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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?

  • What were the readings? Did you try measuring the voltage at the pin on the connector for the ECU? – rpmerf Jun 2 '16 at 20:00
  • @rpmerf I took a variety of readings. Typically the difference was between 1/4 to 1/2 a volt. – Robert S. Barnes Jun 2 '16 at 20:10
  • .5v is more than you would like to see, Generally .25v is acceptable. – Ben Jun 2 '16 at 20:24
  • @RobertS.Barnes I tend to start with grounds since they are easiest. Did you always use the battery for a ground, or did you ground to random location on the body? If you always use the battery, only the positive side is a variable. After that, follow it to the fuse box, ignition switch, ECU and see where it drops off at. – rpmerf Jun 2 '16 at 21:00
  • It would also help to write something about the status of the car while measuring. Was it running? Any heavy load like blower, lights on? – sweber Jun 2 '16 at 21:54
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As I commented in your very similar question about alternator voltage, where you are measuring voltage and where the ECU does are two different locations. Voltage drop test all the main battery cables and wiring.

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Well, it seems that part of the reason the voltage didn't match is because I was using a very cheap multimeter. I got a good Uni-T meter and with the engine off my cheap meter was showing 12.92v and the Uni-T was showing 12.74v. This was after I'd been driving around a bit, so I'm sure there was still a bit of surface charge. I checked again later and can pretty much chalk this up to an inaccurate reading by a cheap multimeter.

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