Electrical ground question, checking at different points

[Guest bastaad525]

Okay... just wondering if this is normal/abnormal, and if it's something I should worry or do anything about.

 

this is once again regarding my SE-R but is a pretty generic question that I"m sure applies to just about any EFI car.

 

Basically, I'm trying to test the ground voltage on a sensor (the MAF). It's supposed to be within a certain range, in this case, below .009 volts, when measure from the MAF connector pin to the engine ground point, with the car running. Now, just like on my Z, many of the grounds from the ECU are bolted onto the intake manifold. So, with a voltmeter, I checked from the MAF ground terminal itself, to the ground point on the intake manifold (with everything connected, probe just stuck in the back of the connector on the MAF, touching the ground terminal). I got a reading of .001 volts... WAY better than the maximum allowed.

 

However, before I knew that I was supposed to test specifically at the intake manifold ground point, I was testing from the sensor to the negative terminal on the battery. Now... I would figure that touching the probe to EITHER ground point SHOULD give the same voltage reading, yes? I dont really know much about car electrical, but that just seems common sense to me.

 

Well, with the positive probe on the MAF ground pin, and negative probe on the battery negative terminal, I was getting what I considered to be really odd readings, it was reading NEGATIVE volts, like .040 or .060 negative... I forget... I figured I wasn't using the multimeter right so I didn't note the reading. I later asked around here at the forum and found out that I actually WAS using the multimeter right at the time.

 

So, just going on a hunch, I figured maybe there was some 'blockage' from the manifold to the battery ground... I decided to run some wire (14 gauge, was the thickest I had handy) from the battery negative terminal, to the same ground point on the manifold that the ECU's grounds are bolted to.

 

Oddly, after I did this (yeah I'm working backwards what can I say ), I took a good look at the ground wire... there is no visible corrosion on the battery negative terminal or the connector, nor on the midpoint of the negative cable, where there is some of the insulation stripped away and the wire is bolted to the wheel well, nor at the other end of the wire, where it is bolted to the block right near the thermostat housing. The wire and the connection points all look clean. So there SHOULD be good 'flow' between the batter, block, chassis, manifold, etc.

 

I then retook voltage readings with the multimeter.... Testing from the MAF ground to the intake manifold this time got me a reading of .002-.003 volts... up a little from the last time I tested... that doesn't seem good, but still well within spec. I then retested from the MAF ground to the battery negative terminal. I got a reading of negative .045 volts. I then decided to try checking voltage between the manifold ground point, and the battery negative terminal (I have no idea why I tried this but it seemed like a good idea), and got negative .050 volts... and I'm left wondering is this normal, does it make sense? Is this because I didn't use a large enough wire? Was it even a good idea to run a wire from the battery to the manifold like that? Common sense keeps telling me the voltage measure from the sensor to ANY ground point should read the same, and all be within spec (less than .009 volts). But again... I'm really unsure about car electrical and how it works.

[Pop N Wood]

Even a straight piece of wire has some "resistance" to current flow. If current (amperage) is flowing though that wire, then you will be able to measure a voltage drop across it. The voltage drop is equal to the resistance (in ohms) times the current in amps.

 

Normally the resistance of a piece of wire is small enough that the voltage drop is not important. But if you try to pump enough current through a wire, then the voltage drop can become significant. That is why you use great big cables to hook up the starter, but small ones for say dash lights.

 

What you were probably sensing was the current flow charging the battery. The alternator is grounded to the engine, then through the battery cables from the starter to the negative battery post. With charging current going from the alternator to the battery, you will see a slight voltage drop from the engine ground to the battery ground. If the battery is being charged (drawing current), then it will be slightly more positive than the engine ground. (Conversely the engine will be slightly more negative than the battery). The more drained the battery is, the more current it will draw hence the larger the voltage difference. The voltage drop will probably be slightly different every time you measure it.

 

The voltage readings you are measuring are really small. They are consistent with what I would expect so I say don't worry about it.

 

Just make sure you always measure the voltages where the manual says you should.

[Guest bastaad525]

Okay I read that like three times and I think I get it Heheh just waking up maybe my brain not fully on yet... but if you say that sounds just about normal, that's good enough for me, I'll leave it alone.

 

Here is the line from the technical service bulletin from nissan regarding this problem, as far as where to check it:

 

"Inspect the MAFS ground voltage between connector F6, Pin "C" (white wire) and the engine ground. Place the Volt Meter's positive lead at F6 pin C, and the negative lead to the engine ground. The voltage reading should be less than or equal to 0.009 volts DC."

 

That was pretty unspecific, to me... "engine ground". I originally thought that checking to the battery negative was what it called for. I asked around on the SE-R forums and was told that meant to check it at the ground lug on the intake manifold, where all the other ground wires from the ECU were run to. This seemed to make more sense as later in that tech service bulletin it recommends running a new ground wire from the MAF to the intake manifold... so it seemed right to measure the voltage from there.

 

 

For what it's worth... doing all of this did seem to make SOME improvement... idle is more stable, gas mileage is up about 2mpg's, and I haven't seen that funky error code from the ECU in a while (ECU kept reporting a leaky injector... the tech service bulletin specifies a bad MAF ground as the cause)....

 

 

I hate newer cars....