Help/advice please ,Alternator charging voltage

[SILVER BULLET]

Whilst out on a very rare drive the other evening I noticed that the volt meter was indicating 15+ volts ,which I thought was a little higher than I had previously observed.
Is this an indication of something going wrong? the battery is the same one which was in the car when purchased in 2007 and is periodically attached to an optimate battery charger/conditioner.

Thanks,

Ian

[TheSebringOne]

I'm sure Kaz mentioned something on this about 6 months or so, ago?

[Kaz-kzukNA1]

Hi, Ian.

There are many factors that can affect the voltage reading including the temperature, battery type, surface charge, etc so it’s not that simple but I would first measure the actual voltage at the battery terminal.

The gauge on the dash has several errors on all of the readings and they are different from one car to the other.
This is the reason why there are adjuster volume for the speedo and tacho/rpm on each gauge.

As you know, the faster you drive, the higher the speedo reads. It’s partially on purposely done by the manufactures but also on our NSX, some of the components on the gauge circuit boards are getting old and going out of their original spec.

The rpm gauge is the same.
While the error is fairly small at the lower rpm range, you could see about 100 – 200rpm higher reading above 5,000rpm.

The volt meter is fairly accurate but not in the order of 0.1V difference.
Your eye position can also alter the reading.

The gauge is just showing the voltage against its own GND level so while it is normally the opposite direction (shows lower voltage), the gauge GND level could be floating and not at the same level as the battery GND if there is bad connection although this is very unlikely to be the case.


Best to measure the voltage at the battery posts using the multimeter first and then compare it with the dash gauge reading.


I don’t know how high your voltage reading was but if the ACG regulator failed, normally it would register really high voltage such as almost at 16V or even higher.
It is not that popular for the ACG to fail on our NSX though.

If the battery fails and created no load condition like running the car without connecting the battery, you could fool the ACG but then you would struggle to crank the engine.

In order to test the ACG and battery properly, you need proper equipments but as a quick check, you can turn the head lights on for about 1min without running the engine.
This will discharge the surface charge depending on the type of battery used.

Switch off the lights and leave the car for about 5min.

Turn the IG switch into P2 (lots of light on the dash before cranking) and read the voltage at the battery terminals as well as at the gauge.
If the battery is healthy, you should see about 12V while in P2 at both the battery and the gauge.

Start the engine and let it running until the operating temperature and the idle rpm will settle at about 800rpm.
Make sure no electrical loads such as A/C, rear defogger, headlights, etc are in operation.
Measure the battery voltage at the battery terminals and you should see around 14.0 – 15.0V at idle rpm but typically, around 14.5V.
Please pay extra attention to the radiator fan as it will suddenly start spinning.

Then, raise and hold the engine speed at about 2,000 - 2,500rpm.
Again, measure the voltage at the battery terminals and one more time, please be careful with the radiator fan.
It should be below 15.1V and if not, further investigation on the ACG and battery required but probably easier to just borrow known good ACG and battery from another owner and test again.

Kaz

[SILVER BULLET]

Thank you Kaz, I will have a go at the above and report back here with the results,

Ian

[SILVER BULLET]

Carried out the various tests and happy to say all readings were as advised above ,thank you Kaz.

Ian:)