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Alternator cuts out at high rpm

10-15-2009, 09:08 PM
On my 2000 Astro, 197,000 miles, the alternator cuts out at high rpm. The vehicles operates and charges normally unless the rpm gets above a certain point and then the alternator goes offline. I can turn everything on and it will handle the electrical load fine but if I increase engine speed beyond a certain point (going up a grade at 70 mph and trans downshifts into 3rd, etc) then the voltage indication will drop way down to 8 volts or so, the lights will dim, etc and when the rpm gets back below that threshhold everything returns to normal. I noticed it doing this Sunday but I have driven it several hundred miles in the last few days. I can even duplicate the problem with the vehicle in park by gradually increasing the rpm until it drops the voltage. There are no other drivability issues, no check engine lights, etc. I did not search for codes. I did check the belt and observed alternator rotation. No evidence of belt slippage, loose conections, arcing, burning, odors or anything apparent. Anyone seen this problem before? The alternator was replaced about 50K milles ago.

12-02-2009, 02:14 PM
The alternator may stop generating power at higher speeds (like from worn or dirty slip rings) or the voltage regulator may cut out at high speed (higher power output) for some reason. I don't know if a bench test (like at Autozone) will fully excersise the alternator at all speeds but I'd say take it there or somewhere and check it.
It concerns me that you drop down to 8V quickly though, almost indicates your battery doesn't have enough storage for a few minutes at high speed operation. Check your battery voltage just sitting (should be 12.3-12.7V or something like that) and then check with engine running (should read 14-14.4V if alternator is charging). Of course you may have weakened the battery with a bad alternator (which is what I think you have) so maybe a charge will do it, voltage will tell you.

Good Luck

12-03-2009, 02:48 PM
Hi. i've seen worn brushes do this. They get too short to keep a good contact on the slip rings at higher speed. But 8 volts is way too low. I'd get the battery checked too.

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