Worrisome voltmeter

[Donald]

Drove to Phoenix Sun-Mon. When I got there it was 116F. 110F in the shade. I noticed that at idle my voltmeter was reading 12. If I reved the engine it would go up to 13.5 or so.

Got home OK and changed my CarChip to track voltage. Coming home today I noticed at a light that the guage was reading 12 on the nose. Outside temp was 100. Dumped the chip contents and it shows it never went under 13.10.

Ideas?

[MENINBLK]

Drove to Phoenix Sun-Mon. When I got there it was 116F. 110F in the shade. I noticed that at idle my voltmeter was reading 12. If I reved the engine it would go up to 13.5 or so.

Got home OK and changed my CarChip to track voltage. Coming home today I noticed at a light that the guage was reading 12 on the nose. Outside temp was 100. Dumped the chip contents and it shows it never went under 13.10.

Ideas?

I would question the accuracy of the Voltmeter installed from FORD.
It seems like your needle is off by 1 volt...
High end should be 14.5v when your gauge reads 13.5v.
Mine seems to be doing the same thing.

[DOHC 4.6]

Drove to Phoenix Sun-Mon. When I got there it was 116F. 110F in the shade. I noticed that at idle my voltmeter was reading 12. If I reved the engine it would go up to 13.5 or so.

Got home OK and changed my CarChip to track voltage. Coming home today I noticed at a light that the guage was reading 12 on the nose. Outside temp was 100. Dumped the chip contents and it shows it never went under 13.10.

Ideas?

Mine will drop down to 11.5 volts with the A/C blasting away and the headlights on. Dealer told me that my electrical system was fine and that the voltmeter was 0.5 to 1.0 volt off. I still think that much of a drop is excessive, but they claim the charging system is working fine. Go figure.

[David Morton]

Drove to Phoenix Sun-Mon. When I got there it was 116F. 110F in the shade. I noticed that at idle my voltmeter was reading 12. If I reved the engine it would go up to 13.5 or so.

Got home OK and changed my CarChip to track voltage. Coming home today I noticed at a light that the guage was reading 12 on the nose. Outside temp was 100. Dumped the chip contents and it shows it never went under 13.10.

Ideas?Yes, well not an idea as such but an explanation of what's happening and why everything's still OK.

At Special Electronics Training class in the Atlanta General Motors Training Center back in '92 I learned that every electrical connection, whether soldered or not, has about a half an ohm resistance that will result in a voltage "drop" across the connection. The amount of the drop depends on how much resistance is in the total circuit. For instance if the total circuit resistance is 1 ohm, 1/2 for the soldered connection and 1/2 for the rest, for a 12 volts supplied, a 6 volt "drop" would result, just from the connection.

I suspect the voltmeter is one of those low resistance meters (they're cheaper) and is getting the reading from the fuse block, 2 or 3 connections down wind from the battery positive terminal. If you want a truer reading, rewire the guage by taking the positive wire directly to the battery positive terminal, and even then you could expect a couple of tenths "drop" from what a shop type voltmeter would show.

[RF Overlord]

I learned that every electrical connection, whether soldered or not, has about a half an ohm resistance that will result in a voltage "drop" across the connection.

David, are you sure you haven't misplaced a decimal point somewhere? ½ ohm per connection sounds high...by that measure, simply splicing an in-line fuse into the power lead of an audio amplifier (for example) would result in a 2 ohm increase which would, if the amplifier draws 4 amps, result in an 8V drop...

[Donald]

I have a new set of guages (real oil presure and a voltmeter) that I have not gotten around to installing. Looks like I will have to get around to installing them and wiring the voltmeter directly to the battery.


Yes, well not an idea as such but an explanation of what's happening and why everything's still OK.

At Special Electronics Training class in the Atlanta General Motors Training Center back in '92 I learned that every electrical connection, whether soldered or not, has about a half an ohm resistance that will result in a voltage "drop" across the connection. The amount of the drop depends on how much resistance is in the total circuit. For instance if the total circuit resistance is 1 ohm, 1/2 for the soldered connection and 1/2 for the rest, for a 12 volts supplied, a 6 volt "drop" would result, just from the connection.

I suspect the voltmeter is one of those low resistance meters (they're cheaper) and is getting the reading from the fuse block, 2 or 3 connections down wind from the battery positive terminal. If you want a truer reading, rewire the guage by taking the positive wire directly to the battery positive terminal, and even then you could expect a couple of tenths "drop" from what a shop type voltmeter would show.

