Swapping an electric speedometer into your Firebird

Swapping an electric speedometer head into a thirdgen Firebird is becoming popular, probably due to the operation of the electric unit being much smoother than a cable-driven one. Hooking it up to work properly (or at all) is the challenge many of us face. Hopefully this will help things to go along just a bit smoother.

Just a quick note beforehand: This swap is from my own personal experience at installing the wiring harness and associated components from an 86 Firebird SE into an 89 Firebird Formula. The Formula was a theft-recovery, and had much of it's under-dash wiring hacked, including removal of the ECM. It is my understanding that only 86-92 Firebirds with guages had the electric units.......
If you plan to install one of these electric units into an 82-85 Firebird, the wiring to the cluster will need to be changed around a bit in order for things to work correctly

What you will need to accomplish this task:

• Speedo head, or entire cluster if converting to gauges ( in which case you will also need to change the water temperature and oil pressure sending units).

• The sending unit (VSS) from the transmission of the donor vehicle, along with appropriate gears to ensure your speedo reads correctly.

• A new O-ring for the VSS.

• The yellow (could be a different color) plastic box which is 'piggybacked' to the Firebird's ECM. A scanned image of the one I used is below.

• The wire connectors from the VSS and the plastic box, along with as much wire as you can grab with them.

• A plastic plug to fill the hole in the firewall from where you will remove the original cable.

• Assorted wire connectors for splicing wires together, power hookups, grounds, etc.

How to install and hook up the unit

To remove the cluster

1. Disconnect the negative battery cable.

2. On models without cruise control, disconnect the speedometer cable strap at the power brake booster. On models with cruise, disconnect the cable at the cruise control transducer.

3. Remove the left and right side hush panels (the ones above your and your passenger's feet).

4. Remove the right and left side lower trim plates and attaching screws.

5. Remove the instrument cluster trim plate and attaching screws.

6. Remove the cluster attaching screws.

7. Pull the cluster out. Disconnect the speedometer cable and electrical connections.

8. Reach beneath the cable connection at the speedo head, push in on the cable retaining spring, and disconnect the cable from the speedo.

9. Remove the old cable from the dash and pull through to the engine compartment, and seal the hole with the plug.

To install the cluster

1. Since I'm doing a complete swap of dash and all, I can't be sure about swapping only the gauge cluster into a non-gauge dash, but that part should be pretty straightforward.
2. If your present wiring harness is not equipped with wires for the power and ground to the speedo, you will need to attach some wires to them now. Use small ring tongues to attach some wire under the three screws at the rear of the speedo head- use different colors to keep them seperated and mark down which one goes to which terminal. The terminals are marked 'E', 'IG', and 'S'. Allow 3-4 feet of length for hooking them up.
3. Feed these wires through the back of the dash.
4. Connect the electrical connections.
5. Push the cluster in.
6. Install the cluster attachment screws.
7. Install the instrument cluster trim and attaching screws.
8. Install the left and right side lower trim plates and attaching screws.

Wiring it up

Purple to sending unit Yellow to sending unit Pink with black stripe to switched 12V power Brown (2) *not used in this application* Green with black stripe to speedo head 'S' Black with white stripe to ground

Find a place to run the purple and yellow wires through to the transmission- the original route was through the plug on the passenger side just ahead of the door hinges. I ran mine underneath the shifter plate boot on the floor, and left enough slack for movement.

Find a place to later tie up the plastic box, and shorten (or lengthen) those wires as needed and splice together. Locate the wires previously hooked to the speedo head. Splice the one from the 'E' terminal to the black wire from the plastic box, then hook these to a ground screw or bolt. Take the one from the 'IG' terminal, splice it to the pink wire from the box, then run a wire from them to the fuse box and hook to a wire which has power only when the ignition key is in the 'RUN' and 'START' position. Connect the wire from the 'S' terminal to the green wire from the box. You can now tie up the plastic box and reinstall the hush panels.

On to the dirty work

Jack up the car and support it securely with jackstands (or use a lift if you are lucky enough). Have a pan of some sort ready to catch any transmission fluid that will drain from the transmission. Remove the cable from the existing sending unit on the tranny, then remove the sending unit itself. It is held on by a bolt and clamp (similiar to a distributor clamp and bolt). A 10mm wrench (a socket may not fit) will remove the bolt. The VSS unit installs the same way the mechanical one was removed. Then hook up the electrical plug, refill the transmission to the proper level, lower the car to the ground, hook up the negative battery cable, and you're ready to go.

This article has been researched and written by Peter Hetherington, AKA Petes 84Z28 using
information and drawings obtained from Chiltons' 1982-1994 Pontiac Firebird repair manual.