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Exactly. That is what I saw yesterday on my DVM at the battery, and that is what I want the voltmeter to show as well. That way, any deviation is something to look into. The way it is any change in load is a deviation.
Having said that, there is one wire in the cab that doesn't have a load on it - the red/orange wire that feeds the ammeter would be a good place to pick up the voltage. There may be a bit of voltage drop across Fuse Link J as it feeds part of the cab and, by definition, the fuse links are smaller wire. But maybe not enough to be a problem. I'll probably put my DVM on the high-side of the shunt and see what variation I get by switching various things on and off in the cab.
I just put a RCCI converted voltmeter (original ammeter) in my '81 F350. I tried using the ammeter shunt wire for the + side and found that regardless of which ammeter wire I used they were powered at all times, not keyed. I ended up tying into the tach for both + and -. Seeing as the RCCI voltmeter doesn't have numbers (being the original ammeter face) all I really want to know is charging or not, and it tells me that. I had the alternator belt come off my '81 F100 and knew right away with the RCCI voltmeter that it was not charging, never would have noticed with the original ammeter.
Good point. Hadn't thought about that source, or the battery for that matter, being hot all the time. Guess my approach either needs a relay or the voltmeter will be on all the time. Wonder how much it pulls?
On the ammeter, I wonder how much current the factory shunt can handle. If I left the shunt in and installed a 3G alternator I'd be able to see the ammeter move for a change. But, would the shunt burn up?
Not sure what the draw is, I was more concerned with how long the gauge would last. I am not entirely certain how the shunt works but I thought it was just to show direction of current flow and didn't see much amperage. I thought the main charge wire took the load. I also recall a tech article where increasing the length of the ammeter shunt would increase the sensitivity of the gauge. It was a long time ago so I may be wrong.
The shunt is a very low ohm resistor, and all of the current goes through it save for the very, very small amount that goes through the ammeter itself. And a longer shunt means more resistance, so that means the ammeter is more sensitive.
What crisis am I solving? Shunt characteristics or where to hook up a voltmeter? The answer is 42. (Bonus points if you know this semi-obscure reference)
The latter is easy. I have my voltmeter connected to fuse #11, the switched power to the radio. I forget the wire color, but it's right in the vicinity of the instrument panel, so access is easy. I have an aftermarket gauge installed on a bracket under the bottom edge of the dash. Had I known about Rocketman's conversion at the time, I'd have done that instead.
Shunt characteristics? How about we create lasting world peace instead, as that is easier. The thing to remember about an ammeter is that it's actually a very sensitive voltmeter. And for monitoring an electrical system, virtually worthless. You have to really be paying attention to catch most failures before it's too late. Lose your alternator while driving and the needle will only show a very slight discharge. It's super easy to miss. That's why I'm much more partial to a voltmeter. You'll see a much greater change in the needle to indicate a failure. My ammeter has been dead as long as I've owned this truck (1992) and I've never bothered fixing it.
Back to that ammeter is a voltmeter thing. The super sensitive voltmeter simply reads the miniscule voltage drop across a shunt of known resistance (an external piece of wire in this case). The face of the gauge technically shows amps, but inside it's still a voltmeter.
I had to put on my thinking cap to think about changing the sensitivity of the "ammeter" display. If you're installing a more powerful alternator, you'll need to decrease the display sensitivity to avoid pegging the needle. Theoretically, you could add a second shunt wire in parallel with the first. If the two shunt wires are the same capacity, the gauge sensitivity would be cut in half.
Personally, I wouldn't want to mess with that shunt if I didn't have to. That shunt handles a LOT of current. A crimp that was less than perfect might come back to haunt you down the road and leave you stranded. Why take the risk?
A preferable option would be to add a resistor is series with the meter. You'd have to play around with the value, but a little extra resistance would decrease the meter sensitivity. You could jumper in a potentiometer and play with that to adjust the meter sensitivity. Once you get it dialed in, you could either replace it with a fixed resistor or just glue the shaft with a dollop (love that word) of RTV. I think we'd only be talking a fraction of an ohm, so the potentiometer might be more practical.
