VCCA Home Forums GENERAL DISCUSSION New User Forum Howdy from Oregon - Gauges/instrument panel

Testing 1964 C-30 CarryAll OEM dash gauge for "battery", I say it like that only because I understood it to be an Ammeter, however I've seen it contradicted eluding that it reads and displays Voltage. Either way my Gauge isn't functioning. I've got the fuel and oil pressure ones working and I'm stuck with this now. I'm not sure if I understand how to troubleshoot an Ammeter or it's circuit to eliminate possibilities?
(The guy whom owned it before me thought he was a mechanic which is twice as bad as someone that knows they're not working on her!) She's been Jerry rigged including wiring so I'm not sure what that circuit looks like?
Any help it'd be appreciated!?

Well..... It is a voltmeter that measures amps.....

Voltmeters, as you probably know measure voltage, and are connected from positive to negative. They have high internal resistance and draw as little current as possible.

Ammeters on the other hand measure flow, and are placed in the wire carrying the current. They are as close to a dead short as possible because all the current they are measuring goes through them. They are connected with big wires to handle all the current. Imagine a water meter in a water pipe.

Most cars that have real ammeters (Chrysler products for instance) have a big wire running from the charging system (probably an alternator) THROUGH the ammeter, and then back out to the battery.

But, did you know that there is another way? And that you can measure amps with a voltmeter? For example, what if I put a resistor (instead of an ammeter) in the wire between the charging system (probably an alternator) and the battery? If I measure the VOLTAGE across that resistor, it can be converted to amps using Ohms Law. Volts = Amps x Ohms. It is always true.

For this example, I am going to pick a 1 ohm resistor because it makes for easy math. If I measure 10 volts across the resistor, there is 10 amps flowing through the resistor. 10 Volts = 10 Amps x 1 Ohm.

So, what Chevrolet did in the 60s was put a resistor in the charge circuit. The resistor can also be known as a "shunt" when it is part of an ammeter system. The resistor (shunt) probably has much lower resistance than the 1 ohm in my easy math example. In fact it is just a piece of metal. It is on the horn relay.

You will notice the big fat wire coming from the charging system (probably an alternator) going to the horn relay. Was the charging system an alternator yet in 64? If it was a generator, then the fat wire starts at the "bat" terminal on the voltage regulator and goes to the horn relay. The long piece of metal on the side of the horn relay is the shunt. At the opposite end of the shunt another fat wire connects. This one goes to the battery.

Two small wires, connected to opposite ends of the shunt go into the cab to the dash gauge. The dash gauge is just a voltmeter with zero at the center. It is not measuring battery voltage. It is measuring the voltage across the shunt. Since it is a voltmeter, it does not need or want big wires.

Since the voltage across the shunt represents Amps, the reading on the dash gauge represents Amps. Since the gauge has zero in the center, you can see current (Amps) through the shunt in either direction. One direction is charge, the other is discharge.

It looks exactly like a traditional ammeter to the driver.

Dexstar welcome to Chatter
All the Chevs I have seen have a ammeter on the dash though I am not sure if there is a shunt system in the later models but up to my 38 the shunt is not needed. I have not attempted to repair a ammeter so will not attempt that and lead you in the wrong direction.
Tony

That actually was a great explanation of how it all works. It may rake a few more steps to enable dexstar64 a successful troubleshoot. So 1964 as far as I know was the second year of Chevrolet using alternators. The original were what are called two wire alternators and I think there were two models. One had an "idiot light" in the dash and the other used an ammeter like bloo described. Now in the last 57 years I can pretty well guarantee someone has replaced that alternator at least once. What they replaced it with is anyone's guess. In my opinion that is the place to start. Figure out what you have then determine what you need and go from there. If you need a little help contact Philbin Electronics there in Portland. I think his name is Tony and he is just up off of Vancouver Ave. and about Knott St. Nice fellow who seems to love a challenge.

Keep us posted as to your progress. What you find and do may help someone else sort through their problem.
Art

I agree with Bloo that the “ammeters” in those 60’s trucks were really voltmeters that read current flow in both directions. Sort of an odd duck.

The key is that it does not function like the ammeters used in our older Chevy’s. Full current does not pass through the instrument.

Thank you very much! That was a perfect way to explain that. I understand completely now. It has 2 poles or posts on it for amps coming in and going out. And not a POS/NEG connections which also explains why my VOM reads 14.7 v on my (yes it's an ) Alternator externally regulated. So if I run a hot batt to it then out of it do I need to have a draw on the outgoing to make the gauge fiction properly ? And or circuit ?

Thanks, I appreciate that. This is a great forum

Ok, Art will do my Alternator has two small wires coming from it's connector which I believe are field and coil. Then there is the POS. Charge return and finally a Gnd.

