code dump - code 2111 speed sensor + RPM & Hall interoperation

[Ben Swann]

Subject: 2111 code on 97 5ktq

Doyt,

You will not find the information on exactly how the Reference and Speed
sensors operate in the Bently.  I have the qlist in copy here so that any
others knowledgable of the MAC-11 ECU internals operation would add
anything I've left out or is in error.

Information here is gleaned from AUDI service training guide which provides
some of the details of the speed sensor, RPM sensor and Hall signal and how
these signals are used an interoperate.  Probably the actual operation with
repect to ECU internals is in some obscure engineering document.

Anyway, in short, the speed sensor as it is called is a magnetic pick-up
which produces 135 pulses per engine revolution - 1 pulse/ flywheel tooth.
 The pulses are used by the control unit to determine engine speed and the
ignition timing point.

The reference sensor senses a pin in the flywheel and generates a pulse - 1
pulse per crankshaft revolution at 60 deg. BTDC,  This signal is used by
the control unit along with the Hall sensor in the distributor to identify
TDC for #1 cylinder.

The reference and speed sensors are located right next to each other and
AFAIK usually replaced together.

The Hall sensor sending unit in the distributor produces one broad signal
(square wave) per each distributor shaft rotation just before ignition TDC
on # 1 cylinder.  Only this signal permits the 60 deg. BTDC signal to be
accepted as a reference mark signal in the control unit.  If the Hall wheel
is out of it's window, the signals are ignored, a code is generated and the
engine won't start.

Once the engine is started, the reference sender pulse is ignored AFAIK and
the Hall signal determines spark/RPM.  I am not sure about the speed
sensor, but it appears to help keep the signals in sync. while the engine
is running.   The reference sender is required therefore for the engine to
start, and is initially syncronized with the speed sensor and Hall  window,
so my best guess is they are all needed for the engine to start, but once
running, only the Hall sensor is actually required (except perhaps to
resync the signals) - I don't know/just an educated guess.

I have seen where the engine can be mad to run when the RPM and Speed
sensor signals aren't present but the fuel pump relay wire(pin 21 on ECU
connector) is wired to ground - aka the charlie mod which disables
overboost cutout.  I think this is because the ECU requires the sensor
pulses to be present in order to operate the fuel pump.

What this boils down to is that your engind will run without proper speed
sensor signal, but it is a good idea to replace it, because the engineers
must have had a good reason to have it.

Ben

Subject: 2111 code on 97 5ktq

Hello listers.......a question about code readings.

2111 indicates "RPM sensor signal not getting to Mac 11C ECU in my 87
5ktq."  Yet my speedometer and tachometer work just fine, as does the dash
computer.  I think suspect that each of these components depends on that
same RPM signal.   ....but maybe not.

If the above is true, and if my ECU isn't receiving an RPM signal, that
would indicate that the wire between the signal generator and the ECU is
broken.  (but not broken to other locations such as tachometer and dash
computer?)

Or, Do those components run off some other signal?

If all these components run off the same RPM sensor signal, and some of
them are getting that signal, then I can conclude that the RPM signal
generator (sensor) is functional and does not need replacing.  But I'm not
sure that they all run off the same sensor. That is my first
question.....do they?

My second question asks how the ECU _uses_ the RPM signal. Does the RPM
signal have anything to do with the way the ECU handles starting, or how it
adjusts timing or fuel delivery or boost?

I've studied Bentley on this, and need a little reassurance about the above
details.

Doyt Echelberger
87 5ktq in OHIO   USA