DC3

Roger and I have been PMing about the ammeters and he was gracious enough to complete some additional experiments for me. Based on that, I think your statement above is best said as "There SHOULD be no significant current flowing through the ammeter leads and the ammeter".

Roger's original experiment has proven these ammeters can handle much more current than we previously thought so the fact that they fail points to significant current getting to the ammeter. How is that so?

Regarding the additional experiments Roger completed for me:

I asked him to disconnect one of the leads inside the ammeter, reassemble the ammeter and then run his headlight test again. The headlight did not light so there is no alternate path for current to flow through the meter (ie a current divider circuit). This showed that 100% of the current in his original headlight test was flowing through the ammeter movement.

I then asked him to test the ammeter plugs on a couple of wiring harnesses he had out of cars. Those plugs tested as being open so there is no internal path for current to flow across the plug. I was making sure the 16 gauge ammeter circuit didn't normally flow current with the ammeter being shunted across that circuit rather than the main 10 gauge charging wire. This shows that when current does flow in the ammeter circuit, 100% of that current flows across the ammeter movement. Since it supposedly only takes milliamps to deflect the needle to full scale, how is enough current getting to the ammeters to fry them?

Yes, if the smaller wired gauges are not capable of carrying the same current as a 16 gauge wire, then it makes sense for the fusible links to be smaller.

DC

7T1vette

The ammeters get "fried" because people do dumb stuff. Like getting in the engine compartment with power connected and "playing" with the wiring (i.e. fiddling with the wires on the horn relay because of a horn problem). In any event, a short occurs which takes out one side of that main buss line and all the current tries to route itself through the ammeter. Pooff!!

Another scenario is that the main buss line sustains a dead short causing instantaneous current spike in the hundreds of amps (very short duration). Even though the ammeter circuit flows a very small percentage of the main line current, when a dead short occurs, even the ammeter circuit would get 10 amps or more (if the fusible link blows, more than 20).

DC3

I tend to agree. It nearly always comes back to Bubba.

And this quote by Roger may be the most telling. Maybe at some point the meters were redesigned to better handle unexpected current until the fusible links have a chance to do their job.

Noonie pointed out a few posts back that the shunt point connections were different depending on model year. I suppose it could be that the location of the connection points had to be changed to help with calibration after the wire size was upgraded.

DC

7T1vette

Very possible.

noonie

It's rare than the circuit (10ga) fails and if it does then the vast majority of the time the ammeter circuit fails with it. This of course excludes tampering. (remember that in most designs the shunt resister is in series in the circuit (10ga) and shunt leads are connected to the circuit (10ga) in most years without very easy access. In short you would have to cut the 10ga wire outside of the shunt resister area.
In another senario, if the fusible link blows by the starter on the 10 ga circuit, the ammeter cicuit is also broken.
In cases such as where the 10ga wire has been left disconnected on the horn relay and the shunt lead left on, then that would be tampering and really not considered by GM in their system operating design.

As far as the shunt lead wire size and fusible links, it seems that the concern and protection would be potential fire risk in the shunt wiring leads alone, not the gauge itself.
It seems that GM changed the coil wiring inside the ammeters at some point and apparantly the finer wired ones failed more (obviously handle less amperage), but none were a fire hazard, since the ammeter and internal coil is totally encased and as seen by the pics in this thread not a hazard. GM wasn't perfect, but these circuits were in gazillions of cars, pretty hard to second guess them.

Hope that makes sense.

7T1vette

Makes perfect sense to me...

DC3

Does the 10 gauge wire itself comprise the entire shunt resistance or is there another resistor inline somewhere between the two ammeter connection points?

DC

7T1vette

That wire IS the shunt. Every kind of wire has a 'resistance per foot' of running length. Yes, it's very small (fractions of an ohm), but it is very predictible. The resistance (between the lead connection points) and the amount of current running through that wire sets up a voltage drop across the ammeter circuit. That voltage is what the "ammeter" reads and converts to "AMPS" on the dial.

70BBvert

Finally got to take apart the ammeter I have & take some pictures. As far as I know it has never been apart. Coil had a dozen or so wraps of a very fine wire and resistance measured 5.4 ohms.







Jim

...Roger...

OK Jim I think yours gets the prize for being the ugliest.

Another one with only a few windings , can't tell what the gauge is but guessing it might be the thinner wire.

Thanks Jim

rabidpanda

Hey everyone, sorry to dig up an old thread, but seemed better than starting a new one since there's so many ammeter threads out there.

Anyways, I'm rewiring my 68 my own way (instead of buying a harness) and have been trying to figure out how this ammeter works so I can still use it. It has proven difficult with all of the mixed up info out there, but this thread has a lot of good info in it and helped bring it together for me.

I just wanted to clarify the whole ammeter/millivolt meter thing. I'm not sure what people mean by a "true" ammeter, but this is not a voltmeter and the statement below is incorrect.

First a volt meter requires a high resistance. Since these ammeters have been measuring around .3 ohms (very low impedance), they ARE allowing current through them. If you have used a multimeter in the current mode you may be aware that the two test leads are shorted together and protected with a fuse. If you try to measure voltage in the current mode set up, the fuse is destroyed.

As others have indicated, this is a parallel circuit so some amount of current goes through even though it's a small fraction.

Second, based on a short thread I read elsewhere that stated the gauge was a voltmeter, I connected the meter in parallel to a .001 ohm shunt and powered a head light. Since the gauge is about 300 times the resistance of the shunt I used, only 1/300th of the current (a whopping total of .0067 amps) was going through the gauge, in other words, I didn't see it move. If it was a volt meter, it would have measured the .002 volts on the shunt indicating 2 amps or so which should have caused the needle to move a bit.

My last point, based on the pictures, the coil induces magnetic force on whatever attaches to the needle causing the needle to move. Without current flowing through that coil, the magnetic force would not be present and the needle would not move. There is something in the EE world called the right hand rule. If you make a thumbs up with your right hand and align your 4 fingers with the coil so that they are pointing with the direction of current in the coil, your thumb points with the magnetic force which should correspond to needle movement.

Another little tid bit, coils tend to be capable of handling more current than just a straight shot. Probably why Rogers test kept running. Take a 20 gauge wire and connected across 12V battery terminals, it will probably melt, but coil that wire tightly around a screwdriver 50 times and connect the ends to the battery terminals and not only will it not burn up, you will magnetize your screwdriver! I wouldn't recommend doing that with a good battery .

So if you're still reading, has anyone ever measured the resistance of the ammeter power circuit in a good harness, not the gauge part, but the starter to horn relay part? The ratio of the gauge resistance to the harness resistance should equal the ratio of the harness current to the gauge current (yes they should be inverse).

Also, testing is always the best way to learn something, you can calculate and speculate all day long, but it's useless until you actually do it. So thanks to Roger for helping us all out!

Hope this made sense, my technical writing skills have gone down hill since school

Clubby99

WHERE IS ROGER? I miss him.

Peterbuilt

Is not a true ammeter or a true voltmeter it's a galvanometer.
Both wires have 12 volts at all times.
The needle going + or - indicates if electricity is flowing to or from the battery.

That's all I know about it.
I miss ***Roger*** too!

BKbroiler

I remember Roger talking about relocating to Florida, I think. Must have lost his computer in the process.

Rally68

Yeah what the heck? We need guys like him on here...

HOOAH

I agree, me too. I was just thinking about him the other day and how long it's been since he's presented us with one of his little scientific contraptions. I also remember he was selling all of his accumulated parts and was going to move to Florida but haven't heard from him since (the big sale).