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Background: Turbo Miata (purchased from a member here). A lot of wiring was done with wire nuts, and lots of extraneous wiring was left in the car from previous modifications. I have been eliminating these pieces one system at a time, this may be relevant, then again it may not be.

Problem at hand: The car blows the ECU fuse (under dash by the hood release pull, 15A), and only does so at very high RPMs and boost (10+psi, above 5,000 rpm). This is the only time it blows, otherwise the car can be driven fine for any length of time.

Diagnostics so far: Checked resistance across coils to see if any look particularly different, they do not. Tried moving the wiring around with ignition on to see if it blows the fuse, and look for any obvious places for fraying etc. none present. Put the Wideband on a 5A fusible link, did the same to the boost control solenoid. No change, still (only) blows the ECU fuse.

I am thinking a faulty injector may be to blame? Would love any other ideas, tests, opinions, etc.
Thanks!

olderguy

iTrader: (19)

What ECU is in the car?

L337TurboZ

iTrader: (1)

Maybe they tied the fuel pump into it somehow? I had a similar issue on my Starion. I installed a Walboro 255lph pump and every once in a while it would suddenly spike in amperage. The fuse for my ECU would pop and the engine would stall. This was only after extreme WOT pulls back to back. Eventually I replaced the wiring for the pump and gave it a separate relay.

They are wired similar to the Miata with the main relay activating the fuel pump. In my Miata the main relay actually got hot over time when I installed a AEM pump which cut the car off. I had to unpin the under hood fuse block and install a whole new fuse block since it had melted.

Repeating the process which corrected my Starion worked for the Miata as well. No more overheating the main relay, no more popped fuses.

Just something for you to check. Not gonna say that's the actual fault.

Reverant

iTrader: (10)

At higher RPM, the injectors and coils pull more current. At higher load, the injectors pull even more current. If you have added any additional load to the engine fuse circuit (such as a wideband and/or a higher rated fuel pump), it is somewhat expected for the fuse to blow.

bar

Thank you both for the feedback So, I would think that the fuel pump would be on a separate fuse and circuit (save for the signal to the fuel pump relay). I will try pulling the fuel pump fuse, see if the pump still comes on. The car does have a higher flow pump. My line of thinking was around coils and injectors since those are the parts that would draw more under high loads.

I pulled an alarm unit out recently that was causing an intermittent no-start condition (starter would click but often not engage). I wonder if it was feeding the ECU circuit in parallel...

I will try moving the Wideband to a separate circuit, and see if activating the fuel pump relay powers the fuel pump with the fuel pump fuse pulled out.

Reverant

iTrader: (10)

The wideband will also pull more current at higher RPM as it will try to keep the heater of the sensor at optimal temp.

bar

It was my understanding that the heater circuit draws less current after start up, since after start up/at high RPM the exhaust gasses keep the sensor hot?

I did place an inline 5 amp fuse for the Wideband and that fuse did not blow, so unless the draw from the Wideband is just enough to push the circuit past the edge, there may be another problem.

I can try powering the Wideband from a different circuit, or could power the coils directly with a relay (or from the cigarette lighter circuit or something).

L337TurboZ

iTrader: (1)

The only time I would ever use any type of wire related to the ECU for wideband use would be to just run a signal wire from the controller back to the ECU.

Example: AEM has an output wire from the gauge to the ECU to simulate a factory sensor
You should run any wideband or gauge power from its own fuse on a completely separate circuit.

Reverant

iTrader: (10)

At high RPM, the exhaust gases are actually cooling the sensor down, and thus the heater draws more current . Source: Me implementing a wideband controller for my ECU and having actual datalogs to back it up.

bar

Thank you for sharing!

Update: Moved the Wideband to a separate circuit for testing (ordered a pigtail to plug into the factory front 02 sensor connector, plan to to get power from there and feed the simulated narrow band signal there for the ECU as well).

Result: Fuse intermittently blows very near redline (currently set to 7800rpm). It does not blow under 7000rpm, and only blows at or close to redline about half the time.

Thinking about powering coils directly with an ignition switched relay.

bar

I apologize I missed this, it has a Link Obi ECU.

olderguy

iTrader: (19)

If memory serves me correctly, a Link ECU should be the only thing fed through a 15 Amp fuse to the battery. I would remove anything else in that circuit and feed it separately.

bar

To be clear, are you recommending I remove the coils from the ECU circuit? The ECU still needs to actuate injectors and turn on things like the fuel pump (just the relay signal). I am not aware of what else to remove from that circuit.

