The Stovebolt Site Forums The Shop Area The Electrical Bay OTC "Shunt" for Amp gauge? (57 3600 w alternator)

Anyone have an over the counter source for a 12V "shunt" to prevent pinning/damaging my Amp gauge?

Followed a few threads, did some web searches, saw mention of a factory Chevy "shunt" but no part number or description that local parts stores could work with.

Tried asking at the parts counters, saw the perfect illustration of the term "Blank Stare"!

Google just comes up with parts for solar panels.

Found a link to instruction for building a shunt, http://millpower-race-engineering.piczo.com/?g=43218292&cr=5 , but Radio Shack cannot supply the necessary resistors.

My Amp gauge works, but I prefer not to overload it via the alternator and heavy loads.

the HD alternator on my 58 has been working the ammeter fine for many years, not sure why you think it would be "overloaded" - the gauge simply registers the output of the alt to the battery, if you're sending so much current to the battery [or drawing from], to peg that ammeter I'd be more concerned about a battery explosion

Bill

Alternator, halogen driving lamps, electric winch.

After using the winch the charge rate can get pretty high, driving lamps also draw quite a bit.

According to other threads, the amp gauge is really only made to handle 40 amps.

I do not want to have to replace it due to preventable damage.

Richard/SIA,

I have seen the shunt circuits talked about but have never played with a automotive one. From my limited experience with industrial types I would think you would need a new ammeter calibrated to the values of the shunt. I don't think you can just stick something in the charging circuit and expect the existing gauge to read correctly.

I know this wasn't the point of your original question, but I'm sure you know a voltmeter could tag into the system without having to pass all that current and with a little understanding, you can read it just as well as a ammeter.

This would of course mean you would have to disable your existing ammeter by kicking it out of the circuit.

Keep us posted if you find out the secret of adapting a shunt to the ammeter you already have in your rig.

Stuart

If you go to the bottom of the linked page from my original post, it gives directions to build the necessary shunt.
It appears to be a proven design.

I guess that since no one appears to know the part number of the unit Chevy used, I will have to order the components on line and build a unit for myself.

I will post the result.

The first thing to know is whether or not you ammeter reads current directly or does it read mV and thus need a shunt.

Shunts normally put out only 50 mV at full current and are rated in terms of output (50mV or 100 usually 50) AND for current in circuit 5 50 500 etc etc.

You can get good instrument rated designed shunts from any good industrial instrumentation outfit (in Seattle Branom inst). Yes you will pay around 25 but it will almost never burn out.

Dan Bentler

What leitmotif is getting at, is that some ammeters have an internal shunt and some use an external shunt. If you find a shunt to buy, it will be for an ammeter with no internal shunt.
I do not have a wiring diagram for a '57, but I suspect it has an internal shunt.
External shunts have much lower resistance than the .3 Ohm that guy used. He is actually using a second shunt, in parallel with the meter shunt.
Remove all wire from your ammeter and measure its resistance. If it does not have a shunt, the ohm meter voltage will peg the ammeter. Oops.!!! If it has a shunt, it will probably read about .2-.4 ohms and no meter movement. Short the ohm meter leads together and subtract that static reading to get the correct value.

If you think you are going to increase the current through the meter by that much, then you need to be more concerned about the size wire and the quality of the connections under the dash.
They stopped using ammeters on cars as more and more accessories were added and current requirements increased. Chrysler stuck with full current ammeters up into the '70s and started having problems with overheating the connections in the firewall connectors.

One way to decrease current through the ammeter is to use an external shunt type ammeter as GM did on my '67
http://www.geocities.com/chevroletc1070/v8engcomp.html
The 12R wire that goes from the junction block at the battery, over to the wire splice (junction) below the alternator and the regulator is the shunt. The 12R wire is the shunt. There are 1 Amp inline fuses connected at both ends of the 12R wire. Look for two small grey, inline rectangular boxes in the diagram. These fuses connect to the ammeter. The 12R wire carries almost all of the current and the meter only milliamps. If the 12R wire were to be damaged, one of the fuses must pop or the meter is toast.

I'm with squeeze on this, you don't really have anything to worry about. The ammeter on the Task Force trucks only reads between the battery and the alternator, namely, the charge rate to the battery. If you're concerned that the stock gauge can't handle the load, you may want to consider running through an aftermarket gauge designed to handle it, rather than adapting the stock one. Check with Autometer or Stewart-Warner.

Hi Richard/SIA,
If the amp meter in your TF truck is the same as the one in my '50 AD, it maxes out at 40A. I found that I needed the shunt with a 63A alternator. On cold startup the initial charge way exceeds 40A.

The .33 ohm 300W resistor array pushes the full scale deflection of the amp meter to around 70A -the reason this isn't a precise figure is that the Chevy amp meter internal resistance and therefore its current reading varies with temperature, the shunt resistors have a relatively flat temperature coefficient.

There isn't any kind of coil internal to my amp meter, it's the so called "Moving Iron" type that relies on the current flowing through the amp meter's metal frame which creates the magnetic field to move the needle by attraction or repulsion.

Here's a US supplier of the 100W 1 ohm resistors you need.
http://www.caddock.com/Online_catalog/power/Power.html
Hopefully they'll be able to sell you retail size quantities.

Stuart.

must not be the same as AD's, a cold start [winter cold] doesn't move my 58's gauge much with a 60amp alternator, although the engine always starts pretty easy .... did a check, engine off, key on, high beams on [4 bulbs], wipers on high [raining today], heater on full, ammeter fell nearly to full D - sit for a minute then start it - immediately back to center, turned everything off [still running] it showed a slight charge ..... isn't that what the regulator is all about? I can't imagine a battery lasting long being shot with 60 amps every time you start the vehicle and I can't imagine using a winch without the engine running, or halogen lights for that matter

the ammeter shows the current being sent to, or removed from, the battery, not the current everything electrical is using

Bill

I have the shunt components on order.
I should be able to build it this week end.