Alternator Issue

[Jimjific 23]

Hello All,

 

Well, I've been hit by the mechanical bug. My truck (2001 GMC Sierra V6) is giving me issues. I hate working on it so I dropped it off at the shop to see what was wrong. Since it was a nice day, I drove the Mustang to work. On my way home, I stopped to see the truck and went to leave and the Mustang didn't have enough juice to start. So I jumped it and got it home. Voltage at the battery read just under twelve volts. So I'm assuming the alternator isn't working properly. Is there an easy way to tell if the alternator is dead?

 

I hate electrical problems.

Thanks,

Jim

[dsc2 10]

One way is to start the car and then take the positive lead off the battery. If the car dies then the alternator is not working.

[MustangGT 11]

+1 on pulling one of the battery terminals.

 

One of the many great things about these older cars iae that alternator problems are usually fairly easy to diagnose. Without all of the other electrical components (i.e. computer, electric ignition, fuel injection, etc) the charging/electrical system is fairly simple.

[Jimjific 23]

So I did the alternator trick and the car still runs. So I reconnected the negative terminal and checked the voltage while running and it read 14V. SO I turned on the headlights and the blower fan since that is how I was driving last and the voltage dropped to sub 12 and when I pulled the negative, the car died. It makes sense that I am only seeing this now since I haven't really done much winter driving in at night. So I basically have an alternator that can only supply the car and nothing else.

 

Jim

[MustangGT 11]

Sounds like the voltage regulator rather than the alternator. You could try and swap it out or pull the alternator and have it tested at a local auto parts store (i.e. Auto Zone, O'Riellys, Pep Boys, etc). I would do the regulator first since it has the easiest mounting screws.

[dsc2 10]

When you turn on the addtional load(s) bring the car to fast idle and disconnect the battery lead. If it still dies then I would look to the alternator being weak and get it tested. Without a doubt the alternator should keep the vechile running without battery help, unless the idle speed is very low.

[Stevewr54 12]

Don't pull a battery cable off of a running vehicle. That is not a valid test. It will cause

a good alternator to spike the voltage off the chart. Use load and voltage tests. Steve

[MustangGT 11]

I have heard the above (about pulling battery cable) on newer cars but I have always pulled them off on "non-computer control" cars to do a driveway check. Never had it cause a problem and never have had a spike [that I knew about] -- but I guess it could happen.

[Stevewr54 12]

I have heard the above (about pulling battery cable) on newer cars but I have always pulled them off on "non-computer control" cars to do a driveway check. Never had it cause a problem and never have had a spike [that I knew about] -- but I guess it could happen.

A simpler,safer test can be had. See if a screwdriver is magnetically

attracted to the rear bearing of the alt. while running. Steve

[dsc2 10]

A simpler,safer test can be had. See if a screwdriver is magnetically

attracted to the rear bearing of the alt. while running. Steve

 

+1 on this method, but if the alternator is weak it will still have a magnetic field. I was fooled by that in the past.

I agree load tester is a more reliable method but the OP sounded like he was looking for more a shade tree solution.

[Fordrevhead 29]

Voltage test before starting, then while running will always tell you and is safe on any vehicle. It wont always tell you if you have a crap battery though. You need to perform a load test for that. I still pull out the old hand held load tester, and other antique equipment like the dwell meter every now and then.

[Stevewr54 12]

It seems like we were under the shade tree so that's why I suggested the screwdriver.

I 100% agree on meters and load tests. A scope will show the spike from cable

removal while running. Your electronics will see it, too! Steve

[danno 128]

If you have a good voltmeter, measuring the voltage on the battery is also an effective method. If everything is fine, it should be about 14.4 volts with the motor running. When you turn on the lights it should not drop very much. If it drops to under 13 volts, then you have a weak alternator.

 

An alternator can have a diode that fails, and when this happens everything in the car will still work fine, until there is a heavy load on the battery. Turning on the lights is a heavy load. Your alternator is putting current into the battery, but the lights are taking more current out than the alternator can put in. When this happens, the voltage goes down.

 

Have you noticed a squeel on the speakers of your radio? This is a symptom of a bad diode in the alternator. The symptoms you describe are typical of a bad diode in the alternator.