Do I understand this correctly: the amp gauge on my 68 XR7 is really just a voltmeter measuring the voltage drop across a shunt wire? In other words, will the car still run if the gauge is disconnected?
That is correct. (It will run if disconnected) Chances are very good that at some point in time the wiring was incorrectly put on the starter solenoid, and the meter has already opened up internally.
I think they didn’t work very well right from the factory anyhow. The one in my '69 xr7 never moved.
Mine works (at least it did once when my regulator failed and fried my battery), but I want to convert it to a volts gauge.
Yet another thing Bob at RCCI can take care of for you!
That is definitely on my wish list!
A shunt is a parrallel current path. The majority of the current goes through the shunt wire, so yes the car will run with the ammeter disconnected. It is reading current, not voltage. They do work fine when in good condition. I installed an NOS one in the Blue GT-E I sold back in 2008 and it worked perfectly.
The guages lose their sensitivity over time and then don’t work at all. The only cure I know of is an NOS guage, or having Bob make yours into a real volt meter which will require a minor wiring change to operate.
I have had one of Bob’s volt meter conversions in my green GT-E for many years. It still works great!!!
I have always preferred (and still do) an ammeter. It tells you so much more than a voltmeter could ever tell you. - means you are losing stored charge, + means the alternator is keeping up with demand and putting additional charge into the battery. A voltmeter can only tell you the voltage, above a certain voltage you infer that the alternator must be charging (unless the battery has one or more open cells in which case the voltage will read high and the battery/charge will be no good in spite of that). However, they sure seem to be popular with folks, not sure why (but bet you guys will tell me!). Simplicity?
I had never seen a Cougar ammeter move off center until I started Q-vert for the first time after reassembly. Guess I got lucky. Neither of my previous Cougars had a working ammeter.
Most Cougar ammeters don’t move because they are dead. The coil inside the ammeter is wound of lots of turns of very fine wire that can burn through very easily. Since it is live all the time, all you have to do is short one of the two wires that lead through the meter and it instantly burns up the coil, or most commonly the lead running from the post to the coil inside the ammeter.
One lug is at the battery side of the starter solenoid and the other side is attached to the harness coming off the alternator, being fed by the main charging wire from the alternator to the battery side of the starter solenoid. It is vcery common for people to confuse the lug at the starter solenoid for the prove out wire for the low fuel lamp in the over head console. (keep in mind that the ammeter is only used in XR7, and therefore always found in combination with the convenience control panel). All you have to do is to touch the lug to the wrong side of the solenoid for a fraction of a second to burn out the ammeter.
I am not sure that the ammeter is really losing its sensitivity if it does work. What I think is really happening is that the total resistance of the circuit has increased over time to the degree that the internal resistance of the meter is swamped by the series resistance of the harness. I will leave it to the EE’s to get to the exact theoretical details, but when I clean up the contacts and connections, it seems to help get the meter working again.
I tend to agree with Bob that the ammeter probably tells you more about what you need to know than the voltmeter, but the voltmeter can tell you things that the ammeter won’t. For example that you are charging at 17 volts! What I find confusing about the volt meter is when you have a reading right at 12 volts or so. It looks too low for the alternator to be working, but too high for the battery to be the only source of power…
To my thinking, a gauge should tell you whatever is happening with your car that’s going to leave you stranded on the side of the road. Last summer I was driving cross-state with a failed voltage regulator. My amp gauge dutifully swung to the right of center and stayed there for about half of the trip. I figured charging was better than discharging and the car seemed to be running fine so I continued on with the trip. Turns out the battery was charging alright— being force-fed about 18.5 volts from the alternator. By the time I pulled in the driveway the battery was sizzling so loud I got scared and yanked it out of the car and literally threw it out into the yard, fully expecting it to explode at any second. I figure a volt gauge would have warned me that the sulfer smell my wife was complaining about wasn’t due to being downwind from a paper mill, as I kept assuring her it must be.
Good 'pernt Bill and I agree. So the true bestest thing would be having both or a combination gauge, there’s a product idea! It wouldn’t be too hard to design such a thing either.
Bob, maybe you can engineer it and Bob at RCCI can build it?
Yorgle, you should be very careful with batteries. It could have blown up in your face. Your story makes the point exactly: charging is good, over charging, not so much! I am happy that it didn’t go horribly bad. I am pretty sure you would have healed but the battery acid would have been really bad for your car… Just kidding LOL!
I prefer a voltmeter if I have a choice. An ammeter won’t tell you if the battery is charging, it only tells you the amount of alternator output. As Bill says, with an ammeter you won’t know if the voltage is too high until the battery expires. All you would know is that the alternator is charging.
Sure it does. The charging current goes into the battery (minus any current used by the car). A voltmeter is the instrument that won’t tell you if the battery is being charged, only the voltage across it. With a voltmeter you infer charging by the voltage being above a nominal value (probably 13.8V), which is meaningless if the battery impedance is higher than it should be for whatever reason.
The charging current is not necessarily charging the battery if it is not above the required voltage. For that reason voltage is relevant to battery charging. Amps reading is only a by product that by itself is not necessarily relevant.
Anybody make a dual meter, amps and volts in one housing? What about a wattmeter, to actually measure the power (volts times amps) at the same time? Does anyone even make an analog wattmeter that has both positive and negative deflection?
That’s actually what I had in mind when posting this thread. My thinking was that if the gauge is actually just a voltmeter measuring the voltage drop across a given length of wire (i.e., the shunt) then wouldn’t it be possible to install a SPDT switch to select between the shunt resistance (i.e., amps) and a second circuit (having a different value) that would reflect voltage. The voltage circuit could be calibrated by means of a variable resistor placed between the amp gauge and ground. Basically, I’d start with the pot at infinite resistance and slowly decrease to where the needle started to deflect and see what value that was at with an ohmeter. That could then be checked against the actual voltage and white paint dots made on the gauge face to mark normal charging voltage and the start of overcharging. Is this crazy, Bob?
I am not positive, but it think that the deflection you see is actually the difference in voltage between the alternator and the load (battery and car). The direction of deflection tells you the direction of current flow (toward the battery or from the battery). So if there is no charging current from the alternator. then the direction of current flow is from the battery to the load. IF the alternator is charging, it is because the output voltage is higher than the battery output voltage, and the current is flowing in part, towards the battery.
What we really need is a battery charge level gauge. What me measure is what we can measure, and we have to make inferences from those measurements about what is really happening.
How about a set of these?? Most Pre-1997 Ambulances used them, so they should be pretty easy to obtain, and they are pretty accurate.
Pete
