911 No "battery light"/No charge: Tips
12-10-2001, 08:52 PM
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911 No "battery light"/No charge: Tips
A number of 911 owners have come to me with this symptom. Some of then have already had the alternator/regulator/belt/battery etc replaced.
So, here are a few tips:
The "battery" light in the dash must come on when you turn the ignition key to "on".
If it doesn't, the alternator will not charge.
This is because the alternator field winding excitation current is supplied through the "battery light" bulb.
If the bulb doesn't light, check:
1. Bulb (burned out?) Swap it with another dash bulb to test.
2. Alternator slip ring brushes (must test continuity to ground through field rotor windings). You can test from the "battery light" bulb holder to ground; should be less than 500 ohms. Another quick test: just ground the bulb holder with the ign turned on. If it lights, you probably just need new alt brushes.
3. Internal regulator (rarely fails this way). Just check continuity. If OK, go to next step.
4. Alternator rotor windings (test across slip ring; less than 300 ohms). Or, just put a powered test light across slip rings. If it lights, you're OK.
5. Alternator ground strap (sometimes left out or disconnected; test from alt housing to engine case; less than 5 ohms)
6. Engine/trans ground strap (check at t'axle. Less than 3 ohms, or 0.5VDC voltage drop when cranking engine.
Most 911 "no battery light/no charge" problems are simply worn brushes or burned out dash bulbs. I'd guess 95%. Can't tell you how many alternators, etc have been replaced in error due to a $2.00 bulb or $ 15.00 brush set.
Always, check the battery cables, posts, and connections when working on charging/starting problems. And, please realize that batteries are wierd; particularly in garage queens which get little use. Check the battery surface voltage; if it is less than 11.8V after charging, bad cell (replace). Check (after charging) while cranking engine. If less than 10.3V, replace (sulfated plates, etc).
If you use only your car only seasonally or occasionally, get a "Battery Buddy" ($50)and leave it hooked up. You will save lots of batteries and headaches.
Hope this helps.
John
So, here are a few tips:
The "battery" light in the dash must come on when you turn the ignition key to "on".
If it doesn't, the alternator will not charge.
This is because the alternator field winding excitation current is supplied through the "battery light" bulb.
If the bulb doesn't light, check:
1. Bulb (burned out?) Swap it with another dash bulb to test.
2. Alternator slip ring brushes (must test continuity to ground through field rotor windings). You can test from the "battery light" bulb holder to ground; should be less than 500 ohms. Another quick test: just ground the bulb holder with the ign turned on. If it lights, you probably just need new alt brushes.
3. Internal regulator (rarely fails this way). Just check continuity. If OK, go to next step.
4. Alternator rotor windings (test across slip ring; less than 300 ohms). Or, just put a powered test light across slip rings. If it lights, you're OK.
5. Alternator ground strap (sometimes left out or disconnected; test from alt housing to engine case; less than 5 ohms)
6. Engine/trans ground strap (check at t'axle. Less than 3 ohms, or 0.5VDC voltage drop when cranking engine.
Most 911 "no battery light/no charge" problems are simply worn brushes or burned out dash bulbs. I'd guess 95%. Can't tell you how many alternators, etc have been replaced in error due to a $2.00 bulb or $ 15.00 brush set.
Always, check the battery cables, posts, and connections when working on charging/starting problems. And, please realize that batteries are wierd; particularly in garage queens which get little use. Check the battery surface voltage; if it is less than 11.8V after charging, bad cell (replace). Check (after charging) while cranking engine. If less than 10.3V, replace (sulfated plates, etc).
If you use only your car only seasonally or occasionally, get a "Battery Buddy" ($50)and leave it hooked up. You will save lots of batteries and headaches.
Hope this helps.
John
