Fuse 27 constantly blows - immediatley

I am troubleshooting a blown fuse issue on my 1998 Ford Ranger XLT 4x4 Automatic.
At the moment, if I insert a fuse and turn the ignition to "ON", the fuse blows immediately. Obviously an over-current situation.
According to the manual, this fuse controls the following:
Electric Shift (I understand this to be part of the 4x4)
Backup Lights
Daytime Running Lamps (Don't believe I have this)
Transmission Control Switch (I understand this to be the overdrive switch)

I have done a lot of troubleshooting - starting with the most common...the wires in the steering column that control the overdrive. I checked those wires twice because I heard you have to check them very closely. So I even took the extra step of tracing them and disconnecting them at the plug near driver's feet. No joy.

I then disconnected the vacuum solenoid under the airbox in the engine compartment. No joy.

Next I went for the backup switch. I believe I located it on the driver side of the bell housing - it was a two-wire connector. Disconnected it. No joy. While I was in there, I disconnected the transmission range sensor too just to make sure the switch wasn't integrated into it. No joy.

I then removed both tail lights and disconnected the wiring harness that goes to the backup lights. No joy.

A trailer hitch was added before I owned the truck. The wiring looked suspect so I cut all of that out. no joy.

Every now and then, the fuse will hold for a few hours or longer - but it's totally random.

With the fuse blown, the 4x4 still works (high and low), the backup lights do not work and the Overdrive doesn't work.

All I have done recently is replaced the entire front end on both sides (upper control arm, lower control arm, sway bar end links, and outer tie rod ends). This had it's own set of challenges since I had never done it before, but the only thing that would have been any bit related to this fuse problem is that I was unable to get the c-clamp to release on the wheel axle so I had to do the job without removing the axle which turned out to be a lot easier. Why is this related you might ask? Well, it probably isn't, but the 4x4 vacuum relies on that hub and I tried in vain to get that axle off. I am 99% sure that there is nothing electrical in that axle and it's all vacuum-based, but I wanted to mention everything I had done.

Thanks for any help!

Put a blown fuse in the slot and test the little pins at the top for power and ground. One should have power, the other ground. Once you have that figured out start unplugging everything that fuse powers until the ground goes away. If you have everything unplugged start looking at inline connectors on those circuits.

Eventually you will unplug something and lose continuity to ground. Once that happens you will have isolated the short. Then just overlay the wire.

I did something similar - I put a test light in place of the fuse. It lit up so I know it was shorting still. Then I unplugged everything listed above and the light never goes out. I'm outbid things to unplug unless it's something not listed in the manual that uses that fuse.

That's why you need to go after inline connectors. It's probably a pinched or rubbed wire somewhere. If you unhook all inline connectors for those circuits, and there are probably a few of them, you can isolate the section of harness the problem is in.

Thanks. I guess I'm not understanding when you say "inline". I've tried disconnecting every connector I can locate for these components. Are you saying to just continue that path?

I am really not sure how the term "inline" is confusing. An inline connector is one that connects two sections of a wiring harness, rather than connecting a component (like the 4x4 module) to the harness.

There is probably one going to the TCS, probably one or two between the fuse block and the reverse lights, maybe one on the way to the 4x4CM. Also unplug the trans range sensor since it is involved in the reverse light circuit.

The Daytime Running Lights connector is located on the front of the radiator support behind the grill and bumper on the drivers side. Check that plug so make sure nothing hit it or it's starting to rust out. I saw many trucks for some reason without the weather plug and the connector was rusting away.

Also to get daytime running lights if you want them. All you have to do is buy the module and it will work. I think it's about $40.00 usually. I personally don't like them here in Cali, but they do help up in Minnesota where my mom lives.

There are two ways to do this; but, the bottom line is something on the output side of the fuse is grounded. Pretty simple right, just not so easy to find the fail point.

But, I did check Chilton, Haynes and AutoZone and there is no fuse 27 listed for chassis wiring. The backup lights are on #26 and the DRL is on #6. Since I could not find a diagram for the 4X4 system, IDK about that. They could be screwed up. The AutoZone stuff is here: http://www.autozone.com/repairinfo/repairInfoLanding.jsp If you are using the owner’s manual, I would believe it.

Anyway, there are two ways to check this, with a meter or with a test light. Meter is the safest, but a test light will do it.

With a meter and the key on, remove the fuse and determine which is the power side of the fuse socket. Now with key off, either remove the negative battery cable or be very careful. ON the meter’s OHMs scale, connect one meter lead on the OUTPUT side of the fuse socket and the other to a good ground. Since the fuse blows, you should get a pretty low reading. Now the hard part, you have to disconnect each component that is powered by that fuse. When the meter reads open you have found the bad component. If, after you have disconnected everything, and still have a reading, you have to trace each power wire back to the fuse socket. You have to disconnect each inline connector as you come to it. When the meter shows open, you have found the problem wire.
To do it with a test light, you leave the negative battery cable connected and key off. Connect one lead of the test light to the positive battery post and one to the OUTPUT side of the fuse socket. If you connect it to the input side it will not light. Since you have short someplace, the lamp should light. Do as above and when the light goes out, you have found the problem.

Good luck!!!

jeffsaidso said:

Thanks. I guess I'm not understanding when you say "inline". I've tried disconnecting every connector I can locate for these components. Are you saying to just continue that path?

Click to expand...

When he says inline, he means continue up/down that wire and isolate everything connected to it...sometimes there are other things that get spliced into wires over the years by people trying to shortcut things like installing a stereo, lights, etc. They find a source that has power and that's where they go and splice in at, nevermind what that wire goes to it has power so they splice into it. Sometimes over the years wires will rub on things and eventually it will wear through the wire sheathing and now you have bare wires hitting metal somewhere causing a short thus blowing the fuse. So trace all the wires from start to end, I'm guessing you have a wire or multiple wires shorting out somewhere.

Fixed

So, I found it. It was pure luck. I disconnected inline connectors and all end devices. Test light still burned bright. I literally sat down in the driver seat from pure disgust and the light went off. I then shook the truck and found the light could be turned on and off by doing this. I then used a rubber mallet to bang around the truck to isolate the source. This wasn't as revealing as I imagined it would be, but it did work in the end. There is a large wire bundle going through the firewall. The wire bundle would move 1/32" of an inch and the fuse would blow. Because it was late, I just tie wrapped it for now but will dig into it when I can. See pic in the link to see where I tie-wrapped it.

Thanks for all the ideas.
http://jeffsaidso.com/wp-content/uploads/2015/09/ranger.jpg