Has anyone got a factory gauge to work ? Also does this gauge run on a wire to the starter solenoid.
The ameter gauge worked fine when new but over time the fine wires that make up the internal coil can open or get “burned” so it will stop working. I don’t have a diagram in front of me but it should not run to the starter solenoid if I recall correctly. I believe that will cause it to stop working. Maybe someone else will chime in on this.
Steven
The ammeter measures the difference in voltage from two different points in the charging circuit: that from the alternator output compared to that at the connection of the battery at the starter solenoid. When current flows from the alternator to the battery, there’s a small voltage difference and that voltage is reflected in the gauge going to the charge side. When the voltage is higher at the battery than at the alternator output, the gauge registers discharge.
Now then…this system measures voltage differences that are extremely hard to detect in the presence of corrosion at various contact points of the wiring circuitry. The system, as designed, requires a voltage output across wires with very low resistance. Stated another way, if resistances increase due to other factors (age, water, corrosion, etc.), the gauge will be unable to adequately measure these small voltage differences. I use a top-of-the-line Fluke multimeter, and it is only accurate to about 0.2 ohms. That is larger than the differences experienced between these two points of measurement. There are micro-ohm meters out there with extremely high accuracy, but that is pointless when contacts between plugs are on the order of 0.1 to 0.2 ohms themselves.
Now some will say but it is an ammeter, measuring current, not voltage. Yes, that is true, but it becomes harder to explain in terms of current.
Randy would the connection to the solenoid be on the battery side rrceiving battery voltage ?
Yes. Some years/models have it via a separate ring connector on the starter solenoid, but most applications it is buried in the OEM splice just short of the main main that goes to the starter solenoid from the headlight (or in 1970, the underdash) harness.
These gauges never worked well even when new - the current flow required to/from the battery to deflect the needle by a significant amount is just too large.
Have a read of this article for more info: 1966 Mustang ammeter wiring | Ford Mustang Forum
They actually work great when all is correct. This is actually a “Shunt Type” ammeter, it is in parallel with a wire that carries the bulk of the charge current. The ammeters are quite sensitive and easily damaged. I have found that an NOS ammeter fixes the problem every time but they are getting harder to find as time goes on.
You would know better than I, but what you say is at odds with the mathematics presented below taken from the article I linked to earlier:
“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.”
Both of you can’t be right…
How does adding series resistance reduce total resistance? If the coil is 1/6th ohm ( and what king of meter gives fractional ohm ratings like that?) and then you put it in series with ANY additional resistance the total has to go up, not down to 1/4 ohm. And then there is the fact that these meters worked fine when new so a fundamental engineering failure like the author describes just doesn’t make any sense.
Royce and I have argued ammeter versus volt meter many times. Royce has always favored the volt meter. After reading extensive history of the development of charging systems and their development, I now understand why the ammeter was the right thing in early generator designs, and a voltmeter is superior in modern alternator designs. The point here is that you really ought to consider getting a voltmeter from rocketman and call it a day.
With the ammeter out of the car how would I determine if its shot ?
Since an ohmmeter has a battery in it usually measuring the ammeter for continuity will cause the needle to move either way. Back in the day I would ‘tap’ an AA battery to them to see if they moved. No movement would mean an open winding.
He’s writing an article based on theory. I am quoting my experience in the real world. Theory and reality are in theory the same. In reality, they are not.
I love this, Royce!
I know my knowledge is not nearly as deep as others on this subject. If your statement is true I would think the Mercury dealerships back in the day would have been hounded by upset customers forcing a lot of ammeters to be replaced. I do remember my first XR7 ammeter moving (working).
Steven
Priceless! I should have kept reading the other replies before I posted my last reply.
Steven
1/6 ohm is 0.1666 ohms
1/4 ohm is 0.250 ohms, and that is larger than 1/6 ohm.
Both statements by Royce and the statement from the other forum are correct; they just come from different perspectives. There are two parallel paths for charging (main, and through the ammeter), and the main section between the two ammeter leads is called a “shunt resistor”. It is a shunt resistor because there is inherent resistance of any copper (or other metal) conducting material. The worst aspect of this design is that the ammeter leads are not fused and can be pinched to chassis ground. Ask me how I know and I won’t tell you the sad story that happened many moons ago.
How to test an ammeter? Use any AA, A, C, or D cell and alligator clips and wire. Briefly touch both contacts of the ammeter with the two poles of the battery and the meter will deflect almost instantaneously.
If you read all his posts you’ll see he’s actually experimented on two different '66 Mustangs that he owns, including posting a practical guide on wiring modifications to make the ammeter more sensitive to register bigger deflections. I’m not trying to argue with your experience, but he has experience as well.
My personal observations (45+ years down the track) on 2 different 1969 Mustangs is the ammeter gauges hardly deflect at all. Neither of mine were ‘burnt out’ as someone here suggests happens, they just hardly register anything under any conditions. Maybe the Cougar gauges are slightly different - my '68 only has an idiot light for the charging system so I can’t comment.
Another bad aspect of the design is as the resistance of wires and connectors increases over time due to ageing/corrosion,it upsets the finely tuned ratio between the resistance of the main charging wire and that of the ammeter shunt. Typically that results in less current flowing through the ammeter with the needle deflecting correspondingly less for a given rate of charge/discharge. Not to mention any significant failure of the charging wire will result in a huge current trying to flow through the shunt, destroying the ammeter.
At the end of the day, manufacturers changed from ammeters to voltmeters for a reason - they provide similar information but are a lot more robust and reliable in the long term.
If you have a new or undamaged ammeter it indeed will read full deflection when the alternator is charging at 25 - 30 amps. The problem with these shunt type ammeters is that they are easily damaged if the ammeter is seeing an alternator charging at 40 or 50 amps, both of which are possibilities with the stock equipment. Even worse, typical auto part store replacement alternators put out as much as 60 amps. Anything over 30 amps charge rate will permanently damage this very sensitive movement, causing it to not deflect as much, if at all.
Bottom line is that who ever that guy is who wrote the forum post that you like so well is simply uninformed and does not have any idea what one of these gages looks like when it is working properly. All the theory in the world won’t show him that.
Royce, I meant no offence - just wanted to present some information from another forum that I found to be an interesting read.
I still think there’s room for both of you (Mr. Theory and Mr. Practice). The other guy is coming from the perspective of looking at 45+ year old gauges and wiring, doing quantitative measurements of what he finds and drawing some conclusions on why things are the way they are now.
I believe you are speaking from first hand experience of how things were when these cars were much younger, the other guy is only able to extrapolate from current times.
In any event, you pretty much agree that these ammeters are sensitive devices that can easily be rendered practically useless by a number of factors including the passage of time and modern replacement parts like high output alternators.
FWIW, my OEM ammeter works great. When the battery is maintained at full charge, the deflection is small, as the alternator is not having to supply a lot of current to charge the battery. So a small deflection from that and pulses higher with the brake lights, turns signals, stereo at high volume, etc.
I prefer the ammeter to a voltmeter that so many people seem to be so fond of. An ammeter tells me more than a voltmeter does (but I wouldn’t mind having both! - idea for Rocketman?)