Engine fuse keeps popping. Reasons?

[99gst_racer]

My Talon died pulling off the freeway today. We towed it home and discovered the fuel pump wasn't coming on. And then discovered the engine fuse was popped. So, I replaced it, but it popped again upon attempting startup.

So, what copuld cause the engine fuse to constantly blow like this? I'm not an electrical diagnosis pro, so let me know what to check.

Could a bad ground track in the ECU cause this to happen?

 

[steve]

Exactly when does it pop? As soon as you replace it, when you turn the ignition on, when you hit the starter, after running for awhile?

That's the MFI fuse (the same function as the MPI fuse on the 1G) It powers the solenoids, ECU, MAF, ISC, Injectors, CAS and CKP sensors. Do you have any of the solenoids disconnected where water could get into the unused connector?

 

[99gst_racer]

Exactly when does it pop? As soon as you replace it, when you turn the ignition on, when you hit the starter, after running for awhile?

That's the MFI fuse (the same function as the MPI fuse on the 1G) It powers the solenoids, ECU, MAF, ISC, Injectors, CAS and CKP sensors. Do you have any of the solenoids disconnected where water could get into the unused connector?

It pops as soon as I hit the starter. The solenoids have been removed, but I'm 99% sure that there's no way moisture could have even had a chance to get into the connectors overnight last night. But I'll check them tomorrow to be 100% sure.

If it makes a difference (I'm pretty sure it will), I did replace my front O2 sensor yesterday evening, and I didn't drive the car again until this morning. And there's a possibility that I forgot to unplug the old O2 sensor before cutting through the wire to get a wrench over the end. I honestly can't remember either way, so it's possible at least.

 

[steve]

I forgot about the heater for the O2 sensors but it looks like they also use the MPI circuit on a 2G.
They shouldn't have been powered if the ECU was off. Are you sure it's not until you try cranking the car?

I can't think of anything in the MFI circuit directly related to to cranking. The fuel pump wasn't working because the ECU wasn't working since it wasn't getting power. Usually these fuses pop when the MPI relay closes when you turn the key to RUN do to a short.

Was the new O2 sensor a direct replacement or did you wire in a universal?

 

[99gst_racer]

I guess I've never really tried just turning the ignition ON without cranking. I'll try that tomorrow and report back.

The O2 sensor was a direct replacement and I have verified that the connection is A-OK.

 

[99gst_racer]

It is popping when the ignition is in the ON position.

I found some exposed wiring and corrected it. I also found a questionable unused connector and eliminated it. Neither resolved the issue though. I covered the entire enginebay wiring harness and everything looks good. Thoughts?


EDIT: I also opened up the ECU and verified that everything in fine there.

 

[92AWDHX40]

Paul sounds like a short somewhere, ive rewired my dsm last year, found some loose wires, but never that fuse, check your grounds. Its gotta be simple.

 

[doubleclutch99]

Can you verify the wiring on the alternator is correct ? Ie. no cut wires , loose connections, wrench laying on the positive post of the alt. i only ask because i did that one once. just a thought

 

[steve]

Sounds like this started when you replaced the O2 sensor. Disconnect them and check.

 

[99gst_racer]

Alternator wiring is fine.

I did try it with the O2 sensor unplugged, and the fuse still pops.

I did get the car to sucessfully fire up last night, so I must have moved the wiring with the short. I'm almost positive the short is somewhere within the wiring right at the ECU. That is possible, right? If so, I'll go through it and cleanup anything questionable.

 

[daddyo]

Did you rewire your fuel pump? Could have a bad relay or a wire could be shorting back there.

 

[99gst_racer]

Nope, fuel pump wiring is all stock and unmolested.

Being that I've only "moved" the ECU wiring and some wiring under the hood and the problem has disappeared, I'm certian the short is within the wiring in one of those locations.

 

[pindlepiper]

I had the exact same problem. Turned out to be a short in the o2 sensor wiring on the ecu connector plug. Sounds like it could be the same on yours.

 

[99gst_racer]

Well, I think I figured it out. I found an exposed wire that had had been tee'd into pin #76 at the ECU (front O2 sensor). I sealed it off and put everything back together, and she fired right up.

So, until the text time...
*fingers crossed*

 

[99gst_racer]

Well, I was wrong again. I've now found a handful of exposed wiring, and none of it was the root problem. But, after today's little electrical fire, I know know where the short is. Turns out it was the wiring from my Kiggly crank trigger setup. The wiring runs from the front balance shaft area, up and to the back of the engine. It travels behind the timing belt cover and rides very close to the t-belt. I've never liked the routing and now I have a great reason to reroute it.

 

[99gst_racer]

I'm still sorting this out... New crank trigger set-up is installed, and associated wiring under the hood has been replaced.


Question for you Steve - Let's say the 20a engine fuse was jumped and there was a growing short in the crankshaft position wiring. If the engine was ran until it shut off on it's own, what would have caused it to shut off? Which component becomes the fuse at that point? Would it just melt the wiring to the point that there's now a massive short that is keeping battery voltage from opperating the car's electrical components? Good chance for ECU damage?

I've replaced everything that was obviously damage under the hood, just the short is still present and still strong. This afternoon, I'm going to follow that wiring into the cabin to see how far the damage goes. I'm just trying to get an idea of what exactly happened and what to look out for.

 

[steve]

If you jump the fuse then there is no telling which part of the system is going to become the new fuse.
You stand a good chance of burning the car to the ground since many of the items connected to the MPI fuse and their wiring pass near fuel lines. If they overheat and melt the problem is going to be far more expensive that a few more 20A fuses.

 

[gofer]

Not sure if this was ever fixed so I'm submitting my input. You need to unplug everything on that circuit and see if its a shorted wire or possibly a sensor. Keep the MFI relay plugged in and unplug the following sensors...

- ECU connector B-53
- Injector Resistor Pack
- ISC
- Purge Solenoid
- BCS
- EGR Solenoid
- FPS
- CAS
- CPS
- MAF
- Front o2
- Rear o2​

Turn the key to the ON position and see if it pops it, if it still pops you'll have to start chasing wires and checking for shorts using Gary's (luv2rallye) write-up. Also check under the fuse box for wiring shorted together as well since its a damn birds nest under there.

Finding a short, partial short, or battery drain

If it doesn't pop after unplugging all that crap start plugging in one thing in at a time, checking the fuse after each sensor, and see which pops it.

 

[Detail]

Also, do yourself a favor. Check the head unit anti-static ground strap on the manifold. Its that little black clip by the stock fpr. That little bugger will pop the fuse. Yes, somehow it dead shorts that much current.

 

[99gst_racer]

As a update, this issue killed my battery badly. It would measure 13.x volts sitting on the bench, but once it was hooked up to anything at all, voltage would immediately drop to about 4 volts and continue to slowly fall from there.

It's also worth noting that the 12v crank trigger wire was burned all the way up to the MPI relay under the radio. But it didn't seem to affect anything that branched off of that wire.

 

[born2tune]

so the thread says resolved but there is no solution? what happened???? maybe i missed the post.
i need to know please, thanks.

 

[99gst_racer]

Post #15 sums up the problem. The power wire to my crank trigger was partially exposed and was arcing to the cylinder head. And it ended up melting the wiring before I discovered it.