With that being said, tell em what it takes to convert the ammeter into a 12vdc meter.I have waved my hands to explain this one several times so I figured it was time to make a posting of exactly how it all works.
The attached JPG shows how the ammeter wiring in a factory 1966 harness works. I have only drawn the wires involved in the ammeter circuit and given their colors and size/length where appropriate. The red and yellow wires go through a connector in the firewall which I have not shown to keep the drawing somewhat simple. The lengths given for those wires includes the wire on both sides of the firewall.
The ammeter is actually reading the voltage across what I drew as the big purple wire in the drawing. (Its actually a black/yellow wire but that's hard to draw.) The little rectangle shown in the middle of the yellow wire by the solenoid is a connector built into the wiring harness. These wires are all wrapped together inside the underhood harness so you have to scratch your head a little while to figure out where these all are and how they connect. The 4-way wire connection down by the alternator is a thick lump you can feel inside the taped harness, its not anything you can actually see without removing some tape.
The ammeter coil itself is about 1/6 Ohm but when you include the red and yellow wires to and from the purple wire the total impedance for current flow through the ammeter wiring is right at 1/4 Ohm. I give example numbers for an alternator charging 100 Amps into the battery. About 1 1/4 A, or 1/80 of that charge current will go through the wires to the ammeter if all the connections are good. Unfortunately, full scale deflection of the ammeter takes about 2.8 A so that means the charge rate at the upper end of the ammeter scale is almost 230 A. Neither 230 or even 100 A will ever happen with the stock alternators which explains why this meter needle never moves a whole lot even if it does move.
On many cars, the connection at the solenoid end of the red and yellow wires are dirty or broken and there will be NOTHING going through the ammeter. That doesn't hurt anything else in the car so few ever take the time to figure out there is even a problem with the wiring.
1966 Mustang ammeter wiring
I have waved my hands to explain this one several times so I figured it was time to make a posting of exactly how it all works.
The attached JPG shows how the ammeter wiring in a factory 1966 harness works. I have only drawn the wires involved in the ammeter circuit and given their colors and size/length where appropriate. The red and yellow wires go through a connector in the firewall which I have not shown to keep the drawing somewhat simple. The lengths given for those wires includes the wire on both sides of the firewall.
The ammeter is actually reading the voltage across what I drew as the big purple wire in the drawing. (Its actually a black/yellow wire but that's hard to draw. ) The little rectangle shown in the middle of the yellow wire by the solenoid is a connector built into the wiring harness. These wires are all wrapped together inside the underhood harness so you have to scratch your head a little while to figure out where these all are and how they connect. The 4-way wire connection down by the alternator is a thick lump you can feel inside the taped harness, its not anything you can actually see without removing some tape.
The ammeter coil itself is about 1/6 Ohm but when you include the red and yellow wires to and from the purple wire the total impedance for current flow through the ammeter wiring is right at 1/4 Ohm. I give example numbers for an alternator charging 100 Amps into the battery. About 1 1/4 A, or 1/80 of that charge current will go through the wires to the ammeter if all the connections are good. Unfortunately, full scale deflection of the ammeter takes about 2.8 A so that means the charge rate at the upper end of the ammeter scale is almost 230 A. Neither 230 or even 100 A will ever happen with the stock alternators which explains why this meter needle never moves a whole lot even if it does move.
On many cars, the connection at the solenoid end of the red and yellow wires are dirty or broken and there will be NOTHING going through the ammeter. That doesn't hurt anything else in the car so few ever take the time to figure out there is even a problem with the wiring.
Electrically this is called an 80:1 shunt circuit. Enjoy.
The attached JPG shows how the ammeter wiring in a factory 1966 harness works. I have only drawn the wires involved in the ammeter circuit and given their colors and size/length where appropriate. The red and yellow wires go through a connector in the firewall which I have not shown to keep the drawing somewhat simple. The lengths given for those wires includes the wire on both sides of the firewall.
The ammeter is actually reading the voltage across what I drew as the big purple wire in the drawing. (Its actually a black/yellow wire but that's hard to draw.
