ammeter wiring question ???

[brawley]

I have a 1968 c10, with factory gauges, It had a new 1 wire alternator installed when I bought it. I am in the process of cleaning up the engine compartment and wiring, any info on how to hook up the factory ammeter using the factory wiring and the 1 wire alt.. thanks

[70LonghornCST]

I moved your post to the Electrical section to get some specialized help. I'm sure some of our electrical experts will be along shortly to help you out. Good luck!

[franken]

Jus put the inputs to the factory gauge about where they were before the new alternator.

Notice that the factory labeled it a battery gauge, not an ammeter...

The factory gauge inputs are 12V near the battery and 12V near the alternator. The meter moves depending on the difference (think subtraction) between those two voltages. Thus it's a voltmeter, but measures just a few tenths of a Volt.

If the alternator is working, the voltage at the alternator is a hair higher than at the battery, and the gauge reads charge. When the alt isn't charging the voltage is higher at the battery and the gauge reads discharge.

No need to believe me. You can check the GM wiring diagram and Ohm's law. The wiring diagram will show the inputs to the gauge, and Ohms law says how to measure current.

You can read a lot of babble about a shunt Ammeter and calibrated wires (for an uncalibrated gauge), but this is the simple and true explanation. Sadly, there's a lot of mythology passed as truth on forums (and everywhere else). Always check multiple sources due to the BS out there.

Rant mode on...
The other one I love is 18 gauge sheetmetal in car bodies. Right. Maybe in a core support or unibody pseudo-rear frame part, but not a body. Bodies are 21-22 gauge metal. HF has a $10 caliper and the difference is obvious just looking at the metal.
Rant mode off...

[ray_mcavoy]

Originally Posted by franken

The factory gauge inputs are 12V near the battery and 12V near the alternator. The meter moves depending on the difference (think subtraction) between those two voltages. Thus it's a voltmeter, but measures just a few tenths of a Volt.

You can read babble about a shunt Ammeter and calibrated wires (for an uncalibrated gauge), but this is the simple and true explanation.

Yes, factory meter is measuring the voltage drop across the wiring that connects the battery to the alternator. That voltage difference causes a small amount of current (close to 2 amps at full scale) to flow through the meter's internal coil. So it can be thought of as a current divider circuit as well ... the majority of current flows through the wire (which is a shunt) and a small fraction through the meter.

But no matter how it's explained, the key to making the factory battery gauge work with a non-stock alternator is the location of the new alternator's output wire. The alternator's output wire needs to connect into the wiring harness at the stock location. For 67-72 trucks, that is a factory splice in the wiring harness near the old voltage regulator location. Search one of the many posts on factory wiring by VetteVet ... he has posted the wiring diagrams and a photo of this splice many times.

Running the alternator's output wire anywhere else (like directly to the battery as is often done) will alter the circuit and prevent the factory battery gauge from correctly indicating the charge or discharge current.

[VetteVet]

Try this one:
.........http://67-72chevytrucks.com/vboard/s...d.php?t=581719

[brawley]

From looking at the factory wiring diagrams the previous owner just used the power wire and ground wire to the new alternator and removed the regulator and its wiring. I see the dark brown wire- that looks to me goes to the ammeter gauge? right now it is wired into the HEI ? but im going to run a new wire from the HEI into where the white resistor wire was for power to the HEI-sound right? now what to do with the brown wire that used to go to the regulator? will my ammeter gauge work if this is not hooked up? all other factory wiring is still intact

[brawley]

I found plug built into the alternator after doing a little research found that I could use the R spade to connect to ammeter- brown wire. hope this works out.

[ray_mcavoy]

That brown wire sounds like it was probably the original alternator field exciter wire. It leads back to the "ACC" terminal on the ignition switch and would have originally gone to the #4 terminal of the old voltage regulator. It's not part of the ammeter circuit ... the factory ammeter wiring used 2 black wires, one of which has a white stripe.

With a "1-wire" alternator, you don't really need that brown exciter wire. Many "1-wire" alternators can still be hooked up in a "3-wire" configuration as you've discovered in your research. The exciter wire should go to the terminal marked #1. The other terminal (#2) is for remote voltage sensing and you can hook that up too if you want all the benefits of a true 3-wire alternator.

[brawley]

thanks for the response! I havent finished yet, hopefully it will work out. I have a question- on the HEI pigtail there is a red, yellow and black wire the runs from the base of dist. and plugs up under the cap. does it matter what direction its plugged into cap - says C- and B+ ??

[ray_mcavoy]

Originally Posted by brawley

thanks for the response! I havent finished yet, hopefully it will work out. I have a question- on the HEI pigtail there is a red, yellow and black wire the runs from the base of dist. and plugs up under the cap. does it matter what direction its plugged into cap - says C- and B+ ??

You're welcome!

Yes, the direction of that 3-wire pigtail that comes from the distributor base and plugs into the HEI cap is important. The original connector should be keyed so it will only plug in one way. But if things have been changed, the red, yellow, and black color codes should match up with the color codes on the coil wires they plug into (you can see those by removing the plastic dust cover from the top of the coil). If I remember correctly, red is B+, yellow is C-, and black is a ground that is in the middle.

[brawley]

thanks buddy !