Last Ditch Solution on 1997 XK8 Inoperable Brake Lights or Owner Will Junk Car
 

Have tested and tried all the usual routes...fuses, relays, brake switch, light module, continuity tests, etc. Nothing is working. Shifter solenoid removed so we can at least start the car without sticking something in that hole.


Owner (a friend) so frustrated with car he wants to junk it. My friend does not want to keep throwing parts at this and chasing rabbit holes of endless tests, removing dash parts, sorting through tiny wires for correct color codes. etc.

So, that being said, I am able to fire the brake lights, all three, by connecting a test light from the negative battery terminal to one terminal on the brake light relay.

I appreciate all the suggestion offered on a previous post that went dark and can assure you I have tried every one of them to no avail.

My idea is therefore to run wires from the brake light switch, under the floor, through to the boot area and connect where I had the test light connected. Is it right? No. Is it Mickey Mouse? Yes Is it half assed? Yes. Will it work? YES.

Thoughts???

Does the brake switch actually work? If so it's certainly a way to fix it.

Does the car have cruise control?

I appreciate your frustration, but the brake lights are a fairly important safety system, as you know, that generally reduces the number of times people run into you. So, that's not a good idea in my opinion, unless you are fixing a wire in the current system rather than bypassing the system. (I take it that the car is not starting?)

I again understand the frustration with these electrical tests, but it's a fairly simple system consisting generally of switches and power routing. For example, as you know, if you have continuity in various places (or everywhere), the next trick is to systematically make sure the switches work (which you have done?) and systematically figure out where the voltage is getting from/to (relay, wire A, etc, which you have not done?), especially since the brake lights appear to work. The available voltage must get to some part of the system and not to others. Either start at the switch and work forward or start at the lights and work backwards with the voltage.

Discussion of the owner junking the car over brake lights (throwing up his hands?) is probably not improving the frustration situation, but there must be other things wrong with the car to suggest this since this is, on its face, not an insoluble problem?

He's talking about wiring the brake light relay directly to the brake light switch - why wouldn't that work if the switch is working?

Depends on what we mean by 'work'. If he's simply running a wire from AC-16 (both switches) to the relay at BT11-9, that would light up the lights if there is voltage at the switch and the switch works (And everything else works but, the continuity from these points is already checked, and was found to be fine?). But it would also potentially bypass everything that goes through AC13-3 and every other connection to the back, for example, into the ECM, etc. For example, is it just that the lights are not lighting, or is the shift interlock or other things a problem?

But, if the continuity has been checked from AC-16 to BT11-9, and is good, it seems that the next thing to check is the voltage at either end with the switch closed before running wires.

He says he has bypassed the shift interlock. To me, if the other alternative is scrapping the car, running a new brake light wire so you can drive the thing is a good solution, at least until he can work out what the actual fault is.

My only question would be; does the brake light switch even work?

Seems like we're on the same page from different directions. The shift interlock has been bypassed for some reason, so that may be a problem in general.

But for the brake lights, first check the voltage on the downstream side of the brake switch at AC-16 to confirm switch operation, confirm continuity between AC-16 and BT11-9, then, check voltage with the switch closed at BT11.9 to rule out short(s). AC-16 to BT11-9 is just a wire path, as you and I have noted, but there are other things in that path that are/may be important, like the ECM interaction.

This is three steps which should be relatively simple requiring only access to AC-16 (or thereabouts) and the boot.

It would work - at least as far as the brake lights, as that is essentially where the blue/slate wire starts & ends:

https://cimg2.ibsrv.net/gimg/www.jag...e4da439019.gif

However, depending on where the break is, the electronics may not know when the brake pedal is pressed. The BPM, ECM and TCM all need to know that state.

I have done everything suggested to no avail. Even dropped the steering wheel cover to test the power at the orange/white wire. Even tested the continuity at the plug for the orange white wire.

The shift solenoid is getting no power under any circumstances. Under a bench test I can't gt the shift solenoid to fire. I simply drilled a small hole in the shift housing and put in a small screw to prevent the lock out bar from locking the shifter so he soes not have to use the insert from a ball point pen to get the car out of park.

I sense the ECM could be a problem, but hey, why spend hours and hundreds if I can wire directly to the NEW 130.00 brake light switch. The owner could care less about cruise or shift lock. He has spent 20 G on this car and can't drive it.

I am merely suggesting that I wire the brake lights as the brake lights on my 65 Chevy, 62 Corvette,, 61 Eldorado are wired. Why would that be bad?

I would try it just as a means of proving that the brake switch and relay ends of the circuit are working - you can always go back and fault find from the switch to the ECM later.

Bear in mind the cruise control might not disengage when you press the brake pedal and you might get other error messages, but the car should be driveable.

Peter, my concern would be the wire in place now. Where to enter that wire would be my question or do you completely disconnect it at both ends and put in place a direct power wire off the switch or a new bracket with the old style switch.
Your ECM has a few functions that depend on readings from the original switch, if that is not of a concern with the owner, then what you are going to do will turn on the brake light. Problem solved.
Well done.

To clarify, you 1. checked voltage at the downstream brake switch and confirmed the switch works, 2. Checked continuity from the brake switch (AC-16 in the diagram above) to the relay (BT11-9 in the diagram above) and 3. checked brake switch closed voltage (at BT11-9), and they are all fine?

If so the lights should come as designed on if a wire directly from AC-16 to BT11-9 makes them come on.

Beyond other functions like the shift interlock (which seems like a bad solenoid?), the only role for the ECM here, in this incredibly restricted circumstance with the brake light, would be to provide a short path (which would make #3 above not true).

But to answer your question, after the early/mid 1990s cars have control systems that use the information in the brake switch closure (e.g. the ECM, TCM, etc) that provide functions in the car. Your wire will bypass all of these. It's not clear to me that's okay, or what consequences this will have (especially with fault states where the car decides to brake but the brake switch is not depressed), but you need to know that's happening if you decide to do this.

Sounds like a cheap way to acquire a car....

Wow, I am surprised at how long it took to get a wise ass comment. If this was a Corvette forum such comments would be early and voluminous.

We all typed them out, then erased them Jag owners are more refined.

Given it is well documented on the forum what happens when the ECM isn't getting a signal from the brake switch, I can't see any terrible consequences.

As long as it does not cause problems with the ABS I would think it is fine.

If you want you can disable the cruise control with another NC relay spliced into the cruise control button circuit that opens when you press the brake. Resume probably would not work but cruise still would and would still shut off if you hit the brakes.

Thanks, the owner has never used, nor does he care about the cruise control.

The ECM is probably the issue. The owner is reluctant to pull the dash apart to find the ECM and then throw more money at a used one that may not be in the right VIN sequence, needing programming, etc. That car has a power drain issue that was addressed by an electronic key fob shut off that kills all power while car not in use. The alarm is quirky, and the driver's window does odd things. The car has over 100K, has a custom paint job that is black with purple flake. It was originally denim blue. The glass is all darkened out. I wouldn't pay 1000.00 for this car, although the motor runs good.

Just a note, the ECM is in the engine bay on the passenger side, very easy to get to, remove, have repaired. Repair of his own ECM (which may be the problem as you note, common on this year) is likely a couple of hundred dollars, probably less than the value of the time you've put into it...

The custom paint job sounds 'interesting'.