PCM Fuse Problem - Blows at Key On

topless98gt

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Mar 31, 2005
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Ok, so I've read almost every single PCM fuse issue thread out there but no one ever really said how they fixed it or what the problem ended up being. So before you tell me to search...I already have!

The normal **** happened when the PCM fuse blows -- the car was running, but it sputtered to a stop. So my initial thought was the fuel electrical system. Turns out after a little research, apparently my problem can be a lot worse.

So now anytime I turn the key position to On, it tries to blow the fuse. (I've gone through 3 fuses and finally decided to hook up the multimeter instead of going through expensive MAXI fuses.

I have a Haynes for 94-03 Mustang and a Chilton for 94-98 Mustangs. So I checked all of the fuses then referred to the manual when I found the blow fuse. Apparently, according to the Chilton, it was my convertible top fuse. But then I realized -- hey, that's not the right diagram for my fuse box! So I did a little research on the fuse box, and it turns out that I have a 99-04 fuse box in my car. WTF? Why would this be?

So I unplugged the CCRM as most threads say, and the short went away. So logically it is past the CCRM, but then again there is only 1 connector on the CCRM, so how can one say it is "past" the CCRM?

The PCM fuse just blew 2 days ago. A little history over the past month -- a few weeks ago, the A/C clutch seized up so I bought a new belt to bypass it. Then a few days after that I was tired of my CEL being on (aka I need to get the car inspected) so I crawled under my car and removed the MIL eliminators that I had before (the connector on one of them melted) -- my car was throwing P0171 and P0174 -- and then I wired up my own MIL eliminators based on some specs online. Car still threw P0171 and P0174. So I said **** it, whatever, I don't want to mess with it anymore. Now 2 days ago, the car sputtered to a stop. My fiancé described it as the feeling of "you run out of gas." So I drive down to Waco (where she is stranded) to see about the issue. I get there, check the fuel pressure (result: none), check the codes (result: P0171 and P0174, but sometimes said "error" [this is because it could not communicate with the computer), and also checked the fuses. My #26 fuse is bad -- the PCM fuse. So I took the 20A right below it and plugged it into #26 spot and BAM, popped immediately (key was in on position). So I said we'll just tow it back to the house tomorrow (1.5 hours away) and then we'll work on it there.

I just started researching the PCM fuse issue after I did my fuse box research -- I still don't know why my 1998 Mustang GT has a 1999+ fuse box. My PCM research led to many different things, but no solutions.

1) I know there is a short somewhere.
2) My fuse box is a 99+ fuse box.
3) The fuse being blown is #26 -- it blows as soon as you turn the key to on position
4) I've unplugged the CCRM and the fuse lasted. Plugged back in - pop.
5) I'd really like to trace the short myself, but not exactly sure where all to look
6) I've unplugged all of the O2 sensors and the short is still there.
7) When I took the fender-well out, I noticed there is what appears to be a break line broken. I believe this to be a whole different issue. I previously thought it was my rear driver-side piston leaking fluid (I had to refill every other day) -- so I have a new one waiting to be put on.
8) I really hope you read all of this, but I completely understand if you don't.
9) I'm tired of typing, and my brain is scattered, so this post may not make a lick of sense.
10) Thanks for sticking with me through my issues -- you are a true friend!

So if you understood that, any help would be appreciated. Any questions you have, please ask. I will answer as best as possible.

P.S. I love you [stangnet].
 
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wmburns

SN Certified Technician
Aug 14, 2009
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Normally I am all over electrical problems. But I don't know how to deal with a 98 that has a 99+ fuse panel. I don't have any reference manuals to work from.

However, there is some things in the past that should be looked at closely. You stated there is a leak in a brake line and a lot of brake fluid has been added over a period of time. Consider that brake fluid is hard on rubber and plastic parts. What if some brake fluid has contamined a wiring harness and eaten away the insulation? This could be the source of the ground fault.

Suggest that you perform a through visual inspection of the wiring harness and go from there.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
If so, then fuse F2.26 in the CJB is listed as:

F2.26 30 Windshield wiper motor (17508), Wiper run/park relay, Wiper high/low
relay, Washer pump relay, front

Clearly that doesn't make sense, right?

Fuse F1.26 in the engine bay is listed as PCM power. Which sounds more reasonable.

