Hondata

INFO:
- Car: Mazda Miata w K24 swap
- serial number of your K-Pro: 40058
- version of software being used: 4.1.5.0
- ECU part number: 37820-PRB-A05 serial number: 2068-503822 0L28
- KManager version: 4.1.5.0?
- when and where you got the K-Pro: Upgrade to Kpro V4 DEC 2014 by Hondata

OK...
Seems like I'm having a daughterboard communication error.
But problem is... it's intermittent, and I'm having some trouble diagnosing the issue.

When it happens, I get a unreadable check engine light and I loose connection to the kpro. I can't pull any codes and display from my computer/phone(bluetooth) freezes and stops displaying all of the engine parameters. Car also goes into limp mode and runs rough and doesnt go past 4000 rpm. It however does NOT stall the vehicle, and once I restart the car, error goes away and functions normal again.

It happens time to time during cold start / under large electrical load i.e. at night with wipers/heater and sometimes during WOT pulls.

If you see my datalog of cold start idling, my voltage fluctuation is pretty severe...

INFO about my car.
-My car has fairly new battery, around 6 month old.
-Refurbished alternator from Denso.
-AEM Underdrive pulley on the alternator
-Miata has the battery in the rear of the vehicle from factory.

Currently I tried
-re-grounding engine ground/injector ground by following these guidelines
https://www.hondata.com/tech-k20-swap-grounds
-battery ground check, I sanded the battery post connectors for better contact, and stripped paint off the chassis ground.
-ecu chassis ground (I made one)
-Swap Harness main relay verification by following Hondata schemetics
http://www.k20a.org/upload/k20awiring.gif
-Ran the main relay constant 12V wire directly from the alternator with a 15A in-line fuse.
-Opened up the ECU and unscrew daughterboard, verified connections

The battery voltage is dropping too low when starting. It sounds like you have already determined the cause is excessive load during cold starts. The location of the battery in this case is not helping matters. You may need a battery with more cold cranking amps. YOu may also need to trace your wiring to see where the voltage drop is occurring.

I got my battery replaced and the voltage drop during start up is same.
Since I cant move the main battery to the front, is it possible to run a small 12V secondary battery in the front?

Would running a secondary battery in the front help?

Describe the battery wiring.
The battery positive wire needs to be a reasonable gauge, depending on length probably 2 gauge, and go directly to starter motor without any other connections, breaks, Ts, fuses or other wires spliced in . Then use a short cable connecting the starter to alternator, which can be 4 gauge or thicker. The ends of both wires need to be both crimped and soldered. For the vehicle 12V supply, I would use another cable direct to the battery. You might get away with running one wire to a junction near the starter, but it would need to use ring terminals and bolts and then you would have to use a multi-meter to measure current flows with the engine running to make sure there are no high resistance joints. I'd just use two cables.
With many cars the ground also needs to be a reasonable gauge wire from the battery negative to where the vehicle electrics ground points are. Grounding through the chassis can be hit or miss. Ground points needs to be bare metal, and again the cables need ends which are crimped and also soldered.

OK,
So seems like I will have to do the wiring again from the scratch.

I'm using OEM Miata wires and they seem to be around 4 AWG and I do have a T in between battery/starter/alternator (alternator wiring is going through OEM miata fuse box)
alternator wire is around 8 AWG and starter wire is off of K20A2 charging wire.

For the T I'm using a distribution block, looks very similar to one of these.


Any recommendation on wire types?

The alternator wire needs to go to the battery. If you give me lengths I can recommend wires sizes.
Even very slight resistance at a connection is a problem when moderate currents are involved. That's why I suggest crimped and soldered connections with no joints.

Will fix that!

I'll fix the wiring to BAT >>STR >> ALT >> 12V

Miata has battery in the rear passenger corner of the vehicle, so I'm estimating I'll need about 12 feet of wire to get from the battery to starter.

Really appreciate all of the support!