Need to know the resistance in the c2

[magicv8]

Need to know the resistance in the c2 ammeter coil

I dug out my original ammeter (that died before I got the car). I soldered the opening in the winding to complete the circuit. One of my digital meters says the winding has a 0.2ohm resistance, and the other says zero.

If someone has the cluster out of their car, could you put a multimeter across the ammeter terminals and tell me how many ohms it reads? I am not sure that the opening in the winding was the only problem. It is possible that the harness fire that caused the open in my old ammeter, also caused a short blocking most of the winding.

Failure mode for the wiring harness at the starter connections can send the current that should be carried by a fat 10ga wire through the ammeter.
If my old ammeter is OK, I will use it to create a bridge circuit to convert it to a voltmeter. It will then no longer be at risk.

 

[vigman]

On an AMP meter

The shorting link needs to be "0" ohms.

You pass a GAGGLE of current to run the interior instruments/lighting/engine power ( except starter) through that link.

The coil around the shorting link ( bar) is to pick up the MICRO AMPS of stray current flowing through that bar.

The polarity of the field in that bar will move the amp meter needle either + or -.

Vig~

Ohmmmmmm....Ohmmmmmmmm..Ohmmmmm

 

[JohnZ]

As far as I know, the C2 ammeter is not in series with any load (as opposed to C1's , which were, and were dangerous as a result); it's essentially a voltmeter in a parallel circuit, reading the difference in voltage between the starter stud and the bus plate on the horn relay (which is the alternator connection), and converting that differential into an "amps" reading.

The wire that goes directly from the battery (from the starter stud) to the alternator and appears to short out the ammeter is a meter shunt. What the ammeter actually reads is the voltage drop across this wire; the resistance of this wire is about 0.1 ohm - there is no physical resistor in the circuit, the resistance is due to the length of the wire required to get from the starter to the horn relay. The horn relay isn't in the ammeter circuit - it's just a convenient connection point for the alternator output and the voltage regulator.

When the output voltage of the alternator is greater than battery voltage, the ammeter shows a charge proportional to the difference in voltage between the two connections; when the battery comes up to full charge, the ammeter drops to near zero. If the output voltage of the alternator is less than the battery voltage, as under a heavy load, the ammeter will show a discharge proportional to that difference in voltage at the connection points.

 

[vigman]

John.. I 've been digging in the books

Every reference I can find states it's an AMP meter.

But I agree ( now ) no DIRECT current path exists ( in circut )

And it does measure between the Horn relay & the battery..

And it WOULD measure the current flow ( difference ) between those 2 points.

But I have to dig more.. but it IS an amp meter.

So it's another ELECTRON MYSTERY!


Vig~

 

[JohnZ]

Vig, on C1's it was an ammeter, and was directly in series with every load in the car, and measured actual current flow in both directions. On C2's, it was really a voltmeter, in an isolated parallel circuit, reading a difference in voltage (not current flow) between two different points, and converting that voltage differential and showing it on a gauge face that reads it out as "amps", in spite of the fact that current flow isn't being measured at all. If alternator voltage is higher than battery voltage (the two points being measured), the meter shows a charge; if the opposite is true, it shows a discharge. Just as accurate as a true series-circuit ammeter, but a LOT safer, as it isn't in the current flow path.

 

[magicv8]

Johnz - I will convert the C2 galvanometer using a shunt harness line into a real voltmeter. What I need to know is the resistance in the galvanometer coil (in ohms). It wont be much 0.2 to 0.3 ohms, but I would like to know just the same. :Roll

 

[vigman]

So then John

Explain the SHUNT on the C2&3...


Vig~

 

[magicv8]

The shunt is a known resistance that is connected across the terminals of a galvanometer to make it into an ammeter. It will be chosen to be low enough resistance value so that most of the current flows through it - rather than the winding of the galvanometer gage. It's value should be set so that the readings on the face of the gage will be accurate. In the c1 a dedicated resistance did that. The c2 galvanometer straddles an engine harness wire between the starter and the horn relay that becomes the shunt.

The c2 design is a poor one, since the meter cannot be calibrated by the supplier, altering the engine harness circuit changes the shunt value, resistance of harness wires vary with age and by manufacturer, and most of all - because the starter wiring harness is notorious for melt down due to heat ad from the nearby exhaust system.

So if you want to know if/where the current is flowing in the engine harness wire between the starter and the horn relay, look at the gage in the instrument cluster labeled ammeter. I would rather know the voltage level of the battery/ignition/accessory lines.