Hi, not sure if it's any help but I have an Alfa Romeo manual for the 164 - similar age with a Bosch ABS system - that I could scan and email the relevant pages to you? There's only 8 pages but there's no trouble shooting guides or wiring diagrams unfortunately..... I may have a wiring diagram somewhere, if you need that let me know and I'll have a rummage!
It's odd that you have an intermittent issue. From my attempt to install ABS on the Stratos replica I learnt that the system on the 164 has a warning light on the dash that illuminates when the ignition is switched on. It then does a check of the sensors and pump solenoids (I think) and if all are present it switches off the light. At speeds over 5kmph the system reads the wheel signals and if it detects variations from any sensor it goes into failsafe and illuminates the warning light.
If you have a light that stays illuminated I would expect that it's seeing a wiring issue at one or more corner? Don't be tempted to check for continuity with 12v, I think that will damage the wheel sensors. If there is no evidence of chafed/damaged wiring (check the semi rigid cables where they enter the sensor body as that's an area I've seen breakages). I'd also be looking at the gaps on each sensor to the trigger rings and the state of the reading surface of each sensor in case they're damaged. The gap ought to be between 0.4 and 0.6mm but it should work with a range between 0.25 and .75mm according to handwritten notes on the manual I have. I think it might be wise to check the gap around 360 degrees of wheel rotation in case you have a damaged trigger ring running eccentrically or out of true? This may be caused by worn wheel bearings? If one is out of tolerance it might explain why it's okay at one start up but not at the next however I would expect a fault such as that to generate a fault as soon as the car is moving over 5KmPH.
Drop me a PM with your email address if I you need me to send anything over.
Guy
