Thread: power to alt?
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Old 08-27-2002, 08:38 PM
JimSmith JimSmith is offline
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,596

The alternator is energized by the battery, as without some current in the stator windings (or, possibly the rotor windings, depending on the design) you have no field in to generate electricity by mechanically having the conductors cut by a magnetic field. So, you need a battery to get the process started.

The best way to have the alternator thoroughly checked out to verify it is working is to take it to a shop, in the car, and have them test it, the battery and the overall system for shorts and proper connections. If you do it your self, the way to make a crude check is to measure the voltage across the battery terminals, first with the car off, and all the accessories/electrical loads disconnected (off). Then do it again with the car running, at idle, no electrical stuff on (no lights, no fans, no doors open, etsc.). Then do it again with everything in the car that is electrical running, with car idling. The readings should be around 12.5 Volts, battery alone, 13.6 to 13.8 Volts, car idling, nothing on, and 13.2 to 13.6 Volts with everything running. Make sure the idle speed is pretty constant, as the engine may slow down as you add load, but you should have a little idle adjustment knob just ot the left of the steering wheel column, where it passes into the dash.

If you get a low voltage reading the first step, chances are you will not do well at the others. The battery is shot or you have a nasty short in the system somewhere. Disconnect the battery from the positive lead in the car and check again. Low voltage again, and you need to trickle charge the battery for a good period, then check again. If the battery is still not up to 12.+ Volts, it is shot, in all likelihood.

If you are ok at the first step (battery) and the second is not ok, the alternator has problems, and you will likely do poorly on the third. Pull the regulator and examine the brushes. If they are worn to nubs, you need a new regulator (really only brushes and there are some souls on this board who replace the brushes).

If only the third step is bad, you probably have a bad voltage regulator, but it is possible you have some other alternator components failing that are making the voltage regulator perform poorly. Pull the regulator and check the brushes, and look to see if the wear is even and smooth on them. If they are worn to different lengths but are smooth, there are likely some diodes or other internal alternator bits failing. If they are gouged, the surface they ride on and the rotor bearings are likely beat, and the rotor needs to be turned and the bearings replaced. In the case of either of the last two, you will need to get the alternaltor or voltage regulator or both fixed/replaced. Same if you ever see much over 14 Volts. I would just get a rebuilt one from FastLane and install that if there are any signs of bad alternator guts.

Good luck and I hope this helps. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Last edited by JimSmith; 08-27-2002 at 09:19 PM.
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