ItIsWhatItIs

Hey guys & gals, I have a 1996 Ford F150 regular cab 8ft longbed with the legendary 4.9L I6 and E4OD transmission.


For the last year and a half I've been trying to figure out why fuse position #4 [15AMP] continues to repeatedly blow on me. I am no electrical expert, but I understand when a fuse repeatedly blows there's likely a "short to ground" somewhere, but to be honest I'm not sure exactly what that means and I'm a little slow on the uptake of technical info, so youtube continues to fail me and I'm hoping some veteran forum members here can help walk me through this to resolution, because I seriously don't know what to do anymore.


Fuse #4, as indicated by the chart on the inverse of the cabin fuse panel door, governs 4 components: "warning buzzer/ chime module; exterior lamps; instrument illumination; trailer exterior lamp relay." For the record, I do not have any audible chime/ buzzer in my truck and have not for the 6+ years I've been its second owner--I know some models have a buzzer that sounds when the key is placed into the ignition cylinder, but mine does not have that, or at least the previous owner apparently deleted the functionality [presumably, out of annoyance]--, so I disregard that first component. This asserts the need for investigation into the remaining three components: (1) exterior lamps, (2) the cluster panel, and (3) the trailer lamp relay, each of which I will hereafter discuss in-turn.


(1) As far as the exterior lamps, I want to mention that I installed an additional high-mount/ third brake light just above the hatch window in my rear glass [several years ago], having installed it via wire-nuts and heatshrinking, which I believe to be acceptable/ proper for a DIYer, and that light continues to function to this day, so I don't believe that's the culprit, but I'm mentioning it to give a more robust contextual picture in case it's an important detail and maybe something I need to consider(?). I also installed some aftermarket headlens with different/ new headlight bulbs but they came with an adapter from the manufacturer so as to ensure compatibility with the existing wire system, so logic would follow that nothing could have went awry there, either. (Besides, the headlights are apparently not part of that fuse's responsibilities because they have been illuminating throughout this entire defective situation with fuse position #4, the one exception being that lately I do notice that I have to jiggle the combination switch a little to get the headlights to come on [after, of course, first activating them by fully extending the light switch ****, which sits to the immediate left of the instrument cluster], but I believe this just to be a signal that my combination switch is beginning to die, so I will replace it soon. But that shouldn't have anything to do with the systems riding on fuse #4, right? Again, up until recently, my headlights had been working without falter, regardless of the truck having no exterior lamps or dash illumination for much longer. Please clarify if perhaps there is some overlap here that I should be aware of/ look into(?).


(2) As for the instrument illumination, I did once follow a friend's suggestion to check the backside of the instrument cluster module for a sure fit of its connections. I removed the cluster and made sure the flexible circuit "film(?)" behind it was in place, which it wasn't: one of the circuit ribbons was misaligned in the channel where the multi-wire harness plugs in, so I straightened it out and made sure everything was sitting correctly before I carefully reinstalled the cluster. At that time (about 6 months ago), a fresh fuse placed into position #4 lied to me by making me think that I had found and resolved the culprit, because suddenly my exterior lamps illuminated again, along with my instrument panel. So, it seemed I have solved my problem. Hooray, right!? NO. Within a few weeks, the fuse went back to serially blowing itself each time I would input a fresh, despite having checked the instrument panel's rear circuit film and confirming that all the ribbons were still in place and making proper connections with the wiring harnesses. I was at a loss, and just decided to rescind all nighttime driving until I could figure out what was going on. (Since the taillights are under the "exterior lamps" category, this frustrating fuse situation means that I cannot drive in non-daylight hours, since my only current exterior lights are my headlights, turn signals, and brake lights. I can drive around with my four-ways engaged to keep something flashing on the back of my truck, but I know that's illegal [not to mention it alarms other motorists and makes them wonder what's going on with my vehicle as I drive around them], so I really hate this situation.)


(3) As for the trailer lamps relay as the third and final category, I feel like I should mention the previous owner had a brake-assist module mounted to the underside of the dash in the cab. Additionally, one other time that my "fuse #4 frustrations" resumed in the past was when I went to connect a Uhaul trailer: upon plugging the trailer's light harness into my truck's rear adapter, suddenly the fuse began to blow over and over. So, yesterday, I finally decided to delete the entire brake-assist module and all its wiring, even though it did not appear to be fully connected during the time I've owned the truck---nevertheless, I did not like how much old wire was involved, and not knowing whether any breakage in its casing might be causing this mysterious fuse misbehavior, etc. I fully deleted the entire system and then also considered the trailer lamp relay which is housed in the engine bay fusebox, since this is listed as the final component in the responsibilities of fuse #4. I know my horn works, which is the neighboring relay, so to determine whether the relay plugged into my trailer exterior lamp port was bad, I simply extracted and swapped them: my horn still worked, which eliminated the suspicion of the trailer lamp relay. Apparently the deletion of the brake-assist module did the trick... or at least temporarily: after removing the system and inputting a fresh fuse, the systems dependent upon fuse position #4 suddenly resumed working, so that means my instrument panel lit aglow again, as did the taillight function of the dual-filament bulbs in the rear, as well as the parking lamp function of the dual-filament bulbs in the front. It seemed I had found my culprit... Hooray, right!?...

