Altenator question

The car always starts on first turn. The gauge will show 9 amps engine idling at 800 rpm,I rev the car and the gauge goes to a little over 13 amps and will stay there even when the idle settles at 700 rev. The belt does not slip and the altenator has 70 k on it after a rebuild, Each time I start the car it does this.

Are you sure you mean amps? The dash gauge reads in volts which is more common on cars fitted with alternators. Older cars did use ammeters which measured the current going into or out of the battery. If it is in fact volts you are measuring 13 seems a bit on the low side.

When mine did this even though I refused to believe it as I couldn’t hear it it WAS the belt slipping. Tighten it up and see if that stops it, then change the belt if it does.

John. yes its volts, can I BLAME AGE 81

You might want to update your info by adding your location and the car(s) you are dealing with. Just my perennial request of posters on this forum.

If it makes you feel any better my 90 XJ40 does the exact same thing. I think the residual magnetism in the rotor is the issue. My belt is fine. It usually flashes around 2100 rpm.

John …

Take out your trusty multimeter (if you don’t have one go to Walmart/Lowes/Harbor Freight/Wing Lo’s dry cleaner and purchase one for under $50).

Set the meter for DC volts and auto scale or anything above around 15v or 20v. Now with the engine off place the multimeter’s black lead on the black (negative) battery terminal and the red lead on the red (positive) terminal. Hopefully you should read about 12.4-12.5v which is a fully charged battery.

Leave the red lead inplace, remove the black lead and start the car. Now replace the black lead, and not changing any of the multimeter settings, check the voltage again. It should be above what your previous reading was usually around 13.5v. This means the alternator is charging the battery. If it’s not reading that have someone rev up the engine while you watch the meter.

This test eliminates dashboard gauge error.


What’s the mileage on the clock?
Can you identify your alternator model? Or amperage at least?

Confirmed - belt slipping, especially on cold - highly

The only thing remaining are the brushes on voltage regulator.

Thank you all for the input, I will check the belt again and report back . I am having problems adding to my profile.When they switched to the new site all my history info was lost or deleted. how can I restore it ?

John. After many attempts to update the info in my profile it is now merged back to 1997, that leaves only the alternator.

No, the residual magnetism isn’t the issue, it is the reason your alternator generates at all. Your initial excitation has failed, and at higher speeds the residual magnetism is sufficient for the alternator to feed it’s excitation. You will probably find your charging light does not light up when you turn on the ignition. It could be as simple as a failed bulb (wich actually is responsible for the initial excitation of the alternator) or a wire broke.


Dennis. I got the multimeter and did exactly what you said, the numbers produced by the battery and alternator are what you gave me. the gauge is faulty, I watched it over a few days driving and it drops and rises for no reason,it drops 5 bars on the readout and then returns to 13.5 v. Thank you all for your replies. John