This thread will document the installation of an MPGuino custom fuel economy gauge in a first gen 1990 (NA) Mazda Miata MX-5. (I couldn't find a comprehensive DIY out there.)
Since it's pre-OBD-II, the 1990-1995 Miatas can't use one of the many plug-and-play fuel economy computers available (eg. ScanGauge).
The good news is the installation of the MPGuino isn't terribly complicated. But you will need to be comfortable finding & splicing a few wires for the MPGuino's 4 input connections:
- 12V (DC)
- Ground
- Fuel injector
- VSS (vehicle speed sensor)
Miata "RSW" = VSS...
There's one small hiccup concerning the Miata's VSS signal.
Typically, a car's VSS can be accessed by splicing a wire at the ECU. But according to the interwebs, the Miata's ECU doesn't use a speed signal, so there's no wire to tap there.
[
UPDATE: WAIT A MINUTE! According to my service manual wiring diagram, the RSW/VSS wire goes to the ECU, which removes the need to pull the cluster. Researching further...
UPDATE #2: Lies! My Haynes service manual lied! There is no VSS wire at the ECU, despite what it says.]
Fortunately, the Miata's speedometer generates a speed signal for use by the (optional) cruise control. Pulses are generated by a simple reed switch, and are accessible via the "RSW" circuit (with handy screw tap) which is clearly labeled on the back of the cluster.
The signal from this RSW/VSS circuit needs a small modification so the MPGuino can use it. This will be documented below.