Hi All!
In my 1968 C3 I have installed a new 140 amp Tuff Stuff alternator (#7102NPK) that directly replaces the original GM 10DN alternator. I used a large 4 gauge wire to the starter plus a hefty ground wire to the engine. I also installed a matching voltage regulator as well.

At idle, my multimeter shows 14.6 volts but my in-dash battery gauge shows a negative 19 reading. With 4 headlights on, the mulitmeter shows 15.16 volts but the in-dash gauge shows negative 40. I think the multimeter is telling me that everything is alright, but what is up with the battery gauge in the dash?
What is this gauge measuring, anyway?
Is it just not working well with a larger alternator?
Must I replace it with a voltage meter?
Is it safe to drive as-is?

Thanks everyone for your input!

 

If your 68 has the original gauge, then it is a pseudo ammeter and it looks like it's not wired in properly. Those Voltages seem a little high to me also (the 15.16 especially -- see here: https://shop.advanceautoparts.com/r/...-voltage-range ). You may end up ok, but keep an eye on it. Perhaps check it with a different voltmeter just to make sure.
The old external voltage regulators were adjustable? Did you get a new external voltage regulator? Or keep the old one? This new alternator may require a new external voltage regulator.

From your description of the issue it appears that you have the 2 leads to ammeter swapped.

Swap the leads and see what it reads. This "ammeter" displays, in a relative sort of way, the load on the system. It just looks like your 2 leads are swapped. You also want to make sure that your battery is fully charged.

I am curious why you went with an externally regulated 10DN alternator? I ask because you were already modifying and the new CS alternators are much better at idle and easy enough to swap in.

 

Why did I choose this alternator? I contacted Tuff Stuff and asked them what alternator they recommended for my C3. This was the one they advised me to use.
It was an easy plug n play installation, even used the same pigtail connector. All I did extra was to install a larger cable.

Yes, I did buy a new voltage regulator, also the one recommended by the Tuff Stuff tech service rep. It supposedly matches this alternator.

I have never touched the in-dash battery gauge in my car. It worked properly before installing the new alternator, but now it does seem to be functioning backward.

Yes, I did fully charge the battery before running my test.

Does any of this help anyone to zero in on my issue. Any comments on the high voltage reading of 15.16 under load?

I dread taking my dash apart to switch the leads on my in-dash gauge. I will try that, but only if no one else has any ideas...

Thanks for your help!!!

 

If it worked before, and now its incorrect, it has something to do with what you did when you upgraded the charge wire.

 

The Factory AMP meter is not really an AMP meter. It measures the difference in a resistance wire and the parallel wire to make it simple. (not correct but close enough)

By going from the Alternator to the starter you by passed the volt meters wiring. Thus the meter is only showing the load on the circuits not the charge.

Some people switch to a volt meter and use that instead.

 

That makes a ton of sense! From your statement, it should be safe to drive the car, correct?
Is there a way to make the current gauge work correctly or do I just need to go to a voltmeter?

 

The square above the battery is the starter terminal. no fusible link between bat and starter on a corvette.

 

To make the gauge function "correctly" you would have to play with the gauge and length of the "by pass" wires.

There are really nice voltage gauges that almost look stock, I would go with one of them.

I have seen people change the face plate on an aftermarket gauge to the stock gauge face. You know it's not real but have an idea of when it's charging and when it's not. They set the face so 0 is the normal battery voltage. I would rather have a real gauge.
I have done this for friends who wanted an electric oil pressure gauge. ( use and electric C3 oil gauge and sender + original face)

 

Is your new voltage regulator adjustable (I doubt it but the question has to be asked)?

Anyway, your pseudo-ammeter IS hooked up to something....otherwise it would not respond at all. From the readings you describe it just looks like it's monitoring in the opposite direction. If you ran a new lead to the starter all you really need to do is trace where the ammeter leads are now run to.

I posted that link earlier and 15.6 VDC is considered too high.

 

Here is what I think is happening: You went from the alternator to the starter terminal. The original alternator wiring went from the alternator to the junction terminal on the driver side fenderwell. then to the starter terminal. There are two wires between the junction terminal and the starter terminal; one is the main wire, the second is a shunt through the gage. When the battery is discharging, the flow of electrons is from the starter to the junction and the gauge reads negative. When the battery is charging, the flow reverses and the gauge reads positive. By going directly to the starter, current always flows from the starter to the junction What you are actually measuring is only current use The gauge never sees the alternator output. It just goes directly to the battery.

