I have a 1940 chevy coupe that all 12v, but I want to go back with the original gauges. How do I make the amp meter read 12v instead of 6 amps. I have a fix for all the other gauges, just not the volt gauge.
The ammeter reads current, not volts. It will work fine with 12v. First thing to do might be to learn about electricity, so you understand the difference between "volts" and "amps".
I thought that it also depended on the generator/alternator issue. Most amp meters go to 30 and if you switch to an alternator it might fry your gauge.
It has more to do with how far the battery gets run down, than with the alternator output rating. If you drain the battery and then the alternator charges it at 40 amps for a while, then the ammeter or it's wiring might get warm...how warm? we don't know. keep in mind that an ammeter set up for 6 volt system is designed to carry twice the current of a similarly loaded (in terms of power) 12v system.
I bought some 12v to 6v voltage reducers off E-bay for my project, up untill the early 70's Ford used 6v guages and needed voltage reducers.
Converting an ammeter to volts is a fairly common thing, but does require changing out the meter mechanism. You can have it done by a pro, but some guys have home-brewed conversions also. There're multiple postings here on the HAMB about this, here's one.... https://www.jalopyjournal.com/forum/threads/amp-gauge-coversion-to-volt-meter.977952/. Some searching will turn up more... If your OEM ammeter has screw posts on it, those are the dangerous ones. If it's an induction type with no physical wire connections, just metal loop that the wire goes through, those are safer to use. The gauge may still fail, but you won't get a major fire. Most OEM ammeters are only rated for 30 amps, 40 at the most, nearly any alternator can exceed that. When alternators became standard, most manufacturers switched to shunt-type ammeters instead of direct-read but those are prone to failure as well as being difficult to wire in while retaining accuracy. This is a bit misleading. Again, generators with output amps beyond 30 are fairly rare, 6V or 12V. The available power in Watts will double (6V x 30A = 180W, 12V x 30A = 360W) but total current available will be roughly the same.
I ***ume amps its a typo as 40 fords i have seen have a volt meter. In which case measure the resistance of the gauge and add in a resistor of the same value in series. That will drop the volts by half at the gauge. You then just need to change the printing on the dial plate or live with the fact that you need to double the value indicated.
@Crazy Steve .... A question ... I'm learning some & do not know. Some people say it can be bad to use a amp gauge on a alternator because of possible fire hazard. Then others say it is how you wire in the gauge .... You do not send the load through the gauge. My wiring diagram I'm going to use ... Bat > starter > amp gauge. .... Then from the alternator to the other side of the amp gauge. Now the amp gauge only monitors the amps in the battery. All the load and charging is on the other side of the gauge & does not go through the gauge .... I plan to add a volt gauge in it also to monitor charging. Is this the correct way to wire a amp gauge safely? I hope to start on wiring soon. Just thinking it is fine for @Kirk H to use the amp gauge if wired correctly?
To wire an ammeter so it shows the current flowing into and out of the battery, you have to wire it to where the battery connects to the rest of the system, as you've shown. Using a shunt type ammeter, where most of the current is carried by a large wire, not the ammeter itself, is a preferred way to make it safer. But you can wire a full current ammeter safely...you just kind of have to know what you're doing, use good wiring practices, a good meter, know what the expected current is, etc. Apparently that's too difficult today.
Ford used the King-Seeley 6v gauges until 1985. In 1986, they switched to the 12V magnetic gauges and had many new problems to fix, but eventually dialed them in. From 1957 until 1985, they used a cluster voltage regulator to reduce the power to the gauges to 6v. Very reliable and simple. I often replace bad gauges in 40's Ford products with later gauges by replacing the modern font gauge face and needle with the early components and have no problems.
When everything is working correctly an ammeter presents no hazard. It's when things go wrong that the fire hazard appears, and it takes specific sets of conditions for this. First, you need to be using a direct-read ammeter with connection posts. Shunt type and inductive ammeters don't present this hazard. Second, the ammeter must be rated for the FULL alternator output, which very few if any generator system meters do. Even most aftermarket gauges are only good to 60 amps, still below what many of the currently-used conversion alternators can output. When everything is working right, all the ammeter shows is the Charging current, generally only a few amps or so, with short-duration spikes after starting. But certain faults can cause current across the meter to go up and if current exceeds the meter capability you can get a fire, a problem if using an alternator that has output beyond what the generator could deliver, although certain faults could be battery-caused. It can also depend on what the actual connected load is. A bare-bones system won't present enough load, but if you have a number of high current devices totaling beyond the meter rating you can have a problem. A failed alternator or battery can cause this, also jump starting another car. Short circuits to ground is another cause. Bottom line, there isn't any way to make an undersized direct-read ammeter install 100% 'safe'. The other thing to consider is the actual function of the gauge. It's there to allow you to monitor the health of your charging system, and these gauges don't work equally well on both types of charging systems for this. A generator system will usually have no issues producing voltage even while failing, it's current that will drop off first, so an ammeter is the better choice here. An alternator is just the opposite, voltage will drop more than current, so a voltmeter is better there.
One problem with a voltmeter is that it doesn't tell you if it's charging or floating or discharging. An ammeter will tell you this....and it's kind of nice to know if the charging system is actually working, or not. An idiot light is probably far more useful than either an ammeter or volt meter, for most drivers. I prefer it.
Certainly removes the "cool factor " of using old school gauges. I suspect a strong whiff of wisdom coming from these words
Gauges are nice, old gauges are cool, no question. In my '50 I used the original gas gauge, no issues ding that but some folks think it's pretty cool the old gauge works. I used a new "volt meter" and have idiot light for charging system and oil pressure that shine right in the drivers face to supplement the gauges that a driver might not notice right away. You can have cool as well as useful functionality in the same instrument cluster.
The volt meter in my Buick shows over 12 volts sometimes 14 volts after I start the car. So to me that tells me it is charging. If it shows less than 12 volts it is not charging, so I don't understand what you are saying.
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