84SE: Common Causes for Blowing "Engine" Fuse in Block?
 

Note: 1985 GSL-SE, fuel injected...

Drove 30mi to work today and pulling into the parking lot, engine shuts off. Coasted to a safe spot and started troubleshooting;
1) Fusible Links - swapped at the shock tower, brown for black, no change. Swapped back.
2) Fuse Block - 20A fuse for Engine on IG1 circuit blown. Figuring this is just a bad fuse, throw a spare 20A fuse in, key turned to ON (*NOT start, no crank), fuse blows. Replace with 30A fuse, turn key to ON, START, engine fires briefly, fuse blows again.

Diagnostics;
A) Pulled all extraneous fuses in case there's a short. Put in another 20A fuse for Engine, turn key to ON, engine starts briefly, fuse blows.
B) Disconnected Alternator + and - terminals. Put in another 20A fuse for Engine, turn key to ON, fuse blows. Reconnected Alternator.
C) Disconnected Fuel Pump under the bin. Put in another 20A fuse for Engine, turn key to ON, fuse blows. Reconnected Fuel Pump connector.

What am I missing? Car has 238k mi, been a daily driver for decades and I've not encountered this before. Nothing else changed, no recent electrical work, no recent mechanical work, didnt even add fuel recently. Stumped for the moment and headed over to Foxed.ca to look at the wiring diagram...

Thanks for any assistance offered, and I can answer any questions you guys might have,

bad engine ground ....i always hav 2...add one it won't hurt

I have a monster ground going from the Battery (-) to the Alternator bracket. Been there for years. I'll recheck tightness, though.

look at your injector harness,follow engine harness back to firewall. Look for chafing,rubbing. Pull up pass side carpet and put eyes on all wiring to ecu,disconnect ecu. Put all fuses back in,turn key on,see if fuse blows,still blows? Leave ecu disconnected. Remove EGI fuse,substitute fuse for a test light,use some ground down 1/4"spade terminals to insert in place of fuse then jumper wires so you can attach light to them and move it to where you can see it as you're trying to determine short location(s).
Turn key on,if short is present light will be lit and will remain lit til short is located and resolved. If it's not lit,plug ECU in,if it lights,likely a driver shorted in ECU.No change? Pull fuel pump relay... while watching test light, tap on ECU and orient in different positions while doing so.
Still watching test light,wiggle,manipulate all wires from ecu up til where passes thru firewall. Give a good tug there. Same same underhood,start at battery,follow harness to engine and work all wires,connectors all the way back to firewall,then inside where harness comes thru to inside up to fusebox and to steering column. Test light blinking or going out and back on lets you know you're close to problem-within inches.

This is where i'd start and is quickest way to isolate or find source of problem. Next step is consulting wire diagram and disconnecting components fed from this circuit til problem found.
After all these(reliable)miles my money's on chafed/rubbed thru harness,ECU,or some component on that circuit pulling excessive current-in that order.
Post back what you find. Happy hunting.

Thank you, man - I appreciate it.
 

Will get some time tomorrow night to setup my test light and start digging into it.

These were my thoughts exactly. +1 for worn harness somewhere.

following for the fix, as stated check wiring, check wires at fuel pump too, i believe that its on that circuit too,maybe something got up there from the road and knocked a wire loose.

I unplugged the Fuel Pump inside the bin, so it's not a short at the pump or underneath, but I'll take a close look this weekend. Thanks again, guys.

Alright - spent some time troubleshooting this past weekend and last night, and here's what I've found (*haven't fixed it...);

Observations on Symptoms - not NEW symptoms, just things I've noticed since looking into the problem deeper;
* Tachometer wasn't registering for about the last month - coincides with cold weather when the tachometer usually sticks at startup but spins up correctly once the car interior gets warm. Recognizing that the Tach is driven by the Trailing Coil signal and is used for Fuel Injection Timing, points me toward a bad Trailing Ignitor (possibly) or Trailing Coil (unlikely).
* Pre-start Idiot Lights not lighting - AFTER this problem started, I've noticed that the idiot lights on the bottom row of dash panel don't light except for "Seatbelt" and "Brake" (e-brake). Normally, with key in ON position - not start - all idiot lights would be lit. Once started, they all should go out. Don't know if that's related, but after blowing the Engine fuse, a bunch of those lights no longer come on as expected...

