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Old 02-13-2018, 05:56 AM   #1
74travelall
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Default Ammeter amp rating?

Sorry if this has been covered before. I searched the forum but was unable to find an answer.

I have a 1974 Travelall 100 with factory AC, electric rear window, tow package, etc. Essentially it has most of the factory electrical items and came from the factory with the 61 amp alternator.

I have just replaced the fuse panel with a new blade panel and cleaned up all connections. I also cleaned up the connections at the firewall and checked the wiring. It charges well and has minimal voltage drop from alternator to battery and testing going into the cabin from the alternator compared to coming out of the interior from the ammeter back to the battery. Essentially it all looks good and is working as expected.

The only "problem" I have is charging at idle when sitting at a long stop light or stuck in traffic not moving. If it is raining and I have lights, wipers, and fan on it is drawing down on the battery and it shows on the factory dash ammeter. If I push the pedal to bump up the idle a little amount the ammeter shows I am charging rather than discharging the battery. The solutions I have thought about are either bumping up the idle speed a little bit to get the alternator moving faster or installing a smaller alternator pulley. I have decided to go the pulley route as the truck runs really well and I don't want to bump up the idle to compensate for low charging at idle. I'm also not worried about spinning the alternator too fast in these low RPM engines. It idles at 700rpm in park or neutral and a little lower when in drive with the auto transmission.

Since I will pull the alternator to switch pulleys, I have decided to just put on a new unit and keep the current as a spare. Since I am replacing it, I am thinking about bumping up the amps a little bit. Not a lot as I don't want to push the factory sized wires and I don't plan on adding a lot of electrical accessories. But it would be nice to have the option in the future. I'll also replace the wires with larger gauge coming from the alternator to the ammeter and from the ammeter to the battery at the same time I switch alternators. Just in case.

The question I have is around the rated capacity of the factory ammeter. I like having the ammeter to watch the charging circuit but not sure of the rating as my understanding is that the alternator output all goes through the ammeter before going to the battery or fuse panel. I don't want to set fire to the truck by sending too much current through the ammeter.

How many amps can I safely put through the factory ammeter in a d-series?
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Old 02-13-2018, 09:18 AM   #2
Scoutboy74
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Default Re: Ammeter amp rating?

The wise-aker in me is tempted to say...none. I don't know the actual rating of the OE ammeter gauges. That specific figure might be difficult to come by. But I can't see it being in excess of 75 amps. I strongly advise against putting any more current through that full load circuit design. It really isn't up to the present task as is with all the accessories you have. IH light line vehicles utilized Chrysler's very antiquated power distribution model and instruments. Ford and GM abandoned that model years earlier because they recognized how problematic it would be with ever increasing electrical demands and higher output alternators to match. I know this may not seem palatable to you, but from a purely practical standpoint, you'd be better off bypassing the ammeter with a direct connection and installing an aftermarket volt meter to monitor charging status. If you just can't bear the thought of life without the ammeter, you do well to bone up on the topic of external ammeter shunts. There's a lot of information online. MAD electrical is a great resource.
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Old 02-13-2018, 09:45 AM   #3
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Default Re: Ammeter amp rating?

From what I've seen 60-65 amps will peg the meter. Scout Boy's suggestion to use an external shunt is probably the way to go. Just a piece of the right kind of wire between the terminals will shunt part of the current flow. I've used Teflon insulated wire for this in the past. Another option would be to run a second #10 wire from the alternator to the amp meter.
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Old 02-13-2018, 03:59 PM   #4
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Default Re: Ammeter amp rating?

The alternator is analogous to the ignition coil...it only puts out enough to get the job done. So a 150 amp alternator by itself does not present any danger as long as the load doesn't exceed the ammeter's capacity. Not saying that's an advisable route to take, but I personally can't stomach altering the stock dash/gauge layout. I have a (advertised) 75 amp unit, and for the record, on a rainy night at a stoplight, I too see the needle dip into the discharge area. And that's running the smallest double row pulley I could find. I am told the modern alternators will do better at low RPMs, but have yet to prove it to myself.
The shunt technique with wire will alter the meter's accuracy, and may possibly heat the shunt wire beyond its capacity. Electrons take the path of least resistance, so if 20 amps can use the small wire rather than the windings in the ammeter, that's what they'll do. A true shunt style ammeter is actually a milivolt meter that approximates the amp load in a safe manner. Sort of like a remote fuel pressure gauge would do.
Unless you're running a winch or some big load, I would go the pulley route and call it good.
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Old 02-13-2018, 04:40 PM   #5
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Default Re: Ammeter amp rating?

True a hi-cap alternator will only charge as much as necessary. If you're running an Optima battery and leave the lights on for a couple of hours, the system will see high charge rates for a while. Been there, done that with the stock 63 amp alternator keeping the amp pegged for over an hour.

I have not dug into the gauge, but I'm sure there is an internal shunt in the gauge. The meter in the gauge could be wired to a 150 amp shunt, mounted in the engine compartment, as the best practice to use an oem gauge with hi-cap alternator.
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Old 02-16-2018, 05:20 AM   #6
74travelall
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Default Re: Ammeter amp rating?

Thanks for the recommendations and thoughts. I want to keep the stock dash and gauges so it looks like I'll run the smaller pulley and keep the amp of the alternator close to factory. My understanding of using a shunt would be that the meter has to be calibrated to the shunt or else it will not read correctly. Does this mean that an external shunt will just make the stock ammeter display less amps in either direction as it only sees the amps that bleed around the shunt?
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Old 02-16-2018, 10:19 AM   #7
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Default Re: Ammeter amp rating?

First off there are no numbers on the amp meter to begin with, so it is just a relative indication of a little or lot of amps going into or out of the battery. Some of the "calibrated" shunts I've used are up to 20% off on their readings. So with all that info what accuracy would you expect out of the system?

If a guy really wanted to expend a bit of time, effort and $ the OEM gauge could be calibrated to show say 150 amps at full swing. Is it worth it?
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Old 02-16-2018, 10:48 AM   #8
74travelall
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Default Re: Ammeter amp rating?

I'm not worried about accuracy. Just the general information regarding charging and battery drain is all I need. Just wanted to confirm that the only impact of adding a shunt on the gauge reading will be a smaller movement in the needle. Is my understanding correct?
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Old 02-20-2018, 09:52 AM   #9
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Default Re: Ammeter amp rating?

Adding a shunt on the gauge will reduce the meter reading. If bolting to the terminals on the gauge be careful that it does not get so hot as to damage the gauge or start a fire.

The best way to do this is to mount a shunt like this https://www.amazon.com/CANADIAN-SHUN...+shunt+150+amp under the hood and wire it into the OEM gauge. This assumes that the OEM gauge is a 50ma meter. If so then the gauge would read 150amps at full scale.
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