This page will describe the conversion of my '77 Nova from warning lights to gauges. This conversion involves replacing the regular instrument cluster with one of the optional factory gauge clusters. This conversion applies to 1977 - 1979 Novas since they all use the same style instrument panel.

Note that there are two versions of the optional gauge cluster used in 1977-1979 Novas. The one I installed in my car (shown below) is known as the "Econominder" gauge package and includes a voltmeter, temperature gauge, and fuel economy (vacuum) gauge. The other version is known as the "Special Instrumentation" gauge package and has a tachometer instead of the fuel economy gauge. Neither one includes an oil pressure gauge so I removed the clock (which didn't work anyway) from my cluster and modified an aftermarket oil pressure gauge to fit in the opening.

Parts Required for the Conversion

• Gauge cluster (includes speedometer, fuel gauge, voltmeter, temperature gauge, and fuel economy gauge or tachometer)
• Under-dash wiring harness (nice to have but not absolutely necessary)
• Temperature sending unit
• Vacuum hose & grommet (for the fuel economy gauge)

Differences in the Gauge vs. Light Wiring Harnesses

• Under-dash harness
  • The plug that fits the back of the instrument panel is the same for both warning light and gauge wiring harnesses. However, nearly all of the wires are re-arranged into different slots in the connector.
  • The gauge harness lacks a wire for the "GEN" light. For proper alternator operation, the gauge harness includes a 10 ohm resistance wire that connects the alternator #1 terminal to the "IGN 3" terminal on the ignition switch.
  • The gauge harness used with the optional tachometer cluster includes a tach signal wire that runs through it's own grommet in the firewall and connects to the "TACH" terminal on the HEI distributor cap.
• Engine harness
  • The engine wiring harnesses are identical for both the warning light and gauge clusters. The gauge cluster still has an "OIL" light (no oil pressure gauge) so it uses the same wiring and sending unit. Both the warning light and gauge clusters use the same wire for the temperature sending unit. The temp sending unit itself is different though.
  • As mentioned above, the optional tachometer has it's own separate wire that is not part of the engine wiring harness.

The Swap Procedure

1. Re-arranged the wires/terminals in the cluster plug
2. Added resistance wire to alternator circuit
3. Remove temp sender wire connection at ignition switch
4. Swap temperature sending unit in cylinder head
5. Drilled hole in firewall and added grommet & vacuum hose

The following table lists the wires found on the cluster plug. The pin number (first column) is the location of the wire/terminal in the non-gauge cluster. These wires/terminals need to be removed and re-inserted into the locations listed in the second column. The third column lists the wire color codes and the fourth column lists the circuit number (as found on GM wiring diagrams). The last column lists the wire's function.

Note that the non-gauge cluster has four ground wires, one +12V feed wire, and one cluster illumination wire. While the gauge cluster has two ground wires, two +12V feed wires, and two cluster illumination wires. I believe this was done (along with the pin re-arrangement) in order to satisfy the design constraints of the printed circuit on the back of the instrument cluster.

The two "extra" ground wires are no problem. Just leave them disconnected or place them in unused pin #'s 1 and 18 on the connector. Adding the additional +12V feed and illumination wires involves salvaging some extra terminals and short sections of wire from a junk car. I took mine from an '88 Chevy Celebrity (many 70's and 80's GM vehicles use the same style of terminals). I spliced these short sections of wire w/terminals onto the existing +12V feed and illumination wires.

The internally regulated alternators used in these cars receive their field current supply from two sources. (1) Through the in-dash "GEN" light, and (2) through an internal connection to the alternator's diode trio. The current that flows through the "GEN" light is what initially energizes the field. Once the alternator is generating power, the field is "self energized" through the diode trio connnection. Since the gauge cluster lacks a "GEN" light, GM used a 10 ohm resistance wire in the gauge harness to take the place of the light. Current to initially energize the alternator's field coils flows through this resistance wire just as it would through the "GEN" light. It is important that this be a resistance wire (or inline resistor) as opposed to a regular wire. Otherwise, the output of the diode trio would be connected directly to the battery (through the ignition switch) and that could damage the diode trio. In other words, the alternator's output current should be flowing through its high-current output diodes (as designed) rather than through the low-current diode trio.

To add the resistance wire (or inline resistor) to the non-gauge harness, start by locating the "GEN" light wire on the cluster plug. This is the brown wire originally located in slot/pin number 12. Connect the end of this wire to a 10 ohm resistor or a section of resistance wire that is roughly 10 ohms. Insulate the connection/splice with some heat shrink tubing. Next, locate the 12 gauge orange wire on the ignition switch connector. Connect the other end of the resistor or resistance wire to the same terminal as this orange wire.

The non-gauge cluster uses a wire connected to one of the ignition switch ground terminals to perform a key-on bulb check of the "TEMP" light. This is no longer needed with the temperature gauge. Simply locate the 20 gauge dark green wire on the ignition switch connector and remove it from it's socket in the plastic housing. Insulate the terminal with some heat shrink tubing so it won't short out on anything or snap it into a spare single-terminal plastic housing.

The sending unit used with the "TEMP" light in the non-gauge cluster is basically just an on/off switch while the sending unit used with the temperature gauge is a variable resistor. The correct sending unit for use with the gauge is Standard Ignition part #TS-6. The sending unit is located on the driver side cylinder head between the #1 and #3 spark plugs. Drain the coolant from the engine, unscrew the old sending unit, and screw the new one in it's place.

The cluster I bought off eBay did not include the vacuum hose and the parts car it came from was long gone. But after posting an inquiry on Steves Nova Site, Paul McCullough e-mailed me some photos and even mailed me the hose/grommet from a parts car. Thanks Paul! Here is one of the photos Paul sent that shows the location of the hole in the firewall:

If you look closely, there will be a dimple stamped into the firewall in this location. The factory grommet requires a hole approximately 5/8" diameter.

The original vacuum line consists of a section of 1/4" ID hose that connects to a source of manifold vacuum. I connected mine to the port at the rear of the intake manifold (I assume this was probably the factory location based on the length of the hose). The 1/4" ID hose runs through the grommet in the firewall and then connects to a plastic reducer fitting. From there, a section of 3/16" ID hose runs up to the nipple on the back of the gauge.

When installing a tachometer cluster, it will be necessary to add a wire to slot 15 of the instrument panel connector as is listed at the bottom of the above table. If you want to duplicate the factory wiring, use a 20 gauge brown wire. This wire runs through it's own grommet in the firewall and is likely located in the same location as the vacuum hose grommet described above. The other end of this wire plugs into the "TACH" terminal on the HEI distributor cap. Or connects to the appropriate connector/wire on an aftermarket ignition system.