ThunderMax Servicing ThunderMax Equipped Bikes - Manual (Page 6)

Bypass Port. The Idle Bypass Port is a passage that bypasses the throttle blade, which unlike a
carburetor, is closed nearly airtight at idle. The plunger moves in or out to increase or reduce air
through the port, working in conjunction with fuel delivery as dictated from the ECM. This item and its
controller also play a big part in the engine warm-up cycle and how an engine descends and returns
to idle.
11. Crankshaft Position Sensor (CPS) – Mounted on the engine case in the crankshaft area.
Crankshaf
t is equipped with “teeth” that trigger the magnetic sensor and relay the data to the ECM,
which calculates RPM based on the information received. Multiple “teeth” allow for precise fuel
delivery and spark timing during engine revolutions.
12. Vehicle Speed Sensor (VSS) – Mounted on the transmission, reports vehicle movement and
speed to the ECM.
13. Electronic Control Module (ECM) – The electronic computer or “Brain” that processes
information received and controls all functions of engine management. The ECM collects data from
all engine sensors and controls fuel and spark requirements based on its map programming.
14. Oxygen Sensor
– Device that is attached to the exhaust pipe, usually 4-6” from the cylinder
head. It is used to measure the amount of unburned oxygen in the exhaust stream of an operating
engine. Two types of sensors are typically used on Harleys®, Narrow-Band (factory) and Wide-Band
(aftermarket).

Narrow-Band refers to a type of low-resolution sensor that is essentially a switch activated by
lean or rich readings.

Wide-Band is a very robust sensor capable of precisely measuring exact air/fuel ratios over
the entire Air/Fuel Ratio (AFR) band.