) The little rectangle shown in the middle of the yellow wire by the solenoid is a connector built into the wiring harness. These wires are all wrapped together inside the underhood harness so you have to scratch your head a little while to figure out where these all are and how they connect. The 4-way wire connection down by the alternator is a thick lump you can feel inside the taped harness, its not anything you can actually see without removing some tape.The ammeter coil itself is about 1/6 Ohm but when you include the red and yellow wires to and from the purple wire the total impedance for current flow through the ammeter wiring is right at 1/4 Ohm. I give example numbers for an alternator charging 100 Amps into the battery. About 1 1/4 A, or 1/80 of that charge current will go through the wires to the ammeter if all the connections are good. Unfortunately, full scale deflection of the ammeter takes about 2.8 A so that means the charge rate at the upper end of the ammeter scale is almost 230 A. Neither 230 or even 100 A will ever happen with the stock alternators which explains why this meter needle never moves a whole lot even if it does move.
On many cars, the connection at the solenoid end of the red and yellow wires are dirty or broken and there will be NOTHING going through the ammeter. That doesn't hurt anything else in the car so few ever take the time to figure out there is even a problem with the wiring.
Electrically this is called an 80:1 shunt circuit. Enjoy.
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OK.
1) Buy voltmeter to fit hole
2) Throw away the yellow wire, just disconnecting it will also work
3) Hook voltmeter + to red wire with the - side connected to ground somewhere under the dash
If you meant to turn the existing ammeter into a voltmeter that's a little more complicated because zero is in the middle of the scale which will make a confusing looking voltmeter scale. Also the meter calibration would be a hit or miss thing so I would suggest using a REAL voltmeter if you want to know what the voltage really is.
Although I am sure its possible to go into the ammeter and turn it into a voltmeter that is a little beyond what I think most folks would want to try. I also have not done it so I can't say how difficult it might be.
1) Buy voltmeter to fit hole
2) Throw away the yellow wire, just disconnecting it will also work
3) Hook voltmeter + to red wire with the - side connected to ground somewhere under the dash
If you meant to turn the existing ammeter into a voltmeter that's a little more complicated because zero is in the middle of the scale which will make a confusing looking voltmeter scale. Also the meter calibration would be a hit or miss thing so I would suggest using a REAL voltmeter if you want to know what the voltage really is.
Although I am sure its possible to go into the ammeter and turn it into a voltmeter that is a little beyond what I think most folks would want to try. I also have not done it so I can't say how difficult it might be.
The reason Ford wired these so squirrelly was to save some pennies on wiring. A real ammeter would need 80x times bigger wires which cost money, are heavy, big, etc. Also, by this trick they didn't even have to worry about hooking the ammeter up to the ignition switch. Its ALWAYS working its just that when the car is off there is nothing to measure.
Its sort of like the trick they used of wiring the Rally Pac in series with the ignition switch. They got to have a tach option and, except for the Rally Pac itself, added absolutely ZERO extra wires to the cars to be able to do it. A 'Better Idea' - at least for the bean counters.
If you want a voltmeter it really should be wired through the ignition switch or else you will be drawing battery current all the time. With the stock meter that means you would be drawing 2-3 Amp all the time if you want a meter needle pointing over towards the right side of the gauge. That would cause your battery to go dead every day if you didn't drive the car. A good voltmeter shouldn't draw much more current than your clock so you could probably get away with wiring it like I mentioned above.
Something like a fuel gauge will fit in the ammeter hole and that would be a whole lot easier to turn into a fairly good voltmeter although the scale markings would still be a bit odd. You would still want to hook it up to the ignition switch since a full scale reading on a fuel/oil/water temp gauge draws nearly 1/4 Amp of current. It would take about a week for that to run your battery down but that's still adds a hassle factor most of wouldn't want to put up with.
Its sort of like the trick they used of wiring the Rally Pac in series with the ignition switch. They got to have a tach option and, except for the Rally Pac itself, added absolutely ZERO extra wires to the cars to be able to do it. A 'Better Idea' - at least for the bean counters.
