My AutoMeter volt meter appears to be inaccurate. I have a 140 amp alternator which should be more than enough for my power needs. Problem is when I turn on multiple power items, my volt meter reads below 12 volts. I have tested the output at the battery and get more than 12 volts on the test meter. There are two addons that seem to severely draw down the power...An electric fan, and a pair of halogen driving lights. I can run the A/C. headlights, and radio etc., without issue...I'm thinking of adding a digital meter at the battery to give me an accurate reading. Any ideas??
If the gauge doesn't match the DVM, then the gauge is off, and would need calibration. But you should have the amperage output, as well as the voltage, tested, just to make sure you don't have a failing alternator.
Where are you taking off the voltage for the meter? An old wiring harness will likely lose 0.5-1.5V in old corroded connections between the battery and dash gauge wiring. If the wires at the back of the gauge are one value vs. what the gauge says, then it's the gauge is wrong. Most likely it's a lousy wiring connection and the gauge is showing you the voltage in your dash is dropped simply because of age. A digital connection at the battery is fine, but you'll still have voltage issues if you don't fix the poor connections.
The factory police gauges in my Crown Vic do the exact same thing. If I have headlights and blower motor on at the same time it drops quite a bit. With no load it looks fine. There isn't anything wrong with the charging system (at least on my car). The way Ford did the gauges was they went in place of the clock so all they did was plug the voltmeter in place of where the clock would on a regular car. It isn't great and it isn't a bad gauge. If you want a quick test, connect the gauge directly to the battery and load up the system and see what you get.
I had the entire wagon rewired from front to rear. I think I'll get the DVOM to confirm the input at the battery. It will give me peace of mind.
Take readings on the posts, then a second set on the connectors, then a third set on the starter solenoid post and the engine block where the negative cable is connected. There should not be variation higher than .1V. If there is, clean and reattach all connections, then retest. If there's still a .1V or larger variation, use a jumper cable to supplant each battery cable, one at a time. Whichever one removes the variance, replace that cable. Once that is done, take a reading at the ignition electrical switch and body ground. It should be within .5-.75V of the battery voltage. If it's more, ensure there's no heat or physical damage of the input wires. Any damage needs repair, and then you'll need to find why it overheated.
Wow! I must admit, electricity is not my friend. One time, a long time ago, someone gave me one of those meter boxes that you test with. I never used it. If it can't be fixed with a test light I'm screwed... What I will do is clean all the connectors "Hobo" pointed out and see what happens. Wish me luck. BTW I got something in the mail today not associated with the power issue. Gonna post that in "What have you done lately"
I took the wagon to my auto electrician. He tested the output and suggested that the problem is within the alternator. Something called a rectifier... I'm having a transmission change done. While it's down, I'll pull the alternator and take it to the shop that built it.
Ah, good tech, it's hard to identify a bad diode without a silly scope. If it was outside of warranty, doing the bridge replacement on GM alternators is fairly easy.
