Ammeter - Shunt question

[RMBLFSH]

If I wanted to wire my factory 1972 ammeter, directly to my late model alternator (70 amp), bypassing the factory wiring, what size shunt would I need?

Also, if this is REALLY bad idea, please advise.

Thanks.

[70cst]

I would get a shunt that would with stand 70 amps. I could be off base with this... other opinions?

[jabborabbo]

I assume you talking about the fused 16-18 gauge wire that comes out of the firewall connector, and instead of unwrapping the harness near the voltage regulator to connect the wire to the 12 gauge red wire three-way or whatever connector there, you want to connect it to the 12 gauge red wire (that runs from that same three way connector) at the alternator?

[RMBLFSH]

I agree on the 70 amp capacity of the shunt, but I believe the other factor is the milli-volt capacity of the ammeter. Does anyone know this number for a factory 1972 ammeter?

[JimKshortstep4x4]

I don't know the number but the factory ammeter is protected by two 4 amp fuses. It reads the amps by reading the resistance between the two connections, (approximately 3' of #10 wire).

Jim

[RMBLFSH]

Hmmm...build a shunt with 3' of 10 awg wire and a couple of fuses...wonder if it would work...

[john]

Will a factory ammeter handle full system amperage?" I don't know, But there must be a reason they used the low amp shunt.

[Longhorn 70]

There is no way the shunt they have with 4 amp fuses in it carries full system current. Could it be a shunt of a shunt? Could it carry just 1/25 of the current and be part of a 100 amp shunt? I don't have a wiring diagram to look at that includes the shunt circuit.

When a fuse blows, the gauge quits but you don't quit charging. Just pushing this to the top so smarter than I will chime in.

[VetteVet]

Originally Posted by RMBLFSH

If I wanted to wire my factory 1972 ammeter, directly to my late model alternator (70 amp), bypassing the factory wiring, what size shunt would I need?

Also, if this is REALLY bad idea, please advise.

Thanks.

Here is the factory wiring for the ammeter. The shunt and the fuses are marked as well as the wiring connections. It's not a bad idea, in fact the 67 to 72 trucks with gauge dashes are all wired this way.

Originally Posted by 70cst

I would get a shunt that would with stand 70 amps. I could be off base with this... other opinions?

The shunt is the battery volts/amps supply path for the entire truck without the engine running. There should not be more than 30 amps go through it. If you realize that the fusible link from the battery positive terminal, which is connected to the junction on the passenger side fender, is inline with the shunt, and fuses the battery power, key off circuit, you know that it is rated at 30 amps.

Originally Posted by jabborabbo

I assume you talking about the fused 16-18 gauge wire that comes out of the firewall connector, and instead of unwrapping the harness near the voltage regulator to connect the wire to the 12 gauge red wire three-way or whatever connector there, you want to connect it to the 12 gauge red wire (that runs from that same three way connector) at the alternator?

I don't know where he wants to connect the two ammeter wires but they have to be connected at the ends of the shunt in order to read the voltage differential between the alternator and the battery. IOW charge or discharge.

Originally Posted by RMBLFSH

Hmmm...build a shunt with 3' of 10 awg wire and a couple of fuses...wonder if it would work...

Sure if you wire it correctly. The factory used 12 AWG with a set length of wire and calibrated the resistance of the shunt wire to the ammeter. The difference between 12 AWG and 10 AWG or even 8 AWG for the higher output alternators, probably isn't going to affect the ammeter much, after all it only measures charge or discharge, so the accuracy isn't much of a factor.

Originally Posted by john

Will a factory ammeter handle full system amperage?" I don't know, But there must be a reason they used the low amp shunt.

No it isn't an internal shunt amp meter like they used on the 62 and older models where all the amperage went through the meter. It is wired in parallel with the battery/alternator output wiring and only reads a fraction of the voltage between the alternator and battery. That's why it only needs four amp fuses. Without the fuses, one of the two outputs would run directly through the ammeter if the other one became open and the meter would be damaged.
The shunt isn't low amp as described above.

Here's a picture of the three way connection in the factory wiring in real time.

This would be the best place to wire the ammeter feed wire because one of the wires is connected to the external regulator which is the red sensing wire for the alternator to help with remote voltage sensing.

[Andy4639]

I have to ask why?
Why do U want to run it straight to the alternator.


I also would suggesting put a volt-meter gauge in the truck.

[RMBLFSH]

I appreciate all the good information.

Shunts can be purchased from an electronics store. Question is: how many millivolts is the factory ammeter rated at?

If anyone has this piece of info, it would be greatly appreciated.

The reason for wiring up my own is that the PO swapped in a GM crate motor and the factory wiring is trashed.
I'm just trying to get all the gauges working.

[Andy4639]

Originally Posted by RMBLFSH

I appreciate all the good information.

Shunts can be purchased from an electronics store. Question is: how many millivolts is the factory ammeter rated at?

If anyone has this piece of info, it would be greatly appreciated.

The reason for wiring up my own is that the PO swapped in a GM crate motor and the factory wiring is trashed.
I'm just trying to get all the gauges working.

OK that makes good since. I was wondering why U would want to redo everything.
I would still install a volt reading gauge.

[VetteVet]

Originally Posted by RMBLFSH

I appreciate all the good information.

Shunts can be purchased from an electronics store. Question is: how many millivolts is the factory ammeter rated at?

If anyone has this piece of info, it would be greatly appreciated.

