I am up grading the alternator on my 68 due to adding a 16" electric fan. I was looking at a 150 amp, but may be to big It's rated at 90 amps at idle. My concern is the amp gauge only goes to 40 amps. Would this alternator damage the amp gauge or the wiring? I don't want to burn out wiring.. Thanks for any help

 

The gauge does not have the full load running through it or it would probably explode and melt down. It has wiring "shunts" which can be thought of as current dividers.

The fan that you are adding won't have its current running through the gauge unless you deliberately do so. That would be particularly unadvisable. the downside is you won't see any measurement on the gauge associated with the fan.

I switched over to measuring voltage instead of current. I'd rather have the amperage but wasn't willing to try duplicate what the Chevy engineers did.

 

Thanks, I think I will also remove the amp gauge and replace it with a voltmeter. I've seen replacement volt meters that look like the factory amp gauge.

 

You'll be lucky if the fonts match. I had my original rebuilt at Palo Alto Speedometer nearby to me. Mentally I move the decimal point one place and that is close enough for me

 

Sugarhillboy
If you really want to keep your amp gauge you can. I ran a new 6-gauge lead from the Horn Relay to the Starter Post and just redid the Ammeter leads such that they're still in parallel with the new 6-gauge lead. I did NOT calculate but just slapped it in and redid the termination points. Works fairly well and let's me know when I'm having a drawdown.

 

90 amps at idle will boil your Flooded Lead Acid battery DRY in a BIG hurry. If you are planning on using any kind of battery other than FLA you will have even bigger issues with them as well. That much current going into a battery is Too High a charge rate. Our batteries used in the C3's were about 100 amp hours in capacity, when you have a charger that makes 90 amps at idle it is too much for the battery. I spent years working with and testing batteries and I am very familiar with this stuff. You really want to keep a conventional FLA battery in your C3, especially if you are going to use a huge charging rate like that. The FLA will simply use more water out of the electrolyte while being charged at that rate. Any other technology with relief valves built into them will blow their valves often and that is a no-no. The water you can replace yourself, the rest of them are throw away after they get cooked by a big alternator like that. The only thing that could prevent the damage is a great voltage regulator that can protect the battery from the overcharging that would occur with 90 amps at idle.

"The golden rule of batteries is to avoid extreme charge levels — neither fully charging (100%) nor deeply discharging (0%). Instead, keeping the charge between 20% and 80% helps reduce stress on the battery and slows down degradation."

I don't understand why many of us think they need to put a huge output alternator on the older Corvettes. I too have a 1968 C3 and I have a 105 amp alternator on my engine along with 2 Spal 11" fans, MSD ignition system and a complete EFI system and other accessories. I have never had any issues and I keep my fans running after shut down for a few minutes.

If you run the wire from the alternator Output directly to the Starter Motor's Positive Post you will be fine. From there the two fusible links supply the power to the Horn relay so the interior will have full power but the majority of the power will be sent directly to the battery. With the factory wiring and the current sent to the starter post from the new alternator you should still have a fully functional ammeter. My fans together can pull more than 40 amps so I have them wired to a separate fuse block that is connected directly to the battery itself. I also have a electric fan controller that starts the 2 electric fans at 50% power and slowly bring the speed up if and when the engine continues to get hotter. No spiking the electrical system at any time. I don't need to see the fan power going through the ammeter, so I don't this way. Nor do I see the power going to my EFI control system but who cares?

I agree that a Voltmeter would be 100 times more valuable than an ammeter. I have one that plugs into my cigarette lighter and shows me the battery voltage whenever I want to know what it is.

One thing that you might think about is putting an inline fuse in the output of the alternator when you have that much power coming out of it. On my C3 I am using Marine Grade circuit breakers which are water resistant and reliable in environments like a Corvette convertible. I have circuit breakers for my Fan power and one for the battery itself.

Running all that current through the 40 amp shunt and the dashboard wiring could get ugly as that much power going through the thin wires will make resistance and that makes HEAT. Heat and the potential for an electrical fire is a great reason to NOT run the excess power through the shunt and dash. I suspect that with all that power being fed into the car's electrical system through the shunt might turn the shunt into a "Fuse". Me, personally, I don't want all the power going through the dash board area and wiring. This is why we connected the power output from the alternator output directly to the battery (+) post on the starter. My C3 was "loaded" for a 1968 and it came with a 63 amp alternator. That 63 amps was enough to keep my 427 running and all the accessories with it. The MSD ignition box needs some power at higher RPM's and then my Cooling fans started to make the system marginal. This is why I went to a 105 amp 1-wire Powermaster alternator and I have never needed anything larger. Even driving with my headlights on for many hours never presented any problems for the original alternator and the battery/

Remember that bigger output alternator will draw more power from your engine while it is running. It takes More HP to make more electrical power and that is a waste when you don't need it.

