Amp Meter & Accessory Wiring

[Racer Z]

How do I add new accessories and have them work correctly with the amp meter?

I added electric fans (radiator) a while back. Tapped from the battery side of the starter relay, using their own circuit. Two fans, two fuses, two relays, one switch and new wires. Everything works fine.

My amp meter never worked so I didn't know there was an issue with how it measured the fan draw. I finally fixed the amp meter and it seems to work normal, except when I turn on the fans. Then it reads full charge to the battery while the engine is running. If the engine is off and the ignition is on or off, the draw of the fans is not shown.

Where can I get 30+ amps from without going through the stock fuse box or ignition switch and have the amp meter function correctly?

Thanks....

[fordman]

the gauge measured the current between two points. the point you have your fans hooked to is one of those points. so you may need to move the wire to the actual positive battery terminal end to get the reading right.

[Thunderfoot]

So when the engine is Off and the fans are On, the Amp meter doesn't show a negative draw on the gauge?

[Racer Z]

So when the engine is Off and the fans are On, the Amp meter doesn't show a negative draw on the gauge?

Correct.

If the ignition is on, it will show a discharge if I turn on the headlights. The fans don't register at all.

[Thunderfoot]

Ok that would be correct from the way the Amp meter works using the "Shunt System" (only measuring the current flow in a small section of wire) and where you have the load connected.
The way this Shunt setup works it is more designed to show that the battery is being charged and the alternator is working then it is to show all the current being used in the system.

See the Diagram Below;
This style of Amp meter is just a Differential Voltage Meter, it is reading the difference in the voltage at the two ends of the short piece of wire to the battery, and shows the reading on the gauge.

(Note): for the explanation below Positive Measurement refers to the Amp Gauge showing a Charge and Negative Measurement refers to the Amp Gauge showing a Dis-Charge.

Definition of wires in diagram for reference;
Wire section being used for current measurement = wire 38A (this is the small section of wire being used to measure current flow to/from the battery it is not showing the current flow to the truck when running)
Negative measurement for amp gauge = wire 655
Positive measurement for amp gauge = wire 654
Main power wire to the cab is wire 37 and you see it is not connected directly to the starter solenoid but in the middle of the power wire to the alternator at the same point as the Positive measurement point, (a truck with the Alt Light has this wire (37) hooked directly to the starter solenoid).

Where you have your fan power connected at the starter solenoid, you are connected to the Negative Measurement side of the Amp meter so it will not read in the negative side as there is no current flowing past the Positive Measurement point for a differential when the engine is off, is not flowing past this point except when the alternator is working.
Also you are getting an bit of an exaggerated positive reading on your gauge when running because of this, because the Amp gauge is designed to read the current flow on that Short part of the wire to the battery to show if the battery is being charged or not. By adding an extra load at this Positive Measurement point it looks to that Amp Gauge that the battery is taking a larger load to charge it (the battery) up when in reality the load is your fans, if your battery was to go bad you wouldn't see it as easy because of the fan load masking it. (most people wouldn't pay that much attention to the amp gauge to notice this)
(NOTE)This is also why when you are starting the engine that the gauge doesn't peg on the discharge side because in is not connected in the Shunt area of the circuit and is getting its power directly from the battery.

You would have to hook it up at the point on the Positive Measurement side of the shunt wire to get an accurate reading of this charge wire to the battery.

The way this Shunt setup works it is more designed to show that the battery is being charged and the alternator is working then it is to show all the current being used in the system.

I could go into more details but I think it would just confuse more then help, may have already done that... but I will gladly try to answer any specific questions or details on this.

[Racer Z]

Ok. The diagram (schematic) really helps.

My first observation is that I have the wrong fuses for the ammeter circuit. I used one amp fuses and they didn't pop. I see it calls for four amp fuses.

It looks like I need to move my connection over to the alternator output. My one wire (10 gauge) feeds a fuse block on the firewall that has unused fuse holders for future accessories.

38A would be the Shunt Wire, correct? What I don't understand is how the ammeter reads the flow. I understand that it measures the electric flow to and from the battery. But all I see is a bunch of wires running back-and-forth in meaningless fashion. In my simple mind, it should be three wires. One from the alternator to the ammeter, one from the battery to the ammeter, and one from the alternator side of the ammeter to feed the entire truck.

