Lucas Multipoint Injection

Hotwire Air Flow Sensor

(applies to Lucas EFI equiped cars only)

Description: Hotwire  type sensor, the actual  "hotwires" taking the form of film type semiconductor resistors.

Location: Located in inlet duct between the throttlebody and air filter, a 3 pin connecting plug to wiring loom is located below inlet duct.

Operation: Essentially the sensor consists of two semiconductor film resistor elements connected together in "half bridge" configuration so that the same current passes through both. The circuit is supplied with a constant voltage. One of the elements is exposed to the intake airflow the other is shielded from the flow. The purpose of the second element is  to provide temperature compensation. The current passing through both resistors heats them both reducing the resistance however the cooling effect of the airflow on the exposed element has the opposite effect. As a result the voltage at the point in the circuit between the resistors will  increases with increasing airflow.

Potential Problems: Because the two elements are exposed to different conditions, ageing has an unequal effect on the resistance of each element this can cause the signal output to  creep upwards, causing over richness. The electrical connecting plug  for the sensor sits below the intake trunking and is exposed to dirt and corrosion. The unit grounds through the ECU pin 12


Wire Colour on Sensor
Wire Colour on Loom
Connected to
Main Relay
Green with Grey Trace
Fuel ECU pin 21
Fuel ECU Pin 12

Testing: Use a Digital Multi-Meter (DMM) or Oscilloscope fitted with "wire piercing tips" to tap into the output signal and earth wires to measure the output and ground voltages. Earth/ground  return should show a voltage of 0.25 volts or less when cranking.

Sensor Output Voltage

Running Condition
My 820Sli
Ignition engine not running
Typical 0.7 Volts
Normal Idle 950 RPM
1.4-17 Volts
Ouput over 1.7 Volts at idle will cause over rich mixture
Light Throttle 2500 RPM
2.2 volts
Full throttle high RPM
4+ volts 
 4.5Volts @ 4000  

Notes:  Made by Hitachi  for  Lucas is an expensive unit (£300) it is fitted to a number of Austin, Rover and MG cars.   It can be a cause of high fuel consumption if the output voltage is too high.