Electrical Guru's help me find a short

[Savage Talon]

I have a short somewhere on my mpi circut, it blows the 20 amp fusable link as soon as the key goes to the on position. It only blows when the center connector of the ecu is pluged in (51-68). And it does not blow until the key is turned to the on postion. Please ask me questions and help me track this down. thanks

 

[steve]

So it blows when you turn the MPI relay on.

Grab the wiring diagram, and handful of 20a fuses and your multimeter.

About half the engine bay is powered off the MPI circuit. Unless you just changed something you don't have much to go on, so you will have to start disconnecting things powered by the MPI until the find the one blowing the fuse. Or you can disconnect everything and than start plugging things in until you blow the fuse.

Steve

 

[Savage Talon]

That sucks, I have done that with a few things. Coils, transistor, Idle motor, ac relay, all with no luck. If I have the key in the on position, I can connect either the small plug or the center pug in to the ecu. But not both at the same time, that is when it blows. So the ecu is telling the mpi relay to power something up that has a short, or the wire is shorted before the elect. part. If that is the case I could disconect all the stuff I want and it will still blow the fuse.

 

[want_a_gsx]

Well when that same problem was happening to me, the shop found out that the previous owner powered the stereo using speaker wire, burning an "ecu wire". You may want to check the ecu wiring itself and go from there, but then again, you could have a whole different problem, since many functions depend on the mpi fuse...

Good luck

 

[steve]

Only the ISC of all the items you listed is connected to the MPI circuit. You really need the wiring diagram.

It's pin 63 or 66 on the bottom row that the ECU uses to activate the MPI relay.
You can make sure the problem isn't the ECU by disconnecting the ECU connectors and ground one of those pins (the connect together on the ECU board).
If the fuse still blows then it's not the ECU.

The small connector has the power pins and the ignition RUN signal that the ECU uses to sence when it's to pull pins 63/66 low so that why disconnecting either of them keeps the fuse from blowing since then the MPI relay doesn't activate putting power on the MPI circuit.

 

[Savage Talon]

I have a manual on cd but I'm not the best at reading the schematics. It's a whole lot easier on my motorcycle schematic. My brain is fried I guess I should just continue tomorrow.

 

[Savage Talon]

With the mpi relay unplugged I found pins 102, 107, 56 are grounded. They are not supose to be grounded are they?

 

[steve]

With the mpi relay unplugged I found pins 102, 107, 56 are grounded. They are not supose to be grounded are they?

Define grounded as your measuring it.
Pins 102 and 107 are the output of the MPI relay and the same wire as pin 2 on the MAF or pin 3 on the CAS or pin 2 on the BCS, etc. It's the power line that gets connected to the MPI fuse when you activate the MPI relay. It shouldn't have continuity to chassis ground but you already knew that there was a short somewhere since you were blowing the fuse when the relay tried connecting it to the battery.

Pin 56 is the output from the ECU to the fuel pump side of the MPI relay used to activate that relay. The other side of the relay coil is the ignition switch and it may be grounding it's output when it's switched off. It does that to the START terminal when you don't actually have the switch in the starting position.

Go find pins 4 and 5 of the MPI relay on the wiring diagram. Make a list of everyplace that wire goes and places others wires go that branch off it.

Hell I'll make it easier for you. According to the 91 wiring diagram the MPI power connects to pin 2 on the MAF, pin 3 on the CAS, pin 2 on the BCS, pins 2 and 5 of the ISC, pin 2 on the Purge Solinoid, pin 1 of both the FRP and EGR solenoids, pin 3 of the injector resistor, and pin 3 of the O2 sensor (heater). This is in addition to pins 102 and 107 on the ECU. The all are red wires.

Disconnect each of these and see if you still have continuity to ground. If you do then your harness is hosed.

Steve

 

[Savage Talon]

Grounded as I define it: Turn the Fluke to tone when probs see conectivity. Ground one prob to chassis, stick other prob in connector. Fluke makes tone.

 

[Savage Talon]

Found it, big thanks! It was the o2 sensor. The bracket that holds the connector rubbed through the wires on the harness side of the connector.