Electrical Fuse E Blowing need help diagnosing

Mike Aggy

Member
Apr 12, 2020
11
1
13
Goshen, NY
Hi all,
1994 V6 Mustang, bone stock electronics. I was driving and the car completely shut off. On the side of the road I had a crank no start. After a tow home I see fuse E in the engine bay is blown and has continued to blow as soon as key is switched on.
It controls
- PCM
- Fuel Pump
- O2 sensors
- Canister Purge Solenoid (A/C)
- Ignition Ctl module
- IAC Solenoid
- Fuel injectors
- Transmission
- Camshaft position sensor
- MAF sensor
^ According to service manual ^

I started systematically unplugging sensors to find the one with the high current draw and the only time the fuse would not blow is with the PCM completely unplugged. (Everything listed above still plugged in and ONLY the pcm unplugged.) At this point I am just unsure of how to proceed so any help would be very much appreciated! Thank you
 
  • Sponsors(?)


jozsefsz

5 Year Member
Aug 11, 2013
1,229
342
114
50
Cleveland OH Area
That sucks man, sorry to hear about your problem. It sounds like you've been systematic about troubleshooting. The pain in the rear is that unplugging the sensors and still having a large draw will tell you that the sensor isn't dead-shorted, but unfortunately doesn't tell you if the wiring running to that sensor (or fuel pump) is shorted. Unplugging the PCM will remove power from some of the sensors you list - many receive power through the PCM. So it could still be the wiring running to any of them that's been shorted to ground, so you can't be sure it's the EEC.

I'd personally go hunting for any major areas where you might have a visible wiring problem. Any place where the old harness-sheathing has cracked away, or anything that gets close to the exhaust. O2 sensors wiring is always suspect as it runs right under the headers on the v6, and fuel pump wiring as well since it sits on top of the tank that gets pretty rusty over time. Anything that's been worked on recently - if you had a new fuel pump installed for example, or any area of the car that's had body-damage in the past (a fender for example).

After that, I'd disconnect the harness at the CCRM (the relay module in the engine bay), find a wiring diagram for it, and probe for any leads that are grounded that aren't supposed to be grounded. Best of luck!
 
  • Like
Reactions: 1 user

Mike Aggy

Member
Apr 12, 2020
11
1
13
Goshen, NY
That sucks man, sorry to hear about your problem. It sounds like you've been systematic about troubleshooting. The pain in the rear is that unplugging the sensors and still having a large draw will tell you that the sensor isn't dead-shorted, but unfortunately doesn't tell you if the wiring running to that sensor (or fuel pump) is shorted. Unplugging the PCM will remove power from some of the sensors you list - many receive power through the PCM. So it could still be the wiring running to any of them that's been shorted to ground, so you can't be sure it's the EEC.

I'd personally go hunting for any major areas where you might have a visible wiring problem. Any place where the old harness-sheathing has cracked away, or anything that gets close to the exhaust. O2 sensors wiring is always suspect as it runs right under the headers on the v6, and fuel pump wiring as well since it sits on top of the tank that gets pretty rusty over time. Anything that's been worked on recently - if you had a new fuel pump installed for example, or any area of the car that's had body-damage in the past (a fender for example).

After that, I'd disconnect the harness at the CCRM (the relay module in the engine bay), find a wiring diagram for it, and probe for any leads that are grounded that aren't supposed to be grounded. Best of luck!
Thank you! It’s nice to know that at least my thought process is right lol. I really hate electrical gremlins. Currently probing the CCRM and the PCM for any grounds that aren’t supposed to be grounded but its slow and tedious.
 

Mike Aggy

Member
Apr 12, 2020
11
1
13
Goshen, NY
That sucks man, sorry to hear about your problem. It sounds like you've been systematic about troubleshooting. The pain in the rear is that unplugging the sensors and still having a large draw will tell you that the sensor isn't dead-shorted, but unfortunately doesn't tell you if the wiring running to that sensor (or fuel pump) is shorted. Unplugging the PCM will remove power from some of the sensors you list - many receive power through the PCM. So it could still be the wiring running to any of them that's been shorted to ground, so you can't be sure it's the EEC.

I'd personally go hunting for any major areas where you might have a visible wiring problem. Any place where the old harness-sheathing has cracked away, or anything that gets close to the exhaust. O2 sensors wiring is always suspect as it runs right under the headers on the v6, and fuel pump wiring as well since it sits on top of the tank that gets pretty rusty over time. Anything that's been worked on recently - if you had a new fuel pump installed for example, or any area of the car that's had body-damage in the past (a fender for example).

After that, I'd disconnect the harness at the CCRM (the relay module in the engine bay), find a wiring diagram for it, and probe for any leads that are grounded that aren't supposed to be grounded. Best of luck!
Thank you again for your pearls of wisdom. Allow me to ask you a question since Im currently working on this lol. The only time that the fuse blows is when the PCM is connected. SO….. I’m thinking if I put the key on and power the PCM pins individually to 12V+ that I should be able to find the problem wire when the fuse blows. Yes/no/maybe so?
 

Mike Aggy

Member
Apr 12, 2020
11
1
13
Goshen, NY
Hi all,
1994 V6 Mustang, bone stock electronics. I was driving and the car completely shut off. On the side of the road I had a crank no start. After a tow home I see fuse E in the engine bay is blown and has continued to blow as soon as key is switched on.
It controls
- PCM
- Fuel Pump
- O2 sensors
- Canister Purge Solenoid (A/C)
- Ignition Ctl module
- IAC Solenoid
- Fuel injectors
- Transmission
- Camshaft position sensor
- MAF sensor
^ According to service manual ^

I started systematically unplugging sensors to find the one with the high current draw and the only time the fuse would not blow is with the PCM completely unplugged. (Everything listed above still plugged in and ONLY the pcm unplugged.) At this point I am just unsure of how to proceed so any help would be very much appreciated! Thank you
Update! After two days of diagnosing and prodding and cleaning connectors I tired the key just for funzies and the bi*ch had the nerve to start RIGHT up. Will update this threat if any more problems arise but it seems that for now my electrical gremlins may have just been a loose wire or old fuse. Hopefully by the grace of something it will continue to run.
 
  • Like
Reactions: 1 user

Mike Aggy

Member
Apr 12, 2020
11
1
13
Goshen, NY
Hello anyone who discovers this thread lol. I just wanted to post a final update. The solution was short lived. The same exact problem happened again and at that point I did the only thing left. Turn to the professionals. Mechanic said it was the PCM, which was the same conclusion I had arrived at. After towing the car home, I replaced the computer and all seemed fixed. Car started right up and drove great… for about 7 mins until it died again. SO this post is here to say unfortunately I cannot continue this journey and I am putting my beloved stang up for sale. It’s been a month and I am at the end of my timeframe and budget. Hopefully soon I will rejoin the mustang owners club. :(