Aux. Gauge installation for 84 and 85 models
The 120 MPH speedometer and aux. gauge package was not offered until 1986. These units can be easily installed in the 1985 models with little trouble. The 1984 models are a little harder, but can be done. This write-up is for the 1985 year with comments and tips for other years. Items needed for this modification are a 120 MPH gauge cluster, the aux. gauge cluster with the pigtail wiring, and a vinyl radio/vent pod that fits around the aux. gauges.
The old instrument panel is easy to remove. I found removal of the steering wheel gives you a little more room to work. Next remove the vinyl piece around the shifter and then the vinyl piece around the radio/vent. Mount the Aux. Gauge cluster above the vent.
Now for the wiring
The problem here is that the 84 & 85 did not come with the wiring harness to plug the aux. gauge pigtail into. Also on the 1984 model you have to change the oil pressure sensor to one used from 1985 through 1988, Here is a neat way to install the required wiring without cutting and splicing wires. The connector behind the dash pod is located on the right hand side (see Gauge Cluster plug). Use five (5) different colored wires, 22 gauge, about three (3) feet long. Remove about 2 inches of the insulation and tin the wire. The wire can be slid up the backside of the plug's pin and put a small spot of solder on the back side so it will not affect the pin when it's pushed into the back of the dash.
Wrap electrical tape around the four wires. Run the new harness through the back of the dashboard. Install the 120 MPH gauge cluster. I recommend you test this part of the installation at this time. Place a small piece of electrical tape over the ends of the new wiring harness. If everything checks out it is now time to finish the job.
I used a five (5) pin connector that can be found in my junk drawer or you could just splice the wires together. Wire this connector to match the leads from the aux. gauge cluster (compare "Gauge Custer plug" with "Aux. Gauge Wiring").
Install the vinyl piece around the radio/vent and Aux. gauges. Last to be installed is the vinyl piece around the shifter, and you're done.
If your Fiero was equipped with an oil pressure switch (1984 models) you need to find or buy an oil pressure sending unit for 85 or later year. This will give you a sending unit that is 0 to 90 ohms verses an on/off switch. Remove the wire connector from the sending unit, remove sending unit, install new sending unit and replace the wire connector.
If you are going to add the Aux. Gauge package and leave the 85 MPH pod in place you will have to remove the tan wire from pin 17 on the C206 (sometimes called C3) connector. You cannot drive two oil pressure gauges at the same time.
Also if you have a 4-cylinder engine you will have to remove the resistor from the your TACH, and solder it back on the GT TACH to keep the proper RPM. (Thanks to John Wells who provided information on 1984 models)
This is only a guide. Check your wiring drawing for your year and model. Use a voltmeter and ohmmeter to look and check for the correct signals.