Alternator tests?

[EDMO 1078]

I think a diode in my truck alternator is draining my battery - is there a simple test to verify this?

Would a battery disconnect be a solution?  on hot or ground side, or does it matter?

This could apply to planes too.

EDMO

[ChrisBolkan 130]

Disconnect either the positive or negative lead from the batter and insert an ammeter in the line (with engine not running, lights off, doors closed etc) and look for current draw. Alternatively, get a resistor (100 ohm should be OK) and place it in the same way you would hook the ammeter I described above and measure voltage drop across the resistor with a voltmeter.

If you see current flow (or voltage drop) disconnect the alternator and see if the current draw goes away. If it does, you have found the faulty component. Replace the alternator.

It's probably not the alternator. Is there a dome light or a bed light that is left on that you are not noticing? Much more likely.

[Av8r3400 1178]

Don't disconnect the battery while running on modern vehicles.  This will damage the charging system.  Same with running without a battery.  Even a dead battery is better than nothing.

 

What year, make, model is your truck?

 

In the old days you could put a piece of metal against the back side of the alternator and if it was magnetized it was charging...

Edited 3 Feb 2014 by Av8r3400

[EDMO 1078]

2002 Ford Ranger 4x4 - If I don't start it at least once a week, battery goes dead.  Old battery was doing same, so bought a new one about a year ago.  all lights and switches off and GPS unplugged.   I will try the tests. 

A battery disconnect would only be used when it is parked - I had one hidden on my Mustang to prevent theft - Works on planes too!

Thanks,

EDMO

Edited 4 Feb 2014 by Ed In Missouri

[Av8r3400 1178]

My sister has a F150 about the same vintage had the exact same problem. The stealer eventually found a rely somewhere that didn't operate properly (I think in the radio circuit?).

[lostman 198]

I have a 2001 F250 superduty that does the same thing. I took it to an electric auto specialist and he couldn't even tell me what was going on. I've replaced the alternator and no joy (it was bad anyway, but I was hoping it was the diode). I still haven't been able to chase down the nasty little draw! It's harder with two batteries since my draw seems to change depending on which negative cable I hook my mulitmeter inline with.

My suggestion is to start by getting the meter in place then one by one pulling the main fuses until you see the draw disappear. Then you have to chase the entire circuit down, one piece at a time, I hate electrical problems!

Good luck with it Ed!

[EDMO 1078]

I have had several electrical courses - just wanted to see if there was a simple way to test alternator - my big day in alternators all ended in Avschool when I had one on the test machine - the instructor left the room - and I smoked the alty!  I passed with an A.. probably because I can write reports as good as King O can make speeches. 

 I may have said, "If you like your alternator......"

 Then they let me rebuild and test mags - What a shocker! - no wonder I don't like mags!

Sounds like this could be more complicated.

I remember back about 1980 - Ford had all kinds of electrical fires in their trucks- I had a Dodge back then.

Thanks,

EDMO

Edited 4 Feb 2014 by Ed In Missouri

[IFMT 36]

Burn It! 

[IFMT 36]

Hello Ed. Disconnect the hot lead onto the alternator. If it still goes dead it's not the problem. Simple test. Check the glove box light to make sure it's out. Get that battery tested. You never know. Cheers B

[Av8r3400 1178]

It's a Ford.  Why bother?

[EDMO 1078]

Thanks for all the input guys - This truck has got to last a few more years - only 127K on it - Have to save up for wife's new suv - gets new every time the warranty runs out on hers. (that way I don't have to work on it, or get yelled at when it don't work!)  (more time to work on plane)

EDMO

Edited 4 Feb 2014 by Ed In Missouri