Hello Dexstar64, we are in Sante Fe this week and I do not have access to any of my antiquated reference sources, ie books. Anything I might be able to tell you other than start with a wiring diagram would probably be fuzzy at best. It has been quite awhile since I have had a mid 60s car with an externally regulated alternator. I think they went to a one wire internally regulated alternator in the early 70s.

There is a section over on "The Hamb", or Jalopy Journal that has some good troubleshooting for older externally alternators. It had a diagram but I couldn't get it pasted.

Thanks Beachbum, if you get back and feel like posting or sending me something feel free. I ordered the factory Manuel (1963 truck man 2 parts) and a used 64 supplemental however in my haste of trying to shop smart and cheap I overlooked the supplemental was for cars.

Ok everyone,
My methods may seem a little odd or you can give them your own colorful name. Having an old cheap RadioShack 12v power supply(quick background....trying to get my dash gauges/instrument cluster functioning the way it's supposed to and get rid of the cheap gauge set that came with her and is screwed under my dash and in the way of my propeller shaft brake actuator. I have a spare dash I swapped the speedo out of and tried both ammeters plugged in to the existing socket and neither worked so with some help from you guys....bloo and Beachbum and others I understood better the difference between voltmeter and ammeter. Now back to my tests...) I hooked the power supply up to a flasher with one of the ammeters in series and had a 0-first mark discharge we can call that 5-10 amp discharge then back to center being 0 as the springsteele inside caused the open circuit. So I then swapped my poles on the back of my ammeter resulting in an equally 0 to charge sweep. I believe this proves I have a functioning ammeter. So now I am going to chase wires and figure out where and why those wires aren't connected properly, or shunt is bad or missing, or resistor is bad or missing.
I'll try and remember to update when I get her fixed!
Thanks again everyone!

I *think* the wires just connect to each end of the shunt. Thats how I remember it. I cant prove it. I have a 66 c10, and it has the shunt because all these trucks use the same horn relay as far as I know. The trouble is, mine was built with an idiot light, so the wires going to the dash are not there. I could get a picture of the horn relay if it would help (let me know) but it would really be a lot better if someone could post a picture of the horn relay on a truck that has an ammeter, so you could see the 2 small wires connected.

Dexstar by your last post I would think you have at least 1 working ammeter with a needle deflection when the lights are lit. But like bloo I dont know the wiring under the dash to advise which wire to connect to where.
Tony

Here's a picture from the web of a Chevrolet horn relay. That piece at the upper right with the 2 screws is the shunt. It is shorter than I remember it, but it works like in my post above.



Here is a diagram from the web. The wire from the battery is a big wire, and connects to one of the screws. The big wire from the alternator positive post goes to the other side. They have the other side marked "rest of car" in the diagram because any wires feeding power to other electrical stuff in the truck also connect to the alternator side. "Rest of car" includes the alternator positive post.



The wires going into the truck for the "ammeter" gauge are smaller, even though they are drawn the same size on that diagram. One goes to one side ot the shunt, and the other one goes to the other side. If the gauge works backwards, swap the gauge wires.

If the truck never had a gauge, the wiring harness is probably completely different. It is completely different on idiot light trucks for sure in 1966, and probably on your 1964 too. There were only so many pins in the connectors and Chevrolet reused the pins for other things. This could get really goofy if the dash harness was from an idiot light truck and the engine harness was from an ammeter truck, or vice versa.

SAFETY NOTE: If by some chance the wires for the ammeter have been added, or you need to add them, or consider adding fuses to them. Chevrolet did this themselves on some vehicles but I do not remember which ones. Here is why: looking at the diagram again, remember that the wires coming in the cab from the shunt are small because they do not carry much current. They are connected directly to the battery and alternator! If one of them got pinched and shorted to ground, it would catch on fire dramatically with the full force of the battery's available current, and those wires are hot 24/7. Some 2, 3, or 5 amp fuses would be fine. The fuses go at the horn relay end of the small wires feeding the "ammeter".

Bloo,
Sorry been busier than a 2 peckered puppy. I have a working Ammeter, temp gauge, fuel tank level gauge, oil pressure gauge, and finally a speedo that works too. The black wire from the batt. Terminal of the starter that goes to the engine harness had been removed from her entirely so it was the other leg of that circuit that was missing. I love having a fully functional accessory dash. I left the mechanical temp gauge hooked up for now until I match the two up some and cross checked with an I.R. point and shoot. I don't trust it 100% yet. Now I gotta find a factory Tach from 64 to put in there and I'll be happy.
Thanks guys for the help.....it was your insight that led me to the path where I found my answers.

That's great, sounds like you found the problem. About the tach, talk to Bruce Abbot with Abbot Electronics to see if he has one.

Art

Glad you got it sorted out!