Datsun Roadster Forum-311s.org

I removed the dash, and from the dash all the bits. As I was pulling the wires on the instruments I noticed something quite strange (to me, anyway).

The Black and the White wire going to the AMP gauge were cut away from the guage and then joined together.
Any ideas as to why anyone would do that?

Maybe bypassed it when they did an alternator swap?

The gauge may have acted as a very expensive fuse and has been burnt out.
Bridging the wires like that is valid to keep the car operating.

Thank so much for the reply!
That makes a whole bunch of sense.

Nice pics of the consequences. Must have been an unintended melt event. You can add an amp back in if you like . Not sure if the amp itself is fixable, anyone try?
Linda

A amp gauge is hooked up (spliced in)on the positive feed to measure the current flowing.
they use black and white to make sure the wires are not reversed causing the gauge to read discharge
if the gauge doesnt connect to pos. You need to connect the wires to charge

The up side of bypassing the amp meter, your car has power. The bypass as indicted probably means the gauge is fried. The down side, your now very expensive fusible link is dead and there is nothing to stop the wiring from completely frying in the event of a short.

I would advise you find a working OEM amp meter, aftermarket meter or get a fuse in there. Even when you replace the meter an inline fuse is strongly recommended.

If you need an ammeter i have good used or NOS, just pm, price depends on condition jt

Linda asked if anyone has rebuilt an amp gauge. I rebuilt mine earlier this year. Below is what I did. It may or may not work for you. I took pictures as I was planning to do a write up, but never did, so this is what I've got. This is from a 67.5 R16 if that matters.
Rustbucket


Remove gauge with the three screws on the flanges. Kind of tight in there for fingers and screw drivers.
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Take out the three screws that hold the gauge cover plate on.
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Take out the two screws that hold the gauge face on
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It should now look like this without the gauge face
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Remove the amp meter electronics from the gauge casing by removing these three screws from the back
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Once out I noticed right away the missing inductance coil. It had broken and melted the solder
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This is the coil placed back together to illustrative purposes. It should be in the gap in the prior picture.
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Remove the two screws that hold the coil cover on
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You now have easy access to remove any remaining coil pieces and to solder a new one in.
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This picture is what the coil should look like if it were still intact and in place (minus the melted break of course).
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The coil wire measured 0.063" or 1.61mm so I chose Remington 14AGW enamel coated magnet wire.
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I made a coil winding template from a wood wedge so the coil was the same shape. This is important or the coil will not operate the same, meaning the actual amp measurements will be off one way or the other.
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Scrape the enamel off the sections of wire where it will be soldered. Solder the new coil in. Make sure it is well soldered. Your whole electircal system is flowing through it.
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Reverse directions to reinstall.
Good luck.

Rustbucket, that is impressive!

Rustbucket,
Pretty inventive. And a nice write up too. Seems from your write up that a auto electric shop could rebuild the unit too. Good to know!
Linda

Too bad the early gauge doesn't come apart as easy as the later gauge.

Just a minor point, the coil is called a SHUNT.
Its job is to pass all the current that the meter coil doesn't require.
Typically the meter coil might only need a milli-amp or so to give FSD [Full Scale Deflection]

I'm not an electrical engineer so I stand to be corrected, but...
The ammeter in the gauge I took apart does not contain a resistor (shunt). The resistor in the circuit is used to carry the load and therefore have minimal load that goes through the actual ammeter. In the picture below, note the Shunt (resistor) is not within the actual ammeter, it is in parallel. This is built into most modern gauges.

The ammeter that I took apart would be considered a moving magnet ammeter. It carries the full load and doesn't have a shunt. It's kind of the opposite of the picture below.

...and our roadsters ammeter design without the resistor (shunt) is the inherent flaw and the cause of the melting.
Rustbucket

Here are pictures of the inside of the 66-67 amp gauge. You can see the shunt coil.