What am I looking at?

Open circuit voltage:



OK, great. Now, from B+ post to disconnected B- cable:



And from B- post to disconnected B- cable:



Sooooooo what am I measuring with the 1.4V and 11.14V? Internal battery resistance?

I'm not even saying there's any diagnostic value to this reading. :silly: Just can't come up with an explanation I'm happy with.

Hmm, maybe try to isolate the battery case from the chassis by removing the hold down and setting it on a block of wood

The only explanation I can come up with is galvanic voltage two differing metals in contact acting as a battery --- The B35T has like most modern DMM an internal ressistance of 10 million ohms so won't put enough load on to pull the voltage down. The way to test this is to repeat the test with Power Probe or old style volt meter both of which put more load on the circuit.

Good thinking. That's why I ask the big brains around here.

As Noah pointed out, you got a current path - in other words a short - from the positive post of the battery to chassis ground/ negative battery cable.

Obviously this path is highly resistive, otherwise the voltage drop between b+ and neg. terminal would be bigger and something would get really hot.

You could actually measure the current flow with an amp meter or clamp and calculate the resistance of that path applying ohm's law as you already know the voltage drop...

I agree with all of these comments. Is that ratchet touching something else? It makes me think of when the top of the battery has some crud on it and you can measure voltage from b+ to anywhere on top of the case. I'm thinking it's something like that, making it through to the hold downs and the rest. Cool voltage drop, dude.

Nissan Technician

Did you find the cause of this? I am wondering if a bad alternator diode could cause this.

Chad wrote: Did you find the cause of this? I am wondering if a bad alternator diode could cause this.

I was just about to go experiment! :silly:

Was gonna start with taking the hold down off and recheck, then wash off the battery. If it's still there... *shrug* Maybe I'll check other cars in my fleet for the same thing.

What was the reasoning behind your alternator diode theory? Should I disconnect the alternator B+ while I'm at it?

Tyler wrote: What was the reasoning behind your alternator diode theory? Should I disconnect the alternator B+ while I'm at it?

No reason, in particular. I just thought it could, possibly, be a path to obtain the reading you are getting.

Looking at this generic alternator diagram, it would take two bad diodes?



*EDIT :oops: Doesn't explain current flow, though. :whistle:

Chad wrote: *EDIT :oops: Doesn't explain current flow, though. :whistle:

Eh, still a good idea!

So I tried several variations of testing based on previous suggestions. If there's anything else you want me to try, let me know today, 'cause I'm back to work tomorrow. :silly:

First, open circuit voltage for reference. Yes, I know my battery isn't fully charged. No, I'm not doing anything about it.



Then, I tried measuring voltage drop across the battery surface:



Not all that much. Then I removed the battery hold down and retook the mystery voltage measurement:



OK, it's still there. I washed the battery AND hold down tray with soap and water. Retook the measurement, minus the hold down:

It's better? :huh: But still there. I tried the Vantage (courtesy of Noah) to make sure it wasn't a meter problem:



Nope, same(ish) reading. Then I tried it on another vehicle in the fleet. Open circuit voltage:



Then the mystery voltage drop:

juergen.scholl wrote: ... you got a current path - in other words a short - from the positive post of the battery to chassis ground/ negative battery cable.

Obviously this path is highly resistive, otherwise the voltage drop between b+ and neg. terminal would be bigger and something would get really hot.

You could actually measure the current flow with an amp meter or clamp and calculate the resistance of that path applying ohm's law as you already know the voltage drop...

The more I picture it the more I think that path must be through the various modules of the car.

You've made a big open switch, and we know that when you measure voltage (or more accurately voltage potential), it is the same on either side of the "component" before the open in the circuit.
(Think 12v on both sides of an injector waiting to be grounded).
So the voltage potential is greatest from the negative terminal (the end of the circuit) to the ground cable, which ultimately terminates via everything on the car at the positive terminal.
Voltage potential is at its least from the positive terminal (beginning of the circuit) because everything after the open ultimately must reconnect to the positive terminal.

The sum of those potential readings, is open circuit voltage.

Or at least that's a reasonable enough justification to fit in my head.

More weirdness! :woohoo:

I took this reading at the B+ post of the alternator on a Dodge 2500 5.9 diesel, meter connected to B-. Dual battery setup. Alternator fuse is popped. Why is it reading 38V? :huh:

If it's with the engine running and a big alternator output fuse popped it's probably just the alternators open circuit voltage.

If it's the control fuse, or the engine ain't running, that's really weird........

I feel like it has to be completing the circuit somewhere for this to happen. Does it matter if the modules have all gone to bed, like you're going to do a parasitic draw test? It would be interesting to get an amp measurement on it.

Nissan Technician