`81 Brake Light Issues

[johnstringer]

NO BRAKE LIGHTS!! Tail lights continue to work well. I bypassed all modules from the brake light switch making a direct connect; Brake Lights remain on wether you tap the pedal or not. This tells me theres no break in the wires. The fuse in the #7 slot is good. That leaves a relay or the lamb control unit unless, theres somthing else. Please help. Ideas and testing methods would help greatly. Thanks for the support.

[Alan in AZ]

Test the X-bus - find it at the input of fuse #7 - is it on when the ignition is on? This is the power supply from the ignition switch for the brake lights. If this fails (and it often does on older cars) you will need to replace the ignition switch (electrical part only).

Wasn't clear to me what you jumpered when you 'bypassed' all this - battery power direct to the far side of the switch?

Alan

[johnstringer]

You're correct on how I bypassed the system; direct power supply. Your advice on fuse #7 was my next step but would have never guessed the ignition switch had anything to do with it. Thank you for everything.

Also, I just discovered my brake booster also needs replacement. Leaking fluid pretty badly. Would this have anything to do with the brake lights? The booster is the only other possible problem I can come up with.

[Rixter]

Originally Posted by johnstringer

Also, I just discovered my brake booster also needs replacement. Leaking fluid pretty badly. Would this have anything to do with the brake lights? The booster is the only other possible problem I can come up with.

the brake switches (there are 2 in your MY) are in the bottom of the master cyl, if fluid gets in them is will short them and can cause the lights to stay on
hope that helps

[Alan in AZ]

So use your same battery live jumper to the input of fuse #7 - try the pedal now - work? if yes your switch is OK and the power supply to the fuse must be the issue.

If it seems to be the ignition switch - try wiggling the key around in accessory and or ignition positions - if sometimes it works in this way - its definitely the switch. They are <$60 and not too bad to change - no messing with the lock part at all - switch just bolts on.

As Rixter noted:

Given you have leaks it could be related to that - on older models the switching is direct from the pressure in the brake circuit (dual redundant switches)... later cars just have a pedal activated switch.

Alan

[johnstringer]

Alan,
I've researched your info regarding the ignition switch. When you stated electrical part only I took that as the Electronic Ignition Control Unit, is this correct? If so, is this part located up in the passenger side fender toward the front of the car? The manual shows the unit inside the car next to the fuse & relay board. My car is a `81 928 Competition S 5spd. Also, the manual shows a Transistorized Ignition Control Unit bolted to the front of the car just under the hood in clear view; my cars does not have one! I haven't performed the test on #7 just yet because other test have led me to believe I'm not getting power to #7 Fuse. Just wanted to get all my ducks in a row first.

The brake booster is leaking fluid onto my floor board. Each brake light switch is dry as is the master cylinder.

Thanks for all the support fellas and ladies.

[Alan in AZ]

No I really meant the ignition switch - the back part of the place you put the key...

The car gets multiple things switched when you turn the key:

Ignition power (15-Bus) - things needed for the engine to start/run
Accessory power (X-Bus) - non-essential items during starting
Starter (50) - the drive to the starter motor
Radio (R) - Power supply dedicated to the radio
Park (P) - Power to the parking lights with key off/out

These outputs are all separate terminals at the switch and are active in different combinations of the key positions...

The most heavily loaded are the X-Bus contacts so they typically wear out the fastest...

Alan

[johnstringer]

Located the problem fellas. It was combination of both fuse #7 and the master cylinder. Now I have power to the brake light switch; believe the brake light switches are bad.

Can't say thanks enough for the support. Another question on the forum!