Hi Guys
I just bought a Tach drive HEI distributor off of ebay to replace the reg. distributor in my 71 Vette. It has a 350 in it. What mods, if any, will I have to do to make this work in my Vette? Is it a hard change over? Any help would sure be appreiated.
Thanks
Dave Prine

 

I just put HEI back in mine last night. The 75's were supposed to be HEI originally but someone put points back in. Anyway all I had to do was hook up the ignition switched positive from the old coil to the big red wire from the HEI that hooks up on the right. My electronic tach wire from the negative side of the old coil hooked up to the "tach" wire on the left on the HEI.

 

I dont believe the HEI uses a ballast resistor. So the switched ignition wire connected to your coil may not be the best choice. That may have a ballast resistor wire in the circuit even if there's no obvious ballast resistor.
Gary

 

That's interesting Gary. I think I'll put a voltmeter on and confirm the voltage coming in. I'm thinking whoever put the points in probably neglected this but I better check.

 

Hello,

Did you buy a mechanical tach drive HEI distributor? Isnt there a difference between the way a tach hooks up from a mechanical gear drive to the electronic signal sent from an HEI unit.

kdf 1973 454
kdf 1986 L98

 

I did buy a Mech. Tach drive HEI. I need to know what wires to hook up to it and what to do with the old coil wires.
Thanks
Dave

 

z76 -
Your points-type ignition system uses a resistor wire in the "+" wire going to your old coil. This drops voltage to the coil to around 8 volts. The HEI runs on 12. You need to bypass the resistor wire when running the HEI by either running a new parallel wire, or by using another ignition switched source for your HEI power feed.

Note that you cannot tell if the wire is a resistor wire by sticking a voltmeter onto the end of the wire: It will read 12 volts whether it is a resistor or not when the wire is dosconnected from the coil. To test the wire, you have to have it hooked up to the "+" side of the coil and run a ground wire from the "-" side to ground. Only with current flowing through the system can you check the voltage on the resistor wire to see the voltage drop.

The only wire you need to hook up on the HEI is this 12-volt power feed wire. No other hookups required.

 

i never did quite understand the concept of voltage drops.... why does that happen... thks bob

 

Think of the wire as a steel tube filled with pressurized water coming from one end of the tube (the voltage, or "push"): there's a pump at the end pushing on the water (the battery or alternator). If you simply cap off the opposite end if the tube (disconnect the wire), the water isn't going anywhere, and the pressure throughout the tube will be the same pressure as the output pressure of the pump.

Now, smash the tube down in the middle and create a massive restriction (a "resistor"). With the end of the tube still capped (no flow through the tube), the pressure on both sides of the restriction will still be the same - it's a static condition, and the pressure will equalize across the smashed section. But if you now uncap the tube and allow the water to flow through the tube, you will note that you will read pump pressure on the pump side of the restriction, but you will now have a lower pressure on the other side of the restriction due to the pressure drop across the smashed part of the tube.

The resistor wire works essentially the same way: If you disconnect the wire from the coil and have no current flow, you will read battery voltage on both sides of the resistor. But if you hook the wire up to the coil and allow current to flow, you will then read the actual voltage drop across the resistor by reading the voltage at the coil "+" terminal. If you read the voltage at the "-" terminal you will read the total combined voltage drop of the resistor wire PLUS the resistance of the coil (two smashed sections in the "tube").

Make sense?

 

yes, thank you, great explanation. thks bob

 

That was great, you took it down to terms even I could understand.

 

Check out madelectrical.com

This is a really great source of information about how your electrical system works. He sells conversion kits to adapt HEI distributors into older cars. Also conversion kits for newer alternators into older cars; i.e. cars with external regulators.

Also, I think lectriclimited.com may sell some harness for the HEI conversion.

Buying a pre-made harness is good because the instal looks more professional.

Check out madelectrical.com's discussion about single wire alternators and also how the Corvette ammeter works. (The ammeter is really a voltmeter that reads amps.) Madelectrical is a great advocate of adding relays to your car to provide current to high wattage devices.

 

just happened to be thumbing through the DEC HOT ROD and on page 140 they talk about this exact hook up, they even reference an HEI distributor wiring harness kit...

www.madelectrical.com