RK-TECT

Happy Thanksgiving! When I bought my '65 convertible 4 years ago, it was already converted to an HEI distributor. Other than the ignition shielding not fitting, I am very happy that the previous owner made the change several years earlier. However, I couldn't understand why my highway RPM's were so high with a 3.55 rear & 215/70 R15 tires. At 65 MPH, my tach shows 3,500 RPM when in fact it should be about 2,900 RPM according to all the gear ratio calculators & charts. I checked the speed with a GPS and it was right on the money. I have researched this issue and determined that the tach is getting too much voltage. I previously tried adding a tach filter, but that didn't change anything. So I now want to try running the yellow tach wire through a resistor to bring the voltage down to about 9.3 V which I understand is what it used to see with the original points & coil ignition. Since it is no longer a cable tach, I assume that it was changed when the HEI was installed (the tach looks a little newer than the speedo). Since the ballast resistor isn't needed for an HEI distributor, I thought about running the tach wire through the BR since it is still screwed to the firewall. However, there are two wires still attached to the top terminal; a black wire and a pink wire. The pink wire runs up into the harness, and the black wire runs down a short distance and blends with other wires before penetrating the firewall and into the cabin. The HEI has a double pink wire connected to the "BATT" terminal on the dizzy and runs into the wiring harness before heading toward the driver side. I assume that the double pink wire splices into the single pink wire in the harness which then connects to the ignition switch. I read that the entire pink wire should be replaced with a new 12 AWG wire in order for the HEI to function at its best... "performance & fuel mileage should improve". My car runs very well, so I will go with the philosophy that "if it ain't broke, don't fix it" for now. I would use the existing BR as my tach resistor, but I read that the single black & pink wires are now getting a full 12 V and are using the BR as a junction point since the bottom terminal isn't being fed from the coil (which was removed for the HEI). I'm not sure if that's true, but they must be connected to the BR for a reason. The conclusion of my research is suggesting that I purchase a 15K ohm, 0.5 watt resistor and splice it in the yellow tach wire in order to lower the voltage to resemble a set of points, thus allowing the tach to read the ignition module's signal correctly. Any thoughts? Anyone experience the same issue before? I have attached a photo of my HEI, BR, and wiring (the black wire is hidden behind the ballast resistor).

Thank you for your help,
Roger

fullcontrol

The original ignition wire doesn't last long, really must upgrade to 12 awg. The BR is of no use at all, however my guess is your two pink wires are the previous owner's attempt to carry the heavier voltage rather than run a new wire.

As you note, the tach would have been switched out with electronic tach glued behind the original tach face. Some tachs have a voltage selector switch on the backside. Perhaps yours is not in the correct position.

RK-TECT

Hi Fullcontrol,
Thank you for your reply. I assume that the single pink wire from the ballast resistor running into the harness isn't doing anything at this time... it must be there for "decoration" . If I disconnect the wire from the BR, I guess I will find out for sure. In addition, I want to know why the thinner black wire is still connected to the top terminal on the BR. This wire runs through the firewall with other wires. Just like the pink wire, I assume it isn't doing anything since there is no wire connected to the bottom terminal of the BR. Other than the pink ignition wire, does any other wire connect to the ballast resistor to reduce the voltage to a device in the cabin?

Roger

mikem350

Electronic tachs responds to "on-off" pulses from the dizzy, like a computer circuit. The HEI has a tach output terminal for electronic tachs (your yellow wire...it is not a DC voltage, its pulses, right?..your DVM is being fooled).

Don't waste your time trying to tweek with resistors. It MAY be possible that the module in the HEI is not putting out correct pulses, but doubtful...somebody with a scope could check this out.

Make sure its getting full 12v switched ign, car running, which you seem to have.

Make sure the switch on the tach is set for 8 cyl..or else you need a new tach. I don't think HEI needs the filter, but try it both ways.

RK-TECT

Hi Mike,
Thank you for your suggestions. I checked the voltage at the BATT terminal on the HEI and, with the car running, it's getting nearly 14 volts. I'm still not sure what the two wires connected to the top BR terminal are powering, but both terminals of the BR are also getting over 13 "fluctuating" volts with the ignition on (the voltage doesn't remain steady). Anyway, everything in the car works so I'm not going to mess with it. However, I would like to see my tach read the correct RPM's; or at least closer to correct. As you know, there is no way of getting to the back of the tach in a C2 without pulling the gauge cluster . That's not gonna happen! The previous owner who installed the HEI was a GM mechanic, so I want to believe that he would have set the switch on the back of the tach for an 8 cyl. engine. I already tried a tach filter which had no affect on the RPM's at different speeds. I read a tech article that recommends adding a resistor in the tach wire when "early 12v signal style tachometers are reading too high". The article claims that a capacitor might also be required in a small number of tach designs. Since I know it isn't reading correctly, I have nothing to lose other than a little time and a few dollars. If that experiment doesn't work, I can always replace the tach in the future .

Roger