The Motronic ML4.1 ECUs support a CEL, but lamps were not fitted for some reason.
This is how to wire up a CEL, and how to flash the fault codes out.
Pin 4 on the ECU is for the diagnostics switch. It's usually a blue wire with a yellow stripe on ML4.1 Alfas.
Connect a momentary switch (push button) between this wire and ground. Pin 5 (black wire) or pin 16 (black) are both grounds, or any other convenient ground point will do.
Pin 17 is for the Check Engine lamp (CEL). It's usually a blue wire with red stripe.
Connect a small lamp (or LED with suitable resistor) between this wire and a 12v power source.
Pin 18 on the ECU (red wire) will have 12v on it so you could connect to there. Or connect to any convenient 12v source on the car.
So look for a plug/plugs that have blue/yellow, blue/red, black and possibly a red wire. If you have a wiring diagram showing the ECU connections, follow the lines from ECU pin 4 and 17 and they will most likely both run to the same plug somewhere.
A wiring diagram would give you an idea where to look for this plug, and how many pins it has.
Alfa often used a rectangular 4 pin plug under the dash for these connections, but some models had a round 5 pin plug in the engine bay.
Once you have the lamp and switch connected, switch ignition on but don't start the engine. The lamp should turn on with ignition.
Then press the diagnostics switch and hold for about 2 seconds.
When you release the button, the CEL should give one long flash, then 4 sets of flashes with a pause between each set.
Write down the number of flashes in each set, i.e. if you see one long flash, then a pause, then 1 short flash, pause, 2 short flashes, pause, 4 short flashes, pause, 2 short flashes, pause then a long flash, that would show the code 1-2-4-2.
It will keep displaying that code over and over until you hold the diagnostics button on for a second or two, then it will move on to the next code.
There can be up to 5 codes for it to display.
If you only get the code "4-4-4-4" it means that no error codes are stored in the ECU.
To then get the ECU into self-test mode, the procedure is something like:
1) switch ignition off
2) hold diag button down
3) switch ignition on
4) keep holding diag button down for about 2 seconds
5) release the diag button, ECU should now be in self-test function #1 (injector self-test)
6) press diag button to advance to the next self-test function, and so on.
Here's a list of the DTC flash codes for the Alfa ML4.1 ECUs.
Fault codes
4-4-4-4 No DTCs in memory
1-2-5-1 ECU/EPROM failure
1-2-1-2 Idle microswitch short to ground
1-2-1-3 WOT microswitch short to ground
1-2-2-1 AFM reading not appropriate for RPM
1-2-1-1 Battery voltage > 16v
1-2-2-5 IAT out of range
1-2-1-4 CTS > 140* or shorted to +
1-2-1-5
1-2-2-4 Lambda sensor voltage < 0.1v or >1.0 v (i.e. out of range)
1-2-2-3 Lambda sensor out of correction limits
1-2-2-2 IAC short/open circuit
1-2-4-3 Fuel pump relay output error
1-2-4-5 VVT output circuit error
1-2-4-4 Evap solenoid circuit error
1-2-6-5 CEL output circuit error
1-1-1-5
Self-test function codes:
1-4-1-1 Injector test in progress
1-4-1-2 IAC test in progress
1-4-1-4 VVT solenoid test in progress
1-4-1-3 Evap solenoid test in progress