Monte4ever

iTrader: (2)

I've been monitoring volts since I've noticed during WOT the volts would drop to 12.5ish. I've done the "Big 3" upgrade and even added a 1/0 gauge ground wire from the 270 amp Alternator mounting bolt to the engine block and it still drops volts according to the scanner. So today I checked the voltage reading off the Alternator mounting stud with the car idling and it read 14.6ish. Then I checked the battery, which is mounted in the trunk and it read 14.5ish. This is where I'm concussed, the scanner is logging Control Module Voltage and it reads almost exactly 1 volt less at 13.5ish. I also have a Autometer volt gauge and it reads 14.5.

My question is why does the scanner (ECM) read less than all the other test locations? I know it's not because of how it's wired because the ECM gets it constant power from the Alternator stud. The ECM is a P10 411 with a 12216125 Flex Fuel OS.

Monte4ever

iTrader: (2)

Looking at some old WOT logs my voltage was dropping down in the upper 11 range with the stock C5 145 amp Alternator almost as soon as I flooded it. Now it goes to 12.9/13 volts with the S terminal hooked up but that wasn't a full WOT run. Considering what it use to do I'll call that a good improvement. I'm also looking into this in case it doesn't hold the current voltage during a full 1/4 mile pass: http://www.casperselectronics.com/ca...&product_id=73

After some searching this voltage drop seems to be a common theme.

Bella Mingo

This applies to both posts. I'm a long time lurker and this is my first reply.

What you are experiencing is normal. Alternators in effect "turn off" at high rpm such as at WOT to protect from overspeed, prevent over excitation, as well as prevent excessive slippage with regards to the field.
The lower voltage you are seeing when at WOT is actual battery voltage itself. Once you come off WOT, the alternator field resyncs and you see the higher normal voltage again.

As far as why the specific 1 volt difference measured by pcm , I can't say exactly but several things can affect it.
Dirty or poor ground or cable condition, distance from battery, and sizing of wire. Thicker wire can actually reduce voltage due to the IsquareR losses (sorry don't know how to put sub and super texts via phone).

I hope this helps.

Luis Corcuera

iTrader: (1)

It's the opposite actually and I just confirmed this with my multi meter, Control Module Voltage is actually 12v Ign voltage which is why it reads different than the alternator post. At WOT I was getting 14 volts at the battery but on my data log on hptuners it was dropping voltage to battery 12v just like you.

I then hooked up the S terminal to the alternator battery stud in the back and now I was seeing roughly 13v on my WOT logs. I know our cars run a single wire to the L terminal but the S terminal is used as an option to compensate voltage drain.

So then I started to think that maybe my Control Module Voltage was 12v ign voltage reading and sure enough it was. Which means that your coils and injectors (which are run off 12v ign) is what's showing on your logs. So then I tried wiring my S terminal to a 12v ign source and what do you know now I get 13.5v steady at all RPMs WOT, there is a draw back to this, since the alternator is compensating I did get a 15v reading at my battery post only during WOT runs only which I dont think itll hurt anything if it's only for seconds at a time