Electrical inconsistencies

[millarg]

Afternoon All,

I have used a cheap (ebay) cigarette type voltage meter in my car for some time (left permanently plugged in - I'm a non smoker).

Bought another cheap (ebay) permanent voltage/ammeter and have installed it.

With both fitted, wired into different power sources, they generally give close readings, but occasionally the new Volt/ammeter will give wildly fluctuating readings, voltage dropping to 3 or 4 volts, then back to 13.

I've checked that the connections to this meter are OK & realise that these are both cheap and of different manufacture,

BUT.... is it possible to see different voltages on different parts of the same system?

Could this be telling me something? and if so what?

Finally, I can't read Mandarin, how should I be hooking up the ammeter - directly to the battery? (I believe that it is internally shunted - but wouldn't bet money on it).

Please use short words when responding, I'm an electrical novice

[Porboynz]

Well you answered your own question, both are cheap. However they are probably useful voltage indicators, better than nothing right? The cigarette lighter voltmeter is going to need to make a good snug contact in the socket to accurately and reliably read the voltage. The dedicated voltmeter needs to be wired in, near the ignition switch is good but the voltage should be the same anywhere you tap in under the dash, within 0.5V or so anyway depending on what's loading that wire. (Headlights might cause some voltage drop on a feed wire) If you are getting wild fluctuations then its tapped across the wrong 12V source or the gauge is a dud or there is a loose connection. A cheap digital volt meter can be used as a reference. The earth connection is good and solid right? Nice solid chassis connection somewhere?

Connecting in an ammeter is a bit more tricky. Its really just a voltmeter that sits across a very low resistance shunt within the gauge. It measures the small voltage drop across the low resistance shunt. It needs to be in the main battery feed to the ignition switch, in series with the wire. Even there it will not pick up all the load. (clock and interior lights, park lights etc all avoid the ignition switch.) Some folk install it in the main current wire between the alternator and the battery so they can see charge or discharge. That's a real issue on a Porsche 911, its a long way from the dash to the alternator and you need heavy guage wire. It cannot go in series with the battery lead as the starter motor needs hundreds of amps and I'll bet the gauge is only good for 60 to 100Amps? Might be a bit too much trouble getting the ammeter connected anywhere except in series with the ignition switch feed wire, someone else might recommend an easy cut in point. (You would need to cut into the feed wire, probably extend it using suitable wire to the gauge then run the same grade return wire back to the cut and rejoin. A nice tightly twisted and soldered joint at a minimum, many have contrary views on how to join wires. I am happy with a strong mechanical join (twisted tightly) then sweat solder into the join and heat shrink to finish. Others prefer crimps or connectors, either way nothing is going to be as reliable as a length of original wire left intact. Not to mention the quality control of the Chinese made gauge shunt and connections. No, on second thought, don't do it.

[fanaudical]

I wouldn't put a cheap ammeter in a 911; the charging system is finicky as it is.

It's entirely possible that two voltmeters in different parts of the system read different voltages.

[millarg]

Thanks guys, I am getting wild fluctuations, confident of a solid ground, but will move to an ignition powered source and see what happens.

Thanks again