The Amp gauges that were common in many old cars are not “preferable” these days, especially when the cars have been upgraded to an alternator. Most alternators put out more amps than the old gauges are meant to see. In my old cars, I don’t really care about seeing the voltage but I would like to know if it quits charging for some reason. I got to wondering if an old amp gauge (-30 to +30) could be wired in such a way as to monitor just a portion of the amperage produced by the alternator, just enough to let me know it’s working. I know very little about electronics, is there a way to “tap” a charging circuit and just see if it’s got current flowing? Mini inductive sensors possibly? I ask because I have a few cool old (-30 to +30) amp gauges that I would like to use. Any ideas?
This might not be for everybody, but I used an original face on a later SW volt gauge. The volt gauge normally goes to the far left with the key off, so I drilled a hole in the face and glued in a piece of tig rod painted black to hold the pointer from going all the way over. It holds it at about the 12v mark, so when the car is running it reads about straight up, when it's going down the road and charging it reads higher. I had to drill a couple extra holes in the face to screw it to the later movement, no big deal with a pin drill by hand. I did the same thing with the fuel gauge so I could use a modern matched SW gauge and sending unit.
Simple answer is yes. You need a shunt. If you put a very small high current resistance that is the same value as the ammeter accross the terminals it will show half the current on the meter. I have used a piece of resistance wire from an electric fire as a shunt in the past.
You can put a shunt across the gauge ,but it will no longer be accurate. However, it will show if you have current flowing. There is actually a formula to figure out the cross section of the shunt, but I only used it once in a physics class maybe 50 years ago. What you can do is hook the ammeter into a circuit that flows 25 amps or a bit more (blower motor is a good bet). Get an inductive ammeter. A clamp type DVOM (digital volt ohm meter) is a good investment. Then try different shunt material to get the meter to read half scale or less and verify the current with the DVOM. A shunt circuit is a circuit parallel to the ammeter (across the terminals on the meter). Start with maybe 14 gauge wire and go up or down in gauge to get the desired gauge reading.
Attached photo shows the loop of wire I added as a shunt between the terminals on my 'AVATAR. If you are working on a Model A with almost no space behind the instrument panel you might be able to add this shunt inside the terminal box on the firewall. Ironically the modern generator puts out about twice the amps of a Model B generator and the ammeter now reads about half so everything looks normal.
If the meter can be opened and the internal shunt can be removed/disconnected, you could rely completely on an external shunt of suitable resistance (chosen to give the reading you want at the current there will be), which for example could be a suitable length of the battery cable. That's assuming the meter is built as I assume, with a meter sensing the voltage drop across the built in shunt.
II used a 100 amp alt from Jegs in a stock '33 that I converted to 12V a couple yrs ago.No shunt.It worked just fine.Needle remained steady after full charge.
You make a "Diode Shunt" I did this to my friend's Patina'd 54 Chevy pickup. This had a 350 and 700R4 conversion but he wanted the body /interior to be stock [including all the gauges working as normal] We connected a Diode Shunt from the Alt direct to the battery. This is absolutely needed if using a 65a or larger alternator, the"Diode Shunt" bypasses around the ammeter. [you can keep this in the engine bay ,and not crawl under the dash] The Diode Shunt uses 2 "forward drop diodes" in opposing directions [parallel in this wire] The diodes have a fixed value of voltage drop needed to open them, whereas the ammeter [acting like a resistor] has a progressive/non-linear voltage drop across it. Below 35A there is less voltage drop across the ammeter than required to open the diode, so the ammeter flows current. [path of least resistance] At about 35A there is a point of equilibrium where internal resistance in the ammeter starts exceeding the forward drop value of the diode so the diode will flow the excess current. This ^^^^ usually happens when the ammeter is "maxxed out" on its scale. [eg: after cranking over an engine] Once the battery recovers [enough] and the charge starts dropping off the ammeter starts reading normal again We mounted 2 x Vishay 150a rectifier diodes back to back on a piece of phenolic plastic [using common studs and spacers] Then mounted a wire with eyes each side on the studs. 1 wire goes to the Alt and the other to the Batt. We hid the diode board insde an old "period correct" regulator box. [you should have one left over from the internal regulated Alt Conversion] These diodes have a 1.13v forward drop which is about the same of voltage drop of an ammeter when its pegged out at 35A [either direction] Also if the ammeter totally fails this will not leave you stranded on the roadside because All Voltage will now flow through the diodes. If you make the normal Resistor shunt [or a simple bypass wire] it will throw the ammeter off scale
A diode bypass can make sense in many cases, but rectifier diodes has a voltage drop of about 1V when conducting any significant current, and a 1V voltage drop on the battery wire is far too big to be acceptable. 14.4V at the generator/alternator becomes 13.4 or slightly less at the battery, and that's not enough to charge it properly. That's barely enough voltage for standby batteries on 24/7 charging, not nearly enough to recharge a starter battery in minutes during short drives.
