chitty chitty bang bang

iTrader: (1)

Ok team, here is my summation of the MTX-L install (with questions). Does below make sense?



Notes:

General:
Yellow to Megasquirt wb02 input
red to power
black to ground
white to headlight for dimming toggle

Calibration for yellow signal: slightly different from instruction book but this forum seems to like Yellow .22v = 7.35
5v = 22.34

Actual Wiring
- Red to factory 12v fuel pump relay. On an earlier NA this is B577 under the dash.
- Black go round. Joe Perez likes to ground to the PNP. " In the case of a Plug-n-play, splicing into the ananlog ground wire of the harness (at a point near the ECU connector) is recommended."

- Others like to ground to the Throttle body - "There's a ground (which is supposed to be the ECU ground) located below the throttle body in between the throttle body and the head. You can't miss it. I ran the green and white grounds there. ". Can I supply ground out through the DB15?

- Yellow to DIYPNP 02 input. Either DB15 to DIYPNP or maybe extend yellow to engine bay and connect into the 02 sensor harness.

Questions

Super n00b question 1: the MTX-L wiring out of the gauge is very short... do I extend them?

Question 2: What's the best way to connect the ground? (A)Throttle body, (B) splice the ground input for the diypnp? For me this will be to splice into the incoming 4D or 4C or (c) connect a wire from one of the GROUNDS near the proto area to a pin on the DB15 and tie the power for the MTX-L into the DB15 pigtail

Question 3: Yellow 02 input - do I (A) extend yellow into the engine bay and "plug" into the O2 sensor harness like I plugged my IAT wire into the MAF harness (B) wire yellow to DB15, connect to another pin, and wire that pin to O2 which is 3C on my connector board.

Question 4: does anyone have a picture of where to connect to the headlight signal

Question 5: How do I get the MTX-L cable from the exhaust into the cabin? I cna only find an unused grommet in the firewall and I am already using that for my vacuum line.

chitty chitty bang bang

iTrader: (1)

Ok, this is what I ended up doing and calibration worked out well.

I connected most everything to the ECU harness (Ground, 12V, o2) so the ground that the wideband sees is the same as the ECU. For 12V, I used an inline 10A fuse. I made a long pigtail to connect to the actual gauge and used old IDE power connectors to mate the connections.

For the sensor, I drilled a new hole in the firewall in front of the passenger side and routed vacuum and the sensor cabling. I left the control unit or whatever the giant lumpy part of the cable out in the engine bay as it looks properly weather sealed.

To seal the hole in the firewall, I used 3M self stick silicone tape to bundle together my Vac hose and cabling and made it thick enough that I could shove the ball of silicone snugly into the hole I drilled.

My wiring setup:

(Please tell me if the 12V connection is acceptable or if not, how I can connect into the fuel pump relay)

noname4me

The dimmer wire (white wire) from the MTX-L gauge should not be connected to the actual dimmer circuit. Instead, the dimmer wire from the gauge needs a full 12v signal when you have the dash lights or parking lights turned on. I tapped into a wire under the dash which feeds the parking lights. Found it using the wiring diagram for my particular year Miata. If you try to use a connection to the interior dimmer circuit, the <12v signal will do weird things to the gauge.

The 12v power feed for the gauge should ideally be connected to a circuit which is only on with the engine running or starting, not one which is hot with the 'key on engine off' state because of the O2 sensor heater. However, I have my MTX-L wired to be powered on any time the ECU receives power with no problems to date. Makes it easier to use the O2 gauge/sensor to tune starting and cold warm up, as you can turn the key to on, wait for the heating cycle to finish, then start the engine without powering down the gauge. This gives you O2 readings as soon as the engine starts. The warning in the manual mentions that with some exhaust systems, condensation could be pushed/thrown against the hot O2 sensor by the engine starting, damaging the sensor. With my RB header, I haven't had that issue and have used the O2 gauge to help with cold startup tuning.

I am going to change the ground of my gauge to tie into the ECU wiring harness sensor ground, as even a good chassis ground does cause a differential in O2 sensor data between the gauge readout and ECU data feed. This is very common knowledge on this forum... Should have done it right the first time.

chitty chitty bang bang

iTrader: (1)

Thanks NoName, my understanding is that one end of the dimmer receives full 12V signal upon LIGHTS ON and outputs a 0-12V signal based on user input / dial selection is this correct? Even so, I like your parking light idea and I will do as you suggest.

I will take your advice about the rewiring my power line. You are right, the instructions say to put it on "IGNITION" not "Accessory Power" I will connect to the FP relay.