MAF Mass Air Flow adjust


I have a MAF from 1990 with an adjusting screw.

how much ohm has to be set between pin 1 & 6?

at the moment it is set to 650 ohms


There is no pre-defined setting for MAF sensor. Those were adjusted on the car to match 1-2% CO at 800rpm (idle). Potentiometer was installed since 87 in XJS (non-CAT). It cheats the heater a little bit. Increasing resistance will bring voltage on the hot wire down making it cooler (more fuel injected as higher air flow is simulated)… Or maybe other way around :wink:

Before you’re going to fiddle with it - use a lot of WD40 or you’ll snap it off internally.
This should be done only with gas probe in your exhaust pipe to get to the required 2% or slightly above that (for non-CAT only).
My advice - run rich.

I have the same Lucas/Hitachi 5AM AFM on my MGB V8 (Land Rover 3.9)- if using the oxygen sensors, you adjust it to 1.8 volts with ignition on, but not running.
If running open loop mapping, it ends up 1.3 to 1.4 volts.
I assume it would be similar voltages as displacement is similar.

Hello. where can I measure the voltage while idling?

At the AFM connector; back probe the black/red (signal ground) and blue/red (idle trim). Have to use sharp probe to get through the weatherseal where the wires come out. I used furniture sewing needles…
Don’t know if your wire colors are the same, I can look later today for the wire positions in the connector.

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thanks for the help,
it should be GK (green / pink)?

then you measure against earth and then when the engine is idling and the engine is warm, there must be about 1.3-1.4volt?

No, idle trim which is Green/red according to your diagram and signal ground (black/yellow on your diagram).
The AFM signal measures .3v with key on, and measures 1.5v idling if I recall.

ahh okay, I slipped. the 1.5v at idle with a hot engine? if I am not connected to the 1.5v then turn the screw on the MAF … do you know whether less volts is fatter or leaner,?

No. The potentiometer only directly adjusts the idle trim (green/red) measured to signal ground. It has a slight affect on the AFM signal to about half throttle.
The AFM signal (Green/pink on your diagram)is in relation to airflow; ignition on not running should be .3v; it will go higher (1.5-1.8v at idle and close to 5v at full load, high rpm) as the engine starts. If it doesn’t, you may have a dirty or failed air flow meter.
As far as idle trim, lower is richer and higher is lean.

okay, and what can I adjust on the screw on the MAF ???

If you have oxygen sensors, 1.8v. If you do not have oxygen sensors, start at 1.4v. If it hesitates, adjust a little lower (1.3)

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