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1990 Civic SI
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Discussion Starter #1
Hello Everyone!

My name is Jeremy and I have available to me a 1990 Civic Si. The owner of the car passed away and other than what I can see on the outside, the mods done to the car are unknown. The person managing the estate (a friend of my father in law) let me pull the car home to check it out before buying it.

What I do know is that it has a D16A8 engine with a PG7 ECU. It has a header, catback exhaust, MSD ignition, cold air kit, and some suspension mods. Its been sitting for several years.

It blows the ECU fuse when I turn the key to the "run" position.

I was able to spend a couple hours today cleaning the car out a little and troubleshooting the ECU fuse popping.

So far I've confirmed-

-Main relay looks good (although I understand that shouldn't cause an ECU fuse to pop). With the main relay unplugged it doesn't blow the fuse.
-With the ECU unplugged it doesn't blow the fuse.
-With fuse #14 (Alternator solenoid valve) unplugged it doesn't blow the fuse.
-Unplugged the fuel pump harness in the car and it still popped.
-Unplugged the 02 sensor and it still popped.

Trying to piece this together with a wiring diagram but finding conflicting info online with the wiring diagrams I've printed. Does anyone have access to an accurate wiring diagram for a 90 Civic SI?

I have pictures but cannot post them yet.


1990 Civic SI
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Discussion Starter #2
Spent some time working on the car again today.

Points of interest-

-I jumped the fuel pump and got that to run.
-I plugged in ECU plug A and it makes the ECU fuse pop.
-I plugged in ECU plug B&C and the ECU fuse does not pop.
-Cleaned grounds under the hood.
-Unhooked several sensors under the hood and ECU fuse still pops.
-Fuse #2 (Starter signal) has .5 volts on it in either the run or crank position. This is with the main relay unhooked. Shouldnt that be 12 volts?

Key off no wires at ECU A have power
Key on, following wires have power-

I dont see anything else obvious. The ECU looks perfect inside and doesnt have a bad smell. As soon as I plug in ECU A and power up the ECU fuse blows.

I confirmed this car ran fine when it was parked several years ago.

I'm by no means asking anyone to solve this for me, but anyone have advice?

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