Alternator field collapse?

[ZsZ]

I had a strange hiccup today.
We are on the last leg of a 3 week roadtrip.
I was driving on the german highway, around 100-110kmph (~3000rpm) when the charge idiot light came on for a sec or two, the thacho dropped to zero then jumped up to full sweep. And then everything back to normal. Did it two times, then I slowed down a bit and came off the highway.
It was OK since and was before.
Tempreature was warm (around 26C ambient) the compressor fridge was running, headlights were on.
Notung obvious is visible, the belt might be a bit looser than it was when we started the trip.
Was the alternator overloaded and the field collapsed?
Any other ideas?
(JX two belt setup with 90A alt.)

[colinthefox]

I have experienced something like this when water has splashed onto an old fashioned alternator V belt. It slips for a second or two with a heavy electrical load, then when it dries out, it's back to normal service. The tacho dropping to zero seems to indicate that the alternator may have stopped rotating.

[ZsZ]

Yes I was thinking about the same, but it was dry weather.
My suspicion that the rad fan kicked in that caused voltage drop in the field circuit that either overloaded the alt or collapsed the field.

[ZsZ]

Just an update:
I recently disassembled the alternator.
The problem was caused by the aluminium shim (strip from a beer can) I added before to reduce the play between the plastic bearing retainer cage and the housing.
The shim walked out and shorted the diodes. But immediately vaporized so the charge came back.
I removed the remains and reassembled for now
It will need a new bearing and a plastic reatiner.

[colinthefox]

Just an update:
I recently disassembled the alternator.
The problem was caused by the aluminium shim (strip from a beer can) I added before to reduce the play between the plastic bearing retainer cage and the housing.
The shim walked out and shorted the diodes. But immediately vaporized so the charge came back.
I removed the remains and reassembled for now
It will need a new bearing and a plastic retainer.

Thanks for posting this. It's always good to get a definitive description of the reason for a fault, even if, as in this case, it's unlikely to affect anyone else. Glad you found it in the end. Beware the home brew modification!
 

[ZsZ]

Replaced the bearings and the plastic reatiner as it was a bit loud.
Now I have no RPM indication on the dash.
Checked the wiring with a test lamp it is fine up to the dash.
How can I test the W terminal without an oscilloscope?
Should I read 8V AC there with a multimeter?

[ZsZ]

It was the instrument itself.
I have a hodge podge cluster with tach from some Mitshubishi or other japanese thing so the PCB is screwed on exposedly on the back of the unit. One of the capacitors was smushed and had a broken leg. I did manage to solder a new leg for it and now works.

BTW the alternator charges with 14.4V measured at the battery and I had 7.34V AC at elthe W terminal and 7.30V AC at the dash connector