Troubleshooting Steps Taken;
1) Shorted Wiring Harness? Built the test light rig to replace the fuse (thanks GSLSEforme) - plugged into the Engine fuse location which keeps blowing fuses, and with Key in Run (*not Start), the light comes on (ECU is plugged in). Then, starting at the Battery / Coils began moving the wiring harnesses around, all the way from injectors to the firewall in the engine bay, across the passenger side engine bay to the firewall, and then under the dash on both sides all the way to the ECU. At no point did the light even flicker. Pulled the carpet back on both sides, removed passenger kick panel and all ECU wiring intact / undamaged. Swapped the wide ECU connectors (2) for a backup N304 computer, and no change in the light. Replaced 20A fuse for the test rig, and when Key turned to Run (not Start), fuse pops. No change at all. Returned ECU plugs to original N304 ECU.

2) Shorted Fuel Pump? Installed SE-EGI Fuel Pump Shunt at Air Flow Meter connector. Unplugged Fuel Pump connector under driver's bin and installed Voltmeter, turned Key to Run and checked for 12v at Fuel Pump connector - no current. Plugged Fuel Pump back in. Obviously, no Fuel Pump operation, as no current is getting to it, so I started looking at...

3) Air Flow Meter (AFM) Shorted? Removed wide connector from AFM and plugged into spare AFM. Test Light never went out. Key to Run, and no Fuel Pump heard (shunt is still installed). Pushed open AFM door to turn on Fuel Pump switch - no change. Installed 20A fuse in fuse block instead of Test Light, turned Key to Run = blown fuse. Plugged original AFM back in. Dead end?

4) Alternator Shorted? Unplugged Alternator Voltage Regulator Plug and unscrewed Positive Terminal lead. Connected both to backup Alternator. No change in Test Light Rig (steady on), and installed 20A fuse in fuse block, turned Key to Run = blown fuse. Reconnected cable and connector to original Alternator.

Other possibilities (in order of likelihood);
A) Unplug ECU and install Test Light Rig in blown fuse position and recheck all wiring harnesses again. Maybe I misunderstood unplugging the ECU before doing this test...?
B) Replace Trailing Ignitor (*I have spares)?
C) Test Fuel Pump Relay for short? I couldn't find it under the dash... is it in the engine bay?
D) Main Engine Relay(s) shorted?
E) Check Relay under dash shorted? Possibly related to the idiot lights not working?
F) Fuel Pump wiring & ground? Unlikely, but getting stumped sucks!

Any tips or notes on my troubleshooting is always appreciated. Thanks for the help so far,

Made a little progress tonight; I unplugged the ECU and did the wiggling of the wiring harnesses with the Test Light plugged into the Engine fuse location, no change. While plugging the ECU back in, I *unplugged* the connector next to it, shown in the pics below, and this made the Test Light go from a bright 12v glow to a dim glow - hopefully indicating the short (less current / voltage due to short?). Here's the connector at the ECU under the passenger side foot panel;

http://oi65.tinypic.com/2qsbig2.jpg
Closer view of the connector showing count & wire colors:
http://oi67.tinypic.com/2db8mly.jpg

I'm not sure where that leaves me, and I'm hoping someone knows what this connector powers. Is my short circuit ON the components for this connector, or somewhere else? Also, now - with this connector UNPLUGGED - the dashboard idiot lights are lit correctly for Key in Run position (*all lit, prior to start). Thanks again,

It's either a short in the CPU or one in that part of the wiring harness and connecting it to the CPU completes that circuit. Hard to tell if the source of the voltage is coming from the CPU or the CPU completes the ground. What I would do is make a pinout and document the voltage of each pin with the connector unplugged. Next would be to plug it in and do the same check by back-probing the connectors. I don't like this part of it because the fuse will not be protecting the CPU at this point. I'll do some digging in the wiring diagram and see what this connector is and what the values are suppose to be.

By "CPU" ^^^, you mean "ECU = Emissions Control Unit" located under the passenger footwell, right?

