Mods, I think that this thread should be archived in the 'Best of Tech' section as the question comes up every few months.
Since it is fresh in my mind - here are the conversion instructions:
Main Harness:
Check to make sure that your original non-rallye harness is in good shape with no burns, melts or broken insulation.
Make sure that all of the connectors look good with no melted parts or cracks/breaks.
General Directions & Precautions
1. Remember that you're dealing with 30+ year old wiring - be careful not to bend or stretch it too much.
2. If the insulation seems fragile/brittle or there are more than a few suspect areas then you may be better off buying a new harness.
3. Connections: For all wire-to-wire connections, make sure that you solder them securely using a good rosin-core solder. Seal and insulate the connection using heat shrink tubing (don't forget to slide it on before soldering).
For all crimped connections, use high-quality brass terminals and expose the bare minimum of wire necessary to get a good connection and tight crimp
4. Connectors: You will need a few different connectors to make the conversion successfully. Many can be sourced through your local auto parts store, dealer or favourite repro place.
Some of the moulded connectors such as the right-angle push-on single terminal ones can be salvaged from a scrap harness.
5. Dimmer Switches: The connectors for these two are the most commonly melted areas on even the best harness. Replacement connectors are available from most parts stores, however, the wires are all black instead of various colours. Be especially careful when soldering and insulating these replacements as the dimmer circuit generates a lot of heat.
6. Form vs. Function:
The steps described below will get you a perfectly functional conversion that is neat, safe and will not be discernable to 99%+ of enthusiasts.
There are slight differences between rallye and non-rallye dashes that would make a conversion like this inappropriate for concours judging.
Depending on how you want to finish things off, you could choose where you make your splices, and re-wrap the entire main harness to make the additions invisible.
Getting Started
Essentially, there are few functional differences between non-rallye and rallye dash harnesses. In order to convert you will need:
1. Both power and signal connections to the tach
2. An 'always on' or Batt connection to the clock
3. A right-angle push-on connector for the oil pressure gauge
4. You may need to lenghten some wires to give yourself more working room or take stress off the wires.
Tachometer
The tach uses two wires:
1. A grey wire with red tracer and a single, female spade terminal at the gauge end.
This is the signal wire and it is connected to the negative side of the coil through the bulkhead harness
2. A dark blue wire with a round, push-on, right-angle connector
This is the power lead and will need to be connected to a switched, positive connection such as 'Acc' or 'Ign'
Signal wire
Take a piece of fresh wire and strip approximately 1/4" from the end. Crimp a bare, brass spade terminal (female) to that end.
On the white bulkhead connector, find the terminal space embossed with '20'. (It should be empty).
Snap the connector into place.
Uncoil however much wire you need to reach from the bulkhead terminal to the tach. (I temporarily tape the wire to the main harness to make sure that the length is close to factory).
Cut the wire, strip 1/4" and attach another female brass spade connector.
Snap one of the black insulating sleeves in to place over the terminal and you're done this step.
Tach Power
Decide where you want to get the power for the tach. The factory splices the wire into the red power circuit for the radio -- this will mean cutting open the wrap on the main harness.
I usually run the wire right down to the fuse box and use an insulated terminal on the 'Acc' connector
Using a round, right-angle push-on connector salvaged from another harness, solder an appropriate length of wire to it. (how long will depend on where you get your power source).
Trim the wire to final length, and either splice it or add a connector for power per above.
Oil Pressure
Conveniently, there is already a wire that feeds the oil pressure circuit. In our case we are going to replace the idiot light with a gauge.
Find the light bulb socket that is fed by both a grey wire and a dark blue wire.
Cut the socket off, leaving enough wire on the socket that you could restore the harness if you wanted to.
Trim the end of the dark blue wire so that there is no copper showing. Permanently insulate the end of it will not be used again. (electrical tape followed by shrink wrap tubing should do the trick).
Using another round, push-on right-angle connector from a salvaged harness, solder it to the grey wire and insulate the connection.
Now you're done this step.
Clock
The clock is quite straightforward. It gets its ground (negative connection) from the dash frame, so all you have to do is get it a positive feed.
Like the tach power, the clock power is spliced into the main harness from the factory. If you go this route, be extra careful to make a good connection and insulate it well. Circuit will be live 24/7 and you don't want to risk an electrical fire or meltdown.
Clock end: Trim 1/4" from the end of a new wire, crimp on a brass, female spade terminal.
Snap a black insulating sleeve into place over the terminal.
Power end: Using the main harness as a guide, decide where you want your power from
Trim the wire to length and either splice it into the main harness or crimp on an insulated female spade connector.
The 'Batt' connection on the fusebox is easy to get to, gives a constant feed of power even when the car is switched off and it's easy to pull the plug on the clock if the car is going into long-term storage so that you don't drain your battery.
Alternatively, you could solder a male bullet connector to the end of your power wire and plug into one of the 3-hole accessory power feeds that are on the main harness. This is how the non-rallye clocks are hooked up.
Final Adjustments
As mentioned earlier, you may need to lengthen a few wires on the other connectors or light sockets as things can be a little tight. If you do so, be very careful to solder and insulate well.
This conversion can be done with the dash and harness still in the car - just pull the gauges out to give yourself some working room.
Clearly, the best option is to have the dash & harness out of the car for easy access.
Last Notes
With the harness out of the car, this is a good time to detail the rest of the connections. Make sure that the terminals and sockets are clean and free from dirt or corrosion. Replace any suspect connectors at the same time.
You may also want to convert the instrument voltage regulator to an electronic one using the instructions elsewhere in the Moparts Tech Archives. This will ensure that your gauges have a nice, constant 5 volts available and will help with reliability and troubleshooting.
If all has gone well, install your rallye gauges and get out there and burn rubber!!!!
