CB650SC/Ignition

Timing

The engine speed is sampled directly from the crankshaft by two pickup coils placed at Cyls. No. 1&3 TDC and Cyls. No. 2&4 TDC. The two "pickups" are VR(Variable Reluctance) type, and together are referred to as the "Pulse Generator" in Honda literature of vintage. Each pickup is a coil of wire, wrapped around a steel core. One end of the steel core is exposed to the outside of the rubber moulded casing of the pickup, toward a sheet of steel attached to the crankshaft. The other side of the steel core is a magnet with the north pole pressed against the steel core and the south pole facing away from the entire assembly. Each pickup has a coil of wire with a resistance of 330ohm +/-10%.

As the crankshaft turns the stamped steel bolted to it approaches, aligns with, then passes the steel core of each pickup. This induces reluctance, and a voltage dip and spike into the coil. At low RPM the crankshaft position is detected at a low frequency AND low voltage. The frequency is of course directly correlated to the engine speed, velocity, or RPM. The voltage is proportional, starting as low as tens of Volts up to hundreds of Volts at high engine RPM.

From what I have read, using two sensors is very important to preventing the engine from running in reverse, as the controller can detect which pickup is triggered first.

Control

The Ignition Controller is a TCI(Transistor Controlled Ignition) type. It is referred to as the IC(Ignitor Controller) in Nippondenso and Honda vintage.

1. Condition VR signal input from the Pulse Generator
2. Calculate Engine velocity and position
3. Calculate ignition advance (see Advance table) from velocity
4. Calculate ignition offset from position and ignition advance
5. Trigger appropriate coil's charge duration (dwell?) and ignition event

Advance Table

RPM	Degrees Advance
At idle min-1	10 BTDC
Full advance/min-1	32 BTDC/3000

CB650SC D-F(83-85) CDI Pinout (Module side)

	Pulse gen/pickup coils plug				Ign. Coils Plug
	3	2	1		2	1
3-2 Pickup ->	NP	Y	W	+	B/W	L	4-1 Ignition coil	12v signal
4-1 Pickup ->	NP	L	W	-	Dg	Y	3-2 Ignition coil
	6	5	4		4	3	Tees off to tachometer signal

Look-up

• Ignition coil primary resistance: 2.8 ohms
• Ignition coil secondary resistance: 13.6-15.5 ohms
• " with spark plug cap: 21-28k ohms
• Spark plug resistance: ohms
• Spark plug gap: 0.8-0.9mm (0.031-0.035in)