I was messing around with electrical loads on my 98 Mazda 626 GF 2L today and noticed that the voltage I was reading at the battery with my multimeter, didn't match the voltage the ECU was report via my scan tool. I'm sure the multimeter was correct.
I took measurements at idle, with no load and with all the electrical loads turned on ( high beams, radio, blower, defroster ) and the difference between the multimeter and ECU readers varied between 0.25 to 0.5 volts.
What might cause this discrepancy? Might I have a voltage drop problem somewhere on the path to the ECU?