Old amp meter (and co.) = useless junk?

Inner Circle said:

I'm reasonably sure it DOES cause a drop

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I doubt that the resistance across that ammeter's terminals has changed since it was new. Ammeters are simple devices - I can't picture what would result in the resistance increasing. If it is a "-60 - 0 - +60" ammeter, the needle wouldn't move very far when installed on an Alfa. So one solution would be to replace it with a "-30 - 0 - +30" gauge, that better displays the level of amperage that you would get in an Alfa.

Also, if your old ammeter had a meaningful voltage drop, that would mean that it had enough resistance so that when current was passing through, it would get HOT. Do you observe or smell smoke/burning when the engine is off but the headlights are on?

Would you just take the amp meter off the wiring and have one more non-working old ornament on the dash, or would you try to make it actually useful?

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The easiest way to disable the ammeter would be to detach the two heavy wires at its back and join them together with a small screw & nut. Of course, insulate that connection very well, since those wires are not fused. You might even do that temporarily to prove to yourself that the voltage drop is the same with and without the ammeter.

Also, there were two red wires running from the battery straight to the fusebox and these had been disconnected so the amp meter would work correctly. If I took the amp meter out of the equation I could reconnect these and hopefully it wouldn't hurt the performance.

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Not knowing exactly what those two red wires are, and where they go, it's hard to say what would happen if you reconnected them. I'm puzzled why there would be two wires between the battery and fusebox. On 105 models, there is typically one wire to the fusebox and a second to the alternator. But I'm no expert on Alfettas; maybe Alfa ran two, redundant wires in parallel.

But sure, if those are the factory wires, and one end is still attached to the fusebox, then taking out the ammeter and re-connecting these wires to the battery "+" post should put the wiring back to its original configuration. Just be sure of where these wires go before re-connecting them - since they are not fused, things will get pretty dramatic if you find that they go to ground.

The oil temp is probably just a bad or wrong sensor. I'd try changing that first. The volt meter could be easily tested by running a test-piece hot wire from the positive terminal of the battery to the hot side of the volt meter - I'd also toss on a test ground wire to a "known good" ground point too. You have a known voltage at the battery that you can compare to the meter's reading. First test with both a temp hot wire and a temp ground. If it's a bad gauge, it'll continue to read wrong and thus you know you need a new one. If works ok that way then see if you can isolate the problem to the hot side or the ground. My bet is a corroded ground. That's always my first bet on an Alfa anyway. In fact on my 105 all the instruments share a common ground.... theoretically a bad common ground might affect ALL the instruments.

In any case I would perform the exact same hot wire/ground wire to the battery test on the ammeter. This was a bad idea - don't do it.

On the red wires: Do you have a volt meter or a least a turn signal bulb that you can use to see if they are hot both ignition off and ignition on?

There's an old saw about never tearing done a fence unless you are sure you know why it was built. Your description is unclear to me, but I wouldn't be inclined to do anything but tape ends and maybe hide the red wires for cosmetic purposes until you've traced them.

Hey guys, I did some work.
The oil temp sensor is built into the oil drain plug. It might well be dead, but I don't feel like messing with it.
The volt meter is practically dead - I took it apart and measured the battery directly, and it does work, but at 13V moves to about 8V. Did the same in the dash. It can't be taken apart and there is no obvious way to adjust it. I could have sworn I had a spare one, but could not find it, so for the time being I set this aside.
Now for the hot topic the amp meter, a 50-0-50 type. Voltage drop is negligable, so it does not do much harm - but it does not do much good either. As it is connected into the cable between the starter and the fusebox, it measures the internal consumption, meaning it moves between -30 or so and 0. No plus values - it would have to be built into the alternator circuit. I found this online:

Original PDF attached. It should be something like this, and I that's what my two wires should do, the ones I told about, disconnected between the battery and the fusebox (call them AB). The alternator is close to the battery, so whoever put the Amp meter in meant the cable between the alternator and the battery should be disconnected from the battery and instead connected to (AB) - both, as they are supposed to carry the current together.
(AB) should run to BOTH one terminal of the Amp meter and the fusebox - here marked as the on-off switch. (Disregard the cable between the on-off switch and the starter, it's the starter relay cable, not the main power.)
I reconnected the wires according to this and voilá, charge show up on the Amp meter!

So this thing works two ways: when power is running from the battery to the electric system, it starts from the (+) of the battery, passes through the Amp meter right to left, exits the left terminal and reaches the electric system from there. The alternator is inactive in this case.
When the alternator is active, power is running from the alternator to the left terminal and onwards to the system. It also enters the Amp meter, goes to the starter motor terminal and onwards to the battery. Internal consumption in this case is not measured. It's below the alternator input and that should be enough. If the alternator wouldn't keep up with consumption, it would not charge the battery, the battery would be forced to supply power to the system and this would show up on the Amp meter (as power would flow from right to left).
Am I correct? Sorry if it's blindingly obvious to everyone, I'm kinda elated for having figured it out. I just hope I figured out OK