Batt voltage displayed by FAST question.

[disco stu]

Does your fast display battery voltage about one volt lower than actual system voltage? I just want to see if these values are typical of what others are seeing.

It is an XFI with an adaptor harness, and the grounds are run straight to the battery. And also no voltage drop on the orange ECM power wire, and power at the CCCI fuse is the same as alt voltage ~14.2.

 

[TurboTR]

Well there will likely be a measurable voltage drop on both the feed wire and the ground lead when the box is at full power; both leads are pretty long. The ECU will be measuring system voltage at the ECU power and ground pins basically. So it seems reasonable to assume that what it sees across its pins at full power will be less than what the alt is developing at its output terminal wrt the battery ground post.

Unfortunately I can't say for sure how much less. The best way to answer this question for sure, and quickly, is to put your DVM directly across (as near as possible to) the ECU power and sensor ground pins and see what it says at WOT.

TurboTR

 

[10SV6]

The old boxes measure voltage off the 12v switched terminal.