If you want a voltmeter it really should be wired through the ignition switch or else you will be drawing battery current all the time. With the stock meter that means you would be drawing 2-3 Amp all the time if you want a meter needle pointing over towards the right side of the gauge. That would cause your battery to go dead every day if you didn't drive the car. A good voltmeter shouldn't draw much more current than your clock so you could probably get away with wiring it like I mentioned above.
Something like a fuel gauge will fit in the ammeter hole and that would be a whole lot easier to turn into a fairly good voltmeter although the scale markings would still be a bit odd. You would still want to hook it up to the ignition switch since a full scale reading on a fuel/oil/water temp gauge draws nearly 1/4 Amp of current. It would take about a week for that to run your battery down but that's still adds a hassle factor most of wouldn't want to put up with.
Thanks for the great explanation and the diagram, Ivy66GT.
The original ammeter in my car still works, but the puny original alternator barely moves the needle about 1/6th across the positive side of the scale, even with all electrical loads turned off. It will show discharge and charge states to some degree, I suppose.
I just purchased a 90A 3G alternator for my 66, but haven't installed it yet.
95A 3G Upgrade for 1965-1980's Mustang.
I also bought a kit that includes a dummy regulator box that allows retention of the original alternator wiring harness.
http://www.paperformance.com/scripts/prodView.asp?idproduct=75
Based on what you have described in your posts, I'm thinking it's safe to put the 90 amp alternator on the car and that it won't blow up the ammeter... it will just make it move further to the right (hopefully).
Any advice or suggestions would be appreciated!
Thanks.
The original ammeter in my car still works, but the puny original alternator barely moves the needle about 1/6th across the positive side of the scale, even with all electrical loads turned off. It will show discharge and charge states to some degree, I suppose.
I just purchased a 90A 3G alternator for my 66, but haven't installed it yet.
95A 3G Upgrade for 1965-1980's Mustang.
I also bought a kit that includes a dummy regulator box that allows retention of the original alternator wiring harness.
http://www.paperformance.com/scripts/prodView.asp?idproduct=75
Based on what you have described in your posts, I'm thinking it's safe to put the 90 amp alternator on the car and that it won't blow up the ammeter... it will just make it move further to the right (hopefully).
Any advice or suggestions would be appreciated!
Thanks.
I wouldn't see any problem. I have a 60A in mine from some later year Mustang. Alternators will only put out what the regulator says they should so a larger alternator will not necessarily make the ammeter move any differently. With a 60A alternator my ammeter needle still rarely moves.Based on what you have described in your posts, I'm thinking it's safe to put the 90 amp alternator on the car and that it won't blow up the ammeter... it will just make it move further to the right (hopefully).
Thanks for the quick reply. Much appreciated!
Ivy66GT - I have scratched my head for years about this and like you said in your initial post I have just left it alone. My ammeter has never worked in the 23 years I have owned my 66 coupe. I would like to get it working now. I have replaced all of the wiring harnesses under the hood but the ammeter has never moved. Where can I begin my troubleshooting?
I would start by disconnecting the red and yellow ammeter wires near the solenoid and using an ohmmeter to see if there is continuity through the ammeter; it should measure less than an Ohm. Actually you only have to unplug the yellow wire and measure from the yellow wire back to the end of the red one. The reason to disconnect the red wire also is to make sure it is making good contact with the solenoid connection when you put it back together.
If that measures OK then the meter may work when you reconnect the red and yellow wires. With those wires reconnected use your meter to measure voltage across the 'purple' wire shown in my drawing while you are discharging some current; such as with the headlights turned on. (Its not purple in real life, black and yellow actually, but since its covered in black tape you can't really tell that.) The voltage won't be much but you should measure a few millivolts. With continuity and a little voltage your ammeter should be working.
Remember it takes a lot of current to move the needle very much so you need headlights, not just a courtesy light or two for the test.
