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MAF-testing

The Mass Air-Flow sensor is connected directly after the airfilter with large rubber boots on each side (one to the airbox, and the other to the DK inlets). There is one on all models, except the V8 and V12s have two because of the dual manifolds and inlets.

01_left_maf
The left MAF on a V12

These readings are performed on my 91 750iL using dual digital multimeters. Please note that the right one (multimeter) had a low battery and thus displayed a little lower than the left one. Grounding the meters at the big jumpstart nut on the left shock-housing, the setup looked like this:

 107_0713_IMG.jpg

To be able to probe the MAFs you can follow this procedure.

Disconnect the MAF-plug to read the pin-numbers located on the plug. These are very small digits and I had a little trouble making sure which was which. But I finally discovered that they are numbered 1 through 6 in a circular pattern, with number 7 (not connected) in the middle of the plug.

03_maf-unplugged
The MAF-connector unplugged

Then when you have located the desired numbers you can pull back the rubber boot on the backside of the plugs and locate that pin for easy backprobing while the connector is pluged into the MAF. The engine will not run if the MAFs are unplugged.

04_probing
Probing setup for the right row on the V12

The mixture can be adjusted by drilling out the black plastic cap indicated in the picture. Behind it is a screw that controls the mixture by adjusting the voltage to pin #6...

107_0725_IMG.jpg
Believed to be correct reading on pin #6 from the MAF-plugs (mixture level)

The above picture displays the reading of pin #6 on the MAFs when the engine is heated up and running at idle, and without having engaged any gears on an automatic transmission. This reading varies and increases somewhat with higher RPM, but should settle around this reading after a short amount of time (just tested to around 2000 RPM).

Below you will see the reading for the same pins with just the ignition turned to position 2 and engine not running.

107_0726_IMG.jpg
MAF-reading on pin #6 with ignition in position 2 and engine NOT running

The reading from pin #3 on the MAFs is a constant +5 volts regardless of engine temperature. It was measured constantly while warming up. This voltage is probably used to supply any electronics within the MAFs their constant operating voltage. +B (constant battery voltage) is supplied through pin #2 on the MAFs. And pin #1 (or 5 as I don´t recall) reads out a frequency of about 1.65kHz (varies from around 1.55 to 1.75kHz) when the engine is running, and hot. This could seem to be about twice the engine RPM, but is just a theory of mine.

107_0716_IMG.jpg
Constant voltage readings (pin #3 = +5v regulated, and pin #2 is +B)

107_0721_IMG.jpg
Frequency reading on MAF (and constant +5v-reading)

The wiring to the MAFs consist of 6 wires which I´ve outlined in the below table.

Pin numberWire Color
LEFT side
Wire Color
RIGHT side
Function
1Brown/White dotsBrown/YellowFrequency? (1.65kHz)
2Red/GreyRed/Light blue+B via ignition position 2
3Brown/Yellow dotsPurple/Yellow+5V regulated
4BrownBrownGrounding
5Brown/White/Yel. dotsGrey/WhiteFrequency? (1.65kHz)
6Brown/Grey/Yel. dotsWhite/BlueMixture voltage (approx. 2.4V)

 

 

 

 

 

I didn't do any extensive probing, and forgot to make a note of which of pin #1 and 5 that carried the 1.65kHz frequency... The Mixture Voltage was 1.38VDC with ignition on, and engine not running. While running, and hot, the voltage was 2.38-2.40VDC.

 

 
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Spent this weekend tinkering with the eletronics. Fixed the brake linings error message and wired up power, ground and signal cables for the new headunit that hopefully arrives soon.
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Date: 04/12/2007 Views: 6212 Owner: Amuso

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