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Discussion Starter #1
Hey all, I previously asked about what brand wiring harness to go with. And after talking to you and the guys at American Auto Wire, I have selected to go with their classic update harness. My problem is that it doesn't support an ammeter. What should I do? I have 67 RS with console gauges and oil pressure and ammeter share the same gauge window. If you are not familiar with the gauge package there are 3 windows and 4 gauges. The outside windows have fuel level and ECT in one and Oil press and amps in the other while the center is the clock. Any ideas? Anyone tried fitting aftermarket gauges in the factory bezel?


 

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You can put in your own wires. It's just two 18awg wires and the ground for the gauge (you can tap into the lights and ground). The positive goes to the horn bus bar and the negative goes to the battery positive or battery positive junction behind the battery ('69 anyway).

This of course requires the system to be wired as original with the alternator output going to the horn bus bar and a charge wire going from the bus bar to the battery. The ammeter is really a volt meter that reads the potential between alt output and battery using the charge wire as a shunt to indicate if the batter is higher (gauge shows discharge) or alt output is higher (gauge shows charge).

I don't know why the harness wouldn't support the factory configuration unless they understand "does it support an ammeter?" as meaning a true ammeter with full system current run through it.
 

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I don't know why the harness wouldn't support the factory configuration unless they understand "does it support an ammeter?" as meaning a true ammeter with full system current run through it.
That was my first thought as well. My hat's off to you for your explaination on how the factory guage works, it's the plainest one I've read.

Jeff
 

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Discussion Starter #4
Hey thanks Steiner for the explanation on the circuit. Now considering with this new harness will have all power distribution happening at the start, yes...batter, alt, and power feeds to fuse block, how would this get hooked up?...From starter lead to battery terminal?...I believe the tell you they dont support ammeters because of the amount of current most cars now are using with electric fans and what not. I don't really know what I'm gunna do here. There is a guy in Missouri that says he may be able to rewire my factory ammeter to a voltage gauge but he needs to see it first.
 

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The factory gauge is a voltage gauge. It does not measure current but instead reads the voltage drop across the charging wire to give you a representation of your charging system state. In a setup like this the meter is calibrated using the ohmic value of the shunt (charge wire in this case) to display the calculated amp draw based on the assumed voltage supply value (battery and alternator). Essentially it's alt voltage minus battery voltage, divided by ohmic value of the shunt being displayed BUT it is a voltage value being read. As long as alt voltage is higher the needle will be positive, if battery voltage is higher than alt voltage it'll go negative, if the two are equal it'll read 0. It doesn't matter how much current the car consumes because the meter is only measuring the voltage difference between the battery and alternator which is going to be a couple of volts max.

A true ammeter would be installed in the main power feed. Basically the feed would be cut in two, one leg would go into the meter, and the other leg would go out to the car. Obviously our meters aren't like that.

Are you going to use an external regulator like original or a newer internal regulator alternator?

Factory setup was:
Alternator output to horn bus bar (also main fuse box feed there).
Charge wire from bus bar to battery.
Ammeter wires connected on each side of the charge wire.
Starter cable from battery.


If yours is not going to be wired from alt to starter to battery, you'd put one line from the gauge on the alternator output and the other on the battery positive. It'll still indicate charge or discharge but won't be as accurate as the factory intended. You may want to fuse one leg of the meter. With a digital meter or something like that the impedance is so high that there is no current consumed by the meter. I don't know the design of the factory meter so if you lost a connection on one side of the charge wire you'd want the fuse to pop if it were to try to supply the car through the meter.
 

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Discussion Starter #6
The new harness also doesn't have wiring for external regulator so I will have to switch to a 1 wire alt. What you are saying makes perfect sense now. I was thinking it was more of a voltage drop gauge than ammeter but wasn't 100% on that. I will have to do some experimenting. Wonder if I only connect gauge to one hot lead (ignition key on feed) if I can get the gauge to move to on side or the other. Like actually make a volt meter where the middle would be 0 and maybe half way to C would be 12 volts...I would probably need to wire in a resister for that as to not overwhelm the gauge with a significant voltage difference?...I dont know...whats ur opinion on this? I'd like to make this a real voltage gauge rather than voltage drop and keep my guage cluster the same.
 

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Just consider your 0 point on the gauge to be 12.6 volts which is battery voltage. It should almost always read above 0 with a good charging system. If it reads over 0 then system voltage is higher than battery voltage which means alt is outputting fine. You can hook the wires wherever you want.....one to the ACC or IGN spade on the fuse block and one to the battery would work but again, the gauge should have been calibrated for the stock system voltages and charge wire values (I assume) so I don't know what the gauge will show on the positive side.
 

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The OE ammeter is a millivoltmeter and indicates the millivolt difference from each end of the wire from the horn relay buss bar to the battery junction block.
The polarity of the voltage when current flows determines whether the gauge shows a discharge or charge.
If there is no current draw in either direction, the ammeter reads 0, middle scale.
Turn on ignition and should show a discharge with alternator off, not running.
If you connect one side of the ammeter to power feed at the fuse panel more action will be shown on the OE ammeter because of added length of wire, length being the wire from the horn relay buss bar to fuse panel along with the firewall connector.

Whether or not you have a one-wire alt, three-wire alt internal reg'd, or OE ext reg'd alt, the OE ammeter is a separate circuit unto itself.
A separate voltmeter can be added and connected to the IGN spade on the fuse panel and a good ground, your choice.
 
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