A parcel arrived this morning containing an MX5 mk2 instrument cluster and a reverse light switch. I ordered the instrument cluster because it should contain a bit of electrickery that converts the AC wave signal from the speedo sender to a DC pulse.
Itchy fingers, and sensing this could be tricky, I got right to work. The thought of heading down to the garage on a dreary day didn’t really appeal so I made use of the indoor workbench. This also doubles up as an ironing board which means the missus doesn’t get angry with me leaving it up.
Researching online showed a slightly easier way to achieve what I wanted to. I originally thought I’d have to find out what each pin on the board does and tap into the appropriate ones. However, there seems to be an easier way. The ECU gets a 5v pulse speed signal from the instrument cluster, so I could just tap into that. Apparently this is pin 2L on the instrument cluster. I started tracing the path to see if it when to the board but this got quite tricky towards the middle. Instead, I broke out the voltmeter. With it on continuity mode, I put one end on pin 2L and then tested the connections on the ribbon style cable at the other end. Soon enough, one beeped!
My plan therefore is to find out what goes to positive and earth, what connects to the speed sender and just wire those up. The one that goes to 2L will provide my speed signal.
The potential hiccup here is I was originally supplying the dash with a 12v pulse and it will now get a 5v pulse. I think it will be ok with that but I’ll keep it in mind. I’ll also get a different number of pulses per mile but that’s fine, that can be configured easily.