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...NOPE. Today, as I was returning to my truck from being in the store, a little voice urged "let's pull the light switch all the way out to turn the exterior lights on just to enjoy the sight of them lighting up again/ check to make sure they're still working," so I humored the nudge, ...went back to my taillights and cupped my hand over the lens casing as I leaned inward to see if the filament was lit (it was high noon). Take a wild guess, anyone? No lights! Ragefully confused, I yanked out fuse #4 and held it up against the sunlight---BLOWN. An immense sense of deep disappointment flooded me all over again, the same disappointment that has plagued me for over a year and a half now as I have tolerated this truck's mysterious fuse #4 problem... and I'm just at my wits end here. I came home and plugged in a fresh fuse, pulled the light switch out and took a look: now they're working again! What the heck is going on here!? I'm sure that tomorrow the fuse will no doubt blow on me again, and this cycle will repeat itself ad naseum because, contrary to every false sense of resolution I've yet experienced, the true culprit continues to elude me. Please be thoughtful & thorough in your suggestions, bearing in mind I have no resources to take my truck into a professional electrical shop where they could run a diagnostic and trace the wire to identify the culprit; I am forced to do everything on my own, strictly DIY. So, I hereby approach the F150 forum humbled by recurring defeat, pleading for someone [whose niche is electrical and knows these trucks well] to help me by providing a by-priority troubleshooting list that I can exhaust, line item by line item, in order to eliminate all things until I finally corner this elusive little booger and exterminate him once and for all. Can some fellow owners of this bodystyle (92-96) chime in if they've suffered the same issue, or give me pointers on exactly what to check and how to do so thoroughly? Many, many sincere thanks in advance---all input will be appreciated. God bless!

Steve83

All the truck's details need to be in your signature so it's easier for us to help you:


(phone app link)

This diagram is more-complete than the list on the fuse panel cover, and the link in its caption is a site with free wiring diagrams:


(phone app link)

The TSB in this caption describes common electrical problems on that truck, and the links in the caption explain more:


(phone app link)

A "short" is a very specific electrical problem. When electricity was just being developed, and conductors (Copper, etc.) were not as refined & pure as they are today, a circuit could be built with a certain length of wire (because of the wire's internal resistance). If the circuit was "too long", not enough current would flow to do anything useful. If it was too SHORT, too much current would flow, and the wire would overheat, melting it, or starting a fire, or tripping the circuit protection (fuse, breaker). That term has stuck, for any electrical fault resulting in too much current flowing, even though nowadays circuits can be built nearly any length (including too short to be seen with a cheap microscope). Circuits are built for current to flow from the negative side of the voltage source (battery, in the typical vehicle; or alternator if the engine is running) through some wire (&/or the chassis ground), through a load (like a light or electronic device), through a control device (switch or another electronic module), through circuit protection (fuse, breaker, fusible link), back to the positive side of the voltage source. The chassis & wire are expected to have 0 (or negligible) resistance. So all the voltage developed at the source is expected to drop across the load (although a tiny bit may drop across the control module or protection; and some always drops across connection terminals). If the load's resistance is somehow reduced (e.g.: a fault in a device, or the wrong bulb), or if the load is bypassed (e.g.: a pinched wire, or improper circuit modification), then too much current will flow because there's not enough resistance. That's a "short circuit". The fuse blows to prevent the wires from becoming as hot as the filament of a light bulb, which could start a fire, &/or damage other wires bundled with the overheating one.

ItIsWhatItIs

Steve83, thanks for your reply. I'm disappointed to find that, of so many more than a hundred views, my post has received only your one reply... disappointing, I really thought members were more supportive on here.

I'm sorry I didn't include an exhaustive description of my truck's identifiers. I will implement this soon if I think I'll be using the forum for a bit longer, but it doesn't seem like a very helpful place.

I appreciate the links you provided, but after digging around on them, I am left with no better troubleshooting suggestions. I have looked at the wires in the affected system and do not find a short. Last night, again, the fuse mysteriously blew after more than two weeks' being fine and the instrument panel/ taillights/ parking lamps having been properly illuminated.

This is becoming very frustrating. Has no one else seriously ever had this problem? No specific pointers of exactly what to try, or what to search for!?

Steve83

You expect others to put MORE effort into helping you than you're willing to put in yourself? Good luck!

mmk

First of all, do not discount anything as a possible source of problems concerning blowing a fuse.
I usually go after things that are already malfunctioning. Fix now what you know is bad, you may fix the problem.
All information below is given with the following disclaimers:
- You get what you pay for
- I do not have a 96, I have a 94
- I am not a Ford mechanic
************************************************** ************************************************** ************************************************
1) The door chime module receives an input (interestingly enough, through the malfunctioning main light switch). Unless you know for a fact that the brown wire has been removed from the circuit going to that module, you need to check it, since it may be the cause of your problem. Sometimes people will pull wires off and do not properly insulate the ends, or the wire may have fallen off and is randomly touching ground. Whatever the case may be, that wire needs to be checked. Additionally, it may be connected and that input on the module may be grounded, which would require removing the wire from the module and insulating the end or replacing the module.
2) In addition to the wire from fuse 4 going to the Main Light Switch, it also has a connection to the dimmer switch, which affects your radio and instrument cluster. The instrument cluster appears to be protected by a 4 amp fuse, so I doubt that is the problem, but the wiring to that fuse needs to be verified, as well as the dimmer switch. If fuse 11 isn't blowing sometimes, I doubt it's your radio.
3) All your marker lights (front and rear) are powered by this fuse, so that's 6 up front and 2 in back. Additionally, your license plate light(s) get power from that fuse, another lamp or two.
4) Power is also connected to the Anti Theft circuit. Although this doesn't supply power to the module, if it's faulty, it may blow the fuse.
5) If you have remote keyless entry, that wiring needs to be checked.
6) It appears that if you have towing, you may have a wire leaving from near the front left turn/park connector. It may go nowhere, but it should be checked if it's there. Double check the wires for that brake assist.

Chris_1

iTrader: (1)

Trailer wiring ?