Need Help: Wiring stock battery gauge to one wire alternator.

[Mick Je]

I recently got a 100 amp one wire alternator for my 70 GMC and I want the stock battery gauge to work. I know these gauges are not true amp gauges, they are called "Differential Amp Gauges". I've looked around at old threads and can't find how to wire these gauges up to a one wire alternator.

[Wrenchbender Ret]

There shouldn't be any difference in the wireing for the gauge then before.

[Mick Je]

But the gauge doesn't work, there's just the battery hot wire on the alternator. Do I have to run a separate wire for the gauge?

[leddzepp]

Moved to electrical. Hopefully vettevette will see this and respond. Your gauge will not work as you discovered. I have never found a way to do it either with a one wire alternator.

[Mick Je]

I was gonna call the place were I got this alternator and see if they knew.

[Wrenchbender Ret]

There are 2 fuses in the harness in the engine compartment. They are in small rubber holders that look like connecters. Check each one. They use 4 amp fuses. With a test light there should be current on both sides of the fuse holders. This is usually the cause of this gauge from not working. Sometimes the fuse holder is corroded. You can replace it with an universal fuse holder. You can put a 6 amp fuse in them if the higher amp alt is blowing the fuses.

[ray_mcavoy]

^ I agree ... the fuses would be the first thing to check if your battery gauge is not working.

However, I don't think I'd use 6 amp fuses. It only takes about 1.5 to 2 amps flowing through the meter movement to peg it full scale. A better way to allow the stock battery gauge to handle higher currents would be to use a heavier gauge "shunt" wire in the charging system.

And the 1-wire alternator shouldn't make any difference so long as the output wire is connected to the charging system using the stock configuration. If the output wire of the new alternator is connected directly to the battery, it will bypass the "shunt" wire and the battery gauge will no longer indicate charging.

[VetteVet]

Here I am after a busy weekend I'll have to go with Wrenchbender and say that the gauge should work if it is wired correctly.
Question is , did it work before the one-wire. If it worked before then you have wired the alternator differently.

The biggest mistake the one-wire guys do is run the output wire directly to the battery positive post, or to the starter, because that's what the guys that sell them say to do.

The correct way to do it is to run the output wire to the same place that the original output wire went to. It's best to use a bigger gauge wire than 12 gauge in order to handle bigger electrical loads on the system.

The one-wires also don't compensate for voltage drop downstream from the alternator because they don't have the sensor wire to tell the regulator what the voltage is away from the alternator so they just put out the current based on what the voltage is right at the alternator.

I'll leave it here for now until we find out how the alternator is wired.

[cleszkie]

Reviving an old thread to get more answers -

So I too have a high output one-wire alternator, and I want my alt gauge to work again (I have a volt gauge that works, but just want all my stuff to function). I ran the output straight to the battery, and just set the old battery wire, along with the other two wires in the plug, safely aside in case I needed them again in the future. After reading this thread, I thought that if I took the old battery wire and reconnected it to the one-wire battery post (keeping the direct line to the battery attached), that I could bring the alt gauge back to life. But no such luck. I guess the electricity takes the path of least resistance and flows through the 4 ga direct line to the battery, rather than through the original 12 ga line to the "shunt". Am I correct in assuming that If I replaced ALL of the original battery line in the original wiring harness with 4 ga wire and ditched the direct to battery wire, that the alt gauge should function?

[VetteVet]

It's like Ray and WB said, you have to run the alternator output wire to the main junction of the power wires.
What the gauge does is compare the alternator output to the battery state of charge. The battery side of the gauge is connected to the positive battery terminal with the fusible link at the right fender junction.Then the other wire that is on the fender junction runs to the main power junction in the harness. This wire is called the SHUNT, it also connects the alternator output wire with the battery. It also carries battery voltage to the truck circuit when the key is off and the engine isn't running.

The alternator side wire of the gauge is connected to the main junction in the harness with the shunt and the voltage sensing wire. Then a fourth wire carries both of these power sources to the cab for truck power.

A good way to picture it is to imagine that your body is the ammeter and with your right arm straight forward your right hand will be the right fender junction where the battery connects.

Then with your left arm straight forward, your left hand is the main harness junction where the alternator output wire connects.

Now if you connect the tips of your index fingers, the become the SHUNT to connect the battery and the alternator. Your arms are the connections to the gauge and they measure the voltage differential between the battery and the alternator and by being connected to the battery gauge, it can show charge or discharge depending on whether the alternator is charging or not.

Here's a diagram to show all this.

[gmachinz]

What is the reason for a 100-amp alternator upgrade? If youre running a lot of interior electrical options Id be highly suspicious of relying on a single 12ga batt feed into the cab via the stock firewall junction-I hope you are upgrading your supply feeds inside as well as outside the truck-Id relocate your common 12V junction to the center of the firewall as a better source to land all of your hot leads. Having said that, a 3-wire GM alternator (CS series for example) is a far better solution than any 1-wire alternator.

[cleszkie]

VetteVet - Thank you very much for the info.

gmachinz - I am running a 320 amp one wire. 180 amps at idle. 100 amps was not enough for my purposes. Currently, I run 4 ga from alternator to battery, and 2 ga from batteries to the inverter. None of the original 12 ga is in use. I have fuel injection, fuel pump, A/C, and dual cooling fans to run. Plus I have a dual battery system and a 2000W inverter, and a 5,000 BTU window mounted A/C in my camper top. The alternator will provide enough power through the inverter to run the 5000 BTU A/C, while still charging the dual batteries, and running all the other electrical in the Blazer. This doesn't even consider my winch. So yeah, I've tried a 100 amp alternator, and it was not enough.

