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I have a 2010 ford focus, I've had it for about 2 years and I've never had to fix anything on this car. Then one day, I went to walmart, went inside for about 30 minutes or so, and when I came back out and tried to start the car the ignition fuse blew so i put another one in and it started no problem for awhile. Then again one day it decides to blow out of nowhere, i put another in and it blew again immediately, I put a 30 amp in (usually it's a 20) just so I can get it home and it started, ever since then it's been sitting, it's been about 2 weeks. I replaced the starter and checked some of the wiring. I tried to start it after I replaced the starter and it blew again. Then the other day i decided to try it the other day just for shots and giggles, surprisingly it worked, I turned it on and off about 20 times and it didnt blow the fuse, so then I test drove it to walmart where it blew on me again. I drove it home turned it off and it worked, I've been driving it to work and back and to the store all week and it hasn't blown another since. I'm at a loss here on what it could be. Any suggestions?
 

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Which fuse number is it in the fuse box? (Lack of sleep making it difficult to concentrate)

Somewhere between the blowing fuse and the load the fuse supplies power to, the insulation has been damaged or a connection is able to brush up against some metal on the car.

Does the position of the steering wheel affect it at all? I’d try turning the key to “ON” (shouldn’t need to be running) and turn the steering wheel lock-to-lock and see if that blows the fuse.

Also check for loose or frayed wiring at the supplied load, and moisture entry.

At least that’s where I’d start.

Edit: Does the fuse only blow when the car has been sitting, parked, and then you try starting it?
 

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@Dan_in_WA it does it pretty randomly, sometimes after I drive it somewhere and sometimes when I come try to start it after coming out of the store. I haven't tried turning the wheel back and forth but I forgot to mention that it also blew a diode shortly after the fuse problem started happening. For the last 2 weeks I've been driving my car to work and starting It up on every break and it hasn't blown at all until tonight when I went to the little store near my house. It blew 2 fuses before blowing the diode again
 

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What diode is blowing?

When it blows, do you smell the stench of silicon toast and it looks burnt, or does it just test “open” and otherwise look ok?

Weirdly enough, my wife’s Focus is the one car I don’t have a service manual for, since I’ve never needed one. So I’m having to work from general knowledge.

Can you be specific on the location of the fuse and diode that are failing?

One last bit, I know a lot of cars have a “fly back” diode in the A/C compressor clutch circuit, has it ever happened when the A/C is turned off?

The A/C control circuit just might be drawing its power through the ignition fuse, I don’t have a manual.

A little background on the use of the diode in the A/C clutch circuit; please bear with me if you already know.

The A/C is turned on and the clutch coil builds up a magnetic field to engage the clutch, but when the A/C switch turns off the magnetic field collapses.

When that magnetic field collapses, that energy has to go somewhere. If nowhere else, it would draw an arc across the contacts that just opened, and that poor switch would have a short lifespan.

If you have a wiring diagram, please note that the diode has been installed in reverse polarity in relation to the battery. No battery current should ever flow through it - if it did, it would look like a short across the battery.

The magnetic field collapses, causing a current flow in the reverse direction of the DC current that created the field in the first place.

That current comes back out of the coil, and flows through the diode it encounters and returns to the coil. No voltage spike is created, just the voltage drop across the diode. The switch is saved from a destructive arc.

Whatever load your blowing diode is connected to is the problem, or in the wiring to it. A/C isn’t the only time these diodes are used.
 

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Hi. I want to add a little more information. I'm related to Mark and have been troubleshooting this issue with him. We have the Chiltons manual for the car which has all the wiring diagrams in it. We live in the Sacramento valley, California where we've had some record-breaking heatwaves (triple digits). Not sure if this factors into anything but it does seem to happen mostly after the car has been driven or at least has been warmed up.

After the initial fuse blowing, the car was driven approximately 45 miles. The inverter was plugged into one of the accessory ports charging a cell phone and was left plugged in while doing business inside an establishment. When attempting to start the car, it blew the fuse again (#27-? ignition fuse in the passenger compartment fuse panel) as well as the 2 powerpoints and the diode under the hood. I should also mention that the AC was left on when the car was shut off and so was under that load when trying to start back up, which is a common practice by the driver. I will note that the ignition system runs in conjunction with the fuel system and when the car blows these fuses there is a crank but no-start situation before the fuse blows. Of course it's almost instantaneous.