Voltage is determined by the supply(Alternator/battery) Current is determined by the load(anything that uses power) What that means is it doesn't matter how much current the alternator is capable of putting out. It is how much current is being used by the load that is read by an Ammeter. Factory Ammeter's are positioned first in line so to speak so that all current used by the entire electrical system passes thru it. If your vehicle had a Ammeter AND has a factory stock electrical system with the only change being an alternator. The Ammeter should work fine because the combined load will be within the Ammeter's range. It is when we add additional loads to the system that we exceed the Ammeter's capabilities and bad things can happen. If your system does draws more than what the Ammeter can handle. Simply bypass any loads that take it over the top. One way to do this is to connect any additional loads so that they bypass the ammeter(connect them before the Ammeter with their own fuses). I have also done this before by connecting the Headlights and blower motor( 2 of the biggest loads) so that they bypass the Ammeter. The Ammeter still reads correctly minus what the bypassed loads use.
I never gave it any thought and hooked up my stock 30-0-30 amp gauge with a modern alternator. in 20 years I have never seen the gauge over 15-20 amps either direction. With a correct charging system, you should never see it put out full alternator amperage. Even with A/C blowing full, headlights, and engine running, my hardly moves the gauge needle.
A non-calibrated gauge is best for this but hey. Mine is the standard 35-0-35 gauge, and the internal shunt measured about 0.3 Ohm. I bridged it with a piece of heavy cable, measuring about 0.2 Ohm. (Yes, the wiring is perhaps a little overkill but hey, you cannot go too big, only too small). The gauge now measures approximately 70-0-70. That's about 15-20 Amps charge after cranking the motor over a little. The maximum draw is a little over 25 Amps which means the needle never moves too far to the DIS side (just about resting against the halfway dot, below with lights, heater fan and radio all on). The alternator is rated 63 Amp and it can make it almost to the full CHG dot if the battery is dead and taking a heavy charge. You can see that the old shunt had been quite warm from the crackling of the plastic panel, and that was only with a 30A dynamo... shunted it hardly gets warm at all. Phil
I plan to use an old 20-0-20 amp gauge with 40 amp Denso alternator. No AC, just lights, heater motor, radio, horn etc.
I replaced the amp gauge in my 49 Buick with a volt meter. I removed the amp gauge, but kept the glass face with the printing on it. I took an aftermarket volt gauge apart and mounted the movement behind the glass where the amp gauge sat. Now straight up is 12 volts, to the plus side is charging more than 12 volts and discharges side is less than 12 volts. And it looks stock from the drivers seat.
I'm not computer savvy enough to do a drawing but I wire ammeters to only measure what's flowing from the battery or back to the battery charging it. The way I wire cars is the ammeter is wired into a 10 Ga. wire that feeds everything in the electrical system off of the battery cable lug on the starter solenoid. As an example if the electrical system is using 20 amps and the alternator/generator is putting out 30 amps to recharge the battery right after starting the engine, my amp gauge would show 10 amps. I never really know what the charging system's output is, rather I just know if power is being used from or returning to the battery. Once the battery is fully recharged after starting the car the ammeter will read "0" indicating that the charging system is supplying all of the power being used by ignition, lights, etc.. Lynn
Well, this is great. Just what I was hoping for, several great solutions to chose from. My Stude pickup has no accessories at the moment, just basic lights, horn, etc. I will probably be adding a heater in the future but no AC or other power accessories, I’m trying to keep it old and simple. In my yout, I ran amp gauges and never considered the details of the electrical side. I just ran the wires as shown in the old PPC Spotlight book and never looked back. At this point, I wanted to know more about how the gauges worked and how to use them properly. Thanks for the info everyone.
I cooked a harness with an amp gauge in the '90s. It was fun actually other then the repair. 110 amp output, 30 amp gauge. I was almost home late at night, I saw the gauge glow then the cab filled with smoke. I rolled the widow down and did the Fireball Roberts shuffle for the last mile. I hit the driveway and flung the hood open. The missus walked calmly over and pulled the ground wire. Then smiled and said, "This is not our first rodeo!" and laughed. You can wire them properly and if you do not put too much amp draw with your accessories you are golden. The problem is not just output, it is also usage that kills ya. The proper wiring is already posted. @bchctybob BTW I found a box full of AMP gauges in he shop if you need one. SW, No Name, Studebaker. So if you are building gauges and not using the stock ones drop me a note.
