BLDun

Not sure if this goes here or in the computer section. In any case, where do the voltage readings for the guage come from? And on my Scanmaster I assume it is getting it's reading from the PCM. Where does the PCM pick it up at? And, is the reading on the Scanmaster the reading of the battery or the amount it should be charging? On my Scanmaster while I am driving down the highway I get a fluctuation between 11.9 and 12. When I check the voltage at idle on the altenator I get almost 14 but the guages (i have two) never read more than 12 to 12.5. Seems to me like I am loosing some voltage somewhere between the alternator feeding the battery and the guages. Can I check at the fuse block and get any help???

shoebox

The voltage for the gauge and PCM come through the ignition switch. Since it is a common point of failure and connector overheating, my theory is that there is a voltage drop right there on most of our cars.

BLDun

So, assuming the switch is working properly I should get the same reading on the guage and through the PCM as the alternator is putting out to the battery? I replaced the switch about 6 months ago when I was experiencing alternator trouble only to find that the pigtail to the alternator was bad. It was an AC Delco replacement so it should be good. I will try to check it again. Any other possibilities??

Injuneer

Based on my experience, the ScanMaster reading does not match the dash gauge. ScanMaster is typically lower. If you think about it, there are 4 connections to the PCM for 12V power, two directly from the battery (PCM BAT fuse to pins B15/B31), and two (pins B30/D3) through the PCM IGN fuse. Hard to tell which one the PCM is reading for its "system voltage".

My aftermarket ECU supplies yet another voltage reading, that doesn't match the ScanMaster or the gauge.

The path for the PCM IGN source is through the ignition switch, through a body connector, through the fuse, then splits to the fuel pump and VATS module, continues toward the PCM through another body connector, then has a takeoff for the MAF sensor. Anything in that path could be affecting what the PCM sees as system voltage.

BLDun

MERCY!! Thanks, Fred, I think. Sure is going to make tracking down the drain difficult. Especially since I'm unsure which ones should read battery voltage - 12 volts - and which ones should show the amount being charged from the alternator.

Thanks
Barry