A word of caution. I am not sure why you put in that big alternator but if you are adding power draws, remember that all the power after the starter terminal will still have to go through the 10 or 12 gage wire between the starter and the junction terminal.

 

Looking at blue1972's wiring diagram is your high voltage caused by where you are taking the feedback to the voltage regulator from? Both the output and the feedback appear to be coming from the same location in the wiring diagram. The way you wired your car, You will have a significant voltage drop between your output at the starter terminal and your feedback at the junction terminal. That voltage drop gets larger as you increase load and your alternator appears to be putting out more power as you increase the load. The alternator output will be higher to compensate for this voltage drop and may be overcharging your battery.

Also, are you overloading the wire between the starter terminal and the junction terminal? In the original configuration, that wire only saw the difference between what the car used and what the alternator put out. Right now it sees everything the car uses. It looks like you are using 40 amps for the lights and the basic car. What happens when you turn the heater on with the lights, and roll down a power window? In my 73 the AC high speed blower and the power windows are fed from separate wires at the junction terminal. Under normal circumstances (car running) at least some of that power comes directly from the alternator and doesn't go through the wire between the starter and the junction.

 

My suggestion is to get rid of the ampmeter. I resisted getting rid of it for years as I like to have certain things original while others I prefer to be modern. Mine worked as it should until I put in the 100 amp alternator, halogen bulbs, stereo system and electric fan. My electrical system now outputs more amps to compensate for the draw and the gauge is confused. So I bought a GM restoration voltage meter with the same face plate colors so it looks like its original and it will bolt into the amps position with the possibility of just having to drill a few holes as per the instructions.

the vendors here should have the same gauge
https://www.ebay.com/itm/1968-1971-C...72.m2749.l2649

 

Interim cheater until you wanna pull out gauges. https://www.walmart.com/ip/4-in-1-Du...xoCN1UQAvD_BwE. I bet you don't use the lighter for cigarettes

 

My ammeter is still installed and I have installed a 140 amp CS-144. It works just great. Our pseudo-ammeters will work just fine --you just have to wire them properly.

I have separate voltage indication on my EZ-EFI and I also have one of those digital cig lighter indicators (but it interferes with my smoking ). I have been considering getting rid of that a/f ratio meter that I installed in place of the clock and installing a voltmeter there, but since I really don't need I'm pondering what else would be a good gauge to have???....anyway....

Anyway- your ammeter will work if you wire it properly. Or you can replace with a voltmeter.

 

Oil temp. AND, you can hide a toggle switch and hook it to a tranny oil temp sender, and get a two fer.

 

Vacuum is also great.

I would like a AIR/Fuel MIXTURE gauge .. esp if you plan on engine mods....

 

Here is the easy solution to replace your ammeter:

https://www.zip-corvette.com/68-71-v...gine-swap.html

68-71 Voltmeter Gauge



I would be doing the same in my '75 but Zip doesn't have one for 75-76 yet.

 

Derekderek: That's handy! Just purchased it from Walmart online for $8, marked down from $18 … in case anyone else wants one.

 

To answer a question: Why did I put such a large alternator in the car? I was told that one could not put too-big-of-an-alternator in a car and that a 140 amp alternator will only generate what the car needs. It does not constantly put out 140 amps. Therefore, change the alternator once instead of constantly buying a bigger and bigger alternator as additional power items are added to the car. Supposedly, the only issue is the size of the alternator-to-battery cable. That must be large enough to handle the amperage. I hope I was instructed in this correctly!

Please comment on this idea to fix my issues:
1. See my wiring diagram attached. ( Have also attached Blue1972's proper schematic for comparison). I would use a 4 gauge wire from alternator to junction block, then back to the starter as my battery charging wire. I would connect the current wiring harness at the junction block to replace the "splice" where Blue1972's schematic shows a "splice" . I would think that the higher amperage would direct to the starter and then to the battery and the rest of the wiring harness would only draw what it needed from the junction block. Am I thinking correctly?

 

Yes, that's the proper way to do it. You can also move the power wire feeding to the firewall connector and into the interior onto that junction block too.

The MAD diagram doesn't really represent how a C3 is configured, since they have the battery in the back. It's more what's expected on the big or intermediate cars of that era. Many cars used the the horn relay as a junction block too.

The ammeter would continue to somewhat work if you connected it to the new junction block and the solenoid. The reading wouldn't be accurate, but it would deflect the correct way.