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Faunsdale, AL | To test if you have properly functioning disconnects, switch them both/all off and then test for voltage between the other terminal of the battery that is not disconnected and the frame of the machine. If the battery connections between negative post and machine frame are disconnected, you should get no voltage reading when you test from battery positive to machine frame. If no voltage there, proceed to test for bad batteries as per Bern’s post above.
If you pick up voltage between frame and battery positive with disconnects open, there’s a connection you’re missing. If it’s less than battery voltage, then look for some sort of bad insulation on a cable where it passes through a clamp or tight against a frame member, layer of damp dirt on batteries carrying current to frame etc.
The better way to test for draws on individual circuits is to test voltage drop across fuses. There’s a formula you can find online that calculates the actual amperage flowing through a certain size and type fuse using the voltage reading across it. A low amperage fuse will be passing less current than a high amperage fuse with the same voltage drop across it. A fuse with no voltage drop is passing no current or it’s very low and your volt meter cannot pick it up.
With modern computer control units on a CAN bus, disconnecting power and then restoring it to a module normally will “wake it up” and it’ll start drawing power again for a while before it goes back to sleep again. That’s why testing voltage drop across the fuses is a good way instead of pulling fuses and replacing them. Same thing happens when disconnecting battery connection to put a meter in the circuit. That’s why it was recommended that you only disconnect one positive cable to put your amp meter in and then disconnect the other leaving the only connection through the amp meter. This supposes that you can accomplish that without ever interrupting power to the system and that the draw will be less than the meter’s current carrying capability. It works if you do it right but the problem of waking up a module by pulling and replacing a fuse still remains.
I have no idea what a DT225b is or what level of technology it would have, so it might not be so much of a problem.
You need a good meter that can read millivolts accurately to do the voltage drop tests across the fuses.
A thermal camera/scanner is also useful to see what is staying powered up on modern vehicles.
Edited by ccjersey 7/7/2025 13:00
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dead batteries
