Since having a new engine put int, I've had the erratic speedometer issue a few times but nothing major or long term.

Now I'm having had new issues after installing my AEM EMS v2 and having it tuned on last Friday. My tuner has tuned many S2000's and other cars. We even installed a tune from a different car that we know runs fine.

First, my tuner said that the wideband reading on the EMS was not matching that of the gauge and thought that was. Then as we were tuning, the A/C stopped working (checked the relays they were fine) and would only work intermittently. Then, the car was hard to start. The voltage at the Alt was in the high 12's and seemed a little low even when reving it. Finally, the starter sounded like it wasn't engaging (like a bad bendix). I was able to get the car home and turned it off and on a few times and the starter worked. I put the OEM ECU back in and when testing the voltage at the alt, it read 13.35 regardless of RPM. The A/C seemed to work fine on the OEM ECU, but the starter still didn't work every time.

Car started fine on OEM ECU after sitting all weekend. Alternator read 13.3/4 with no accessories on but between 11.9 and 12.5 with lights, a/c, and radio on.

I've read that the erratic speedometer can be caused by a bad ground (specifically the one on the rear of the head). That seems tight but It's tough to get back there to check more.

If anyone could point my in the right direction, I would greatly appreciate it.

 

Ended up being a loose ground wire on the bottom of the block.

Problem solved.