AFM adjust!

I have found the values should be between 1v- 1.5v but I have 4v and need to adjust it. I have been turning a Allenkey screw on the side of top and it makes no difference at all? I have pulled this out and found a little flat head screw behind it, is this what I should be turning?

sible88 said:

I have found the values should be between 1v- 1.5v but I have 4v and need to adjust it. I have been turning a Allenkey screw on the side of top and it makes no difference at all? I have pulled this out and found a little flat head screw behind it, is this what I should be turning?

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This doesn't sound right.

The voltage value you quote - where were you measuring this and was the engine running?

The screw you have removed is for adjusting a variable resistor inside the meter.

As a base setting, remove the plug from the air flow meter, use a multi-meter and measure the resistance between the two outside pins. The value should be 300 ohms. The value can (or should) be changed by turning the allen bolt - clockwise makes it richer, anticlock leaner.

If you are measuring between sensor ground and air flow signal, the voltage will be something like 1.7v at tickover rising to 4.5v at full load. With engine off and ignition on, the value should be between 0.2v and 0.7v.

However, CO trim value is different and should be 1.0v - 1.5v at tick over.

Make sure you know what you are measuring. All of the above assume a 3.9 with a 3AM air flow meter.

Do you have an efi warning light? is it on? What happens if you disconnect the air flow meter?

Yes I have a 3.9 efi with no warning light as it's in my 90. When I dos-connect the plug it runs sweet but 6mpg :-( I done the check between ground and co trim with engine running and it was 3.97v

sible88 said:

Yes I have a 3.9 efi with no warning light as it's in my 90. When I dos-connect the plug it runs sweet but 6mpg :-( I done the check between ground and co trim with engine running and it was 3.97v

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I suspect you need a replacement air flow meter if you can't get the trim value down.

I had a similar problem a few years ago (before I knew how to check the AFM) the EFI warning light was off but it ran like a bag of nails. Disconnected the AFM, it ran better but very rich (6mpg). Had AFM and ECU tested, both came back no faults found. I subsequently had the ECU plugged in (on the car) and the diagnosis was the ECU was not reading the AFM. Changed ECU and not looked back but oddly enough the previously faulty ECU now works fine - go figure!!!!

So try a different AFM if no change, try a substitute ECU.

I have 3 afm all read different values so looks like I need new ECU

sible88 said:

I have 3 afm all read different values so looks like I need new ECU

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Can you set any of your AFM's to 300ohms between outside pins? ECU can't affect the internal components of the air flow meter, but if the ECU can't "see" the AFM it will default to limp home and use a fixed value that will enable the engine to run. This is a rich mixture, hence your 6mpg.

Have you checked that all the connections to the AFM are good. Continuity tests might be a good place to start, then check for good clean earthing. As I said earlier, the AFM can be given a base setting off the car by measuring resistance between the outside two pins on the AFM - value should be 300ohms, adjust with allen screw. If you can't get the value to change then the AFM is duff.

Usefull info will try this when I get home.