I have a 1994 ford ranger, 3.0 v-6, standard transmission, air conditioning, pw/w & dl. The truck runs good.
My electrical voltage meter/gage has started cycling. It goes from say 12 to 18 volts continiously. If I start the AC fan it goes faster. It does it all the time, whether i’m at idle or driving at 60 mph. It doesn’t effect the performance of the engine. When i have the lights on you can see them cycling also, bright-brighter-bright etc.
I have had the battery tested and it was good. I had the alternator out twice and bench tested and the voltage was steady, not cycling. I have also checked the ground straps and installed additional ones to see if that would cause it to stop.
Recent maintenance: Pulled the engine and replaced the freeze plugs, and also pulled the fuel pump for cleaning. After the items were completed the voltage was fine.
Have the alternator tested in the truck with a charging system check. The bench test will not add a load. The alternator needs to be tested under load to rule it out. I suspect a bad voltage regulator, which is built into the alternator, and is replaced with the alternator.
Verify by checking voltage at battert with Multimeter.
Something is affecting your alternator while its on the car and not on the bench.
Heat
Connections
Battery
Can the regulator be replaced seperately?
Load
Mounting fources
Would like to disassemble alternator and inspect.
The alternator’s voltage output is regulated by a device cleverly named the voltage regulator. This regulator should NEVER allow the alternator’s output to reach 18 volts.
Verify the actual output of the alternator by using a quality voltmeter, measuring from the voltage from battery terminal to battery terminal. See if it fluctuates with the gauge. Measure also at the alternator output terminal and to the engine block. If the readings are any different you’ve found a bad connection.
One last thing to check: the ground for the alternator/voltage regulator. The regulator has to have a reference voltage to base its regualtion on. That reference is the ground. With a bad ground there is no reference and the voltage regulator hasn’t got a chance of delivering a stable output. This ground may be the connection to the engine / battery, or it may be somewhere in the wiring harness. Consult a Haynes manual.