Even though we knew it most likely was not the starter (we jumpered the starter before removing it), we went ahead and pulled it and had it tested (the first time it tested bad but then tested good the next 4 times) and then subbed in a starter from another focus but still having the same problem. I looked over all the wiring I could, from the lock cylinder, ignition switch, to the fuse box and all around the engine bay and nothing looks visibly out of whack. I checked all the grounds and the battery terminals and all looks good. I tested the voltage on the battery both with the car running and with it off. It tested fine. We tried wiggling wires while the car was running (ignition coils, etc.) and nothing. I haven't checked anything at the fuel pump yet. I can't tell you which diode was blown because I wasn't involved in that. However, after the last incident where the ignition fuse was jumpered to get home it was noted that the diode that had previously blown was hot.
We have not pulled the fuse boxes out yet to check the bottom/back side. I did try to check behind the passenger compartment fuse box with a mirror. I couldn't see a lot but didn't notice anything that looked problematic. My guess is it's not the fuse boxes as the current makes it at least to the starter before the fuse blows.

From what I can glean, there are several systems that can affect this making it extremely difficult to pinpoint. Also, other than testing voltage at the battery, we have not tested voltage on the wiring between each component. To be honest, we're not super saavy when it comes to vehicle mechanics but I would mention that we're a family of focus drivers and know the focus pretty well. Mark is on his third focus, this problematic one being his second (he just bought a 2017 titanium - sweet car!). One last thing. The car has been having transmission issues and I believe I read that there's a sensor on the tranny that could possibly be a problem that could also potentially blow the fuse.
If you need, I can upload photos of the wiring diagrams and fuse boxes. Thanks for the help.

Edit: I forgot to include the fact that the car has a relatively new battery and it holds a full charge, so we eliminated the alternator.
 

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Not sure what may be going on, as you pointed out, there can be several culprits. However, to rule something out, try not using the inverter, kill the power to it if possible, it's a shot in the dark, but it can rule out one item.
 

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The inverter plugs into the accessory port (cigarette lighter). It was only plugged in one time and that time the ignition fuse, starter diode, and the 2 PPs blew. It was blowing the ignition fuse both before and after that but it's only been the ignition fuse blowing every other time. Thanks for the reply but I think the inverter only plays into the PPs and diode blowing which only happened that one time.
 

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What diode is blowing?

When it blows, do you smell the stench of silicon toast and it looks burnt, or does it just test “open” and otherwise look ok?

Weirdly enough, my wife’s Focus is the one car I don’t have a service manual for, since I’ve never needed one. So I’m having to work from general knowledge.

Can you be specific on the location of the fuse and diode that are failing?

One last bit, I know a lot of cars have a “fly back” diode in the A/C compressor clutch circuit, has it ever happened when the A/C is turned off?

The A/C control circuit just might be drawing its power through the ignition fuse, I don’t have a manual.

A little background on the use of the diode in the A/C clutch circuit; please bear with me if you already know.

The A/C is turned on and the clutch coil builds up a magnetic field to engage the clutch, but when the A/C switch turns off the magnetic field collapses.

When that magnetic field collapses, that energy has to go somewhere. If nowhere else, it would draw an arc across the contacts that just opened, and that poor switch would have a short lifespan.

If you have a wiring diagram, please note that the diode has been installed in reverse polarity in relation to the battery. No battery current should ever flow through it - if it did, it would look like a short across the battery.

The magnetic field collapses, causing a current flow in the reverse direction of the DC current that created the field in the first place.

That current comes back out of the coil, and flows through the diode it encounters and returns to the coil. No voltage spike is created, just the voltage drop across the diode. The switch is saved from a destructive arc.

Whatever load your blowing diode is connected to is the problem, or in the wiring to it. A/C isn’t the only time these diodes are used.

I'm told but haven't checked for myself that it's the starter diode that blew. It only blew the once with the inverter plugged into the accessory jack. What you say makes a lot of sense as to why the diode and the 2 PPs blew. How likely is it that the problem lies somewhere underneath or inside the fuse box itself? The diode and 2 PPs are under the hood. The fuse that has blown multiple times is fuse #27, ignition, passenger compartment fuse box
 

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I’m suspicious of the starter solenoid at this point, possibly shorted turns or the plunger inside is sticking.

Would you mind posting the schematic of the circuit from fuse 27 to the starter solenoid?

It’s unlikely (to me) that the problem is in or around the fuse box, unless something like a screw came loose, dropped down and lodged itself in there causing the occasional short. There is enough current available in the fuse box that if a short happened near there, I think you’d see the flash and have smoke, depending on where it landed. The factory generally does a pretty decent job of tying everything down under the dash... there hasn’t been an aftermarket stereo installed, has there?! But that’s another horror story.

I don’t remember on the Focus, is the starter solenoid part of the starter motor assembly, or mounted separately somewhere on the inside of the fender? Look carefully at the starter motor and solenoid connections.

Good luck,
Dan
 
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