ok, Ive got my 97 disco-1 pretty torn up right now trying to track down what I understand to be a short in the brake light circuit. Electrical (kinda like math) makes my head spin sometimes, but I think I'm on the right track. Here is what I've done up to this point:

The other day my friend said my Drv side brake light was out, so I picked up a bulb.
-Later that night I went to pull into the lighted part of the driveway and found we were stuck in PARK with a blown 15A brake light fuse.
- A new fuse blew as soon as I stepped on the brake with the key turned to 2nd (accessory)

-I started to disconnect everything I knew was on that circuit:
* left and right tail light harnesses
* center brake light
* brake light switch

I also did a visual of these things and thier connectors:
--trailer wiring harness (never been used)
--the brake/shift release solenoid (clean & intact)
--The rear window defroster connections (they were pretty gross, but not touching so I cleaned them all up and left them disconnected)


Then with a 30a circuit tester plugged into where the 15a fuse would normally go I began to reconnect things, adding one-at-a-time to see how much juice was being pulled when I stepped on the brake with the key turned to accessory/position 2.

- first the switch - ~5a
- RR tail light harness - ~8a
- LR tail light harness - ~10a
- center brake light - was ok if i stepped gently on the brake but anything harder spiked the reading to >25a then blew the 30a tester.

- Then I replaced the fuse in the tester, unhooked the center brake light- it spiked and blew again.

So now I'm like blonde-girl confused, WTH just happened, and what does it all mean?

 

sounds like the center brake light may be the problem. might be connected through the rear defroster grid to ground?

 

Thanks for the reply Jim, That is what I thought when adding the center brake light blew the tester with 25+ amps when I stepped on the brake. But then I disconnected the center light and the same circuit blew again... it's just really confusing.

Also, looking at the circuit diagram I cant tell what the rear defrost grid and the center brake light share. they are not on the same circuit, and don't share a ground. However anyone standing there looking at the inside of the rear door can tell you that the two appear to be intertwined all over each other back there.

Any advice on what I should do next?

 

I assume you opened up the center brakelight and checked for corrosion and checked the grounds, maybe start looking for a wire rubbed through in the harness. try removing the bulb and see if that makes a difference. What happens when you turn on the rear defroster grid?

Wiring can be a real pain sometimes which is why they charge so much for it. Usually they charge straight time as you end up crawling all over to trace wires.

 

Yessir! I did open her up, looks all shiny and new in there, no corrosion or anything melted. I went ahead and took everything down as far as I could, the bulbs, holders, harnesses, grounds 400 & 401 then I cleaned up the connections anyway, just to be thorough. I have not tried the rear defrost however. In fact I have never used the rear defrost, lol, or the rear wipers for that matter... pretty odd for the pacific northwest I should think.

Anyway I will try the rear defrost here in a bit, soon as I get some daylight so my neighbors don't give me "the look". And maybe start over testing one component at a time on that circuit.

Might have to order up a bucket of 15a fuses from Costco tho. I have a bunch of different meters and testers here, but no experience using them yet. After a bunch of tutorials and videos on the subject of electrical test equipment the only thing I was sure about was having a headache, lol.

 

Ok... so I figured i ought to update this in case anyone else experiences the same thing and needs the info.

I went out and decided to START OVER troubleshooting the brake light circuit. Since I had everything already opened up, inspected, and cleaned it went pretty fast. This time I disconnected everything on that circuit including; the brake light switch, both Tailgate harnesses, the center brake light at the light itself, and from the connectors on the window glass next to the defrost grid. Also laid out the trailer harnesses just to be sure nothing was touching anything else, anywhere, and proceeded thusly;

-1- With a 30a digital tester plugged into the fuse holder
-2- I turned the key to acc2 -3- waited for the ding dongs to complete
-4- turned the tester on: It read zero
-5- stepped on the brake, varying the pressure several times: still zero

***so there was no draw on the circuit (because everything was disconnected).

Next I reconnected the brake light switch to it's wiring, and began again from step 1 above
This time in steps 4 & 5 the tester read between 3 and 5 amps. ***since the switch is the only thing connected to the circuit reading seemed appropriate.

I was about to rule out the switch and try the next thing, but I kept on (step 5 above) the brake pedal a few more times. That's when the tester suddenly jumped to 19, then 23 amps . I noticed that with only slight pressure on the brake pedal the current was within the parameters for it's 15a fuse, But if I stepped hard the current spiked way over that number.

Diagnosis... fubar brake light switch. Update: Brand new switch on the way, $16.52 from RockAuto.

Q. So, why didn't the switch fail the first time I tested it by itself?
A. Eggnog, because x=4,760* Pi

I have no answers. Things like Electricity, and The Maths are right up there with VooDoo, and Cait Jenner on the list of things I don't need to know any more about in this lifetime.

Rover On Everybody!

 

Wait... you got 3-5 amps with just the brake switch connected?? A switch is just a set of contacts, it shouldn't draw any current ( no amps) When you get the new switch, test it, I am sure you will see no amps from it. any draw from the switch alone with no bulbs connected will be from a bad connection, unless it is reading the shifter lock mechanism (possible)

These things are notorious for bad connections. Most of my problems electrically have been from burned connectors. They get burnt from bad connection and then don't pass electricity too well.

P.S. good job finding the problem!!

 

the solenoid for the shifter lock was my though also

 

If the brake light switch was still hooked up to wiring harness, and it spiked. Doesn't mean necessarily it is switch. Since the switch is hooked up to harness, and all lights were disconnected, chances are, there is a wire shorting to ground. Check continuity of all light wires at socket and see if any of the positive wires show continuity to ground. They should not, whereas the negative side should.

 

I have had a few brake light switches go bad and it can be pretty flakey when they go. Not a bad idea to just replace it and eliminate it as a problem. There are 2 different switches, one for manual trans, and one for automatic. Last time I went to get one, they had them labeled wrong in their parts book at the parts store.

There is a second switch on the trucks with cruise control, but that one has a vacuum hose attached to it. It does not have anything to do with the brake light circuit.