[alfa] RE: 86 Spider no start

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi Don,

Your AR Spider has the Bosch Motronic system, a derivative of the L-Jetronic used on
earlier vehicles.  It so happens that I know an awful lot about L-Jetronic & Motronic
especially the AR ones.  The only difference between the Motronic used here and the USA
version was the inclusion of an O2 sensor in the USA spec vehicles.

You have no fuel injector pulses because you have no spark!

The L-Jetronic used the pulses from the coil -ve terminal to clock the injector ECU to
give engine speed information.  If the ignition is bad the fuel will not be pulsed.

The Motronic design built on the L-Jet design with a number of subtle changes, the main
one is that is there is no switch in the AFM to energise the fuel pump.  Bosch changed
this when the Motronic was designed, instead the ECU has to see valid crank sensor signals
before the pump relay is turned on.  The fuel injection side is identical to the earlier
L-Jetronic, except the speed signal which used to come from the ignition coil -ve terminal
now comes from the coil driver trigger.

The ignition portion of the Motronic ECU needs two signals from the crank sensors.  One is
the rpm or speed signal which is the magnetic pickup that sees the starter teeth on the
flywheel.  There are 131 teeth on the starter ring gear, so for every revolution the ECU
expects to see 131 pulses.  When it has "counted" 131 pulses it knows that 1 complete
revolution has occured, but it has no idea what the crank shaft angular position is.
This is gets from the second magnetic pickup.

The second signal is called the "Reset" transducer and has an identical
magnetic pickup but this one sees a single pin or lump on the flywheel.  This gives
crankshaft angular position information.  On the '86 Spider the pin occurs at 57.5 degrees
before #1 TDC.  The Motronic computes the ignition advance using this "epoch pulse" to
define where #1 TDC is and from the rpm signal to compute how fast
the engine is turning.  (There is a limited built in default mode where it can provide a
fixed idle timing and injector pulse width if the rpm signal is out of specification but
this does not allow the engine to be revved up).

In order for the ECU to start looking for pulses several things need to happen, apart from
being powered up by the ignition switch being turned on.  There is no "Keep-Alive" power
connected to the earlier Motronic ECU's such as yours, this only occurred with the later
versions.

1).    The starter solenoid activating signal is routed to the ECU Pin 4.  When this line
goes above 6V the ECU switches on and starts looking for rpm signals from the starter
ring gear.  (This fixes the ignition timing at a low advance and a fixed injector pulse
width - computed from water temeraure - until the engine speed exceeds approx. 500rpm).

2).    If the rpm signal is present and above 100rpm the ECU then goes and looks for
Reset pulses which occur once per revolution.  If these are both within given parameters
the fuel pump relay is pulled in and the ignition coil driver stages are powered up.
Note: Cranking with a flat battery sometimes won't boot the ECU/fuel pump as the starter
signal is below 6V and the engine is turning too slowly!

3).    If the rpm signal is too slow the ECU assumes the engine has stalled and shut down
until the starter signal is again present.  This kills the spark and the fuel pump relay.

4).    There a limited start stategy designed in that allows the vehicle to be push
started.  After seeing rpm AND reset signals for longer than 1s the ECU will start
computing the spark timing and then fire the coil.  Note: There does not need to be a
starter signal present for this.

The most likely cause is the magnetic pickups.  These give endless problems through poor
connections and AR is not alone in this.   BMW & Porsche cars use the same Motronic system
and 99% of all non-start cases I have worked on the magnetic pickups connectors are the
culprit.  The actual transducers rarely give trouble but the 3 pin connectos are a PITA
when they become corroded.  (Recently I had to travel to Mozambique to try and get an old
Porsche 944 running.  On this car both magnetic sensors were dead, because the swinging
casting that sets the correct clearance between the sensors tips and the flywheel had been
swung too far and the flywheel nibbled the ends away so damaging the sesnsors.
Fortunately AR don't use such a stupid setup!)

Also if you accidentally get the two connectors mixed up the ECU doesn't understand the
incorrect signals and will stay in shut-down mode with a resulting no-start.  A very
common problem especially if the engine has been pulled and the connectors are incorrectly
attached to the wrong sensor.  Unfortunately Bosch didn't use different connectors for the
two so it is easy to get them mixed up.

The possibilty of putting the flywheel back in the wrong place will not result in a
no-spark condition, you will get spark but it will be in the wrong place.  As you have no
spark it isn't that.  IIRC the Alfa flywheel can only go on in one position.

I hope this helps to get your Spider running.

I have the official Bosch/AR service manual on the Motronic system,  like most service
manuals, it contains a lot of errors!  I have verified the system on the bench (I have a
test jig for trouble shooting the Motronic ECU's I built some years ago) and the
measurements I made on a running engine to confirm the system parameters.

Incidentally you may not have noticed this but Motronic actually does this.

The VVT solenoid on the inlet cam is controlled by the Motronic ECU.  It switched over
at approx. 1500rpm.  If the engine speed is increasing it pulls in the relay that powers
the solenoid.  If the engine speed is falling at 1500 rpm it drops out the relay.  But at
the switching point it switches off the coil driver whilst the VVT relay is operated.
This I noticed as a sudden drop in rev counter reading and then a sharp recovery.  The AR
rev counter is very damped and so you don't normally see this effect.  On my race car I
had an after market rev counter with not much damping and I wondered what caused the
needle to twitch, even tho' I was not using the VVT cam as I had Euro cams fitted to the
2L engine. The VVT relay was disconnected but the ECU still cuts the spark whilst it
switches over!

If you need pinout details please shout, I have this for the '86 Alfetta which uses the
same Motronic ECU, but no Lambda sesnsor.

best regards

John
Durban
South Africa
Alfetta 1.8L turbo
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