Adjusting MAF Sensors.

tblo760 · Post by **tblo760** » 31 Jul 2010, 10:24

Most of the early 200,700 and 900 series have an adjustment screw on the sensor,and can be used to reduce the emmisions if required.These MAFs have six wires,the White wire is the signal wire which is connected to a small potentiometer in the unit.By simply altering the resistance the mixture is changed.Other six wire MAFs that do not have the adjustment screw can also be adjusted.When weakening the mixture of a system that does not have a catalytic converter,there is no problem with the ECU.Altering the mixture when there is a cat fitted should only be done very small,this is because the Oxygen sensor will recognise a weak mixture and automatically richen it.The only way to "fool" the ECU,is to install an Electronic Fuel Injection Enhancer,(EFIE)which will add a small voltage to the circuit,the higher the voltage from the O2 sensor the weaker the mixture.

: MAF#1.JPG (25.03 KiB) Viewed 11614 times

: MAF#2.JPG (38.36 KiB) Viewed 11614 times

: MAF#3.JPG (33.01 KiB) Viewed 11614 times

Post by **billofdurham** » 31 Jul 2010, 11:49

Verrry interesting - as Wolfgang the German soldier used to say on the Rowan and Martin Laugh-in.

Thank you for the information.

Bill.

IC53 · Post by **IC53** » 15 Apr 2013, 20:16

Hello, can anyone advise on these? :
What would be the voltage for the following condition beween ground and the white wire:
1/ Smoothest operation:
2/ Lowest emissions:
3/ Best Power/performance:
4/ Best mileage:
Thank you in advance.

Post by **jimmy57** » 16 Apr 2013, 00:10

The adjustment screw only affects open loop mixture and can also offset base fuel trim so it is rich or lean but only when throttle switch is closed (idling).

The purpose of the screw was to give a remote adjustment to MFI ECU idle fuel trim. The MAF sensor signal at idle is not too accurate due to inherent error at low mass at idle. Once the oxygen sensor is warmed up and MFI uses it for fuel trim adjustment the screw is less important. Volvo procedure for adjustment is to do it with fully warned up engine, a/c off, out of gear. Adjust it have a high low voltmeter sweep or an even LED on -off time if using LED. This sets the base fuel trim to where the fuel trim compensation is working equal time lean and equal time rich. Rich and lean are relative, and in this case a small range. The best results were usually to find the proper rich-lean balance and then turn the adjuster screw 2 turns clockwise. The two turns offset makes the engine have better cold running behavior before fuel trim correction begins as engine warms up.

This screw is IDLE ONLY and therefore has no effect on MPG. IF you adjust it and get MPG improvement it will be pychosomatic improvement because you drive easier to maximize the effects of the "magic" screw you adjusted.

The 89 and later (90 for turbo) models got MFI ECU's with adaptive memory that "learned" the warmed engine fuel trim range center value and that became effectively the mixture screw set adjustment. The screw was dropped on these. The screw never adjusted the MAF sensor output. It was just where they placed a remote potentiometer for varying a voltage to signal fuel trim adjustment value.

Post by **98T5** » 16 Apr 2013, 08:07

Wow. I will look into this tonight. thanks

IC53 · Post by **IC53** » 16 Apr 2013, 19:43

Jimmy27, thank you for the detailed explanation.

I am one of the few people that logs all fill ups and calculates the mileage. I did play with the adjustment, the only thing that I found was that in one position the engine idles the smoothest.
I agree with your statement, I saw absolutely no effect on my mileage, even though a few times it appeared to be better, however a longer term tracking showed no difference. I am in Noert East USA, so I see Winter and Summer driving. I get just under 22mpg in Summer and around 18 mpg in Winter for an all around driving. The car is 1989 Volvo 740 Turbo at 180k miles.
Is there anything that can be done to improve the mileage on these cars? The car starts and runs well, so there is nothing that needs to be repaired or replaced [ other then several oil leaks that it sprouted - some of which I already took care of.]

Thank you again.

waynegarbo · Post by **waynegarbo** » 15 Dec 2015, 17:38

Hi Jimmy,

The voltage reading on my 86' 740 MAF Signal wire(mine is the very 1st one, white, on the left in the harness)is 2.68 volts. Should I gradually ea day
lower it by 0.2v until I get to 0.9-1.0v? Hope to hear from you

Waynegarbo