weak alternator issue

Hi to all

this year truck usually runs during nightime (low traffic). And I can see that when engine idles and headlamps are one voltage drops down to 12.1V

At over 1200 RPM it is 13.5V. When all light is off - about 14.2V (normal)

I checked tentioner, installed new belt, but.... May be I need smaller pulley?

Alternator is not hot, just normally warm.

So before I park truck on truck stop I have to let it idle for 10-15 minutes with light off, else if I use bunk heater low voltage unit wakes me up four hours later!

Any opinions plz)

 

about -10 dgr C

I have brushless alternator

Voltage regulator and diodes are OK. The only problem that alternator generates less power then light system uses at idle

I rechecked output current with clamp meter and I see that at lower 600 RPM I got about 60Amp

at over 800 RPM all is OK, so I think I need smaller pulley

 

today I checked other trucks with series-60 engines and found that all other alternators are weak at idle speed. If you turn lights on (+fog lamps), turn fuel filter heater and blower in cab - it discharges battery. Only at over 850 RPM alternator makes more power to let voltage rise up to 14V.

But EU trucks other - they have 24V electric system and alternators with brushes. 28V at idle with all lamps on!

 

Alternator removed and taken apart. Two bad diodes found. Rectifier replaced (part 10511353). As I can see avalansh type diodes are used.

Ok, I changed pulley with twice smaller and got over 80 amp at idle. Manufacter says that maximum alternator speed is 10 000 rpm, so I get over 1000 at idle, but not more 4500 at max engine speed.

 

gauge 2 is like a starter cabel, I use 20 mm2 and I did upgrade to remote sense.

BTW, what rating must be diodes for 135 amp alternator. Stock diodes look like 40 amp, I wanna purchase 60 amp ore 80 amp press fit diodes, mashin holes for diodes in aluminium radiators, press in with thermal paste and drill vent holes on rear cover of alternator