However, we are back to the original issue of not knowing what the correct wiring diagrams are for your MY.

For the 96-98 MY, the power for many sensors is routed through the PCM. So for example, a fault in the O2 heaters will blow the PCM fuse.

On the 99-04 MY, a fault in the O2 heaters will blow fuse F2.8. The PCM will remain powered.

IMO, many of the wiring changes done in 99+ MY's were to improve separation between the various systems and improve diagnostic ability.

I am clueless on how to help you further without wiring diagrams. Again, suggest starting with a visual inspection of the wiring harness. Next look at the O2 heaters esp the MIL's. Check for burnt/chaffed wiring harness.
 

topless98gt

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Mar 31, 2005
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The fuse being blown is EJB 26 (F1.26)

I understand exactly what you're saying though. I have the O2 sensors unplugged (one of the first things I checked) and it still happens. I unplugged every visible sensor/connector under the hood and in the trunk and nothing resulted in a fix.

Here's how I tested the "short" with my VO meter:

I pulled the blown F1.26 fuse out and placed my red lead into the "right" side of the fuse connector ("right" being the side furthest from the front of the car). I then connected the black lead to a confirmed ground position. Before turning the key on, the circuit is open. As soon as I turn the key on, it drops to 0.2 or so and slowly drops to 0.02 or 0.01 from there. When I unplug the ground connectors below the EJB (the connects that I am lead to believe feed the rest of the car's ground) and the resistance jumps to 0.14.

I kept my VO meter connected to the fuse inlet and the ground while I disconnected every sensor and connector that I could find under the hood. The reason I did it this way is because I assumed as soon as I unplug the faulty component, the resistance will become open, ie: no ground fault. I checked this theory with every other fuse in the EJB and no other fuse has closed resistance to ground.

Please let me know if my theory is off before I continue any further.
 

topless98gt

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Mar 31, 2005
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Here is a picture of my EJB (see attached image)
 

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wmburns

SN Certified Technician
Aug 14, 2009
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Did you disconnect ALL of the COP connections including the radio interference capacitors?

OBTW, as best as I can tell, fuse F1.26 powers the ignition. If this is true for your car, we have to isolate if this is a wiring problem in the ignition VPWR (fault) or a bad PCM coil driver(s).

With the key off, test the return line (Not red wire) on each COP to a known good ground. Post. If you test any ground faults, the test will need to be repeated with the PCM removed.
 

topless98gt

Adhesive Feces
Mar 31, 2005
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I'm not saying that you are wrong, but where did you get information that suggests F1.26 powers the ignition in my car? If this were true, I would assume the car wouldn't even turn over.

My car doesn't have COPs, it has the 2 coil setup. I'm not exactly sure what the radio interference capacitors are, but am assuming it is the small gray clip on each set of coils.

I unplugged both gray clips and both coil connectors and the short did not go away. Am I testing for the short properly? Or should I be doing it some other way?

Also -- where exactly is my PCM located? I have yet to find any information leading to the location of it in the car.

I sent you a PM, please check your messages.

Thank you again for your time and effort in helping me solve these issues. I really do appreciate it.
 

players02GT

New Member
Nov 14, 2007
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Did you ever figure out what was wrong wtith your car? Mine is doing something very similiar and I am not sure what the F is wrong with it.
 

topless98gt

Adhesive Feces
Mar 31, 2005
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Arlington, TX
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Did you ever figure out what was wrong wtith your car? Mine is doing something very similiar and I am not sure what the F is wrong with it.
Yup, sure did! It was a bad fan...anytime the fan came on, it would blow. So after searching long and hard for months, through junkyard and junkyard, I found a 1998 Mustang GT Convertible (exact match) with 27,300 miles on it...the fan seriously looked brand new after I got it home and dusted it off. I picked that up for $20 including the connector. $20 FOR A BRAND NEW FAN!!!!!!!!!!!!!!!!!!!!!!!!!

It was extremely easy to change...a lot easier than I thought it would be. Just had to unbolt the reservoir and wiggle it to the side, then take off 2 bolts from the fan and it slid right up. Putting it back in was a little bit more difficult, but the overall process took less than 20 minutes.

I was without a fan for about 3 months...it sucked! But yeah, so far so good on the new fan (I'm on day 5 with the replacement fan). Soldering those wires was a bitch though...