I also studied the Wiring Diagram tonight, and this connector is called the "F - E" Connector, and connects the Front Wiring Harness to the Engine Wiring Harness. the Front Wiring Harness runs up the Indoor Fan Blower Motor, along the top front of the dashboard, over the center Air Vents and then to the Steering Column area where it splits off to the Combination Switch and other stuff under the driver's left foot.

As additional troubleshooting - under the belief that it's not a wiring harness short, but a bad component somewhere along this circuit;
1) Unplugged the Cruise Control Unit (B-03) - no change in Test Light, so plugged it back in.
2) Unplugged the Circuit Openning Relay (X-07, aka Fuel Pump Relay for EFI) - no change in Test Light, so plugged it back in.
3) Unplugged the Pressure Sensor (B-14, aka Altitude Compensator) - no change in Test Light, so plugged it back in.

Followed the wiring harness back to the driver's strut tower, and started checking connectors;
A) When unplugging the Vent / Vacuum Valve Relays (the two round relays attached nearest to the strut tower) - the Test Light went dim when I unplugged the front one. As I unplugged it, I could hear the relay click off, and the Test Light bulb got significantly dimmer than before. Plugged back in, the Test Light gets brighter.
B) Same as above when unplugging one of the connectors under the heavy rubber boot right next to it, as it gets/sends signal to/from the Vent / Vacuum Relays and the ECU.

Does any of this indicate a problem with either the V/V Relays? I don't have spares, but I do have spare Vent / Vacuum Solenoids. Thanks again for reading... this is really frustrating, and I miss driving my car!

More troubleshooting today...
 

Still unable to get the Test Light to go out, but found that unplugging the Alternator plug just below the RE-EGI air Box drops the intensity of the light bulb considerably to barely a glow. To recap, unplugging the Harness connector shown above drops the Test Light from BRIGHT to dim, then unplugging the Alternator connector which includes a black block bolted to the front corner of the RE-EGI air Box, and having the Positive lead which then connects to the Positive on the Alternator, drops the intensity of the Test Light to barely glowing.

Does this indicate a bad Alternator? Still troubleshooting,...

It would be easy enough to just take the alt to your favorite parts store for free testing. This might sound silly, but try adding an additional ground running from the big alt bolt to the shock tower or battery.

Are you doing this test with each successive component you unplug left disconnected or plugged back in as before?
Regarding possible damage to ecu,small test lamp bulb about 3 watts and filament in bulb can't pass enough current to damage anything.

Regarding alternator,did it charge properly previously to current complaint,did all warning lights at base of dash cluster go out completely especially the battery light when engine started and running? If so,not likely alternator is culprit here. Is alternator original? Are you disconnecting the output wire of alternator and the field plug. Try disconnecting field plug 1st and see what happens.
For a quick test take out test light jumpers and with everything still disconnected,put a rated fuse in and turn key on. If it does not blow,start connecting one component at a time and checking fuse as each is connected,leaving alternator field plug for last. Wherever the fuse blows,go back to last plugged in component and remove it and start checking that components' circuit. If all components that are connected don't blow fuse but it does blow when alternator connected,disconnect field plug,put in a new fuse and start car and let run for a few minutes.Plug in field connector,if car shuts off,fuse is blown,have a closer look at alternator or swap another in if you happen to have a spare.

Per your prior advice, this is leaving connectors DISCONNECTED as I unplug them.

Alternator was working fine and charging, is the 2nd Alternator I've put in this car, and is a used - SE part from my spare engine. Thanks for the advice on how to sequentially hook stuff back up, and will try that tomorrow, as well as Alternator Field Plug, if it will start and run. I would love nothing more than to hear it rev, as it's been weeks without.

On the Alternator ground, I have a spare Negative battery cable run from th Battery Negative post to the adjuster bolt for exactly that reason, to provide a solid ground. I actually removed that today to see if it changed anything. It didnt.

And, thanks. for sticking with me on this fiasco. This is the most frustrating thing,...

All part of owning these old cars and keeping them on the road,helps if you can repair it yourself. Imagine the cost at a shop to fix it,especially if not familiar with 1st gens...

