My car is making some problems regarding it's electrical system, lights get dim after some WHILE, and csr starts to decrease it's fuel efficiency, blower works weak, and when radiator fan turn on automatically engine seems to work harder.

When car is on idle voltages are : 13.8 to 14.2

When car is on load and on 2000 rpm: 13.5 to 13.8

But i think for whatever reason alternator loose it's efficiency after it gets hot or something because it keeps undercharging my battery, sometimes when lights are on and tries to start the car it won't start.

Is it possible if car alternators bushes starts to stop working correctly after the alternator gets hot because of temperature?

When after a long drive i come home and check the voltage on load they goes under 12.6 too and for some times they drop to 12.6 to 13.4 etc, should a alternator drop voltages to 12.6 ever?

  • Welcome to StackExchange! Please post your Year Make and Model so we can help you better!
    – NitrusInc
    Apr 24, 2018 at 20:59
  • @NitrusInc it is suzuki cultus car which is only available in my country i think so that's why model and make should be worthless to you.
    – ZBT248
    Apr 24, 2018 at 21:01
  • This largely depends on the style of charging system the car uses do you have wiring diagrams? How many amps is it putting out and what does the AC ripple waveform look like?
    – Ben
    Apr 24, 2018 at 22:10
  • What does the multimeter read when you have every accessory on (blower motor, dome light/s, headlights, etc.)? Apr 27, 2018 at 17:08

2 Answers 2


The alternator itself could be faulty.


You could have a loose or damaged belt. The voltage regulation could be bad. Sometimes a bad battery can cause issues that appear to be a bad alternator.

  • I have changed the battery twice in two years, and that's what make me worried about the alternator in the first place, i charged one battery with external charger and the car works very smooth or at least it felt like smooth for 2 days (200 km) and then again to the weird feeling of things getting dimmer and slow.
    – ZBT248
    Apr 24, 2018 at 21:00
  • OK - time to find a shop / part store that can test the alternator
    – mike65535
    Apr 24, 2018 at 21:08
  • it is hard to find someone in my country to do that, i am not trying to save money by self diagnosing it, can you believe electricians of car in my country don't know how to operate digital volt meter?
    – ZBT248
    Apr 24, 2018 at 21:19

Obligatory remove, clean, and tighten your Battery Terminals and Grounds.

Check the power wire connecting the alternator output to the battery. Check for bad connections at the battery and alternator. Look for any repairs (splices) in the wire.

I would check your serpentine belt for deflection. If your idler and/or tensioner pully(s) are in good condition providing proper tension on the belt and it does not have any deflection, I would suspect that the alternator is going bad.

  • So under the load when alternator requires the mechanical power most, the belt slips and keep it from making current? Then how come when starred cold it regulates the voltage fine?
    – ZBT248
    Apr 24, 2018 at 20:58
  • If the belt is loose enough (or missing damaged teeth - if it has them) then as the engine heats up, the belt stretches some perhaps causing slippage.
    – mike65535
    Apr 27, 2018 at 16:18

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