alternator/regulator, 1980 240 DL

carlitos · Post by **carlitos** » 18 Dec 2007, 18:57

This reading off the battery was about 11 or 12 volts. Even though that's not an excess reading, I decide to change the regulator, external to internal. The reading of volts, when running, jumps to 15! Hurry! That's good, no? No, or I'm not sure, because the battery gets drained again. Any idea why the 15 volt reading didn't hold? (310k miles)

waynej · Post by **waynej** » 19 Dec 2007, 09:15

Anything between 13.5 and 14.8 volts is considered good for the alternator output. 15 volts is close enough as your meter may not be that accurate.

The 15 volt reading will never hold. Within 30 minutes of shutting the engine off the battery voltage will go down to around 13 to 13.5. If yours is going below 12 either you have a very old tired battery that can't hold a charge, or something in your electrical system is draining the battery.

carlitos · Post by **carlitos** » 19 Dec 2007, 09:59

Thanks WayneJ. So are you suggesting that the initial reading is proof that the alternator/regulator was/is working again? And that something else is likely draining the battery? I re-charged the battery, took it to an alternator store which showed the volts back down below 12. (The new battery was replaced from the previous one when it drained was recharged and then wouldn

waynej · Post by **waynej** » 20 Dec 2007, 09:17

Start the car, put your voltmeter across the battery, turn on the headlights and heater fan, bring up the rpms to a little above idle, if you still have at least 13 volts your alternator is ok. If not you may have a bad diode in the alternator. Have a shop put the scope on the alternator to see if all diodes are ok.
If the alt. is ok then you need to check for what is draining the battery. If you have a good multimeter you should be able to measure amps up to about a half amp or 500 milliamps. Turn off the engine, take the key out, turn off interior lights and radio. Disconnect one battery cable, set the multimeter for amps and connect between battery post and cable. Newer cars will show a drain of maybe 50 milliamps for the CU memmories but I don't think a 1980 with K jetronic will show anything. If you have a newer after market radio you might show a small drain, but if you have much over 50 milliamps you need to trace it to the source. Pull one fuse at a time to see which one eliminates the drain. Then please post your findings here.
Good luck.