If that measures OK then the meter may work when you reconnect the red and yellow wires. With those wires reconnected use your meter to measure voltage across the 'purple' wire shown in my drawing while you are discharging some current; such as with the headlights turned on. (Its not purple in real life, black and yellow actually, but since its covered in black tape you can't really tell that.) The voltage won't be much but you should measure a few millivolts. With continuity and a little voltage your ammeter should be working.
Remember it takes a lot of current to move the needle very much so you need headlights, not just a courtesy light or two for the test.
The ammeter will only register the amount of net charge or discharge. If you put a load of 90 amps on the battery and the alternator is putting out 90 amps it will read ZERO!
Now, if you turn your lights on and leave them on (with the car off for say 20 minutes then when you start the car the ammeter will read the charge going back into the battery - 10 - 30 amps with the stock alternator - with a 90 amp alternator you might see 60 amps - which would peg the ammeter in your car.
Your Ammeter is supposed to be rated at slightly more amps than your alternator can put out. If you push more amps than the ammeter is rated at it becomes a fire risk and eventually a fuse. It usually becomes a fuse just after the under-dash wiring has melted and begun to burn.
I doubt you can find an ammeter with the capacity for a 90 - 200 amp alternator in a form that will fit in the dash. That is why newer cars have switched to volt meters instead of ammeters.
Now, if you turn your lights on and leave them on (with the car off for say 20 minutes then when you start the car the ammeter will read the charge going back into the battery - 10 - 30 amps with the stock alternator - with a 90 amp alternator you might see 60 amps - which would peg the ammeter in your car.
Your Ammeter is supposed to be rated at slightly more amps than your alternator can put out. If you push more amps than the ammeter is rated at it becomes a fire risk and eventually a fuse. It usually becomes a fuse just after the under-dash wiring has melted and begun to burn.
I doubt you can find an ammeter with the capacity for a 90 - 200 amp alternator in a form that will fit in the dash. That is why newer cars have switched to volt meters instead of ammeters.
That describes a NORMAL ammeter, but the Mustang ammeter after 1965 was NOT normal. The charge/discharge current doesn't even go through the meter so if you are going to burn anything out it will be the wire I drew in purple which is a 12 ga copper wire about 21" long. With 100 Amp through that wire charging the battery (very unlikely) there will only be about 1/4 Volt across the wire dissipating not quite 8 Watts in a long wire. That will get warm, but unless its damaged it won't burn anything out. To burn out the '66 ammeter the purple wire has to be broken or there is a short somewhere.Your Ammeter is supposed to be rated at slightly more amps than your alternator can put out. If you push more amps than the ammeter is rated at it becomes a fire risk and eventually a fuse. It usually becomes a fuse just after the under-dash wiring has melted and begun to burn.
The '66, and later, ammeters are lame voltmeters measuring voltage drop across that section of 12 ga wire. If you have an early, '66 or later, Mustang ammeter that will peg at 60A, which BTW would be very useful, then the ammeter is not the original one Ford installed.
The reason modern cars have voltmeters instead is because the makers are cheapskates (pardon my French), like Henry Ford was, that don't want to install wire that is heavy enough to do the job required of an true ammeter. A voltmeter can use a tiny, single strand of copper (the '66 uses 16 ga), a true ammeter would take something closer to 10 ga wires. Since most drivers don't know what either one of the gauges would be telling them its easier for the bean counters to install a cheap voltmeter.
I haven't had a chance to look at the wiring yet but I did have my alternator tested today and it failed. I don't know how long it has been bad. The car usually stays on a trickle charger when not in use and that could explain it not dieing all of the time. Would a bad alternator cause a malfunction or problem with the ammeter gauge?
Anything is possible but I don't see any reasonably common way that the alternator croaking should take out the ammeter especially since it not wired as a real ammeter.
I've been trying to find out if anyone makes a retrofit voltmeter for an ampmeter. And can't find anything, you'd think with todays technology someone would have come up with one. And if they have, would ya'll fill me in on where to get one. Thanks.
