I'm rewiring my 78's harness before the engine goes back in. The original harness was hacked to pieces/spliced/taped and twisted. On the alternator, there is the main lug which takes juice back to the battery (via the starter), there is also another two prong plug (labeled 1 and 2) that takes power to someplace? Mine was all cut up, one wire was powering electric fans. The other wire goes back thru the harness to the firewall block (#2 pin). What are these two wires used for?

One more: There are no thermo links anywhere as shown on the 78 Vette wiring diagrams (mostly going to the starter), are these needed? Can I buy them?

 

ok my point of reference is my 77....

you have two red #10 wires, 1 black wire #12 and 1 brown wire #20 and 1 black or dark blue wire that goes to the waer sender unit maybe an 18#


thermo links are commonly available at auto parts stores

heres a link that says what the wires do it may help you figure out what you are looking for...read about three wire alt....

http://www.madelectrical.com/electri...reewire2.shtml

heres the jist of it from them....got to link for more info...

How the THREE-WIRE Voltage Regulator Works
(the Turn ON & Warning Light circuit)

With the “THREE-WIRE” alternator model built for cars and trucks, one of the voltage regulator terminals is wired to an ignition switched OFF/ON source. And the OFF/ON circuit may also be used to operate a warning light at the dash. (The THREE-WIRE type can be wired with or without a warning light at the dash. The warning light is an option–not a requirement.)

REMOTE VOLTAGE SENSING, a THREE-WIRE advantage
The other of the two plug-in wires at the SI series alternator is the “voltage-sensing” terminal for the voltage regulator. Through the sensing-wire, the voltage regulator monitors electrical system voltage and makes adjustments to alternator output. The regulator will adjust alternator output as needed to maintain the place where the sensing-wire is routed (at about 14.2 volts). And the sensing-wire can be routed to a place remote from the alternator (downstream).

This “remote voltage-sensing” feature is often a big advantage when running with a factory type wire harness from the Muscle car period (60’s and early 70’s period cars and trucks). Then the alternator output will be adjusted according to what goes on far downstream from the alternator–at the main power distribution junction, which operates the whole car plus charges the battery!

 

Typically the two wires are either red and black or red and white (if you get a replacement plug from say Autozone). The red wire which is probably the one you state is hooked to electric fans should be an ignition hot wire. This is used to charge the field of the alternaotor to get it into power production mode. The black or white wire should go back to the generator/alternator idiot light. Do not ground this wire or the alternator won't ever fire up and start charging the battery. If you don't need the idiot light, just cut that wire off at the plug and be sure to hook up the red wire to an ignition on source.

 

Alternators and generators work by spinning a coil of wire within a magnetic field. In a generator, the magnetic field is typically supplied by permanent magnets. In an alternator, its supplied by a seperate coil of wire that surrounds the spinning coil. These are called the "field" windings as they provide the magnetic field.

The way the system is supposed to work is that the red Batt wire and the case ground are used to transfer power from the alternator to the battery. One of the two noted pins on the plug activates the field windings. This then allows the field to be activated or deactivated depending on the needs of the battery. Because the headlights are typically the biggest draw (before the advent of electric fuel/water pumps, fans, high-powered stereos, etc.), the other terminal kicks in a set of auxilary windings when the headlights are switched on. Ever watch the amps guage when you turn on the headlights on a properly working system? In some cases, the guage actually shows a momentary jump in charge rate.

In many cases (and in the case of 1-wire alternators), people simply wire the field windings to the batt terminal and don't use the activation lug. In this case the alternator is continually putting out power. An internal regulator is then used to keep tabs on the amount of juice coming out.

In your case, it sounds like they are simply tapping the field input to the hot on the electric fan, and I would imagine that there is another hot wire going to the fan somewhere in the system. Alternatively, if the field is also wired to the batt terminal (very common), this may be the power transfer point to get juice to the fan.

 

Very good info. My 78 wiring diagram shows the # 1 wire going to an idiot light...and the other wire does go to the ignition switch via harness pin #2. So can (or should) I run my electric fans off of the "sensing" wire (normally connected to the low voltage idiot light - which I don't have)?

 

Your electric fan should be tied in to the battery or at the positive high current horn terminal through the contact of a high current relay. The relay should be activated via the ignition so that it comes on with the key, or through some circuitry (thermal or delay) that will allow the relay to remain closed for a time after the key is shut off.

 

i don't know about that... so you are saying the #1 wire is the brown #20 wire....and the other wire to the ignition switch is the red #10 .....from reading those articles, i got the impression that the "sensing" wire regulated the voltage output from the alternator.....seems to me that you would not want to have any voltage drops on this circuit, and if you are talking about running the power off the #20 brown wire i would think that guage wire is way to thin...