1989 brake light troubleshooting

[colinsco]

Driving today and another driver kindly let me know my brake lights don't work. I fell out of the habit of checking my rear lights in reflections some time ago and have no idea when they may have failed. It's my daily drive, but working from home i don't actually put on many miles these days.A few weeks ago i came back from vacation and found that the glove-box light had come on and drained the battery; recharged and has been fine since. Last weekend did a 1000 highway and backroad miles between Carnation, WA and Penticton, BC so ran in some 90 degree plus weather. Either event might be causal. Fuse is fine and at 12v, 12v at brake switch on master cylinder, no voltage at either side brake lamp or center light. Noticed the cruise control wont turn off by depressing the brake pedal some time ago and this could be related. Neither Bentley, Haynes or Waynes books cover troubleshooting this problem so i am hoping someone here can help. 1989 Carrera targa w/3.2 etc. Thanks!

[aj88cab]

Correction...they might be related. The brake light switch releases the cruise control IIRC. The center tunnel switch discussed below is operated by the clutch pedal.

For the brake lights start with the switches on the master cylinder. This is a common problem. Step on the brake pedal hard and see if the brake lights come on. If they do with a lot of pressure it is most likely the switches. You can replace the switches or the entire master cylinder if it is old and if you are so inclined If that doesn't do it, pull one of the connectors on the MC and jumper the plug and see if that works, if it does it is definitely the switches. If not then it is something in the circuit, start with the fuses and follow the wires, use a meter to check voltage & continuity, also check all the grounds, be methodical. Bentley has a wiring diagram to help trace the circuit.

Check the CC cable at the throttle linkage, the fitting at the end of the cable can break and the cable hangs up. The cruise control has a switch in the center tunnel right next to the gas pedal, it is common for these to slip, bend, or go bad. There is a section in Bentley to check and set this switch. You will have to remove the gas pedal for access. If the switch and cable are OK then the it might be the cruise control module itself. I believe there is a troubleshooting guide in Bentley to check that. The 87-89 Carreras have an electric servo for the CC so it is not a vacuum line issue.

[colinsco]

Thanks Andrew. This morning I again verified that with the ign on and the brake pedal depressed there is 12v at the leads to the MC switch. However, there is no continuity between the contacts on the switch itself with the harness off. Does this make sense or is this an indication that the MC brake light switch as shown on p971-10 in Bentley is faulty? What is really puzzling is that all three brake lights are out, which suggests to me more a switch than wiring/ground issue. Any help appreciated.

[aj88cab]

It does sound like the switches are bad. When you say there was no continuity you were pressing on the brake pedal, correct?

Try a jumper across the plug on the harness. If the brake lights come on, that confirms it is the switches.

I am about to leave for the airport for a well needed vacation. So if i do not reply for a week don't think I was ignoring you

Good luck...I think you have it under control.

[colinsco]

Thanks again Andrew, and enjoy your vacation. After my last reply i remembered waking at 4am thinking that the last time i went out the brakes seemed to 'pop' at the first stop but after playing a bit i figured it was my imagination. So now i got braver and pulled the hoses out of the way so i could see the other switch at the back of the MC and when i pulled the harness i found brake fluid on the contacts?! So, now pretty sure this is the problem. As luck would have it, John Walker is hosting a PCA swap meet today and i can fake my way there and hopefully find my switches. If not i'll order the switches from the host later today.

[aj88cab]

That should fix your cruise control issue too...

...signing off here and heading to the beach!!!!

[DRACO A5OG]

Remember the torque is 11lb per on the switches.

[86 911 Targa]

Originally Posted by colinsco

Thanks again Andrew, and enjoy your vacation. After my last reply i remembered waking at 4am thinking that the last time i went out the brakes seemed to 'pop' at the first stop but after playing a bit i figured it was my imagination. So now i got braver and pulled the hoses out of the way so i could see the other switch at the back of the MC and when i pulled the harness i found brake fluid on the contacts?! So, now pretty sure this is the problem. As luck would have it, John Walker is hosting a PCA swap meet today and i can fake my way there and hopefully find my switches. If not i'll order the switches from the host later today.

Just replaced both of the switches on our '86.

An offset wrench will make the job a lot easier.

Easy, inexpensive fix.

Good luck,

Gerry

[Carrera3.2LG50]

I did this earlier this year and wrote up a "how to" that I posted on Rennlist.

Replaced my brake light switches. (It’s all about having the right tool.) - Rennlist Discussion Forums

Might help you or someone else who also needs to do this.

Paul