1991 Honda Civic Si Hatchback. The fuse is hot with car off. It gets an immediate ground short when I turn it on. Wiring schematic says this wire goes directly to ECU. So if it were shorted against body somewhere, it would blow fuse key on of off.Tomorrow I will trace wire directly to ECU and clip it off. If it is ungrounded until I turn on key and then goes to ground when I turn on key, is that sufficient proof that my ECU has an internal short and is junk to be replaced.I hate replacing parts then finding that was not the problem.
odds are it's not your cpu. Check your engine wiring harness for shorts. Power can go through you ecu, and back out another wire where it shorts out and blows the fuse. Most likely locations to find a short are anywhere the wiring harness is too close to the engineespecially the exhaust manifold. Ultimately I would mark every wire and remove the whole harness from the engine leaving it connected to the ecu, and have a couple of fuses ready. Try the key with the only thing connected being the ECU if the fuse blows it's either your ECU or wiring harness. If not, begin reconnecting the harness one device at a time till you find the one that blows the fuse, and you've found your short, replace that component. If you determine it to be the ECU or wiring harness, you can un tape the harness and look for a burned wire. Failing that replace the ECU.
Check main relay. It may be faulty. Below Driver's Dash.
Sound like you know way too much to be on here! Your on the right track. Trace the wire and you will probably find a short. The easiest thing to do since you have the schematic is to just go straight to the ECM and overlay the wire. This way you don't have to take the whole harness apart. If it fixes your problem then make a permanent connection and zip tie or tape the wire over the factory harness. Unless you just HAVE to know where the wire is bad. But that could take awhile.
Be careful bec some schematic pgs will only indicate a splice number and not show you that the wire you're trying to trace splices into other wires feeding other devices etc and any other device connected to that same feed either before or after the ECM could cause the fuse to blow.A real good way to usually that either too much load is on a fuse or it is the wrong amp fuse or there is a short is when the fuse blows black.From what you're saying I'am gathering that in fact the fuses are blowing black.Remember that a relay that is over loading or shorting out internally would also cause the fuse that feeds it to blow. Like in any circuit if a certain pin of a module is a input or output voltage the internal circuit would read a certain resistance if you used a ohms meter to the pin and ground and if you got a ohms meter reading that showed a fully closed circuit this would suggest that the ECM has a fault in that circuit.Tests like these are hard to perform bec you have to have the factory specs on such circuits and most don't offer such info in most manuals. Hope that helps and best of luck.By the way trace any wires going to any related relays and going to the ECM bec on an old vehicle like that the wire might be cracked/dry and have bare spots or melted spots especially the harness runs close to any hot spots like the engine etc.Remember that even the voltage regulator can be shorting out. Note if I was trouble shooting that problem I'd use a reset the same amperage as the original factory fuse and I'd use alligator clips/ mini jumper cables to temporary attache the reset to the fuse socket, plus you might use a male plug in flat connector so it can fit into the fuse socket connectors, plus you could solder the wires with the connectors to the reset.Try moving/slightly tugging lightly the different harness's one by one and then then try starting it, if the rest goes it will rest once it cools down a bit.
