I have a 1990 honda civic si the problem is that it cranks but doesn't start. So far this is what i have found out, the fuel pump doesn't prime there's no spark. I bypassed the main relay and it turned on then I replaced the main relay and it still did not turn on or even clicked I believe that's why there is no fuel pressure or spark. I tried a different ecu, checked all my fuses. I bought a multimeter and tested the main relay plug following the steps from a thread here on honda-tech, all testes passed except this one.

https://honda-tech.com/honda-crx-ef-...-pics-2535047/

"7. Attach the positive probe of the voltmeter to the BLUE/WHITE wire (pin 4) and the negative probe to the BLACK wire (pin 2).
Fig. 35 Main Relay Harness Test (pins 2 and 4)
8. Turn the ignition switch to the START position.
a. Approximately 10 volts should be available.
b. If there is no voltage check the #2 (10A) fuse and the wiring between the ignition switch and the fuse box, and from the fuse box to the main relay."

Instead of receiving 10V I get around -36.6mV. I Followed the blue and white wire to the under dash fuse box checked the voltage and i still get -36.6mV. First time trying to follow a wiring diagram so I think it's saying that the blue/white wire is connected to the black/yellow wire leading to the ignition switch. I attached pictures of the wiring diagram from my chilton manual. Tested the black/yellow wire and i get around 11V. I then replaced the fuse box and nothing changed. Could the ignition switch be the problem? If not what else should I check. Sorry for the big pictures I don't know how to shrink them. Thank you in advance.

 

Sounds like your fuse box is the problem, the blue/white should read 10-12 volts, it is a start circuit, your starter works, so we know the ign. switch is good.

Try replacing the fuse, #2-10A just in case it is defective, [not blown, but defective] also test fuse in the fuse box, test both sides of the fuse as you crank the engine. 94

 

the blue/white does not connect to the black yellow - look at the diagram, if the blue/white was connecting to the black/yellow, you would see a black dot where the intersect - does the Check Engine Light come on when you turn the ignition on? - sounds to me like you need to check the ground wires at the thermostat housing - make sure it is connected, clean and secure