Help, No brake lights

[falcon]

Car is an 88 cab. I just got a tail and I traded out the low 3rd brake light on my cab for the taller "ET" types. Hooked up the black/yellow and brown wire and connected the engine bay light and tested the brakes. Nada...nothing on the left, right or 3rd brake light.

To be honest. I didn't test the brake light BEFORE I replaced the 3rd brake light.....So I could have been driving around with no brake lights for awhile. Car starts and drives, just no brake lights.

Took out the Bentleys manual and looked at the schematic on page 970-66. The circuit is hot when in the "on" or "acc" position. Tried it...Nada.

read across the #4 fuse (#1 panel per the book) and it reads a voltage.

Not sure what going on here. According to the schematic there is a brake light switch connected to the brake cylinder. I see it along with some metal lines. I didn't want to poke around in fear of damaging it or unecessarily removing it. I read a previous post that described it as the switch for the idiot light. Funny, I didn't see any warning lights in the cabin.

There is a mechanical switch located at the pedals. Can this switch go "bad"

Any help appreciated.

Randall

[grant lyon]

Have a '76. If your newer car is at all similar, the switch on the master cylinder is to detect if you have a leak in the brake (fluid) system -- and turn on the brake warning light on your dash.

(Again) if yours is like my more vintage car, the brake lights are motivated by the switch at the pedal cluster. It switches fused power through the stop light bulbs to ground.

Sorry I don't have schematic (wish I had the need) for the newer car. Someone else should chime in soon.

Good luck.

[JDaniel]

Is the fuse blown? IF you keep blowing fuses for the brake lights, make sure you check all the wiring for that 3rd light and the trunk light. There's a tendency to short out along the A/C condensor.

[falcon]

JD, I ran continuity across #4 fuse and its checks out. No blown fuses.

Grant, you mentioned, "the brake lights are motivated by the switch at the pedal cluster. It switches fused power through the stop light bulbs to ground"

It seems the whole circuit is not grounded (Hard to believe I lost a ground point at G302 (located at front right xmission mount) or the switch went bad at the cluster.

I also made sure the both right and left bulbs are seated correctly in their sockets.

Can anyone tell me more about this switch and how it affects the switching on and off of the lights?

I'll take a digital pic of the schematic tomorrow and post.

[falcon]

Here's the external wiring for an 88 carrera. The pics had to be divided into 3 since I did not have a macro lens.

I don't see where the foot brake switch comes into play in the circuit. I'm going to turn the lights on and see if the rear red lights go on dim. If so then the brakes circuit is not right. If the no lights then maybe the parking light circuit is bad. I'll go from there. Any thoughts are welcome.

Randall

[grant lyon]

Switch:
Maybe I'm missing something, but it looks to me as though the switch at the pedal is the dual ganged "Stop Light I" and "Stop Light II." These are normally open, meaning the supply ("+") isn't connected to the rear brake lights until you push the brake pedal and the switch closes (becomes conductive.) Does this make sense? If not, I can try to explain again.

BTW, if the rear side lights don't come on (when you pull your light switch out), then that would be two circuits (both the brake lights, and rear taillight assy lights) with a common, possibly bad, ground connection. Coincidence, or do you have bad ground connection(s)? Suggest you start with checking continuity at the brown (BRN) connections to Ground. Ground "G302" is just a reference point, any chassis ground point should do.

Power:
Not sure I understand the symptom you describe in:
>The circuit is hot when in the "on" or "acc" position. Tried it...Nada.

If you get no voltage reading at the fuse with the ignition switch "ON," then you need to look further upstream in the schematic for a fault -- probably a fuse. Note, upstream could be nothing more than the supply ("hot") wire leading into the fuse you've been poking at-- if it's loose and not connected to the fuse... . Don't have your schematics further upstream, but you've got to have more fuses, for instance to protect the flasher.

Good luck with the diagnotics.

[falcon]

Grant, thanks for the input. Yesterday I just turned on the headlights and the tailights lit also which indicates that the grounding to the tailights is good and as you say the problem is the 12V supply to the lights.

The schematic and illustrations shows some switches that correspond to the sensor switches for idiot lights located in the trunk. If "Stop light 1 and 2" are ganged at the pedals then those are the ones I should be looking at.

Thing appear to be pointing to the brake pedal switches. Thanks for the description. I'll be checking those switches this weekend.

Randall

[KTL]

The switches are on the master cylinder, not in the pedal cluster.

[emcon5]

Originally posted by falcon
If "Stop light 1 and 2" are ganged at the pedals then those are the ones I should be looking at.

They are not at the pedals, they are screwed into the master cylinder, and are triggered by brake fluid pressure. Here is the cartoon from PET.



The switches are #23

Tom

[KTL]

Thanks for the pic assist Tom (got you by one min.!).

How do you snag a clip of the PET diagram?

[emcon5]

There is a "Illustration Zoom" feature built in, select that, center on the detail you want, then alt+PrintScrn to capture the active window. Paste it into a graphics program, (I like Paint Shop Pro). Crop the the stuff you don't want, resize down to reduce file size, and save it as a jpg.

It actually took longer to explain than to do.

Tom

[falcon]

I do see the two switches on the master cylinder.

