What Voltage and current readings should I be reading with the engine off when I disconnect the positive cable and put the positive VOM probe on the battery positive terminal and the negative VOM probe connected to the Positive Cable?

Of course before I check I'll pull all fuses and disconnecting everything else except the ECM on my
'90 GMC Sierra, 5.7L TBI, 1/2Ton, 2WD
Sincerely Jon

 

If you check it before pulling all fuses you should read the same as putting the negative probe on the negative side of the battery. It should be 12.6 vdc or more or what ever the battery reads. If you pull all the fuses I think you will still read 12.6 or more because the alternator is still connected. The current draw would be very small even with the fuses in and key off, less than an amp. I did have an aternator that was draining my battery if I let the truck set for a couple of days.

 

You are right, if one thing is drawing current then nearly all of the battery voltage will read minus a voltage drop. Also the ECM is always drawing a current.

it took a while for my brain to wake up to the task but I shouldn't be concerned with the voltage but the current.

Thank you for your help in solving my problem, I am off to the next one.