moss500

I have a stock 92 Civic DX and every time I turn the ignition to ON(ll) the 15amp ECU fuse blows. Slot 31 I think in the fuse box under the hood.

The fuel pump is new. I unplugged it and still blown fuse.

I unplugged the 02 Sensor, still a blown fuse. I also unplugged the VSS (speed sensor), still a blown fuse. The VSS is also new.

Anyone know what could be causing it? The car won't start. I know this topic has been covered, but usually no conclusions were made. I almost replaced the ignition coil until I realized the fuse was causing my problem.

Former User

Also unplug the fuel injectors and EVAP purge solenoid, and carefully inspect the O2 sensor wires on the wire harness side.

moss500

Thanks for the leads Ron. Should I unplug the injectors 1 by 1?

Where can I find the EVAP purge solenoid? (Noob here) I believe it's behind the intake manifold, but there's a few things back there.

Former User

Unplug everything I mentioned, including the O2 sensor, and then check whether the fuse still blows.

moss500

Useful diagram. I'll update you with my results.

moss500

So I unplugged the 02 sensor, all 4 fuel injectors, and the EVAP solenoid.

Still burned a fuse. I didn't hear the fuse pop as quickly, so I removed it and it was burned (not snapped like the previous ones). Tried a second fuse and heard it snap right away.

Also, the 02 wiring appears to be okay.

Former User

Oops, forgot to mention to unplug the IACV.

moss500

Unplugged, still blew right away.
I'm out of fuses til tomorrow, but open to suggestions.

Former User

The last two components on the fuse 31 circuit are the main relay and ECU. Unplug them next.

moss500

With the main relay unplugged the fuse isn't blowing. I tried a couple 15 amp fuses to make sure.

Is it as simple as replacing the main relay? I'll have one in an hour if so.

Former User

I doubt the main relay is the problem. Did unplugging the ECU have no effect (with main relay plugged in)?

The most common cause of your problem is a shorted Yel/Blk wire for the O2 sensor.

moss500

It's not the relay. I can't figure out how to unplug the ECU. I have the carpet back and can't find any diagrams or photos online.

There's a black box with wire running to it from the top, but the wiring is taped as if it doesn't unplug. Do I unbolt the ECU and look on the bottom? I was under the impression it plugged in on the top or bottom, but the bottom is covered by a metal cap.

Looking at the 02 sensor, I don't really see any black or yellow wires. I see some running to what appears to be the starter?

Bare with me, I'm no mechanic. I appreciate your help.

moss500

Is there only one 02 sensor?

Former User

Only one O2 sensor for 92-95 Civics. It should be plugged into the exhaust manifold. One of the wires on the wire harness side of the connector is Yel/Blk.

nub

You only have 1 O2 sensor.

The plugs on your ecu are on the lower side behind the metal bracket. Unbolt the ecu to access the plugs.

moss500

Thanks guys. Just double checking about the 02. I do see a yel/blk wire on the harness side.

With the ECU and main relay unplugged, the fuse doesn't burn.

So what's next?

Edit. BTW the rest were plugged back in during that test (EVAP, IAVC, 02 & Injectors)

tech8

Try it with the main relay plugged in; but, unplug the ECU; does the No. 31 fuse blow?

Former User

This^

moss500

Fuse doesn't blow with relay plugged in and ECU unplugged.

tech8

There may be a short in the Yel/Blk wire along the main wire harness between ECU connector C405 and the main relay (connector C437). Another possibility would be a short inside the ECU. A short is an accidental connection of a wire to ground or to another wire or the short can be a component contacts are shorted.

Visually check the connector's terminal pins and wiring, and wiring along the main wire harness.

moss500

Just to confirm, C405 is the 16 pin? I only ask because in the second diagram it appears to be the first connector (which is 26 pin). I see no damage anywhere so far. I only see some electrical tape by the ECU connectors, but it could be factory.

Would I have to trace the ylw/blk wire from C405 to C437? Removing the dash?

Just to add, the car is low mileage (60,000) and no rust or water damage. It's never had the dash removed, even for a stereo install. What are the chances it's the ylw/blk wire?

tech8

Yes, C405 is the 16 pin connector.

Just look under there first, without trying to remove much, to see if there is anything obvious.

If everything is plugged back in, except for C405 at the ECU and the fuse doesn't blow; the short is likely along the yel/blk wire there or it is the ECU.

tech8

If you want, you can do a simple test to re-verify: Plug everything back in, except for ECU connector C405; does fuse 31 blow? If it didn't blow, then plug C405 back in, does it blow afterward?

moss500

Okay, thanks for the info.
With C405 unplugged and C404 & C406 plugged in, the fuse blows.
Edit. I'll try plugging C405 back in and see if it blows. One minute.

moss500

With C405 plugged back in, the fuse blows.
Edit. I realized you said if it didn't blow. My mistake. It did, so me plugging C405 back in was irrelevant. I was thinking that as I did it, but that's okay.. it's only a fuse. *Puts beer down*