Last night as I was driving my 93 Diablo I noticed the voltage reading gauge was just above the 12 volts (between the red line/ green line)! As I slowed down to park I noticed the voltage dropping more below the 12 volts mark and just above the 10 volts mark!

If the alternator and battery are in good shape what should the voltage needle read?
I have replaced the battery two months ago so I guess it must be the alternator
Am I correct?

I did a search for the same topic but I am a bit confused about the Alternator part number and if the Porsche 928 Alt is the same?

TIA

Probably the diodes are gone/going in the alternator. Over here and I suspect where you are there are many shops which rebuild and test alternators. There is nothing special about the one in the Diablo. I seem to remember there is a Bosch replacement. Sorry donn't have the number.

on my 91, the gauge has been showing 11 volts since I had it, (4 +years)
we checked the output voltage from the alt. and its normal, so for me maybe its just a bad gauge, or maybe a ground wire

it's the way the gauges are grounded...freak me out when I first saw mine.

One thing about Diablos....grounding and ignition!

__________________
1992 Lamborghini Diablo
1992 Ferrari 512TR

yup those are the 2 reasons u get those wierd electrical ( Gremlins ) problems... i had so many in my 92 it drove me crazy....

I will check the output voltage from the alternator side and will take it a part if I have too. Also I will apply a variable voltage to the gauge from a battery charger.
The needle should increase as I increase the voltage this should verify the gauge operation/tolerance.

Thank you for the pointers

On my 91 D my volt gauge always was on the low side. I finally ran a wire from the battery itself to the cluster with ignition activated relay. I also noticed the temp gauge now shows a little cooler.
I did have my alternator rebuilt and hopped up at Alterstart. It now has the ability to put out 150 amps (110 at idle).

Interesting idea, so did that give you a more accurate reading?
By the way why the temp gauge was showing cooler? Is this is due to poor grounding?

TIA

Works great. 1 hour install.
The temp gauge uses electrical resistance. The lower voltage the gauge is receiving (say 10-11 volts) will give an inaccurate reading. Did you notice the gauge will fluctuate when your turn signals are flashing? The gauge will jump up as the lights blink on.
There are several posts about bad grounding. My problem was that by the time the power finally got to the cluster, it was only receiving 10.5 volts.

Thanks for the info