dogboy

I am going with a 105 amp/ 70 amp at idle powermaster 3 wire alternator. I'm just not getting the idle power I need with the new a/c & radio. I had the old 63 amp which I did upgrade to a 80 but never changed the wireing. I believe I need a 10awg wire. What is involved? I followed the alt wire to the dash which plugs into the rubber block that I assume goes to the back of the fuse panal. Than another harness leads out from the other side to the starter. I guess these are the ones that need changing. How do I change them out at the fuse panal?

Larry B.

I don't know if this is your problem but... Corvettes use a Corvette specific alternator, Not a Chevy generic as most aftermarket / auto parts stores sell. That is because the crank pulley is smaller so the alternator turns too slow. (most noticable at idle) Corvette alternators put out more amps a lower RPM. I went through this with autozone a few years back with a 71. I had no electrical power at idle with the A/C running, lights and wipers on... 20 amp discharge. I tried 3 from autozone because they refused to believe that their books were wrong.. I bought a delco ... Problem solved.

carriljc

I installed a 94 amp 12si in my 68. Wiring is simple as shown below. I got the alternator at Kragen...
here's the part number: 7294-12
I just checked and the part number is still good. It's ~106 bucks for a lifetime warranty.
http://members.rogers.com/mgillespie620/alternator.htm

ROD NOCK

First off, to determine your amperage needs, you need to add up all of the demanding circuits. So if your A/C is 20 amps max, your radio is 10 amps etc. This will give you a total demand figure, then we add some overhead for battery charging and reserve. This final figure will tell you what amp alternator you need to use.
With that aside, you're wondering if the existing wiring off the alt will safely serve your needs!? For this, we need to know the existing gauge of the wire, and the length of it. Now, if the existing wire qualifies, it has to be in top shape without any splices, damage, etc.
In the mean time, I hope you incorporated some properly placed protection (fused) in your new accessory circuits! :eek:

mrvette

OK, a normal alternator WILL work just fine in an early shark, BUT you have to maybe redo one wire from the alternator output to the horn relay/tiepoint for power distribution....I feel it's the only one you need bother with....whoopie doo, about a 8" length...

the cooling fans are best wired directly off the alternator, I feel, both ground and power.....

GENE

Larry B.

Gene... I have had very poor results on sevral C3's (this only applies to small blocks) Using generic chevy alternators. They all work fine above 1000 rpm but at idle they just don't get the job done when you run the A/C ,lights and wipers. I could not put the power windows down at 600rpm idle on the last car I had. I know there are many variations out there but if you have the stock crank pulley and alternator pulley a chevy alternator just doesn't put out enough power a low speed.

dogboy

I have the alternator in shipment , about 1 week delivery. I did a quick run down of accessories that need to be utilized at idle in the most severe conditions.:A/C, radio, headlights, tail lights, wipers, brake lights, & instrument lights, also I would add HEI power needs. This would be at a stop light on a hot rainy night. The constant demand would be the a/c, radio, all lights, wipers, HEI. I would think at least 60 amps for that. I have not measured the existing wire yet and assume it is the large batt wire that is attached to the back of the alt with a nut. It has the original 63 amp wiring set up for the a/c cars. It is in good condition without splicing. So what do I need to replace ? Is it just that wire? and where does it travel. I believe it ends at the starter motors large batt terminal with a inline fuse.

norvalwilhelm

I ran into this same problem and went to a high amp truck alternator. The shop quickly changed it over to one wire for easy installing. All it needed was one heavy wire directly to the main power cable, in this case the heavy wire on the starter and one small 12 volt wire that turned off with the key. This eliminated all the power going through the amp meter in the dash. In your normal system for an amp meter to work ALL power must go through it so a rewire would require a new heavy wire through the amp meter.
To me a volt meter is better anyway. A amp meter can show 100 amps going through it but if you are drawing 110 you are still loosing. With a volt meter as long as the voltage is above 12 volts you are winning. Well not quit but close.

dogboy

Yes I have a volt meter as original with the 77. The one wire set up looks like the easier of the hookups. I'm just confused as to whats between the heavy batt wire from the alt once it goes into the firewall and emerges again. Is it hooked into the fuse panal at this point? there must be a junction at this point.

mrvette

Blue, Norval, Several years ago on a '70 GTO/Lemans I had, the car came equipped with an external regulator type old 40 amp alternator....
and so I changed it all over to the SI17 type....like came in later 70's GM cars....ok, NOW, the problems with early sharks, like mine that came with the same nearly identical alternator, is the same thing really....that is, there HAS TO BE a dash light bult hooked up to the alterntor regulator circuit, and the other end wired to a SWITCHED HOT, not stready 12 volts....the heavier red regulator wire is connected to the harness tie point on the horn relay...
IF that light is not hooked up, you get kind of what we call in electronics, known as a 'crow bar' action....called that because if you followed the currents/voltages on a 'scope' the pattern looks like a crow bar....

at any rate, the presence of that light bulb wired as described, and the regulator red wire hooked up as indicated will allow the alternator to charge at lo rpm's as well as higher up....the entire regulator action is smoothed out...eliminating that light bulb from that circuit will allow the alternator to act as if it was a 'one wire' alternator....alll that's done in the famous one wire alternator is wiring the 'red' wire part of the regulator to the output stud of the alternator....this was done originally for MARINE useage only, because when the boat is going, the alternator is most certainly in the charge stage regardless of the light bulb presence, BUT with car engines at idle, you need that lightbulb......
The factory wiring diagrams on my '72 are screwed up in my book printing, an old original for model year '72, covering the entire chebby lineup....

I have proven this to myself on two separate cars over the years, beleive me when I say.....you need a dashlight in the circuit wired as I say....OR the alternators of the SI series will NOT put out at idle.....
a ONE WIRE alternator is NOT a good idea for street driven cars.....

GENE

dogboy

Mrvette
You are saying its a short line thats needse to horn relay? I can't see the relay from the engine compartment so it must be under the dash next to the fuse panal on the 77. I would think than that the other end would need to be changed also. that is from starter motor to the horn relay. Can I bpass the relay and just run it straight from the alt to the starter with a inline fuse.

norvalwilhelm

Gene I must have got lucky. I went to the one wire alternator and it is from a truck with a 150 amp output. I have one heavy #8 wire going from the alternator to the heavy battery wire on the starter and the one keyed wire was just a hot wire from under the dash, any wire that is switched on by the key to the 12 volt side of the alternator. I also installed a volt gage.
Whenever the car is running I get close to 14 volts regardless of speed. My car does idle around 1000 anyway but no problems with charging.