Battery-alternator question

[M30 Steve]

I had a dead battery on my M30 yesterday, 1990 with 140k miles. Installed a new one and it apears to be OK. I drove 50 miles with the new battery and it held a charge. Should I run a test to see if the alternator is OK?

The old battery was 6 years old. I do not have an Amp meter on the dash. Can I check voltage with my meter at the battery or should it be done on a diagnostic machine.

Thanks, Steve

[Q451990]

You can check it at the battery terminals with your volt meter. The reading should be about 12VDC with the car off, and something like 14.4VDC at idle.

I'm not saying a load test wouldn't be better - but absent of any other issues, I woud just check it with your meter and be done with it if it appears that it's charging ok.

Heath

[Q45tech]

Turn everything on lights on bright, fans, defroster, radio........make sure you have > 12.8 volts across battery.

[M30 Steve]

Ran a test with all electricals on high, reading was 12.01V. Ran another test, engine only running, reading was 12.40V.

I notice in my FSM the spec is called out for regulated output voltage at 14.1-14.7V but not exactly sure what that means.

Thanks for your comments, Steve

[DrewQ45]

If this is the original alternator, at 140K you are sure to see degraded performance and failure down the road. The brushes will have a lot of wear at this point. All the same, just ride it out till it quits. You should see visual indication before all this occurs.

...Drew...

[Skibane]

Ran a test with all electricals on high, reading was 12.01V. Ran another test, engine only running, reading was 12.40V.

Both readings sound VERY low. With absolutely no load on a fully charged battery (engine off, no accessories turned on), the voltage at the battery should read at least 12.7 volts.

With the engine running, a good alternator will increase this voltage to at least 14.1 volts (which is where the 14.1 spec in the FSM comes from), although this voltage may drop somewhat at idle when a lot of loads are connected.

Two possible explanations: Either the battery is almost discharged (which should have been remedied after a few hours of driving), or your alternator isn't putting out what it should.

In all honesty, it sounds like your alternator isn't putting out anything at all, and the car is just running on whatever charge is left in the battery.

[M30 Steve]

I think I will order up a new alt. today, sounds like its on its way out. We shade tree mechanics really appreciate all the great response from the Pros here.

The only reason my old M is still running is because of great advice from NICO folks and all the good posts here.

Thanks again, Steve

[Q45tech]

People don't comprehend that an alternator is actually 3 THREE units in PARALLEL [3 phase windings] thus possible to have 105 amps, 70, or 35 ampere capacity depending on how many PARALLEL circuits are working.

Why you must use a loading test unless it is 100% defective.

[DominickJ30]

People don't comprehend that an alternator is actually 3 THREE units in PARALLEL [3 phase windings] thus possible to have 105 amps, 70, or 35 ampere capacity depending on how many PARALLEL circuits are working.

Why you must use a loading test unless it is 100% defective.

Are you recommending he simply take it apart, test it and rebuild with working parts?

A stator for my Js alternator cost close to 200 dollars, so its probably more economical just to get a new unit.