The reason for wiring up my own is that the PO swapped in a GM crate motor and the factory wiring is trashed.
I'm just trying to get all the gauges working.

The ammeter is rated at 1.5 volts but why does that matter? You won't be sending much amperage through it if you wire it correctly. The shunt you need is just a length of wire from the battery to the alternator connections as I stated above.

I have covered this subject so many times I know it by heart. If you check the electrical section you can find all the info you need. Most of the threads on dash light to gauge conversions will cover it.

Here's one of the threads......http://67-72chevytrucks.com/vboard/s...d.php?t=581719

Here's a picture showing the fusible link and the shunt with the ammeter wires and fuses. The ammeter wires go through the firewall block to the dash cluster plug on terminals 1 and 12 and then to the ammeter via the copper strips.




Here is the dash cluster diagram. Note terminals 1 and 12.








Here's one with the internally regulated alternator conversion with the ammeter wires.

[RMBLFSH]

VetteVet, I appreciate your vast knowledge, and especially that you are willing to share it. I am sure that you have forgetten more than I will ever know.

It matters because I think it may be better to simply wire-in a self contained shunt (if possible), rather then tap into a suspect wiring harness.

By the way, is that 1.5 volts or 1.5 millivolts?

Thank you again for all the info.

[VetteVet]

Originally Posted by RMBLFSH

VetteVet, I appreciate your vast knowledge, and especially that you are willing to share it. I am sure that you have forgetten more than I will ever know.

It matters because I think it may be better to simply wire-in a self contained shunt (if possible), rather then tap into a suspect wiring harness.

By the way, is that 1.5 volts or 1.5 millivolts?

Thank you again for all the info.

The website below may interest you especially the last paragraph.

.........http://www.bracketracer.com/nova/shunt/shunt.htm

[ray_mcavoy]

Originally Posted by RMBLFSH

It matters because I think it may be better to simply wire-in a self contained shunt (if possible), rather then tap into a suspect wiring harness.

I wouldn't really look at it as tapping into a suspect harness. You need to have a wire running from the junction that VetteVet posted a pic of earlier over to the battery whether you use an ammeter or not. If you don't trust the condition of the existing wire, run a new one. Even without the ammeter in place, that wire still carries all of the current to or from the battery (except the starter current of course). When you use that wire as the shunt for the factory ammeter it doesn't really change much ... the vast majority of current still flows through that wire. And only a small fraction goes through the meter itself.

Originally Posted by RMBLFSH

By the way, is that 1.5 volts or 1.5 millivolts?

It's 1.5 Volts.

I just tested a few spare factory ammeters I had laying around to be sure. They varied a little bit but 1.5V gave pretty close to a full scale reading on them.

Like JimKshortstep4x4 said earlier, the factory ammeters are protected by 4 Amp fuses. Out of curiosity, I used my multimeter to measure the current flowing through the meters I was testing. They all had close to 2 Amps going through them at full scale deflection. That's the current that's flowing through the factory ammeter itself ... not the shunt.

[ray_mcavoy]

Originally Posted by ray_mcavoy

It's 1.5 Volts.

I just tested a few spare factory ammeters I had laying around to be sure. They varied a little bit but 1.5V gave pretty close to a full scale reading on them.

I take that back ... what I did yesterday was just a "quick and dirty" test with some 1.5V dry cell batteries. Although they are 1.5V open circuit the actual voltage drops off quite a bit under load. I neglected to measure that ...

So I did a bit more testing with a power supply and a rheostat to vary the current and voltage to the factory ammeter being tested. Full scale was at about 1.8 Amps through the meter. And that resulted in about 180 milliVolts across the ammeter's terminals.

I don't have a micro-ohmmeter to measure it directly, but using Ohm's Law, that works out to about 0.1 Ohms for the factory ammeter's internal coil.

[RMBLFSH]

Ray, thank you for the further investigation. From what you have learned would you agree that a shunt with a rating of 70 amps and 180 millivolts would work in this application?

Also, assuming those exact values are not available, how close does the shunt need to be, would a 100 amp, 200 millivolt shunt work?

[ray_mcavoy]

Originally Posted by RMBLFSH

Ray, thank you for the further investigation.

You're welcome!

From what you have learned would you agree that a shunt with a rating of 70 amps and 180 millivolts would work in this application?

Yes, based on what I have measured, a shunt with a 70A, 180 mV rating would in theory give a full-scale reading with 70A flowing through the circuit.

I say in theory because I believe most commercially available ammeter shunt ratings are calculated for use with a high impedance meter movement. As I posted above, the coil inside the factory ammeter is only about 0.1 Ohms. That will allow a considerably higher amount of current to flow through the meter itself than what you'd get with a high impedance movement.

Also, assuming those exact values are not available, how close does the shunt need to be, would a 100 amp, 200 millivolt shunt work?

As far as how close it needs to be, I don't think it is highly critical. Like VetteVet said earlier, the factory ammeter is more or less just a charge/discharge indicator and the original shunt was just a set length of 12ga wire. If the shunt resistance is too low, you won't get very much needle movement on the ammeter. If the shunt resistance is too high, too much current will flow through the meter itself and blow the inline fuses. Basically, I'd consider anything in between those two extremes to be acceptable.

A 100A, 200mV shunt will have a little lower resistance (as compared to a 70A, 180mV shunt) so I'd expect that to give a slightly lower reading on the ammeter for the same current. But again, those shunt ratings are most likely calculated for use with a high impedance meter, not the 0.1 Ohm factory ammeter so that's going to change the calculations.