Okay, here's what I found; using advice here, I unplugged everything that I knew had an effect on the Test Light brightness. Inserted a 30A fuse in the Engine location - rated at 20A (*instead of the Test Light), and then started plugging stuff back in;

1) Alternator - Field Connector = Fuse OK
2) Vent / Vacuum Relays = Fuse OK
3) "Circuit Openning Relay, X-07" (aka Fuel Pump Relay) = Fuse OK...
4) Main Harness: "E-F Connector" = Fuse OK for a minute and then started to see smoke while I was poking around. Immediately turned off ignition, and here's what I found when I unwrapped the section of the E-F Connector harness where it goes to the X-07 FP Relay that felt warm to the touch;
http://oi65.tinypic.com/25yu1k2.jpg

Off of the E-F Connector, there's a thin Black Wire, White Trace, Red Dashes that started to melt in several places visible in the photo. I unwrapped this back up the FP Relay harness, and saw where it's crimped to the bigger Black Wire, White Trace, Red Dashes with a brass crimp terminal (factory). The melted portion tells me there's a short on this circuit. I cut the thin wire from the E-F Connector to FP Relay because I'm going to have to repair that wire anyway, and turned ignition to Run (*not Start) - no heat anywhere, and the Fuse was OK.

On a whim, I decided to start the engine. Dash lights looked good (Alt plugged in), Fuse OK, turn key to Start and it fires right up. Runs for 8-10 seconds, but then the engine dies. At least this tells me I have spark, fuel, and air. Now we're getting somewhere (figuratively...)!

Put a meter on the thin black wire I cut, and it's getting 12V to ground. Normally, this would be powering the FP Relay after engine start. A few more start attempts confirms;

A) Fuel Pump is pressurizing the rail prior to start, (I had the test plug shunted at the AFM),
B) Engine Starts and Runs until fuel pressure drops at Injector Rail, and then,
C) As soon as the engine is running, the FP Relay loses signal, shuts off FP and engine dies. Elapsed time, 2-4seconds. It runs longer (8-10s) if you let the FP run a bit prior to starting the engine, but as soon as you hit the accelerator to keep it running, it dies (fuel pressure decreasing, as pump is not running continuously after start).

Progress, but now I'm going back to study the Wiring Diagram to try and figure out what's going on. Bad FP Relay - what Mazda calls the Circuit Openning Relay? I'm almost positive I could bypass the relay and run 12v directly to the Fuel Pump and I'd be back on the road... but that's not safe.

So close,...

Putting a larger than rated fuse in place of correct fuse is always a bad move,whether trying to diagnose a fault or not. Damage other than what is immediately apparent can occur adding more complexity to diag and repair. Hope you track down the source of problem.

this is great advice. i would get a reset-able fuse for trouble shooting so you do not eat through fuses. i use them all the time at work. good luck atleast youre making progess, car wiring can be a night mare until you go over them enough times and learn the lay of the land so to speak

Luckily, it was only a wisp of smoke and I was right there feeling the various harnesses and moving them around so I noticed it right away and shut down the ignition switch. I've since put the 20A fuse back in and it's holding.

Do you have a part # for the resettable circuit breakers? Are they reliable enough to replace a fuse for good?

Thanks again, guys. I'm still studying the wiring diagram for pin #13 on the E-F Connector.

Stay with the fuses,much more predictable and melt more quickly than the circuit breaker kicks out,much like scenario described in above post,you don't want that.

Try running a wire with fuse from the battery to the fuel pump and see if it blows this fuse.

Are you saying just wire 12v directly to the connector in the DS bin - ensuring correct polarity - and see what happens? I can do that, and I would expect it will just spin the Fuel Pump. I can hear the FP running when I use the Test Shunt at the Air Flow Meter, but the problem is that once the engine starts, the Circuit Openning Relay de-energizes, which shuts off 12v to the Fuel Pump, killing the engine.

The Black / White Trace / Red Strip thin wire from the E-F Connector is what energizes the Circuit Openning Relay (i.e., Fuel Pump Relay) after the engine is started and the key moved from Start to Run.

I'm wondering now if I have a short in my Ignition Switch...

Thanks, guys - I'll keep trying these things and report back. I think we're getting close, and my study of the Wiring Diagram @foxed.ca has given me a few new leads which are on the circuit for E-F Connector AND the FP Relay; Starter & Ground, Ignition Switch, and Transmission Harness to look for shorts.

I'll stick with Fuses to avoid causing more problems - and appreciate the ongoing advice.