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2005 Mercury Mariner Suspected Alternator Issue

Have a 2005 Mercury Mariner Premier, 108k miles, 3.0L engine. Was driving last week and the ABS light came on. Then slowly the dashboard lights dimmed to black and lost electricity. All this within a couple miles of driving.
Limped home and tested the battery - dead. Recharged and tested the next morning - still fully charged. Put another known good battery in and drove about an hour out and on the way home same thing happened. I suspect the alternator, but on testing, the alternator is pushing 14.3 volts on a good battery.
My fear now is the tear down necessary to change the alternator if it is not actually bad. Anything else I should look for / test before I start pulling the alternator out? Any other tests of the alternator that I can do while it’s still installed?

Any help is appreciated!

Gee I would have thought someone would answer. At any rate if the battery is running down while driving, that would certainly mean that its not being charged again. So if it is actually putting out that voltage and you didn’t just get the wrong contacts, maybe the wiring or connections themselves or voltage regulator? I dunno otherwise except sometimes the testing is not always accurate.

Everything you said points to a bad alternator, except the part where you say the alternator is putting out 14.3 volts, which is consistent with a good alternator.

hmm … well I suppose it is possible there’s something electrical-wrong elsewhere in the car that is drawing a lot of current only when the engine is running. An alternator current measurement test would show if that was the problem or not. Another idea, it might be possible your measurement of the voltage isn’t catching a bad alternator diode. If a diode is bad there could be a lot of AC on the alternator line. Try measuring the battery with the engine running, with the DVM set on AC volts. A third idea, whatever is failing doesn’t fail until you start driving the car. Maybe hook up a volt meter to the battery terminals via long wires, so you can watch it from the drivers seat.

@cliffdodge Are you reading 14.3 volts at the alternator or at the battery?

How old are these batteries . . . the first one AND the “known good battery” . . . ?

The battery is the foundation of the automotive electrical system

db4690 above makes a very good point. Have the battery load tested.

I’d guess that it is corroded connections at the battery.

If you probed the post terminals and not the posts themselves, you may be getting a false reading. You could still get enough amps to start the car, because the high load to start can overcome this…to a point…until some day when it can’t and you are 50 miles from home.

Yet the lighter load that the alternator put’s out is not enough to overcome the resistance, and does not charge the battery.

Also, follow the ground cable from the battery to be sure the far end is not corroded too.


One thing more about corrosion issues. Even a very thin layer of corrosion that looks to be okay visually that gets between the battery connections can cause a problem. Whenever you have problems like this it is best to get a battery post cleaning brush and clean the battery connections, even though they may ‘look’ okay. Internal wire corrosion at the battery cable connections is another thing to be aware of.

@cliffdodge - did you ever solve this issue? Having the same problem with my 2005 Mercury Mariner. Yesterday while driving down the road, the ABS light shows up and then minutes later radio shuts off, then the dashboard, wipers going real slow, then lights went out. Car seemed to be running OK. Made it home and now the car will not start.

Did you find the solution?


Does the Mariner have a amp meter on the dash? I came across a situation where the alternator showed 14 volts but the battery always ran down. Come to find out that the terminals on the ends of the wires connected to amp meter were corroded and the amp meter posts were ohmic so a new meter and cable end replacement solved the problem.

On second thought, I came up with a simple check. With the battery discharged a little bit (leave the head lights on for 15 minutes or do a flood clear start of 10 seconds); start the engine; and measure the voltage directly from the battery post to the alternator post. If the voltage drop is more than 0.1 volts, there is high resistance in the wiring from the alternator to the battery.

Hope this helps

@Mtb-2018. It indeed was the alternator. Ended up putting it in a shop as the location of it meant pulling the wheel and axle assembly. Good luck!

@cliffdodge - Thank you so much! From what I’ve read replacing the alternator will not be much fun - was hoping you would tell me it was a bad ground somewhere…

@researcher - Thanks for the thought…