Alternator diodes dead?

Hello,

I'll try to write in my best english...

I had a problem with my car 2 weeks ago: when driving at fast speed, the battery light randomly flash before staying continuousely on, then other warning light appear, steering assist go away, speed indicator make 0-200 kph twice in a second, dashboad bips.... I stopped the car and it does't restart  The repairman did a quick test on the parking and accuse the alternator. Some days later the garagist tell my that nothing is to be repaired because everything is ok: he did a decharge/recharge of the battery (ok), alternator is charging, warning leds are off, he drive with all electric load at max usage and everything was looking fine.

Nevertheless I sometime experienced some battery light flashing but not the same nightmare as the first time.

The battery is old (2010) but appear to be working fine :
- 12.6V car OFF
- 12.45V car ON, motor OFF
- 11.7V car ON, motor OFF and light+ventilators ON
- 14.3V motor idle
- 14.4V motor at 3000rpm
- 8.5V during motor boot (voltage monitoring attached)
Battery voltage, starting motor (1s):
 
Battery voltage, starting motor (10s):
 

The voltage drop across wire between alternator and battery is fine (20-30mV)

The ripple at alternator output looks more suspicious. I'm not sure to well interpret them. Spike are from ignition, but ripple and general waveform are not very clear.

Car is Skoda fabia II (2010) 1.2TSI
Alternator is Valeo 03F 903 023 E

Alternator output voltage, 20ms snapshot:

Alternator output voltage, 10ms snapshot:

Alternator output voltage, 500µs snapshot:


Any help appreciated, thank you!

Can you check the voltage while running on an AC scale?

Not sure to understand what you mean: my alternator voltage captures are already AC-coupled?

Sorry I should have looked at your voltage scale closer. On page 8 of the SD book he says: maximum allowable AC voltage is .3v (300ma). Was this test done at the battery or on the alternator stud. Either way that looks like a bad diode to me. 

ok, so I'm very close to the maximum... The voltage is taken at alternator stud.

I don't think you have an alternator issue I would suspect you have a loose arcing connection some where.

wrote:

I don't think you have an alternator issue I would suspect you have a loose arcing connection some where.

Exactly what I was thinking. Alternator ripple doesn't account for a no crank/no start. Maybe, if the ripple was extreme? But it'd need to be a lot more than what you're seeing.

Also, keep in mind that higher-than-normal AC ripple can be caused by a weak/discharged battery. If your battery is from 2010 like you stated, it's time for a new one.

I just had one of our work trucks that had an intermittent alt light. Junk ground was the cause {G101}.