Author Topic: AFM Pigtail / Extension - what is this thing ??? HELP (Read 3738 times)

dcroteau · « **on:** January 08, 2010, 09:11:09 AM »

My 91 318 with of course the M42 engine starting dying on me, luck was with me as I was just about pulling into my garage. An examination of the check engine fault code indicated the the AFM.

Looking for the cause I was suprised to find that in between the factory connector and the AFM there was an extension . Not pin for pin this thing looks factory and has male/female round connectors. But when I peeled back the rubber boot it has about a 3/4" square black encapsulated juntion where some alteration of the wires going from point a-b occurs. One of the pins at the AFM becomes a no connect and there is now some measured resistance betweeen two other. The cause of my problem was a broken wire in the factory harness/connector which I repaired.
The car runs fine again and the check engine light is off. I reconnected the pigtail extenision since it has been there all along. But what the heck is this it. I'm thinking maybe a factory upgrade or it's some kind of aftermarket item.

Can anyone shed any light on this thanks

Justonegear · « **Reply #1 on:** January 08, 2010, 09:53:26 AM »

I think it's in case your buddy looses his, you have a spare. I to would like to know what it's for.

Wizard · « **Reply #2 on:** January 08, 2010, 10:05:38 AM »

It had something to with "boosting signal to the AFM...This was posted on another site:

Quote

The wire extension as mentioned is a factory retrofit. It serves as a signal amplifier for the AFM signal. In some case the M42 experienced bad idle and running issues/problems and it was discovered that the AFM signal to the ECM was weak. The pigtail you have their is the fix for that problem.

I have been running without it for a few years now and never noticed any difference..Really not sure how it boosts the signal though.

DesktopDave · « **Reply #3 on:** January 08, 2010, 04:30:39 PM »

I have a spare too. I pulled it because the seal wasn't water tight and it was giving me the 1215 code. Cleared up as soon as I pulled that harness, and I can't tell it's missing.

It's a resistor pack from what I've heard, an OEM factory recall part from BMW to improve fahrvergn�gen with the flapper. There is a small sealed circuit board in one of the boots.

Wrench · « **Reply #4 on:** March 06, 2012, 10:17:57 AM »

I just opened up the little black box. There's a small circuit board in there with an 8 pin chip on it and some other components. It looks to me like an "op-amp" circuit meant to condition the signal from the AFM. It basically takes the signal and gives it a boost and keeps it stable. Op-amps are all about signal conditioning.
Mine was not connected when I bought the car. It was in a box of parts in the trunk. I connected it and it didn't help with my current no revs past 2K problem.

bmwman91 · « **Reply #5 on:** March 10, 2012, 08:22:21 PM »

If there is an opamp & some resistors & capacitors, then it is a low-pass filter. There is no need to "boost" the AFM signal. It is just a big glorified potentiometer that will output ~0-5V. At very low flow rates, the sensor is very very sensitive (more so than at high flow rates), and intake pulses/resonance could make the output signal a little too noisy for the ECU. So, they retrofitted an active filter (opamp + R & C parts) to deal with it. That would be my guess anyway.

Wrench · « **Reply #6 on:** March 10, 2012, 09:21:35 PM »

You're right. Makes perfect sense.