Due to this information from Tom, I will Identify the black w/yellow wire and monitor it as the brakes are engaged. I can use the other "assumed good" switch as an example of how the switch should react to braking.

Question: If I do remove a switch, do I just "unscrew" them? Do you think brake fluid would leak out?

Thanks for the input guys!

Randall

[emcon5]

Even though the switches are on seperate brake circuits, they are redundant. If either switch is good, (and the wiring is intact) the lights should come on when you push the pedal.

I don't know about later cars like your 88, but on my 82 there is some extra wiring that will light the parking brake warning light if one of the 2 switches are not in agreement. The idea is a warning if you have a failure in one of the 2 circuits, front or rear. I wonder if this is what the "alarm system relay" is on the diagram you posted. (the earlier wiring diagram can be found here.)

The switches unscrew from the master cyl, and brake fluid will leak out. You should bleed your brakes after changing the switches. This is the first thing I fixed when I got my car.

Tom

[grant lyon]

Sorry for the bum steer -- no way to know about the fluidic stop light switches. Happy I don't have deal with another source of brake bleeding projects.

I'd be very interested in that Alarm System Relay function.

With the parking brake set and the ignition "On," does your parking light light up? Did it ever? Don't know (obviously) about your particular case, but the warning/idiot lights sometimes serve some not so obvious purposes. Wondering if that alarm system + light pulls down your brakes light circuit.

[emcon5]

Originally posted by grant lyon
Wondering if that alarm system + light pulls down your brakes light circuit.

It didn't on mine, my brake lights never quit working. The theory was to warn you that one of your brake circuits was failing, but in practice, it is a warning that one of your switches needs to be replaced (well maintained brakes don't fail that often). The switches can get gummed up from old nasty brake fluid, and stick.

Earlier versions of the switch have 3 poles, later ones only had 2, and 84-86 could have either. The 3 pole can be replaced with a switch from a VW for less than 1/2 the cost of the Porsche part, I don't know about the 2 pole.

Don't forget that Brake fluid is hell on paint, if you have to replace them, have rags handy.

Tom

[falcon]

Here's what I found so far

Grant, when the key is on the on or accessory position there is NO parking lights and NO brake lights when the pedal is depressed. When I turn on the lights the red lights in back turn on but still NO brake lights when I depress the pedal. As you may know the bulbs for the brake lights have two elements for the two circuits. It does tell me that the bulb is grounded since the parking lights work but no 12V from the brake circuit.

For my own info, I partially removed the wooden floorboard to the brake pedal. I couldn't see to well but I did see a brake fluid line running into a piston-like-thing-of-a-jig. I had seen that blue line before in the trunk. Sure enough the reservoir feeds the blue line. The info I got out of all this was that indeed it appears that in an 88 the brakes are activated by fluid pressure as Tom mentioned.

Up in the trunk area I removed some hoses to get at the two brakes switches. They are of the two contact type. With the car in the "on" position, I did read 12V to both switches. I shorted one of the connectors so that the 12V runs directly to the brake lights. The brake lights lit up which means something is impeding the 12V source to the bulbs.

I'm still not sure if it is the switches. Both are in parallel. According to the schematic if one works then that's all thats needed. I can't believe both are bad. It is possible that one went out and I was running on one good one and finally that went out.

I sanded the contacts and tried them again with no illumination. I swapped the switches with no illumination.

Looks like I will be ordering up two switches (two pole). Luckily they are much cheaper than the 3 pole switch.

I'll post my results when I get the switches in and installed.

[emcon5]

You should be able to test the switches, by checking continuity across the 2 poles with the brake pedal pushed.

It is hard to see both switches failing at once, unless your brake fluid is really nasty, and gummed them up.

Did you try jumping the two poles in one of the connectors to see if turned on the lights?

Tom

[falcon]

Tom,

I've been meaning to change the brake fluid since the PO didn't indicate whether he ever did change the fluid and the history doesn't specifically say anything. I have the moto bleeder and will do so after swapping the switches.

Instead of monitoring conductivity, I monitored the voltage. The bulbs will light given 12V is passing thru the switch since I measured 12V to the switch and actuallyshorted the switch in which the lights illuminated.

I shook the reservoir and the fluid moved quite easily so maybe the switches are gummed up as you mentioned. I hope so, because then it I can stop scratching my head if the swapping out of the switches does not work.

[KTL]

Originally posted by falcon
Here's what I found so far.........................

For my own info, I partially removed the wooden floorboard to the brake pedal. I couldn't see to well but I did see a brake fluid line running into a piston-like-thing-of-a-jig. I had seen that blue line before in the trunk. Sure enough the reservoir feeds the blue line. The info I got out of all this was that indeed it appears that in an 88 the brakes are activated by fluid pressure as Tom mentioned..........................

Randall,

The cloth braided blue line you discovered is the reservoir feed line to the clutch master cylinder in the pedal cluster. This line has nothing to do with your brake light operation!!!

[falcon]

I should have added more words to the post. I was trying to convey that I could not see any mechanical "switches" at the brake cluster and could only see the blue clothed fluid line running to the trunk. That particular blue clothed line does not relate to the brakes but another fluid line does..... somewhere, which prompt me to look at the pressure switches in the trunk.