Petrol from the Air Filter? (1988 8v)

[rubjonny]

The ECU controlls only fuel
There is a seperare ignition module which handles the spark, and the coil gets a signal from the hall sender so it knows what the engine is doing.  If the ECU is fried the injectors will just be stuck open flooding the engine.  Stuff to check:

4 Switch unit - testing
1 When making this test, the coil must be in good condition.
2 Remove the plastic cover on the right-hand side of the plenum chamber for access to the switch unit (see illustrations).
3 Disconnect the multi-plug from the switch unit and connect a voltmeter between terminals 4 and 2 (see illustration).
4 Switch on the ignition and check that battery voltage, or slightly less, is available. If not, there is an open-circuit in the supply wires.
5 Switch off the ignition and reconnect the multi-plug to the switch unit.
6 Pull the multi-plug from the Hall sender on the side of the distributor (see illustration), then connect a voltmeter across the low tension terminals on the coil (see illustration).
7 Switch on the ignition and check that there is initially 2 volts, dropping to zero after 1 to 2 seconds. If this is not the case, renew the switch unit and coil.
8 Using a length of wire, earth the centre terminal of the distributor multi-plug briefly. The voltage should rise to at least 2 volts. If not, there is an open-circuit or the switch unit is faulty.
9 Switch off the ignition and connect the voltmeter across the outer terminals of the distributor multi-plug.
10 Switch on the ignition and check that 5 volts is registered on the voltmeter.
11 If a fault still exists, renew the switch unit.
12 Switch off the ignition, remove the voltmeter and reconnect the distributor multi-plug.<p>

5 Hall sender - testing
1 Check that the ignition system wiring and plugs are fitted correctly.
2 The coil and TCI-H unit must both be in good condition.
3 Pull the HT lead from the centre of the distributor cap and earth it to the engine or bodywork.
4 Pull backthe rubber boot from the switch unit and connect a voltmeter between terminals 6 and 3 (see illustration).
5 Switch on the ignition and turn the engine by hand in its normal direction of rotation. The voltage should alternate from between 0 and a minimum of 2 volts. If not, the sender is faulty and must be renewed.

[Mirage-]

Hmmm if the sensor/switch in figure 4.6a was broken would this prevent a spark from being produced even when testing that a spark is coming from the coil lead?

Just wondering as I seem to remember the last guy that owned this car saying something about that switch cracking.  I've been meaning to replace the cap/arm and sender for months but never got around to it.

Won't be able to do the tests till tonight now as I'm at work.

[rubjonny]

I'm pretty sure it will, as looking at the diagrams it looks like it provides the only signal to the coil/ignition module from the engine.

I don't think you can replace the hall sender seperately though, I think its a dizzy out job!  Not 100% on that though I've never really looked into it.

[mk1]

If this is a PB engine, isn't the ign amp switched by the ECU thru the green wire?, this is how it controls the ign timing as the distributer has no advance mechanism in it. (now putting my asbestos pants on in anticipation of being shot down in flames )
Steve.

[rubjonny]

Yeah apparently the ECU does control the ignition advance, you are correct there is no mechanical/vacuum control on the dizzy.  I'm pretty sure you should still get a spark even with a fragged ECU, but I could be wrong

[AudiA8Quattro]

I thought the digifant system and motronic all have ignition controlled by ECU, as Steve said, there is no advance mechanism in the distributor. So buggered ECU will mean no spark.

[mk1]

I think there is a good possibility the ECU is toast due to the earth problems, the engine would not crank due to the cylinders filled with fuel and hydraulicking from the injectors being held open by the ECU.
Steve.

[AudiA8Quattro]

So the verdict is its f**ked then 

[rubjonny]

Well after reading the Haynes manual again, the transistorised ignition system is controlled by the TCH-H switch. on Digifant engines ignition advance/retard is controlled by the ECU as we have said, but it is not clear wheather or not having a fried ECU would prevent a spark completely. 
Personally I would think not as you can still get the engine running on a carbed engine with the transistorised type ignition when the vacuum pipe to the vacuum unit is disconnected.

But this is only theory on my part, if I had a digifant GTI I would be outside trying it right now

[AudiA8Quattro]

I wouldn't think it would spark if the ECU is f**ked, it would be the same as unplugging it.
Of course a carb engine will work with the vacuum unit unplugged on the dizzy, this is because ignition advance is also controlled by centrifugal weights in the dizzy.