That is the whole purpose of using the diode ! you use the voltage drop so the current takes the path of least resistance [via the Ammeter which has less voltage drop] When the current rises the voltage drop across the Ammeter also increases because it is non-linear [the diode remains fixed] and there is a point where both have the same voltage drop, so both flow current. The voltage drop of an Ammeter at maximum current can exceed the voltage drop of the diode.[but it will never reach this because the shunt flows the extra current when in place] Without a diode shunt, you still get a voltage drop across an ammeter [these things get warm] The only time the voltage drop you mentioned above is of any concern, is when the ammeter fails and there is no flow via the ammeter[so in normal driving circumstances with low amp flow there would still be a 1.13v drop] . But this ^^^^ situation won't leave you stranded on the side of the road if an ammeter fails @bchctybob
That was the old cheat method when playing with old Chryslers. You connect all the accessories etc to the battery side of the ammeter, but the charge wire to the other side [this doesn't show discharge] but will only show what the generator is putting to the battery. This method will stop recharging to the battery if the ammeter fails but you will still have ignition etc [from the battery side] The other cheat method was to connect the charge wire to the battery side of the ammeter so excess charge current wont destroy the ammeter. This method only shows discharge ,from all the accessories etc [the charge doesn't go through the ammeter] That is why I prefer the diode shunt..........the ammeter reads charge/discharge both ways as normal, but in excess current situations the shunt acts like an "electrical pressure relief valve"
I am absolutely not trainable when it comes to electricity. I have loved having this ancient ammeter in my roadster for at least 25 years. (It had probably been out in the rain in a junk yard for ten years before I got it). I have read that using an ammeter with an alternator can cause a fire. I think I’m understanding that if I don’t use much electricity in my roadster that ising an ammeter is probably okay. (I have only the engine and lights) . And because I think that a roadster cannot have too many gauges I also have a modern volt meter. Am I okay in keeping and enjoying my ammeter? Thanks, HFH.
Yes, but losing >1V just going through an amp gauge (regardless of having a diode bypass or not) without anything being wrong seems far too much to be acceptable even in a 12V system, in a good old 6V it would be horrible. Maybe that was normal for shunts 80 years ago, today a few tens of millivolts is more expected - you need some voltage drop to have something to measure, but all voltage drop is a loss so you want as little as possible. If all the old amp gauges come with an unavoidable large voltage drop (yes, I count 1V as huge) perhaps it's time to retire them and seek different solutions that won't mess that much with things like the batteries charging voltage vs. current. Little point in having a good size alternator if you restrict it so the voltage reaching the battery drops into basically not charging territory any time the current goes up, or cause a low voltage in the rest of the system any time you need to take a large current out of the battery.
Its a mixture actually. A big spike can weld one then everything fries. Or big amp draw will do the same or a combination of both. I personally prefer a volt meter, but I am me. You run what makes you smile and make sure its done right. I would not wish my quirks on anyone.
If I understand the "problem" it is a "could" thing. The original generator output and ammeter range were sized for each other. Now, an alternator, or even a generator, with a significantly larger output is installed. NO problem as long as all works well. All your needs are met at less than the range of the ammeter. UNTIL the battery drains all the way down. JUMP the rascal! Now the higher output is flowing through the ammeter as the alternator charges the battery. Ammeter CAN overheat. Ben
When I measured across an Ammeter with normal current draw [idling engine] it was approx 0.2 to 0.3 volt drop. The 1.0+ volt drop across the ammeter is usually when it is at maximum capable flow [the ammeter is a resistor so there will always be voltage drop] The ammeter can get an even higher voltage drop in very high draw situations but it will usually "Chernobyl" itself. When the current through the ammeter meets a point of equilibrium with the diode they both share current flow. Below that , it is all ammeter. This comment doesn't make sense, All Batteries and Charging systems have both Voltage AND Current. That is why you buy a eg: 12 volt alternator rated at 65A, or a 12 volt 72A/H [700 CCA] battery You need to read the voltage at the Alternator, and also at the battery [especially with trunk mounted batteries] there is voltage drop, the regulator allows for this to maintain 13.5V at the battery. Try and restore a valuable early 60's Chrysler muscle car these days [they had 45a alternators and also an ammeter] Send the Alternator off to get it rebuilt and it comes back as a 60A [or 70A] What do you expect them to do in this situation........Disconnect the gauge??? Leave a hole in the dash??? A hidden diode shunt allows them to keep everything working as normal without risking an electrical fire. You might find holes in the dash or cheap mismatched "Walmart" gauges acceptable
With a resistance in series you get a voltage drop, meaning the voltage drops as current goes up. Great for an ignition ballast resistor, bad for a voltage regulated charging system.
Blah blah blah blah!! You're just in "Face Saving mode" now If you are dead set against the "big scary ammeter" just pull it out ........and use an idiot light. My friends pickup has been going for 70 years now with the "BAD" factory installed ammeter which causes a voltage drop in the charging system. With a small output alternator you can leave the whole system as is. And in most cases this means there is no need to butcher a factory dash [which is usually the motive] The shunt is merely protection from overcharging/discharging. [from larger alternator etc] And as I mentioned before, under normal circumstances the voltage drop is a moot point [0.2 V] Some people don't understand this concept, So they try and baffle others with their bullshit [and their almighty superior knowledge] I've done this mod..........."and it works" AND the battery charges fine [three cheers for voltage regulators]
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