[gmachinz]

Originally Posted by cleszkie

VetteVet - Thank you very much for the info.

gmachinz - I am running a 320 amp one wire. 180 amps at idle. 100 amps was not enough for my purposes. Currently, I run 4 ga from alternator to battery, and 2 ga from batteries to the inverter. None of the original 12 ga is in use. I have fuel injection, fuel pump, A/C, and dual cooling fans to run. Plus I have a dual battery system and a 2000W inverter, and a 5,000 BTU window mounted A/C in my camper top. The alternator will provide enough power through the inverter to run the 5000 BTU A/C, while still charging the dual batteries, and running all the other electrical in the Blazer. This doesn't even consider my winch. So yeah, I've tried a 100 amp alternator, and it was not enough.

Wow!! You need a damn sub station!!

[gmachinz]

I assume youre using deep cycle marine batteries?

[franken]

The stock battery gauge measures the difference between 2 voltage sources. One closer to the battery, and one closer to the alternator. If the alt side V is greater than the battery side, the gauge shows charging.
Regardless of the rating of the alt, it delivers what the load requires. If the load requires more than the alt rating, voltage starts to decrease.

[cleszkie]

Originally Posted by gmachinz

I assume youre using deep cycle marine batteries?

I am running two deep cycle AGM batteries (red - primary, and yellow - auxiliary Optimas). I have an isolator system so I can use and charge both primary and auxiliary batteries in parallel, or I can separate the batteries such that I use and charge only the primary battery.

[cleszkie]

Originally Posted by gmachinz

Wow!! You need a damn sub station!!

I doubt if I ever draw 320 amps. So the alternator is pretty much coasting along most of the time. But I like knowing that I have the capacity. I am also going to be installing an on-board welding system that the alternator will be supporting.

[cleszkie]

In case anyone is interested, I have gotten my alt gauge to function with my one-wire alternator. Attached is the modified wiring diagram (thanks Vetvett for the original diagram) . Note that I have removed all of the original wiring associated with the external voltage regulator, and the original three-wire set-up. I ran a Red 4 ga wire directly from the alternator to the battery. I did leave the original Red 12 ga wire from the fender junction block to the firewall harness to provide power to the cab wiring harness. I basically just removed the Black/White 18 ga wire from the original wiring junction and ran it directly to the alternator. This way, the Alt gauge gets the battery voltage reading from the Black 18 ga wire on one side, and the alternator voltage reading from the Black/White 18 ga wire on the other side. The Alt gauge now registers the relative difference in voltages with the needle slightly to the charge side when the truck is running.

[VetteVet]

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Originally Posted by cleszkie

In case anyone is interested, I have gotten my alt gauge to function with my one-wire alternator. Attached is the modified wiring diagram (thanks Vetvett for the original diagram) . Note that I have removed all of the original wiring associated with the external voltage regulator, and the original three-wire set-up.

I ran a Red 4 ga wire directly from the alternator to the battery. I did leave the original Red 12 ga wire from the fender junction block to the firewall harness to provide power to the cab wiring harness.

so what you are saying is that you are charging the battery with a 4 gauge wire and feeding the rest of the truck with a little 12 gauge wire. I think I would have done it the other way around. As long as you don't add a lot of amp draw loads it will be OK but if you add electrical fans or HD lights that little wire won't cut it.

]I basically just removed the Black/White 18 ga wire from the original wiring junction and ran it directly to the alternator. This way, the Alt gauge gets the battery voltage reading from the Black 18 ga wire on one side, and the alternator voltage reading from the Black/White 18 ga wire on the other side.

What you have done is to change the SHUNT wire from the 12 gauge red wire that runs across the front of the radiator to the main junction, to the 4 gauge wire that runs between the alternator and the battery. while it will still indicate charge, it won't be very accurate, not that they were very accurate anyway.

\
The Alt gauge now registers the relative difference in voltages with the needle slightly to the charge side when the truck is running.

[cleszkie]

Originally Posted by VetteVet

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The 12 ga wire to the firewall harness is only used to power stock low draw items (cab/dash lighting, windshield wipers, cig lighter, etc.). All the high draw items (A/C, fuel injection, fuel pump, cooling fans, headlights, winch, inverter, camper power, etc.) have dedicated, appropriate gauge power lines directly to the batteries. I agree that if I attempted to power these items through the 12 ga line, I would have a flaming mess in no time

I understand that the gauge is not accurate. But as you stated, it never really was. All I want it to tell me is that the alternator is working, and that the batteries are taking charge properly with the needle slightly to the charged side. Basically, I didn't want a dead gauge in my cluster, and I was able to clean out some unneeded wiring from my engine bay.

[VetteVet]

OK I feel better now. I had lost track of the rest of your post and forgot you had all the rest. VV

[cleszkie]

Vettvet - Just wanted to close this thread by thanking you for ALL of the electrical system knowledge that you provide on this board. I've lost count of all of the diagrams and discussions of yours that I have either used to improve my projects, or saved in my files for later use. Your knowledge is truly appreciated.

[Daze57]

^ x 2

[VetteVet]

Thanks guys , it feels good to be appreciated.