my bat reading when I first start the car is just over 13 and the battery gauge reads just above the mid mark which I assume means me that I have 13 volts across the battery.

while driving on the highway this afternoon ~ 2300+ Rpms, no accessories or radio on, the bat reads 12.4-12.6 range and the dash gauge reads just above the red hatching.

So the question is - is the bat reading telling me the volts in the battery or is it reading the output of the alternater at the battery?

in other words can I tell from the bat reading if it's my battery or my alternater that's on the way out.

Thanks

 

update

I measured volts at the battery today both when cranking and while idleing and the voltmeter reading at the battery measures about 1.1 volts higher then does the scanmaster.

Guess there is some resistence on the line between the battery and the PCm and sensor for the gauge.

Anyone know where the battery sensor is?

 

The "battery sensor" is your volt gauge. It is just tied into the power wiring and monitors whatever voltage is present. When the engine is not running, you see the voltage of the battery. When the engine is running, you see the voltage of the system as being supplemented by the alternator.

 

I've noticed that the ScanMaster "BAT" reading looks a little on the low side at times. IT reads lower than the dash volt meter. I got sort of worried about the low voltage readings from ScanMaster, and check the "system voltage" reading in my MoTeC ECU, and it is much closer to the dash gauge readings, and always higher than the ScanMaster reading. I think I might have posted this same question about 1 or 2 years ago, and never got an answer.

The PCM gets "PCM BAT" power directly off the red junction block next to the battery, through a fusible link, and then through the "PCM BAT" fuse. The same I/P panel lfuse feeds the instrument cluster and the theft deterent module. The power connects to the PCM at pins B15 and B31.

Still not sure why SM reads lower.

 

Thanks guys - I guess my batt and alt are atill ok.

I did also measure the alt at the red connection box and it was also ~1 volt higher then SM reading.

Guess the reason why will remain one of lifes little mysteries.