Blown ECU fuse.

[wildo]

About a month and a half ago, I'm driving down the road and the car just dies. Literally just shuts off. I didn't even realize it shut off until I pressed the gas to accelerate up a hill and nothing happened. Tracked it down to a blown ECU fuse (the fuse marked "Engine" in the cabin fuse block.

Replaced the fuse, and was good to go for about two weeks when it happened again. Replaced fuse. This has gone on and on for a few more weeks now until just today I'm driving home from work and blew the fuse three times! This happens totally randomly- not when I hit a bump in the road, not when/if I'm driving aggressively, it is totally random. In fact- one time I was driving about 50mph, saw a traffic jam ahead, pressed the clutch to coast a bit- and it died while coasting with the clutch in!

So I'm thinking my (MP3-flashed) ECU is biting the dust... Thoughts??

(I did find this fairly recent thread while searching, but he hasn't mentioned a reoccurrence of the issue.)

 

[tweety2300]

The only way to find out is to get a junkyard ecu and see what happens have you checked all the grounds?

 

[wildo]

Yep- I've checked all the grounds I can find... I just saw a standard P5 here in the FS section. I'll try to pick that up and test. I don't think that any sensors should be able to cause a high current load on the ECU- they should all be isolated. So if the ECU is blowing amps, it must be bad itself. That's the best logic I have... Ugh. What a pain.

 

[tweety2300]

A pain would be if the ecu is good... a shot in the dark but could the vtcs cause it? The mp3s don't have vtcs so it getting a signal from a sensor its not supposed to have could have something do with it

 

[MazdaSpeeder]

The next step is definitely to swap out ECUs and see if it still does it. It does sound like the ECU has an issue, as odd as that sounds. Maybe Spicy would be willing to send you an ECU for a deposit for you to check, then send back. I know he always has a few lying around...

 

[RABID_MP5]

Have you tried moving the harness between the fuse box and ECU, while it's energized, to try and blow the fuse? You might also try and jury rig a new fused 12V to the ECU to rule out the harness. Next I guess is to pull out the ECU and look for obvious short possibilities etc.

 

[wildo]

Thanks for the advice everyone. I have not wiggled the wires around to try to force a a short, though I have disconnected the entire harness from the ECU and reconnected it. I also just scored (thankfully!!) a P5 (non-flashed) ECU from the FS section. I definitely agree that's the next step. I was just not looking forward to having to buy a new ECU if there wasn't one currently for sale here. The "keep a box of fuses in the car" method worked pretty good until it started blowing every other min...

 

[P5Rydah]

I have a friend who was having that problem on his car (not a P5) and it ended up being a wire that was shorting out. Don't quote me on this, but I think he said a wire came loose and would hit somewhere on the exhaust pipe, short, and blow the fuse.

 

[wildo]

I have a friend who was having that problem on his car (not a P5) and it ended up being a wire that was shorting out. Don't quote me on this, but I think he said a wire came loose and would hit somewhere on the exhaust pipe, short, and blow the fuse.

How does one go about tracking down a shorting wire?? Is there an actual method, or do you just start looking around?

 

[wildo]

Actually, I suppose a better question is if anyone knows where to find the wiring diagram for the car, or more specifically- for the ECU power circuit. I don't think there are wiring diagrams in the manual easily found online. Is there an electrical manual or something?

 

[njaremka]

the wiring diagram supplement should be avaialble from the same link as the shop manual.

the "Engine" fuse is connected to the cooling fan system, the ECU, the anit-lock brake system, and the airbag system. so, theoretically, it could be any of those components causing the fault. maybe one of your cooling fan connections has a short in it?

 

[wildo]

What!? The 10A cabin fuse is powering all that!? I'm really shocked by that. And gotta say- you are a lifesaver to at least give me some explicit places to look. I did get a new (used) P5 ECU in the mail today, installed it, and haven't had a failure yet- but that's not really saying anything. I will say- I am so pissed that I feel NO difference between my flashed MP3 ecu and the P5 ecu. I never did feel like it did much at all.

Looks like I have some systems to check... Thanks njaremka

 

[wildo]

Alright- I found the electrical diagram. I am not sure where you are seeing that this circuit supplies all that stuff. Perhaps I am looking at the wrong thing... I've included a screencap where I have traced the circuit in green. I'm seeing that this 10A fuse powers the main relay, a capacitor near the coil packs (according to the component placement), the coil packs, and finally the O2 sensors.

My thought on the O2 sensors is that if they were going bad- the car would run like crap. I'm not putting a lot of stock in them being the issue. Additionally, if the coil packs were going bad- I'd equally think that the car would run like crap. So while I think it's more likely to be the coil packs than the O2 sensors shorting out- I still don't particularly think that's the issue. That leaves the main fuse and capacitor.

What is very, very interesting- when this issue first happened, I had found that the capacitor wires were pulled out of their harness. I didn't put much stock in that because I didn't realize that was a cap, and it didn't seem to be a sensor of any kind. Now, I think I will definitely go check my solder job and ensure it is no longer shorting. I might replace the cap as well.

The main relay is also a trouble point. A relay is just a resistive coil that creates a magnetic field. If the coil gets a short, the resistance drops. When it shorts bad enough- the resistance will go to near zero ohms and that's a direct short to ground. So I think that a new main relay would also be a very good choice (and probably pretty cheap).

Lastly- I can't find any indication in this schematic what circuit is actually powering the ECU, but I am questioning that it is the circuit in question...

 

[wildo]

Wow. I blame the heat wave. It must have gotten so hot under the hood to desolder my new solder connections. Like I said, when this issue first started happening, I noticed that cap had somehow had the wires pulled out of the harness. I added some extensions and soldered it all back up. But look what I just found:

And I tested my theory by intentionally shorting those wires and sure enough: blown fuse. Bingo! (I hope)

 

[MazdaSpeeder]

Well, THERE'S your problem. Holy crap. Glad you found it!

 

[njaremka]

awesome, glad to help!

 

[pasadena_commut]

It must have gotten so hot under the hood to desolder my new solder connections.

No, it did not get that hot. If it did all the plastic in your engine compartment would have melted. Correction, ignited.

It looks like the solder just cracked, probably from the vibrations, and either there was no physical connection other than the solder, or the wires snapped too. When you repair this, slide some shrink wrap down the wires, strip the wire, put it through the holes (I hope there are holes) on the metal pegs, twist the wire around itself (to make both a good electrical and good physical connection), solder, then slide the shrink wrap up and shrink it onto the connectors. The shrink wrap will help stabilize the solder junction and it should last longer.

 

[wildo]

HUGE update here! After this many blown fuses....

...It was time to find the damn problem.
This is the bracket that was causing the problem. Also note the busted wire tire.

Here you can see the rubbed wire harness.

And here you can actually see the wire rubbed down to the copper.

Unwrapped the tape to expose the bad wire.

All wrapped up, AND wire loomed. I used the highest rated black tape I could find- rated for 130* C

All fixed.

 

[njaremka]

awesome