Tachometer doesn't work! :-(

[Orin Eman]

> On Sat, 15 Dec 2001 20:11:27 +0100, you wrote:
>
> >Someone here said distributor hall sensor, how do I test this better
> >than the method in the Bentley? Does this provide the tachometer
> >signal?
>
> I have to add some more information. When the engine is running, I can
> unplug both the hall sensor and the flywheel TDC sensor without
> difference. The only sensor needed for the engine to run is the
> flywheel RPM sensor, but it needs all 3 to start.

Bad things are going to happen if you unplug the timing sensor if the
RPM sensor should miss some teeth.  I wouldn't do that.
The timing sensor is actually 62 degrees before TDC on Cyl 1.
If the car starts, the hall sensor is working.  On later cars,
I don't think anything bad will happen if it fails while the car
is running.  Not necessarily true on earlier (MAC11) cars which have
a bug in the cylinder counting code if the hall sensor fails.

> Checking the Bentley wiring diagrams, it appears that the tachometer
> is connected to the ECU.

Yes, the ECU provides the tach signal.  It mimics the signal that
a conventional ignition provides so the tach is a standard unit.
If the ECU is happy with its sensors and firing the ignition,
you should have a tach signal.  If not, I'd guess the driver
in the ECU might be blown or the wiring broken or shorted.

> Is it possible for the distributor to become loose and move so much
> that the ignition timing gets screwed up? It does not seem loose now.

Timing does not depend on distributor position.  It depends on
flywheel teeth and the timing sensor.  Distributor position does
affect the ability to start since the ECU needs to identify the
compression stroke on cyl 1 - and of course, the rotor needs
to be pointing to the right cylinder.

> Another strange thing is that the cars runs just as good as it used to
> over 2000 rpms, it's just hard to start, runs bad on low rpms and with
> a non-working tachometer.

I'd check the temperature sensors for the poor running.

Orin.