If you want to tell me I don't understand electricity much, I will say, "You are right, I don't".

[Thunderfoot]

38A is part of the main power wire to the battery from the alternator it is just the section of wire that is being monitored by the Amp meter. The shunt wire(s) are 655 and 654, as they are what is measuring the current by not being part of it (shunting around it).

These two wires (654&655) are separate sense wires (voltage meter wires measuring the voltage at their connection points), then the Amp Gauge (aka differential voltage meter here) shows which end has more or less voltage on it. This is done because a length of wire has resistance in it and you will have more voltage closer to the power source then the load being used, so when the alternator is running its output (13.5-14V) it is supplying more voltage then the battery at 12.5 or so... the Amp Gauge will show a charge (not much because a good battery wont have much of a load to charge it). You'll get the opposite results when the alternator is off and you have the headlights on as the battery has all the power and the headlights are the load.

Unfortunately this whole design is not a good one as it is only measuring the current flow of that short section of wire (38A) it is not measuring the alternators total output as it splits out on to the main power wire to the cab (37), which isn't being measured in that short section of 38A... The only time it really measures the total current load of the truck is when the engine is off and it is getting the power from the battery through wire 38A. This all makes it so really the only good thing the stock Amp Gauge is good for is showing that the alternator is working and charging the battery, or if you have a drain with the engine off.

A side note: this is also why these Amp Gauges don't show hardly any movement normally, because when you have a good battery that doesn't need much charge from the alternator there is no current flow there to show...

Personally I would leave your wiring like you have it, as even if you move it to the other end it will only read accurate when the engine is off...

On the fuses the only reason they are there is for short protection (like any fuse ) but it keeps the power from trying to go through them if the fusible link in that 38A wire were to blow. 4amp does give you a little more room for any surges it might see and would be a good idea.

This is all very good information that I have been wanting to put together, as there have been question on the Amp Gauges and why they don't work well, so hopefully this will help with those questions as well.

[knightfire83]

This site looks promising, the guy will retrofit your gauge to read volts instead for $30. It's the forth product down on the list.

http://www.rccinnovations.com/Volts.php

[dcbullet]

This site looks promising, the guy will retrofit your gauge to read volts instead for $30. It's the forth product down on the list.

http://www.rccinnovations.com/Volts.php

That is slick! Thanks, I need to have him do that.

[Racer Z]

I fixed it. I moved the wire from the solenoid to the alternator. Now it registers the current of the fans in the same fashion as the headlights and other stuff. I need to lengthen my lead still. It's a bit short, but will be OK for a few weeks.

Thanks Thunderfoot.

Even though I don't really understand how the shunt and the wiring works, I understand what the amp meter (ammeter) is telling me.
When everything is working and the battery is fully charged, it should read zero. Zero because the alternator is supplying all the current needed to the system.
If the alternator should fail, then the battery would begin discharging to power the system and the ammeter would show a discharge.
If the battery fails in such a way that it begins drawing too much (and that can happen), then the ammeter will show a charge to the battery.
Normally, the battery discharges while cranking the motor, then charges up quickly. The ammeter will show this charge and quickly (or slowly) return back to zero. Turn on the headlights and we see the needle flick as the battery supplies the initial burst followed by the alternator/regulator catching back up. Some vehicles are sensitive enough to show the pulse of the flashers while some are not.

[Thunderfoot]

Yep you got it figured out on how it tells you what is going on with the battery, and hooking it up like you did now will also show when you put more load on the alternator then it can handle, as the Amp Gauge will show a discharge as the battery will be trying to make up the difference.

[bjde0b]

Where should the ammeter be pointing when it is not hooked up to anything (wire harness not installed)? Mine is showing a little higher than neutral to the charge side, is that normal?

[Thunderfoot]

Where should the ammeter be pointing when it is not hooked up to anything (wire harness not installed)? Mine is showing a little higher than neutral to the charge side, is that normal?

With nothing hooked up it should be centered...