I've been adding fuel injection to my Dart, including two fuel pumps. So I thought I'd also upgrade the alternator. But I want to add all that and keep the original ammeter working correctly if at all possible. Is there a good way to modify the ammeter to take more current, with a shunt across it, perhaps? If so, what kind of wire do I use for the shunt?

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

You're thinking along the right track with the shunt idea. The ammeter already has a shunt built into the gauge. You're going to need to disassemble the ammeter, isolate the shunt and measure its capabilities, determine the nature of the actual gauge movement (which, like all "ammeter" movements, is actually a milliammeter), and do some electric calculations which I can't help you with, but 440_Magnum on here can. Then make your shunt (which must be of precise current capacity/resistance) out of copper or brass bar or suchlike, insulate it, and install it across the two legs of A-1/A-1A (main circuit) where they enter/exit the firewall...on the engine side.

FWIW, starting in '71 (except A-bodies) '76 (A-bodies), Mopars went to external-shunt ammeters. So, perhaps an alternate method would be to find a cop car made after '71, grab its ammeter and shunt (no idea where they hid the shunt) and swap the movement into your car's gauge housing, if possible. Needle compatibility is probably going to trip you up on that venture.

There are gauge-rebuilding services advertising in Hemmings and elsewhere; perhaps they could offer some advice.

There is also that yutz going by "MAD Electrical", a Chevy-head who babbles ignorantly about how unreliable Mopar gauges are and says the ammeter must be replaced by a hacked-in voltmeter. Guess you can tell how highly I regard his cute little opinion.

I'll see if I can get Steve (440_Magnum) to duck in on this thread and contribute something more specific than my theoretical jabber here.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

There are a couple of options...

One that has worked OK for me on my '66 Polara and '73 Satellite is to not do anything to the ammeter, but beef up the wires and connections through the firewall. Both of those cars now have Mopar 100A copcar alternators (which Wes Grueninger measured at well over 120A actual output).

But I really don't recommend this- there's a weakness. Normally the battery inteself protects the ammeter. A healthy battery WILL NOT absorb a big charging current for any length of time. The first few seconds after a start-up, it may take in 50 amps, but that drops down to 10-15 amps very quickly. Even a fairly discharged battery will not draw more than 20-30 amps for more than a few minutes. T Since the ONLY current that flows thru the ammeter is either battery charging current or battery discharging current, the battery keeps the ammeter protected, and that's true whether you have a stock 40A alternator or a 200A alternator, because the battery generates enough back-voltage to force the regulator to limit the alternator's output The WEAKNESS comes when you offer someone a jumpstart, or if you have a sick (stone-dead or worse yet shorted) battery. In that case, the ammeter could see a big chunk of the 150+ amp starter current (jumpstart) or the current going into the sick battery. An ammeter bypass switch is a good way to go if you know you might be offering a jumpstart, but it won't automatically protect you against a sick battery- you have to notice excessive indicated charge and throw the switch manually.

Another good option and one that I've been intending to test is to put a big power diode inline with a wire that bypasses from the alternator directly to the battery. Put it in so that it is *FORWARD* biased when current flows from the alternator to the battery. Since diodes have a threshold turn-on voltage of about 0.7 volts, that means that no current will flow in the bypass leg at all until there's more than a 0.7 volt drop across your existing ammeter wiring, which should be plenty enough to make it indicate a charge or discharge. Beyond that point, ALL added current flows through the diode and bypass leg. If you need more ammeter indication, just use two diodes in series. Use BIG power-rated diodes (at least equal to your alternator output rating) and mount them to an appropriately insulated heat-sink- that's the biggest part of the job.

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I used the diode idea to protect the stock ammeter - I put 2 100A diodes in series (to get ~1V drop) under the hood w/ a heavy bypass. The article describing it was in last month's Octane magazine (www.octaneonline.com). It seemed to work fine, but I didn't test it extensively.

The factory ammeter is not a conventional millammeter; it uses a small magnet attached to the needle and another beneath the bus carrying the current; the sum of the fields determines the needle's orientation. If you increase the strength of the fixed magnet, you decrease the deflection for a given current. A small magnetic shunt can be used to reduce the field from the fixed magnet; I've done that to increase the deflection for a given current.

