1999 Mercury Villager Estate. The brake lights fuse keeps blowing immediately when inserted

Frustrated. 1999 Mercury Villager Estate. The brake light wouldn’t illuminate. Found that the brake light fuse (20A) was blown. Put another in and it blew as I was inserting it. Figure there’s a short somewhere, so I’ve disconnected both brake/tail light units in the back, the towing light connection, and have even completely disconnect the 3 brake light switches under the dash (yes, 3… 2 directly attached to the brake pedal and one attached to the e-brake). Still, when I insert a 20A fuse (which is what is called for), it immediately blows. Since I’ve disconnected the brake light switches, it seems to me the short can’t be past that on the way to the back of the vehicle. Maybe I’m wrong though. It seems the short must be either between the switches and the fuse box, or somewhere between the fuse box and the battery. Unsure if the wiring routes through any relays under the hood, but I’d think it would at least have to go through one of the relay boxes since that’s what eventually gets directly connected to the battery. I’ve tried using my multimeter to measure the voltage and resistance on either side of the space where that fuse goes, but I’m unsure what to expect from a reading like that, so those results are inconclusive. Any ideas?

We just had the exact same issue last week. The wires were shorted in th lift gate area. The woman used a cheap 15 amp fuse and burnt up her wiring harness, lots of smoke.

The fuse may power more than the brake lights. What else doesn’t work? Tail lights? Dash board lights? Anything else on this circuit could be the cause.

If it is only the brake lights and you disconnected the brake light switch, then the short has to be between the fuse box and the brake light switch. It cannot be before the fuse, it has to be after the fuse. But I suspect that it is something else.

Modern era cars can be difficult to sort out when it comes to electrical glitches. I have no schematic for this vehicle so I can only offer a few bits of advice.
One is that seeing as how the brake light switch is powered all the time it could be that the short lies in some other area that is also powered up constantly; the hazard switch, cigarette lighter, etc.

Two is that you might consider the purchase of a Helm electrical manual for the vehicle. These are used by Ford dealers and can often be found on the cheap on eBay.
The electrical manual alone is not the answer to every problem but it does lay all the wiring out in a simple, easy to use format with color codes, wire connectors, and so on.

I’ve bought some off of eBay for 15 bucks and invaluable is the word I’d use to describe them.

OK4450 has, as always, provided excellent guidance. You’re going to need a schematic and a wiring diagram (which are NOT the same) to get to the bottom of this one.

Another option is to have it towed to a shop that specializes in automotive wiring. They’ll have access to the proper documents as well as the expertise to find and repair the short. I’m suggesting towing because you have a very real fire risk here. You clearly have a short to ground, and cars have become conflagrations from shorts to ground.

Here are the brake light schematics. I would still be looking at the liftgate wiring.

Hey guys. Thanks for all the replies. Sorry for my delayed response. I didn’t realize that anyone had responded since I didn’t get any kind of notification email.
I’ve still been trying to troubleshoot this every evening after work. One thing I tried this morning was I went ahead and cut the yellow/black wire that goes towards the rear of the vehicle about 2 inches from the brake switch connector. The fuse still shorts as soon as I try to insert it. Pretty sure that tells me that the short must be in between the fuse box and the brake switch connector. I wonder if there’s a relay in between those two points though? There’s several relays right next to the fuses under the dash, but they’re not labled anywhere, so I don’t know what they’re all for. I’m guessing the problem can’t be in between the fuse box and the battery, else the short would simply burn the wire in two, right? This is being a real pain to figure out!

Based on the diagram presented by kfenimore and with the yellow/black cut near the brake light switch any short should be between the switch and the 20 amp fuse shown OR in the red/white splice that goes off to _________? The diagram does not show that and with most schematic manuals they can sometimes require the use of the mechanical shop manual also as they tie them together.
Without knowing where that red/white splice wanders off to it’s impossible to be precise.

As to shorts, the answer as to whether or not a short would burn a wire into is that it all depends. In most cases they do not unless it’s a bad one involving a lot of current and is sustained.

The Helm manual is still the best option. However, it is possible that a service writer at the Ford dealer could possibly run you off a copy of that circuit for little or nothing.
Some years back I ran into an issue with wiring on a Ford for which I had no schematic and the SW at the local Ford dealer Xeroxed a copy in a few minutes at no charge.

I’m looking for a Helm manual…

There’s this…

I’ve never used Chilton on-line so I can’t speak to their accuracy. However, I have a number of their later paper manuals (last 25 years or so) and find them to be horribly lacking in content and accuracy. Maybe the on-line version is better.

The old Chiltons paper manuals from around 30 years and older are very good.

I’ve gotten the ones from auto stores (I think Haynes) and they weren’t very helpful at all.

Is this what you’re talking about?

pay for the 1 year subscription to www.Alldata.com It’s $26.95 and I find it worth it. This is what the mechanics use and it is very accurate. Any time I have a problem I use it.

In the second diagram provided by knfenimore, there is a lead going to circuit S239. I don’t know what that goes to but whatever it is, it won’t be working either and that is where the problem is.

The only other place for the problem to be will be on the backside of the fuse panel, flip it out and look at the fuse socket on the backside to see if the wire going to the car is chaffed or frayed anywhere.

Right now, you are simply looking in the wrong place and as long as you persist in looking there, you are not going to solve this problem.

knfenimore, it’s the AllDataRepair S3000 you’re talking about? They have several different products.

Keith, I’ve tried removing all the fuses under the dash, and all fuses and relays under the hood, disconnecting all 3 brake light assemblies in the back, disconnecting the 4 connector trailer hitch, cutting the yellow/black wire the routes to the back of the vehicle, etc. Perhaps if this AllData subscription can tell me where that red/white wire is splitting off to, I can find my short.

This is why I prefer the Helm manuals. They will show where that S239 is located along with where and what it goes to.

That hot all the time fuse could lead to dome lights, cigarette lighter, hazard warning, etc, etc,.

I find something else odd. Perhaps it seems odd because I don’t know what I’m doing. But when I set my multimeter to the audible continuity setting, and I touch one lead to what I’ll call the “negative” side of the fuse block where the right bar of the fuse touches, and the other lead to the red/white side of the brake light switch connector, I hear an audible. That’s not the odd part. That’s expected since I’m directly touching both ends of that red/white wire. What’s odd is that if I move the one lead from the “negative” side of the fuse block where the stop lamp fuse goes, and touch it the the “negative” side of a couple of other fuse blocks that are near that one, it still audibles. Once I get 2 or 3 fuse places away from the stop lamp fuse, it stops. I wonder if the short is on the back side of the fuse block?

I must be looking at the wrong thing knfenimore. I show the AllData Repair costs $169 per month.

Figured it out. It’s the http://www.alldatadiy.com, not the other one for pros.