Setup and wiring notes for the 1989–1995 1.6 MX-5 (NA) running a DigiTune ME221 or ME442 ECU.
Wideband AFR gauge/controller
ME442 ECUs have an inbuilt wideband controller, so they do not require an additional gauge or controller. The ME221 does not have an inbuilt wideband controller, so an external wideband gauge is required. I recommend the AEM X-Series wideband AFR gauge:
AEM X-Series UEGO wideband sensor and gauge
Or an AEM wideband controller without a gauge, if preferred:
AEM X-Series wideband controller
Recommended wiring:
| AEM wire | Connection |
|---|---|
| Red | 12V switched live |
| Black | Chassis |
| White | OEM narrowband signal wire |
| Brown | ECU case |
TPS
The 1989–1995 MX-5 did not come as standard with a variable TPS. The standard TPS is a switch type that is not compatible with aftermarket ECUs such as the ME221/ME442. A BMW TPS conversion is highly recommended. This allows the ECU to respond faster to changes in throttle position, enables the use of closed-loop boost control, and improves idle entry and exit by responding to your throttle inputs. Good sources for these kits are:
Motorsport Electronics – MX-5 VTPS conversion
or
BOFI Racing – BMW E36 variable throttle position sensor
Although these BMW TPS conversions are often sold as plug and play, to work correctly they do need two wires swapping on the OEM connector. See the diagram below:
This can be done either by removing the pins and re-inserting them in the correct positions, or by cutting and splicing the wires.
After correcting the TPS wiring, please remember to re-calibrate your TPS sensor in MEITE. This can be done by going to ECU → Throttle Calibration.
Knock sensor
The Mk1 MX-5 did not come from the factory with a knock sensor, but one can be retrofitted and connected to the ECU. A knock sensor is recommended for forced-induction or high-compression naturally aspirated engines. Without a knock sensor, I can still tune the car, but I will need to use lower ignition timing in order to guarantee that it will not knock/detonate.
We build and sell knock sensor kits that mount to the Mk2 OEM knock sensor location:
The early Mk1 cars with analogue oil pressure senders need the oil pressure sender spaced out from the block in order to provide room for the knock sensor. This can be achieved with a 1/8″ BSPT extension, or a 1/8″ BSPT to 1/8″ BSPT and 1/8″ NPT T-piece adapter. The use of Teflon (PTFE) tape is recommended when installing oil fittings.
Grounding
The stock wiring on a Mk1 1.6 MX-5 creates a “ground loop” that causes current to flow through the sensor ground. This current flow can cause offsets and noise on analogue measurements.
To avoid this, we recommend cutting the signal ground wire that runs to the back of the head. This is the black wire with a green stripe shown in the photo below.