Interesting! I don't see a place on http://www.octaneonline.com to download last month's issue. Any chance you can scan in or post electronically? I'd like to read the detailed info.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks, that's some very helpful info. The power diode solution seems very promising.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

Thanks for the interest Dan! Apparently Octane doesn't keep the old issues online.

I've put my draft of the article (it was edited slightly by Octane) online at:

http://www.tidewatermoparclub.com/TECH/

and it is file "active_shunt5.pdf".
Enjoy, K

Thanks!

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

The pdf below has information about mounting this diode.


Part: 100BGQ030
Description: 30V 100A Schottky Discrete Diode in a PowIRtab package


http://www.irf.com/technical-info/appnotes/an-1010.pdf

Again I visit this forum while sipping my morning coffee and again I find something very useful to improve the function & reliability of my old Mopar.

Thanks for everything you folks contribute.

-Mac

Thanks for the tip! I have one questin tho... in the first article (active shunt) there's nothing about how to connect the shunt itself... and maybe I'm lacking some imagination here but I can completely figure out how to wire it. Yeah, I'm probably entitled to ass hole of the mont with this question, still interested in learning how to wire it since I'm using a 80 amp alternator.

The only difference I've seen so far is that sometimes the ammeter would kinda "peg" when charging.... but it goes away quickly after less tan one minuute or so.

No melting wires whatsoever.

_________________
Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

It looks like you run an additional 8 ga wire from the alternator to the active shunt.
Then a 6 ga wire + fusible link from the shunt to the starter relay.

So your bypassing the ammeter and the bulkhead connector.

Is that right?




BTW, I'm running an 80 amp alternator with beefed up wiring in my 63.
The ammeter almost pegs for a few seconds in the morning if I left my lights on.
But try this: Hook up jumper cables to a car with a low battery.
Tell the guy not to try to start, but just let the battery charge for 5-10 minutes.
Explain that it will damage/destroy your ammeter if he tries to start.

Then sit in the driver's seat and watch as he tries to start it anyway.

That's what happened to me last sunday.

The active shunt was put near the location of the destroyed alt-lead-to-harness connector was near the rear of the engine. It was just a convenient place.

The orig. path leads went into two ends of one aluminum block; the heavy #6 run to the starting relay (via a fusable link) into the other. Steel set screws held the cables in place (see Fig.2).

The white cover just protected the assembly from shorts and was held in place with a nylon screw.

The active shunt doesn't change the ammeters calibration until it turns on. To reduce the calibration, you need to provide another current path. Dealing with ratios of small resistances reliably is tricky, which is why many Mopars from '75 on which had that trick never had their ammeters very well work at all.

The factory ammeters I've disassembled are not a milliammeter with a shunt; it uses 2 permanent magnets; one on the needle, one behind the shunt, and the combination of the field from the shunt current and the fixed magnet orients the needle.

The only way to easily make them more sensitive it to reduce the fixed magnet's strength using a magnetic shunt; to make them less sensitive, use a electric shunt (preferably not to close to the internal shunt).

I am leaning towards this single diode shunt for a 60 amp alternator in a 63 Valiant.

Here is the plan:
Mount an NTE6050 to a heat sink.
Mount the heat sink-diode and a terminal strip in plastic project box.
Short wires to diode cathode terminal and heat sink (anode) suspend the
heat sink away from a 2-terminal terminal block (terminal A and B)

An 8 ga wire from the alternator connects to terminal [A]
A 18 ga fusible link runs from terminal [A] to the ALT bulkhead connector
An 8 ga wire from terminal (anode side of diode) to battery positive terminal, with a 12 ga fusible link inline

The NTE 6050 silicon diode has a Vf of 0.95 volts @70A
(but I don't know at what forward voltage it begins to conduct - more than 0.7v?)
Maximum current is 70 amps

A 30mm fan will fit in the side of the box, but will 3.7 cfm cool the heat sink noticeably?

The plastic box will be mounted on inner fender in corner near voltage regulator.
The box will be open at each end to protect the hot connections but allow air flow.

NTE6050: http://www.nteinc.com/specs/6000to6099/ ... 048_73.pdf
HEAT SINK: 2 inch piece of 1 x 1 x 0.125 aluminum angle
BOX: http://www.hammondmfg.com/dwg2XX.htm (flame-retardant ABS)

Do I have the diode forward-biased in the above circuit if the anode connects to the battery?

Answer: No
To forward bias a diode, the anode must be more positive than the cathode.