'73 911 with charging problem
11-11-2003, 06:02 PM
#1
Track Day
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'73 911 with charging problem
My battery lost charge a few days ago. I recharged the battery (12.7V on the meter) and reconnected it to the car. After two more days of driving the car, the same problem. I recharged the battery, started the car and measured the voltage at the battery to be 12.2V. I expected the reading to be >13.5V to show battery charging. I suspect either the voltage regulator or the alternator. The red light in the dash board is on when the ignition is switched on (before starting the engine) and off at all other times. Any tips on what to check next?
11-11-2003, 09:27 PM
#2
It's not unusual (apologies to Tom Jones) for output voltage to read low if the car is only at idle. My suggestion is that you rig a cheap multimeter to read off a cigarette lighter plug...go for a drive, read output voltage as you drive at various engine speeds. If voltage doesn't climb to 13.5 to 14 range, you've got troubles. Battery recharging to 12.7 could be marginal? Mine (early 911 dual batteries) exceed that when fully charged, a static measurement. But not by much, and they are fairly new. So, I think you're on the right track...could be regulator or alternator...or, heaven forbid, a short somewhere that drains the battery. A short is often easy to fix, but a female dog to find!
11-15-2003, 10:20 AM
#3
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More trouble shooting...
With a 12.7V battery charge, I started the car and checked the voltage at 2500 rpm and read 12.3V. I then drove for 20 minutes, parked at work for 9 hours, and drove another 20 minutes home. With the headlights on and at idle, the voltmeter read 8.8V and at 3000 rpm, 12V. I shut the car down and it read 11.8V. Any suggestions on what to try next?
With a 12.7V battery charge, I started the car and checked the voltage at 2500 rpm and read 12.3V. I then drove for 20 minutes, parked at work for 9 hours, and drove another 20 minutes home. With the headlights on and at idle, the voltmeter read 8.8V and at 3000 rpm, 12V. I shut the car down and it read 11.8V. Any suggestions on what to try next?
11-16-2003, 01:30 AM
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I think I'd be tempted to replace the regulator and check the brushes. The only problem with that is if the alternator is shorted it could blow out the new regulator. If that happens you'll have to have the alternator rebuilt and replace the regulator again. If your alternator is really old it might be a good idea to just replace the whole thing with a rebuilt unit. I see them advertised for around $180. You might get yours rebuilt locally for less. Good luck.
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11-16-2003, 09:09 PM
#8
Originally posted by RoninLB
if a seperate regulator, replace both the reg and alt with brand new
if a seperate regulator, replace both the reg and alt with brand new
play it safe, replace both?
11-16-2003, 09:16 PM
#9
Originally posted by pwd72s
I concur. The voltage readings suggest one or the other, so why not
play it safe, replace both?
I concur. The voltage readings suggest one or the other, so why not
play it safe, replace both?