This might help... they can convert your Ford ammeter to a voltmeter... not cheap though. ($175)I've been trying to find out if anyone makes a retrofit voltmeter for an ampmeter. And can't find anything, you'd think with todays technology someone would have come up with one. And if they have, would ya'll fill me in on where to get one. Thanks.
Speedometer Repair Tachometer Clock and Gauge Restoration for Musclecars and Vintage Automobiles
e-mail: [email protected]
I'm thinking it can be done with some ingenuity and a modern inverted needle voltmeter as a donor. This one from Sun is cheap and looks like it might fit... proceed at your own risk, though.
Sunpro®
My amp meter shorted out on my 67 and caused a nice fire to erupt in its wiring harness. Ford didn't use any kind of circuit protection with the amp meter for some crazy reason which probably has economics tied to it. Whatever the case, as it sits, it's an electrical fire waiting to happen.
I now use a volt meter instead. It's just a safer and easier way to let the driver know what the charging system is doing. If I need to know what the amperage output is, I reach into my tool box.
I now use a volt meter instead. It's just a safer and easier way to let the driver know what the charging system is doing. If I need to know what the amperage output is, I reach into my tool box.
The danger of a short in the original ammeter wiring is no worse than it is for the other 50(?) wires in each car which have no fusing such as the fuse block or the courtesy lights. If the wiring is bad in any car it needs to be changed for safety purposes.
For updated information and even a method for increasing the sensitivity of these ammeters to make them truly useful see my newer thread: http://www.allfordmustangs.com/foru...s-testing-making-them-useful.html#post5036921
OK IVY66GT, I also need help, but my challenge is exacerbated by the fact I want to keep the car looking original when it is certainly NOT. I am looking at your drawing and as I recall from schematics, most of the 66's electrical load connects where you show the yellow and purple connection. I have a 160 Amp singe wire alternator that connects directly to the solenoid through a really stout wire (not 3 ga but larger than 10). Thus, my ammeter is only indicating current flow through the purple wire, which is still there but not connected to the alternator, when I energize a load downstream from the purple/yellow junction. After studying the schematic and reading your and several other explanations of shunt circuits, my plan is to open the original harness, remove the purple wire, and replace it with the larger gauge wire recommended by the alternator manufacturer. I think this will leave my shunt circuit intact and my ammeter functional without risk of overloading any of the original wires. What do you think?
Hello Fred,
Yes, you have the basics correct. The power feed for the passenger compartment is where the purple and yellow wires come together. When you connect your larger alternator directly to the solenoid (i.e. battery cable), whatever current goes directly to charging the battery will not go through the purple ammeter shunt so you will never see the ammeter needle go to the plus side.
Your solution of replacing the purple wire with a larger one will work to a point. The downside is that with a larger wire replacing the purple one, the sensitivity of the ammeter will be reduced and you may not see much needle movement at all in either direction.
You still have a basic problem in that the power feed to the car is only a 12 ga wire. If you try to force 160 Amps through an 8+ foot long 12 ga wire that will require a voltage drop along that wire of a full 2 Volts. In other words, except to charge your battery you can never use most of the potential 160 Amp output without completely rewiring your car. That is the reason I don't see such large alternators as being all that useful as a replacement item.
What size wire are you thinking of using to replace the purple one?
Yes, you have the basics correct. The power feed for the passenger compartment is where the purple and yellow wires come together. When you connect your larger alternator directly to the solenoid (i.e. battery cable), whatever current goes directly to charging the battery will not go through the purple ammeter shunt so you will never see the ammeter needle go to the plus side.
Your solution of replacing the purple wire with a larger one will work to a point. The downside is that with a larger wire replacing the purple one, the sensitivity of the ammeter will be reduced and you may not see much needle movement at all in either direction.
You still have a basic problem in that the power feed to the car is only a 12 ga wire. If you try to force 160 Amps through an 8+ foot long 12 ga wire that will require a voltage drop along that wire of a full 2 Volts. In other words, except to charge your battery you can never use most of the potential 160 Amp output without completely rewiring your car. That is the reason I don't see such large alternators as being all that useful as a replacement item.
What size wire are you thinking of using to replace the purple one?
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