel pump, & Motronic Relay control coils
--Kill Switch
--Tachometer
--other relays and indicators
Looking at the list there isn't anything that is going to be too seriously upset by the voltage spikes. But during the initial moment of cranking, when the coils are momentarily left on for maximum dwell and the starter has dropped the battery voltage to 9-10 volts, the voltage dips, at the key switch could reach 6 volts. Far from ideal.
When some engineer at BMW realized that adding the stick coils had increased the current 5X through the key switch, and that some starting and running problems could be traced to it, they decided to reengineer the wiring harness and add a "2nd Load Relay" and then route the stick coils through that relay, directly to the battery. I don't know why they didn't also route the lower plug coil at the same time.
I've built a wiring harness and relay (done), switched by the key, that connects all three ignition coil directly to the battery.
From reports, the measured resistance of the secondary coil is less than 2 ohms. My measurement of the stick coils shows a primary resistance of less than 1 ohm. That means that the saturation current (maximum current flow) through each stick coil is 12 amps (24 amps total) and for the lower-plug coil is 6 amps--those current levels are only achieved momentarily, for a few thousandths of a second. At saturation current, the total momentary current is 30 amps. The parallel resistance of all three coils is about 0.4 ohms.
I measured the contact resistance of the key switch on my twin-spark at about 200 milli-ohms, or 0.2 ohms. Do the electrical engineering math on this and knowing that it takes time for an inductive coil to charge and reach saturation current, if the battery voltage was 12V and when all three coils reach saturation, the voltage at the top of the coils will be 8V and it will be 8 volts for anything connected to the key.
What is connected to the key circuit:
--two stick coils
--the lower plug coil
--Fuse 1
--Load Relief, Fuel pump, & Motronic Relay control coils
--Kill Switch
--Tachometer
--other relays and indicators
Looking at the list there isn't anything that is going to be too seriously upset by the voltage spikes. But during the initial moment of cranking, when the coils are momentarily left on for maximum dwell and the starter has dropped the battery voltage to 9-10 volts, the voltage dips, at the key switch could reach 6 volts. Far from ideal.
When some engineer at BMW realized that adding the stick coils had increased the current 5X through the key switch, and that some starting and running problems could be traced to it, they decided to reengineer the wiring harness and add a "2nd Load Relay" and then route the stick coils through that relay, directly to the battery. I don't know why they didn't also route the lower plug coil at the same time.
I've built a wiring harness and relay (done), switched by the key, that connects all three ignition coil directly to the battery.
