When I let my truck idle, i notice that my voltage will decrease dramatically. With no other accessories on, this happens. could it be the alternator is going bad?

 

Could be go to your local auto parts shop and see if they can test it for you. Later

 

If you have a volt meter (multi-meter) set it for at least 20V DC.

1.) Test Static Battery Voltage. Static means NO LOADS (lights, etc. radios are OFF!) You should get btwn 13.0 & 13.5. Any less than this put it on a charger over night and test again.

2.) With Meter hooked up to battery, turn your headlights on for 1 minute. This will remove any gassing charge from the battery and stabilize it for the rest fo #2. The voltage will drop to about 12.2 - 12.4V DC. Turn headlights off and wait 1 minute. Check voltage again. Should be above 13.3.

3.) With meter hooked up to battery, start engine and let idle. Check yer voltage and listen near the alternator for a whine. If you hear a whine then the alternator is more than likely putting out. If not THEN take it down to shop and have them check the alternator. Check yer voltage. Anything about 13.9 is ok. 14.6 to 14.9 is lots better, but those numbers will not come into play until the engine is revved up.

4.) Rev the engine to about 1500 rpm. Check yer voltage now. It should be above 14.3. -14.5. With NO LOADS!!!!!!

5.) with the engine running, turn on headlights, heater fan, and everything else lightwise and accesory wise you can and run at about 1500 rpm. Your voltage will take a drop but it should stabilize above 12.V. This is good. If it keeps dropping then yer alternator is bad.


If your not having any problems cranking in the morning or getting dead batteries everyother day and the truck runs fine otherwise I'd say your voltmeter is probably overly sensitive. As long as it stays in the NORMAL range you're probably ok. But check those numbers.

Larry

 

I had a similar problem with the volts gauge dropping at idle, and returning at higher rpm's. I replaced my brushes in the alternator and that cured it. They were only about 8 bucks at the dealer.

 

I've been fooling around with this too. I ran heavy new grounds from the cab directly to the battery and block and ran a new additional 10 gauge wire to the two yellow terminals on the ignition switch and that propped up the voltage in the cab a bit. Now the voltage in the cab is within one or two tenths of a volt of what it is at the battery, where before it was a solid couple of volts down with the heater and lights on.

I still get the low voltage reading at hot idle, accessories or not (though they make it just a little worse). The lights dim a little when the needle drops. I'm sure it's the alternator, you bring up the idle just a little and the needle pops right back up and the lights/heater run brighter/faster. So I know it's not the gauge.

I found that the "O" in normal's just above 12 volts and the "L" is a touch above 14. Mine's near the "L" except at hot idle. But who knows if this is the same for all of these trucks.

I know the battery and the wiring is good so it's time for me to rebuild the alternator now.

 

I get the same thing on my truck....Sometimes it gets so low it almost stalls......

 

A rebuild kit at Autozone was about 30 bucks. It came with new brushes, voltage regulator, bearings, and one more thing. It took about an hour to take out the alternator, repair it, put it back together the right way (try using a center punch or chisel to make alignment marks) and reinstall it. That cheap voltage regulator only lasted about two months, so I just put the stock one back on. From your symptoms, I'd bet it's your brushes, but you could rebuild it if you want.

If you're thinking of just buying a rebuilt alternator, be careful. I've had bad luck (0 for 7) buying the Kragen / Autozone one's. It looks like they sandblast them, put in their rebuild kit, and resell it for 2-3 times the price. The one's you get at Ford for 180 are actually rewound.

 

Guys: Remember one other thing......HEAT and Electrical components are not mutually compatible. Heat causes increased resistance in electrical wiring and reduced performance in motors, generators, alternators, etc.
This is why at hot idle the voltage goes down below what you would consider normal. In this case it's not a critical problem just one that's more annoying than anything else. This happens in all brands of vehicles. I run a 250V DC Motor Generator Set on a Diesel Electric submarine and when it heats up it takes more DC amps and voltage to run and get the required 60Hz A.C. voltage out the other side. It's normal!

The alternators don't really start putting out amps and volts until about 900-1000 rpms anyway. It will float voltage at idle speeds.

Another analogy is CPU chips on your computer. Guys that are into hardware modifications will cool down the CPU with circulating water, freon systems, liquid nitrogen, and other coolants to enhance the performance of their computer CPU's. HEAT is an enemy to electrical components. Not much in the way of getting rid of the heat in the engine compartment other than airflow, so that's the reason behind the fan on the pulley. The alternator is not as sensitive as a CPU but the premise is still the same. Cool the unit and it will perform to new spec's far longer.

Larry