Astra J - Weird electric behaviour

[pscocoa]

Bad alternators seem to be able to manifest themselves in many ways but would high revving at idle as opposed to low revs /cutting out (which seems more the norm for an alternator issue) be a sign when coupled with other electrical issues?

It depends on the ECU. I don't know if your ECU does it, but, ECU's do monitor battery voltage. They also control idle speed (via the idle air control valve). Normally idle speed is mostly dependent on engine temperature - so on a cold engine it'll be 1000+ rpm, and then as the engine warms up it slowly reduced to 600RPM ish. However, it's entirely possible that your ECU also has a mode where it increases RPM if it detects low battery voltage, in order to increase the charging current generated by the alternator. If it's actually the alternator that's faulty, then battery voltage may never increase, so you get permanently high idle speed.

ECU's also often increase idle RPM when A/C is enabled - to compensate for the extra load the A/C compressor puts on the engine.

I don't know if the Ashtray ECU does any of these things, but it is at least possible, and could explain what was going on.

Thanks for this interesting perspective - seems logical if my ecu would behave in this way in the event of a failing alternator. I may get a chance to pose the question to Eden and VX as a "case file" - has been opened.

[LC0112G]

Bad alternators seem to be able to manifest themselves in many ways but would high revving at idle as opposed to low revs /cutting out (which seems more the norm for an alternator issue) be a sign when coupled with other electrical issues?

It depends on the ECU. I don't know if your ECU does it, but, ECU's do monitor battery voltage. They also control idle speed (via the idle air control valve). Normally idle speed is mostly dependent on engine temperature - so on a cold engine it'll be 1000+ rpm, and then as the engine warms up it slowly reduced to 600RPM ish. However, it's entirely possible that your ECU also has a mode where it increases RPM if it detects low battery voltage, in order to increase the charging current generated by the alternator. If it's actually the alternator that's faulty, then battery voltage may never increase, so you get permanently high idle speed.

ECU's also often increase idle RPM when A/C is enabled - to compensate for the extra load the A/C compressor puts on the engine.

I don't know if the Ashtray ECU does any of these things, but it is at least possible, and could explain what was going on.

Thanks for this interesting perspective - seems logical if my ecu would behave in this way in the event of a failing alternator. I may get a chance to pose the question to Eden and VX as a "case file" - has been opened.

It would require an intimate knowledge of what the software/firmware in the ECU does to know whether my suggestion holds water. No chance anyone at Eden, and unlikey anyone at Vauxhall have that knowledge. Someone at Bosch perhaps (assuming it's a Bosch ECU), or one of the owners groups who have successfully disassembled the ECU code.

I wouldn't hold my breath on getting a sensible answer.   

[countrywoman]

Sounds right, faulty alternator or battery will confuse ECU on most modern cars. My uncles Merc auto box was playing up , turns out it was a faulty battery even though it started and ran okay.

[pscocoa]

so - 3/4  weeks on - all fine in terms of air con, starting and no erratic rev counter etc but the service esp light has been on twice and disappears on restart.

This may possibly be the same issue as Tunnie had with Zafira in 2018 being a broken pick up washer on the hub I think. Will get codes read in next few days but would not think plugs or coil pack at this stage due to fact that after restart  it drives ok.