[mdlug] Automotive technical info wanted (only slightly off topic)

[Drew]

At 11:12 AM 5/9/07, Robert Meier wrote:
>Drew,
>
>Perhaps this is a good subject for this Saturday meeting.
>Bring your automobile, and ...

Easy enough to do.

The van can easily seat a bunch of people, since I have not yet 
removed the seats. Not
as many as I've seen at the meetings themselves, but probably enough 
to handle the
Panera crowd. We could, in lieu of said after-meeting meeting, have a 
rolling demo. I'll
load up the Palm m105 and bring it and the serial cable in any event.

That way I might even be able to get some readings of instantaneous 
air mass flow versus
speed, which might be enough, along with a coast-down test that I 
plan to do at some point,
to decide what exactly something needs to be done *about*.


>I can bring my OBD-II modem and voltimeter to the meeting this Saturday.
>If I get my laptop working,
>I will also have my ScanTool.net software available for demo.
>
>Without looking in the service manual, I suspect my MAF sensor, like
>most of my sensors
>         o is buried in my car out of reach without impractical disassembly
>         o is closely attached to or potted with a PIC-like chip
>         o has no analog output externally available
>         o has only OBD-II signals externally available

Looking in my Chiltons at the engine control system schematic: The 
MAF sensor itself is
drawn as a box into which go four wires, and inside of which is a 
resistor symbol and a
three-lead variable resistor symbol. Out of the sensor, there is a 
black-and-white wire to
ground; a red wire to a host of other devices, probably leads 
ultimately to the positive
battery post; a light blue and red wire to the MAF sensor port on the 
Powertrain Control
Module; and a tan and light blue wire also to the PCM, labeled MAF return.

MAF sensor general information and testing 6-12.
Replacement 6-18.

           The Mass Airflow (MAF) sensor is installed in the air intake duct
           on 3.0L 
engines...........................................This sensor uses a
           hot-wire sensing element to measure the molecular mass (or 
weight)* of
           air entering the engine. The air passing over the hot wire 
causes it to
           cool, and the sensor converts this temperature change into an analog
           voltage signal to the PCM.

Expected voltage readings:
     Switch on, engine not running, sensor disconnected: >10V on harness.
     Sensor reconnected, engine idling: 0.5 to 0.7V MAF+ signal to ground.
     Increased RPM (car still sitting in driveway): 1.5 to 3.0 volts.

<Pops the hood...>

And there it is! Sitting right on top of the tube between the filter 
box and the manifold,
Ford Part no. F72F-12B579-AA (a 1997 part, hmmm...), with a four wire harness
plugged into it. Two of the wires (the signal wires) are different in 
color. No PIC chip
anywhere to be found.

It looks like I can construct my own GPH gage after all. Only issue 
is how high an
ohm voltmeter I need - I'd like to use an analog so that I can put my 
own markings
behind the needle...

Additional info: I've determined experimentally that (1) the trip 
odometer reads 15%
short. That is, for every mile I travel, the trip odometer advances 
0.85 of a mile; and
(2) the speedometer agrees with the trip odometer at the 60 MPH mark. That is,
when the speed gage reads 60, it takes 60 seconds for the trip 
odometer to advance
by what it claims to be a mile. It seems that I am pretty damned 
close to having a
nautical speed/odometer. (Almost exact, according to the HP48 emulator.)

I'll bring along some lengths of wire, knife, and wire tool. I'll 
bring the Chiltons along
too.

----

- Drew.



Footnote *

   I hope this is a wording error on Chilton's part. It seems to me 
that the "molecular
   mass (or weight)" of air should be around 29 grams per mole at all 
times and under
   all terrestrial driving conditions. Hopefully they meant to say "mass flow".