L337TurboZ

iTrader: (1)

Think about this, something is causing excessive current draw at high RPM wether it be your coils, injectors, or the driver's in the ECU. The components powered by that circuit should match whatever the OEM diagram is. Your LinkEcu should have its own separate power fuse to power on/off. If you're powering the ECU off the circuit that's shared with the injectors and coils then you could be causing that spike.

It obviously isn't a hard short otherwise it would pop immediately. Have you done any resistance checks of the wiring itself or any loaded voltage tests of that wire to make sure it's able to carry current properly?

bar

That is my line of thinking also, something to do with high engine rpm/load causing excessive current draw on that circuit...

I was under the impression that the coils are powered by the same circuit as the ECU from the factory, and obviously injectors are driven from the ECU as well. I can check to see if there's still power at coils with ECU fuse out.

I have not checked resistance across the wiring as of yet, will keep that in mind if I do not find something else obvious causing the issue. I am planning to check resistance across injectors as well.

L337TurboZ

iTrader: (1)

Is the fuel pump stock or aftermarket?

bar

It is an aftermarket high flow pump.

PLOT THICKENS UPDATE:
Spent some time with the factory wiring diagram. It appears that on the 97 Miata, the 15amp fuse labeled ECU does not actually power the ECU. It instead powers the ignition coils, cooling fan relay and the fuel pump relay (which then powers the ECU). I have confirmed this. The ECU is powered on (along with various other circuits including fuel pump) with the 15 amp ECU fuse pulled out, if the main relay is jumped. So my understanding is that the problem has to be an ignition coil, wiring or relay with too much draw. I don't see any obvious problems with the wiring and between the coils and the main fuse block there is negligible resistance. That leads me to think the aftermarket coils are drawing more than the circuit was designed for at higher RPMs/load.

I could move the power for two of the coil packs to the front O2 sensor port. I was going to do this for the Wideband since it is a switched and fused 12v system currently unused and I have the pigtail for it already, it would be easy to power half the coils through the same circuit.

curly

iTrader: (25)

I do a **** ton of wiring at work, so your problem peaked my interests. To be clear, it's not the ECU fuse, it's the Engine fuse. It powers, according to the power distribution diagram, ABS, Air conditioning, Supplemental restraints, Engine controls, and Cooling fans.

The ABS, AC, airbags, and cooling fans are all just the coil side of the relay, not the device's power source. This coil side uses MAYBE a tenth or two of an amp. This is simply used as a "wake up" signal for these modules/relays. The only other thing it powers is the coils themselves, which I find interesting. So in total you have the Engine fuse powering 5-6 relay coils, so maybe 1amp. Rest is coils, so if you're blowing this fuse, I would for sure remove the aftermarket coils from it. At that point you could even lower the fuse to a 2.5 or 5amp fuse.

To do this, get a new relay. Permanently ground pin 85, put the old coil power to 86, attach pin 30 directly to the battery through a 20amp fuse, and pin 87 will go to the coils. I know on aftermarket BMW coil harnesses they give the coils direct battery power (just fused, no relay), and their own dedicated ground as well. So you could potentially not use a relay at all, and as long as it's not causing a draw with the key off killing your battery, you'd be fine.

Edit: if you're curious, AC fan gets power from AD fuse, cooling fan gets power from the Cooling fan fuse, ECU is Fuel Inj. fuse, ABS has a 30 and 40 amp fuse, and Airbags has it's own fuse as well.

bar

I really appreciate another perspective! I was pretty surprised at what this fuse powered up as well, energizing a bunch of relays... and the ignition coils directly, seemed odd. Anyway, the coils are the Toyota Coil On Plugs that a lot of people seem to be running. I have yet to find another problem in the system, and this does only happen under very heavy load, it does seem most likely that it is the coils drawing too much current (simply revving to redline and bouncing off the rev limiter does not cause any issues, only high boost at high RPM).

I like your idea of running the coils separately and using the original coil power as the trigger for the relay. However, I also really want to keep aftermarket wiring to a minimum and there is another switched/fused line coming right up to the coils via the factory front heated o2 sensor (and currently that circuit powers nothing at all), with a nice sealed pigtail ready to connect to that port. So I think I will run two of the coils from this port. Another benefit would be that if I do have a coil go bad, this would let me know which of the two sets of coils it is! -Still batch ignition with the OG Link Obi ECU-.

Of course I will post an update!

curly

iTrader: (25)

That's the Meter fuse. It also wakes up the airbags, powers your reverse lights, blinker module, antenna relay, cruise control switch, seat belt indicator, warning unit, CEL, rear defog indicator, oil pressure gauge, coolant temp gauge, tach, fuel gauge, and yes the o2 sensors you're not using. So to say it's not powering anything is sorta incorrect. I would probably just give them direct battery power or through a relay if they're already blowing a different 15 amp fuse.