The red battery light came on bright two days ago but would go down when I picked up speed for a while. After two hours it stayed on all the time. I just replaced the voltage regulator and that did nothing (the old one looked good when I took it out but I put in the new one anyway). Guess I will go to Pep Boys in the AM and ask them to test the alternator.

Anything it could be other than the alternator?

Pep Boys quoted me $340 for a new one installed which is a bit much. *********************** has them for $68 but most places here want about $140.

Yes a visual inspection of the voltage regulator for short brushes is important. Unfortunatly the electrical regulator circuit that is built in to the voltage regulator is not really responsive to visual inspecton. So subbing it with a known good one is always a good ideal as you have done.

Well, it now has a new voltage regulator but that did not solve the problem.

Have the alternator checked. Most places do it for free. As you are well aware there is little out there that is free.

Replaced the alternator yesterday and no red light today. However after running errands for about an hour, I put the battery on the charger to check its charge. It reads low.

If the new alternator is charging properly, shouldn't it read in the good area? Or is this the way a battery performs over its life - reading good, the falling farther and farther over the years into the low range? I had just assumed they all stayed in the good range until they went bad.

Did I just learn something?

What condition was your battery before all this started?

If it was drained quite a bit, that will shorten it's life.

Auto part stores will check the battery for free too.

__________________
83 SD

84 CD

Battery was perfect but I never put it on the charger. It still cranks fine, very strong. This was more a theoretical question.

Not sure that I have a volt meter although my 40 year old Sears charger is supposed to be able to check the battery. From what I remember you just attach the two leads to the battery and do not plug it in to the wall. The needle then shows good, low, weak. Can you use a multi-tester to check the battery; if so, how do you set it?

Always charge the battery before installing/using a new alternator. Not doing so can smoke the new alternator. :-)

Make sure the battery cells are full up the markers if it's not a sealed battery. Put the battery on charge overnight, remove the charger, check the battery voltage. Start the car, check the voltage across the battery, compare the two readings. Should be about 13.75-14.5 at idle. Now load up the alternator by turning on high beams, rear defrost, etc and rev engine to about 1500-2000rpm, voltage should stay between 13.75-14.5

__________________
I choose to be happy, even when the Mercedes is being awkward, lol

meter in the charger is not accurate enough to test the battery right.
check with a digital volt meter and report the exact decimal voltage from the battery. 12.66 is supposedly fully charged. 12.75 is totally 100% charged.
anything less is not.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

A mutli tester can be used and should be set to 20 DC VOLTS. Putting it across the battery terminals with the engine off will give you the battery voltage. With the engine running, you get alternator output voltage.

__________________
I choose to be happy, even when the Mercedes is being awkward, lol

That's good info about the multi tester; will try it later. Here is a pic of the car back on the road after a new tranny; took pic this afternoon.

Attached Thumbnails

 

could be a wiring issue

It ended up being a bad alternator. Put in a new one and I am back in business.

Just checked the battery with the multi tester. Tested 12.54 with the engine off; so it must be just a little down.

Do I leave the multi tester on the same settings when I check it running? Seems like 55 amps should require some different setting on the multi tester.

You are measuring voltage at the battery terminals, not amps. 20 volts on the meter is above the alternator out put so the meter will read it and not peg.

__________________
85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.