New battery only or new battery and alternator?


1998 Subaru Outback wouldn’t start suddenly and 2 successive jump starts lead me down a path to where 3 mechanics gave me differing solutions, so I wonder if any of you can help me decide the best action to take.

After I replaced the 5 yr old battery with a new battery, my 85 amp alternator registered 55 amps with 100% load and the engine reved up.

I wonder, should it have registered closer to 85 amps?

One mechanic said that my alternator wasn’t working and needs replacing, one said that it is weak and needs replacing, and the third said that my alternator is fine.

Please help.

Thank you in advance.


Your problem is the “not starting” part. For the time being, forget about the alternator.
If enough current reaches the starter when you jump-start (the engine cranks and starts); but, it doesn’t when not being jumped, check the battery posts and battery cables for corrosion. Remove the corrosion with a knife, or file, or round wire brush. Smear petroleum jelly on the battery posts, and reconnect the battery cables.
Do any of your mechanics have an ammeter to measure the current during cranking? The draw would be over 100 amps, momentarily. The amperage draw will indicate if the connections are clean, or not. If they are not, corrosion will be low ( IF the battery has the power).


If they loaded and measured it correctly, then yes, an 85 amp alternator should have put out 85 amps. However, if they went to the trouble to put a carbon pile load on it, they also checked the diodes. If the diodes are good and the connections are clean, it should work to capacity.

Alternators don’t usually get “weak” unless they have a corroded connection at the field connector or a failed diode. Otherwise, they work until they stop working completely. This one has an internal voltage regulator, right?

If it were mine, I would make sure that all my connections, including grounds, were clean at the battery and alternator, and keep driving. I would simply assume that there was something wrong with the way that they measured alternator output. (because that is a lot nicer than assuming that they were trying to sell me an alternator that I don’t need)


Alternators never put out their rated capacity, if you get 75% you are doing fine. But like you have been told your no-crank condition is not related to your alternator.


Thank you, everyone.
I tried using my car today and it worked so well :slight_smile: The new battery is keeping me on the road and on time. I wonder, if the alternator actually is bad, and two out of 3 mechanics were correct, could the alternator’s poor performance ruin my new battery?