Or just ditch the ammeter altogether and go with a voltmeter. That option gets my vote.
Stupid question, but why not hook the VM via a relay off the battery? The trigger voltage for the relay would be something in the cab that only has power when the key was on. Run the VM wire in and make sure you have a good ground. It seems to me the readings would be much closer to actual volts at the battery vs getting it somewhere inside the cab.
Stupid question, but why not hook the VM via a relay off the battery? The trigger voltage for the relay would be something in the cab that only has power when the key was on. Run the VM wire in and make sure you have a good ground. It seems to me the readings would be much closer to actual volts at the battery vs getting it somewhere inside the cab.
Not a stupid question at all. However, this would be more complex than really needed.
Ask yourself why you're adding a voltmeter in the cab. Is it for precision calibration of the charging system, comparable to hooking up a $300 Fluke directly at the battery? I doubt it. You're doing this to monitor for problems while driving, right? So what if the all-important reading at battery is slightly different compared to the gauge in the cab? Just learn the difference, and keep that in mind.
That's how the aftermarket voltmeter behaves in my truck. With 13.5V directly at the battery, a perfectly good value, my voltmeter might only show 12.75V because it's so far downstream, but I know that's good. If my actual battery voltage drops to 12V while driving, indicating a charging system failure, my dash gauge might show 11.25V and that will still catch my attention.
If I really wanted to get fancy, I could change the numbers on the gauge. Instead, I'm lazy, and just know what is normal and what is not, even if not entirely accurate compared to what's at the battery. Remember, although laziness often gets a bad rap, it always pays off right away.
I agree with Jim. Any power in the cab is going to fluctuate with what is running. I want to know what the battery voltage is, and don't really care too much what cab voltage is.
However, I have the aftermarket voltmeter that Vernon installed in addition to the factory ammeter. I want to have a functioning gauge in the factory spot, but don't need two voltmeters. Having said that, kr98664 is right that the ammeter isn't worth much as a diagnostic tool. I have had an alternator fail and the slight discharge the ignition system pulls wasn't enough to get my attention. And if I'm going to install a 3G alternator I can't make the ammeter any more sensitive in order to see the slight discharge or the gauge will peg when the battery is being charged.
So, perhaps the better thing to do is to go to 2 aftermarket gauges, an oil pressure gauge and a temp gauge, and then use the Rocketman voltmeter I have as a replacement for the ammeter. I could use the little plastic insert I made for the pocket in the dash, as shown here: Gauge Pocket - ???Gary's Garagemahal.
Not a stupid question at all. However, this would be more complex than really needed.
Ask yourself why you're adding a voltmeter in the cab. Is it for precision calibration of the charging system, comparable to hooking up a $300 Fluke directly at the battery? I doubt it. You're doing this to monitor for problems while driving, right? So what if the all-important reading at battery is slightly different compared to the gauge in the cab? Just learn the difference, and keep that in mind.
That's how the aftermarket voltmeter behaves in my truck. With 13.5V directly at the battery, a perfectly good value, my voltmeter might only show 12.75V because it's so far downstream, but I know that's good. If my actual battery voltage drops to 12V while driving, indicating a charging system failure, my dash gauge might show 11.25V and that will still catch my attention.
If I really wanted to get fancy, I could change the numbers on the gauge. Instead, I'm lazy, and just know what is normal and what is not, even if not entirely accurate compared to what's at the battery. Remember, although laziness often gets a bad rap, it always pays off right away.
True. But, if I use the Rocketman voltmeter as an ammeter replacement there are no numbers on the face. So I really need the reading to be consistent if I'm to detect a change. Seems to me the easiest way to wire it then is to use the existing high-side wire as the positive and take the low side to ground. And in order to not have a draw when the key is off just put a relay in, as discussed. In other words, trade running wires to a less unstable spot under the dash for wiring up a relay.
Or, to put it another way, I've rarely been accused of taking the easy way out, so why start now?
