Voltage Reading

Asked by Oct 30, 2016 at 09:14 PM about the 1990 Ford Thunderbird Base

Question type: Maintenance & Repair

What should the voltage of the battery read when
you use a multimeter while the car is running when
i did it read 15.05 but my battery keeps going dead
but will run as long as it is jumped and last the day
but not over night the alternator is good

8 Answers

If you read the voltage while the engine is running then the 15V is good, it is reading the output voltage from the alternator. Check the battery volts with the engine off. It should be above 12. Sounds like your battery is not holding a charge and should be replaced.

96,665

My book shows voltage should be from 13.8 to 14.2 for your year make and model, if it is reaching 15 that is enough to fry a battery, you may have to replace the voltage regulator as well as the battery and then recheck the voltage output.

Thats what im leaning towards but i was told if i check the battery with no power and the amperage still drops i have a power draw somewhere then i was told to disconnect the neg cable and if it keeps dropping the battert is bad is that true

96,665

If you disconnect the battery and the voltage keeps dropping on the battery then you have a bad battery for sure, the way I tested batteries is too make sure they were fully charged, then turn on the headlights for 15 minutes with the engine off, if after that time the battery would not start the engine then the battery was bad. Remember do not disconnect a battery cable with the engine running, I had a customer that did that to his Cown Victoria and in just a second he was 1500 dollars poorer, it took out every electronic system that was operating at the time. Now if you suspect there is a draw on the system, you can remove a battery cable and put a test light between the battery post and the cable end and if the light lights then you have a battery draw somewhere in the vehicle, 8 out of 10 times it is the alternator, a diode fails and allows power back into the windings. HTH

I can just disconect the negative or do i need to totally disconnect the battery

96,665

Just one cable will work.

I just ran every multimeter test everything came back good even with turning everything on high nothing is drawing any extra power so now im cleaning connections

So i pulled my battery out of my car and every so often i check it and the volts are dropping really slowly so i assume thats saying this battery is junk so im getting a new one and hopefully thats the end of it

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