2.9V Diff between 12V in aft compartment and engine room

I don't think so. I think I've noticed slight differences at most, though I've done that double check very rarely, maybe just once, since it's more hassle digging down to the battery itself. I had one or two tenths variation at most, and have an foggy recollection that it was actually lower (slightly) at the battery, which seemed improbable.

 

That’s not right. Your not measuring correctly somewhere.

Battery. Post to post
Jump. Charge tab under red plastic to strut tower nut right above black fuse box

 

and make sure you have a good ground in the engine bay.

 

Agreed. Are you measuring through paint when you check it up front?

 

An assumption that may not be true.

There also could be a bad connection somewhere; typically where the negative battery cable is connected to the frame.

 

This was the cause of some failures to start on a previous car of mine. In that particular case, a mechanic had disconnected a ground strap to the engine block during work, then failed to properly reconnect it. The bolt had been put through the tab and given enough turns for the theads to catch, but then never even hand-tightened, let alone wrench tightened. The intermittent connection worked most of the time, but not late on a Saturday night four hundred miles from home. Though it took a couple more recurrences before it was finally identified.

If OP has this general problem, the particulars will likely be different. There are many ways to create this general type of problem.

 

First off, unless there is a meaningful electrical load on the system, a "12 volt" measurement ANYWHERE should be nearly the same as between the B+ and B- terminals of the battery; anything else indicates the measurement was not performed correctly (not a well grounded object, etc). The allowed difference between measured voltage at the battery and anywhere else - Power-to-Ground, is 0.10V.

With a circuit under load (headlight, etc) drawing a few amps, the allowed voltage difference increases to 0.50V.

The only way to know where the problem might be, if there is greater differential than 0.50V, is to take the ground of the meter, connect it securely to the Battery (-) terminal and start measuring. Allowable difference on the Ground side is 0.30V and Positive is 0.20V; yielding a TOTAL of 0.2 + 0.3 = 0.50V.

 

SIGH.
NO.....while that might be the likely cause of an incorrect reading, it is not the ONLY cause.
There actually could be a problem that needs to be fixed.

 

Not.

I was VERY specific, under NO LOAD conditions, there is "12v" EVERYWHERE that is battery connected, PERIOD. I have done these tests thousands, if not hundreds of thousands of times and the only exception is when you are not testing power or ground circuits or the circuit is under a load.

 

And because you have never seen it, that means it does not exist.

NOT yourself.

 

Actually it does.

Time to educate yourself on proper use of a meter and the interaction between voltage (potential) and work (current flow).

 

Actually it does NOT.
Unless you are referring to your posts indicating that you have an inflated ego problem.

And I've got plenty of "education" thank you.

50 years as an Electrical Engineering Technician and 20 or so years in IS and a few years at Bell Labs probably means I know how voltage and current and meters work.......probably.

Just stop please.

 

Then go back to school.

Let's review, since you refuse to educate yourself, I will do it for you... kids now-a-days, geeze.

To set the stage for everyone, and I repeat, this was VERY specific and is a no-load condition.

Have you heard of "Ohm's Law"? I guess not, because you fail to realize that on ANY connected circuit, the voltage MUST be the same everywhere. How? Easy, because V = I * R, or V = C * R; as in Voltage drop = Current * Resistance.

Now, the voltage drop is equal to exactly and only zero when amps is Zero (no-load).

As, by definition, there is ZERO voltage drop, the voltage measurement MUST be exactly the same throughout the circuit.

Anyone who cannot understand this, or argues with it, requires education.

You do not need an advanced degree to know this, it is Physics 101 and a fact of every-day life.

 

BTW, someone is going to say that if the resistance is *high enough* it can/will cause a bad reading. It will not and here is why.

Volt (drop) = Current * Resistance

V = 0*1 ohm = 0
V = 0*10 ohms = 0
V = 0*100 ohms = 0
.
.
.
.
.
V = 0*1x10^24 ohms is still 0

 

Self serving rants ignored.

In order for a meter to GET a reading, some current must flow.

Bye.

 

Take the battery post clamps off and with a piece of sandpaper clean the posts real good. Then lubricate with synthetic grease.
Do the same where the ground is bolted to the chassis. Sand spot off chassis where the ground is bolted to and sand the ground eyelet. Grease.

Tighten up the nut on the positive jump point that holds the vertical tab the jump lead grabs onto in the front plastic fuse box. Make sure the battery is disconnected while wrenching on that nut.

Btw, never measured it but there probably is a tiny voltage drop from front to back.

 

Another possible bad connection point is where the ring terminal is crimped to battery negative cable.

 

I know that you won't like any disagreement with your suggestions but...............

Unless it is special conductive grease, one should NEVER put grease on the conducting surfaces of an electrical connector.
Smear it all around the outside if you wish but NEVER on the mating surfaces.

The one exception to that is the connector pins that actually BITE into the mating flat surface and thus penetrate the grease.
But even then it is not a really good idea if high currents are involved.