brake light electrical help!

[stomper]

When i turn on my ignition switch, my brake lights and plate light automatically come on. They won't shut off without turning off the key. And of course, they don't react to pressing the brake pedal. Does anyone know where i should start searching for a short? Electrical issues are not my strong suit.

[siii8873]

what vehicle SIIa, SIII ? Sounds like you have some wires crossed or touching.
What is suppose to activate your brake light a switch on the brake tower that makes when you press on the brakes?

[printjunky]

You don't mention vehicle specifics, and I can only really speak to SIII issues. Others can chime in on earlier models. So you may need to ignore some or all of this if you have a Series I.

As far as the brake lights go, you might first try disconnecting the brake light switch on the brake booster. There are two wires there, the one you want, if I recall, is Green/Purple. If that turns off the lights, it's possible the problem is with the switch or the actuating mechanism. (it means the lights likely aren't getting their power form somewhere else in the harness)
That might save you some wire tracing effort.

That's just one simple, shot-in-the-dark test. Next (if the switch appears to be working), you probably should pull the wire cover panels in the rear corners. That seems like a likely place for some kind of improper contact. Again, you should be able to determine some more symptoms by unplugging some of the connections systematically and observing what happens.

That's where I had some trouble with variations in the results I was getting when rewiring the Rover. Bad grounds and unintentional shorts in those rear corners.

Working from either end of the circuit (the front at the switch, or the back at the lamps), you should be able to start narrowing down where (and maybe why) it's happening. eg: if the switch seems to be working (You can test it by hooking a bulb to the tab where the green/purple wire goes (and grounding the case of course) and pushing the brake pedal. If the bulb lights (and goes out)) then move down the line and try unplugging the mid-harness connector somewhere down below the oil bath filter in the middle of the passenger-side of the engine bay. See what happens when you do that. Just keep narrowing it down.

I would record what happens when you do these tests, and if you don't figure it out as you go, report back on what happened

Similar process for the plate light circuit, except that should be on a separate circuit, wired to the headlight switch.

[Les Parker]

Methinks you could have a crossed wire in the LH rear corner of the rear body, behind the alloy interior panel. You may find a rats nest there and the wires are tracking, bearing in mind that the lights are live with the ignition switched off.

[SafeAirOne]

Well...there's no power back there unless something up front is supplying it erroneously. All those circuits are dead unless the light switch is activated or the brakes are pressed or the turn signals are switched on (or there's a fault up front where all the electricity is kept).

All the wires can be shorted together back there and it should not cause the brake and plate light to go on with just a turn of the ignition key.

I suspect that it's not the BRAKE lights that are coming on, but rather the tail lamps, since the plate light is fed off that same power wire back there, so unless you have confirmed that it is, in fact the BRAKE light you see, I'd be examining the rear lamp circuit for a short/faulty switch up front.

Just the way I'd go about it. Reality might be something completely different. :)

[stomper]

Thank you all for the great advice. Here is where I am so far.

The lights that came on when I switched the key on were indeed the parking lights, and the plate light. The prior owner plugged the wires into the ignition switch incorrectly, and when I moved the spade connector to the correct terminal, the parking lights and the plate light work correctly again. (thanks Mark) There is no short at the rear, as I pulled the lamp covers in the tub, and everything looks good.

The brake lamps still don't come on though. I have a hydrolic brake switch, and when I pulled the wires, there is quite a lot of oil covering the switch area. the connectors were cleaned, but there is still no lights. Do these hydrolic switches break? how do I test it? If I unscrew the switch, do I need to rebleed the brakes? I'm not all that familiar with this type of switch.

Thanks everyone, I think I have a better grasp of the actual problem now, when I had no idea where to start before!

[Nium]

Don't know if the key needs to be on or off but take the wires off the pressure switch and touch them together. If brake light comes on switch bad if lights don't come on switch good (maybe) and problem elsewhere.

[stomper]

Thanks Walker, I was actually testing it while you were typing, as I found an old thread that mentioned how to bypass the switch. It is a broken switch, as I have lights when I bypass it.

Now to find the part on the site, It isn't under electrical, or brakes. I guess I will just give a call in the morning, and order one up. Then I will need to begin bleeding the brakes. I don't know if I need to do each wheel, but I guess it wouldn't hurt. I found some brake fluid seepage in the 4 way junction, so I must have some air in there already.

Funny how one problem leads to another!

[siiirhd88]

I think the switch is:


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[Terrys]

Since you confirmed that it was the tail lamps, not the brake lamps, going on wit the plate light (Mark and I are thinking alike) Are you front marker lamps also going on? Since you have a hydraulic stop lamp switch, I'm guessing your truck is early enough that you have the turret headlamp/ignition switch also. That's where I'd be looking. The outer ring of connectors on the turret are the light circuits, the inner 3 or 4 (CRS) are the ign switch circuit. Check that there isn't some frayed wiring from the outer ring, touching the inner contacts.

[SafeAirOne]

Stomper...Do your brakes work well?

Reason I ask is that it takes a certain amount of pressure in the brake lines to activate the switch. If the system isn't developing enough pressure due to leakage or air in the hydraulic system, you won't be able to get the brake light switch to activate, even if it's brand new.

[stomper]

Thanks Terry, The prior owner just hooked the spade connectors to the wrong terminal on the ignition/light switch. I guess I never realized the lights were on in the back all the time. That is wierd, as I have owned the truck for over a year, and never noticed, and I haven't even opened the dash till now.

I have been driving the truck all week, as my wife's car was totaled in a fender bender. the brakes are good enough to stop the vehicle with one pump even if I need to stop quick. I do get a firmer brake if I double pump the pedal, but the switch still doesn't come on.

Bought a new switch for $11 and change$. couldn't find it on RN's site though, so I had to go to a competitor. (saved $1 too).

To Les: it would be helpful to have the brake switch listed under the brake section, AND the electrical section. I still can't find it on the site, and since I didn't have the RN part number, I couldn't search for it either. Just some feedback to make the site that much better!

[SafeAirOne]

OK, you have sufficient line pressure to activate the switch then.

I suspect that if you had an accurate thread pitch (or had the old switch in-hand) you could probably pick up a replacement hydraulic switch from your local auto parts store, providing it's not some crazy obscure English thread pitch.

...And when I say auto parts store, I don't mean AutoZone or Advanced Auto, etc. I mean a REAL auto parts store where the employees know something about auto parts. ;)

[Terrys]

Stomper, you shouldn't even have to bleed the system. Make sure the reservoir is full, unscrew the old one and put the new one in and there won't be any air admitted.

[stomper]

Thanks guys, The green bible says to bleed after replacing the switch, but I will try what you suggest Terry, and see what my results are. The Green Bible says to have the new switch ready to switch it quickly, so I guess I will wait on a new one, rather than take the old one out and hunt for a new one. It would be more of a mess, and then I really would need to bleed the system then!:thumb-up:

[Donnie]

Stomper...Do your brakes work well?

Reason I ask is that it takes a certain amount of pressure in the brake lines to activate the switch. If the system isn't developing enough pressure due to leakage or air in the hydraulic system, you won't be able to get the brake light switch to activate, even if it's brand new.
If U have air in the system U will have a soggy petal feel. Like a rubber ball between your foot & the petal