Battery light on?

[Bobthebuilder]

I drive long miles AM and PM with my 85 300SD, so most of the time my lights are on. This AM, after 20 mile warm-up, I hit the freeway and got up to about 75. After only 4 miles the Battery light began a dim pulsing which got brighter for about a mile and then stayed on. I pulled off to make sure the belts were intact even though it wasn't heating up. No noises, no smells and pulleys were spinning fine. I continued for another 100 miles and the headlights are still bright. Starts fine and went another 30 miles tonight, headlights normal. The Battery light gets very dim at idle and bright red when running. Is the alternator out or am I getting some other message?

[Knight Rider521]

I had a similar problem with my 1985 300sd. It turned out to be the alternator. I took it to SEARS to have the battery tested thinking that it was the battery. They told me that I needed a new alternator and that I would have to leave the car overnight a pick it up the next day. Said it would cost 326 dollars. There was no way I was going to leave my car and let SEARS do any work on my car besides changing the tires! So I drove the car to my mechanic and he too concluded that the problem was in the alternator. The difference is instead of telling me I needed a new alternator for 250 or so dollars, he just replaced the alternator brushes and I was as good as new. And the brushes only cost 50 usd, so I ended up only spending about 125 usd for the job.

[barry123400]

With a little luck just a loose alternator belt. With engine off see if you can turn alternator pulley fan blades. If you can turn them belt is loose and just retension alternator. Problem gone. But if you cannot turn the pulley with a little pressure borrow voltmeter if you do not own one. With engine off read voltage across battery terminals. should be 12-12 1/2 volts. Start engine and measure again voltage should be higher by a volt or so. But again I just suspect belt a little too loose by your description.

[barry123400]

With a little luck just a loose alternator belt. With engine off see if you can turn alternator pulley fan blades. If you can turn them belt is loose and just retension alternator. Problem gone. But if you cannot turn the pulley with a little pressure borrow voltmeter if you do not own one. With engine off read voltage across battery terminals. should be 12-12 1/2 volts. Start engine and measure again voltage should be higher by a volt or so. But again I just suspect belt a little too loose by your description.

[300SDog]

Dunno about the '85, but earlier models had an external voltage regulator. It's about a $20 part..... Check it first before you opt for an alternator. Worn out regulators will have teeth worn down to nubs.

And with no lights, radio or heater fan being used you might get 3 days starting and driving in between out-of-car battery charges.

[trontek]

the female contacts in the alternator connector.

[barry123400]

If alternator has small, usually round black plastic item on back of alternator about one to one and one half inches in diameter. That is your voltage regulator as well as brush mount and brushes it costs like 20.00 or so. Two screws affix this item and if brushes worn bad just change unit as comes off pretty easy for inspection usually. (with alternator off of car) Kind of a miantenance item anyways and should be checked once in awhile as brushes are a wear item. Should have mentioned this last night but when you quoted everything looked to be functioning my first thought was even with a loose belt everything looks normal with engine running. Thanks to members in previous threads jogging my memory. On older cars everything electical I clean and put some sort of protective coating on contacts prior to reasembling as a habit in my climate. Kind of depends if you want to get your hands dirty but just checked out a car in my garage and it indeed has the borsh external regulator mounted on alternator. Funny one problem have not had is alternators on these cars. But think will heed my own suggestion. Pull all my alternators. Pull and check regulator/brush assemblies and check them as it is pretty easily done. Judging by my cars high or unknown milage actually will be preventitive maintenance in nature. Now have something more to add to my list of getting roundtoit things to do. Nothing like making work for yourself.......