I recently bought a 93 miata with a blown motor, and swapped a good motor into it. The ECU, dash, and engine are all from a 90. The car is electronically stock.

From what I understand, there are no compatibility issues with any dash, ecu, motor, and chassis configuration from any combination of different NA6's.

At the main power for the ECU (red/blue wire) I get 12.5v, and at the main 12v power output of the ECU (red and white right next to the other wire) I get only 1.3v. At the solid purple wire (also a 12v power output i think) I get only 9v.

Does anybody know what on earth could be wrong here? I have already swapped to a tested ECU and all voltages tested exactly the same.

 

get an ecu for your year chassis?

 

I called the mazda dealer and verified that the part numbers for the ecu for 90 and 93 miata are the same

 

sounds lame but, 100% sure tha battery is fully charged?

 

Its at 12.7v with the car off.

 

I should also probably mention that the previous owner soldered the main fuse when it broke so that could potentially have something to do with it.

 

R/W is the main power input. Sounds like your main relay is shucked.

 

Oh ok I was misunderstanding the wiring diagram then. Thank you.

 

While that is likely the case for your car, that is not 100% true. Some 93 year cars that were destined for california had sequential injection from factory and had a few more wires running to the ecu to control the extra injectors.

 

Update: I replaced the main relay with a brand new one with no luck. Gonna try replacing my battery cable.

 

Are you still getting 1.3V on the R/W wire KOEO?

 

Im getting 1.3 on the R/W wire and with the relay out one of the pins reads 12v

 

Bump. I still have the problem. Battery negative to ground cable has been replaced and the main power cable from the batt. It has to be either ground B which I cannot find, or the 12v turn on signal from the ign. switch. Power to relay tested good.

 

You need to be methodical here. Replacing parts and testing partial functionality isn't the best troubleshooting method.

Get the schematic.
Confirm or deny that you have 12V on the WHT-RED ECU pin 1B KOEO
If you don't...
Trace forward from the battery or backward from the ECU to find where the fault is.

Power sequence is

Battery
INJ fuse
Main relay

Battery
MAIN fuse
IGN switch (KOEO)
ENGINE fuse
Main relay

Main relay
ECU pin 1B

You need both the INJ fuse path and the IGN switch path for the main relay to work. Confirm the functionality of each element in-circuit.

 

W/R is coming off the main relay. Is the INJ fuse intact? You need to probe all the pins on the main relay - 2 of the pins should be hot with the key on.

 

If power to the relay tested good, then the relay is either not being triggered, it's bad, or there's a short in the wire between the relay and the ECU. Your ECU is fine, it's just not being powered on.

Relays have 3 inputs, and one output. Two of the inputs are smaller pins for the coil. One will be 12v, the other ground. If both of these are present, the relay will complete the circuit by connecting the third input (12v) to the output pin. So test all of these with the batter on, and tell us which one you don't have. To test that output, put your multimeter on resistance and check from the ECU's R/W wire to the R/W output pin of the relay, you should read less than an ohm.