A "Loaded" 1968 C3 means PB, PS, PW and a push-button AM/FM single speaker radio. The 1968 C3 was not made to be a "comfortable" Corvette with tons of accessories that suck power. It was a Big powerful engine, four speeds and great brakes, all made for one purpose, To Go Fast!

 

Sugarhillboy
The alternator will only supply the amps demanded of it. Can't really be "too" big. Just ensure that have adequate fusible links and/or fuses installed for protection. There lies the rub.
I recommend going with a the DR44G. You may have to modify/create the bracket but the voltage control available is quite wonderful. Let me find my long windy link where I installed mine.
Here you go. See link below. I've been upgrading for decades..... first to a 94 amp 12si, then a 140-amp CS-144, and now the ~140-amp DR44G. I really, really, like the voltage control I have over the DR44G.

https://www.corvetteforum.com/forums...xperiment.html

 

You can't just replace a C3 ammeter with a voltmeter. The C3 ammeter is really just a galvanometer which measures voltage drop over a "shunt" in the main voltage buss line of the system. I think you will do no harm to the existing C3 ammeter by upgrading your alternator. But you WILL have to upgrade your main power line to handle the current that the new alternator will produce. If you do that, your ammeter will not read accurately--but it will still tell you when you have electrical problems.

P.S. If you completely replace the main power line in the car, your ammeter will no longer function. If you ADD an additional power line in parallel to the original, you will have the power handling capability you need, but still retain a working ammeter. Or, if you go the full-replacement rounte, you can install the voltmeter with appropriate wiring to it.

 

Sugarhillboy
As I mentioned above you can keep your ammeter after you run a new lead......if you want to. It ain't rocket surgery. If you really want to know how just let me know.

https://www.corvetteforum.com/forums...post1608694782

 

You need to check with the manufacturer of your new alternator.
Some require that you change from an ammeter to a voltmeter.
My Powermaster alternator required that I swap to a voltmeter.
Corvette Central makes voltmeters to exactly match the design and face of your original ammeter.
Pay particular attention to the wiring requirements of the new alternator and voltmeter.

 

The Ammeter in my 1968 has the small current shunt mounted on the back of the gauge. Current shunts are great devices but I have seen some turn into "fuses" on C3's that were not wired properly. Current shunts are frequently installed on the ground leg and not the positive side of the battery. I have used hundreds of them in my career and they work great providing accurate current readings.

The power coming from the alternator's output needs to go to the battery thru large enough wires not to make things overheat. This is why we run the alternator output directly to the starter motor's Battery (+) post and secure it there. If you took that large cable and tried to push the power through the shunt then there will be problems. You should not push 150 amps through a 40 amp shunt.... or through 12-16 gauge wires.... If you try you might not like the results.

The horn relay still has battery power going to it from the battery (+) post on the starter motor through the fusible links. Without power going to the horn relay your car will have no power at the fuse block or in the interior of the car.

What would "require you" to install a Volt meter over the existing Ammeter when using a Powermaster brand alternator?

"For FLA (flooded lead-acid) batteries, a common charging rate is around 0.15C (15A per 100Ah) for the bulk charge to 80%, followed by a reduced rate (3A to 5A per 100Ah) for the finish charge, which is crucial to prevent overheating and gassing. If this is the correct and proper method to charge an FLA battery then what happens when you have 150 amp alternator on a 100 amp hour battery? Too large an output of an alternator will cook a battery and boil it dry of any electrolyte. The charger in question charges at 90 amps at idle so it claims, and the typical C3 battery is about 100 amp hours in capacity. Not a good match between the alternator chosen and the battery used."

"The charging rate for flooded batteries is typically expressed as a percentage of the battery's capacity (C-rate). The recommended charging rate for flooded batteries is generally less than 10% of their capacity. For example, a 100Ah flooded battery should be charged at a rate of 10A or less."

A battery should be charged at a rate of approximately 1/10th of its capacity (C/10) for lead-acid batteries, while lithium-ion batteries can handle faster charging, often up to 1C or even higher.

All this is information is readily available at the Battery University or just in Google searches.

 

Get rid of the ammeter..its useless. Get a voltage gauge, use one of the black wires on the positive. Tape or somehow keep the other from contacting ground. Add a ground wire to the new meter.
Now you will know what your alternator is doing.
If you go with an 100 or 150 A alternator, by the plug and play bypass for the voltage meter. Also run an 8 gauge wire down to the terminal your alternator used to go to on the starter. Tape off the old wire or remove it

 

You can use EITHER wire on the back of a C3 ammeter as the "hot" lead for a voltmeter. Both of them connect to the same main power line. BOTH will measure at battery/system voltage with a simple DC voltmeter. If you measure voltage between those two wires, there will be some level of millivolts on the scale, proportional to the current flowing in that main power line.