[GordonB]

Donald, DOHC 4.6, and others
My 2003 MM factory voltmeter behaves like i would expect, but the 2004 voltmeter seems to be reading low or showing excessing drop with accessories (towards 12). Had dealer check out charging system before I picked it up 2 weeks ago and they said everything is fine.
Question, has anyone put a trickle charger on their NEW MM just to be sure the battery is fully charged?

GordonB

[David Morton]

David, are you sure you haven't misplaced a decimal point somewhere? ½ ohm per connection sounds high...by that measure, simply splicing an in-line fuse into the power lead of an audio amplifier (for example) would result in a 2 ohm increase which would, if the amplifier draws 4 amps, result in an 8V drop...A two ohm increase in the supplied voltage to an amplifier would only result in a 8 volt drop if the internal resistance of the amp itself is only 1 ohm. Get yourself a Fluke(R) high impedance multimeter (prices start at around $100) and see for yourself. It's not VD=ohm x amp. The VD is a % that equals the % of the ohms encountered at every step along the circuit. For instance if you had a circuit with twenty four connections only, there would be a total of twelve ohms resistance and you could measure, with ground attached to 12v battery negative, a 1/2 volt drop after each succesive splice with the positive probe, until you got to the battery negative terminal where you read zero volts.

Since I'm here I've got another neat thing you might enjoy doing with your new Fluke(R) meter. Put a wet paper towel on a piece of steel and put a penny on top of it. Now with your meter set to read voltage you will find a 1 to 1-1/2 volt reading between the penny and the steel! A good reason to solder terminals on sensor wires like O2 and MAF sensors that send small voltages to the computer. Nothing like a little corrosion between the copper wire and the steel terminal to gather the moisture necessary to create a little extra voltage and confuse the PCM with bad data. Solder today is for keeping out moisture, period. There is no structural or electrical value in solder on electical circuitry today.

Maybe if we meet someday I can tell you about my old friend that worked for NASA back in the '60s on rockets and the things they could do with a half wrap strand of wire and a one milligram dot of solder.

[TripleTransAm]

Both of you are correct, but one small detail is different in each example:

RF is right in that placing a 2 ohm resistance in series with the power feed of the amplifier would see an 8 volt drop across that resistance if the amp continued to draw 4 amps of current even with that extra resistance in line with the power feed. What the total voltage drop would be across the resistance and the amplifier would be equal to whatever voltage the power supply itself was feeding (assuming a constant voltage power supply and not a constant current unit, of course).

Example:
12 volts DC feeding a theoretical audio power amplifier... In RF's example, the amplifier might have been drawing 4 amps of current during some point of its operation, and we take for granted that we have 12 volts at the actual feed point of the amplifier: we can see that the amplifier appears to have an internal resistance of 3 ohms. (all theoretical, because internal resistance can change based on operating characteristics such as current, heat, etc. but we'll keep it simple).

Using that 1/2 ohm-per-solder resistance, one would think an inline fuse (with 1 solder point per end) might place a 1 ohm resistance in line. With the same 12 volts feeding the amplifier, you get a total resistance seen by the voltage source of 4 ohms. The resulting current is now 3 amps instead of the previous 4 amps being drawn. The voltage drop across the resistance of the inline fuse would then be 3 volts, though... as RF implied, pretty damned big. The amplifier itself would only see 9 volts at its terminals. All because we put a fuse in line, which should have been transparent to the voltage source and current flow!!

Here's the shocker (no pun intended): with current flowing through a resistance (and the ensuing voltage drop), there is power dissipated across that element. Power is current x voltage, so across this fuse and its solder joints, we'd see 3 x 3 = 9 watts!!

Consider the sheer number of soldered connections inside a typical power amplifier, and it would appear that 99% (guesstimate) of the power dissipation would be across solder joints!!!


Doesn't make sense.


I'm not doubting your information, David, but I think there's a few details missing. I suspect this 1/2 ohm resistance across solder joints might be a result of using perhaps an automotive-grade type of solder (ie. perhaps heavy on the lead %-wise) made to handle very high currents. Or perhaps the nature of the terminals being soldered (dissimilar metals, perhaps oxidation on the surface of weathered automotive terminals, corrosion even... who knows) due to their automotive application...

But I can assure you that I can go home right now and solder up a whole string of connections and I will not be able to detect a resistance with my good Fluke... possibly detectable with a Fluke, but probably negligeable compared to the internal resistance of the wires themselves!