Question - Brake light on, but why??
 

I have a 1970 911T and the upper red light on the center of the oil temp guage seems to remain on even if the emergency brake is fully released. I played a bit with the adjustment for the switch on the emergency brake lever, but was not able to get the light to go off. Does the sensor on the Mater Brake Cylendar also cause this light to go on??
Any thoughts on how I might diagnose what's causing this light to go on so I can repair it??


Any help appreciated.

You have a 1970; mine is an '83. However, I had your issue which started when I replaced my brake pads. I checked the oil level, and it was fine. I played with the brake lever adjustment and it still would not go off.

Try this: Insert your key, turn to ignition (do not start the car). While the key in 'ignition mode', disconnect the battery. Wait. Turn the key to off; reconnect the battery. This solved my problem.

If this does not solve your, probably someone else could chime in.

Good luck!

Most 911s have a pressure switch on the master cylinder that will turn on a light somewhere, if a problem exists. In the earlier cars it was a light in a gauge; in the later ones it was a light on the dash. I can't recall what year this started, so if you have a switch on your master cylinder, that's probably the reason. Your owner's manual should tell you.

In my experience, the light comes on more oftern as a result of a bad switch than any problem with the brakes, though it makes sense to have someone check them out, as they are a wee bit important to driving the car. If it's the switch that is turning on the light, usually disconnecting the battery will turn it off (although it will probably come back on.)

JR

My '75 914 has a reset switch on the master cylinder. My brake light stayed on after I opened the brake system. Take a flashlight, turn your wheels all the way to the right, and look in the left wheel well under the tie rod. You should see the master cylinder. There is a plastic plug that has a little black pin poking out directly toward you. Take a screwdriver and press it back in. Should make your brake light go out when the parking brake is off.... My $.02.
--Jaybird

Just another idea to toss out there - your 4-way flashers are wired through that switch too. There was something screwy with my 4-way switch, so that I get no turn signals if my 4-ways are off, so I just disconnected the light so it wouldn't flash when the 4-ways are on. This means I don't have 4-ways, but I do have turn signals.

Early cars with no brake boosters have different master cylinders than those with power brakes. As far as I know there are no switches on them at all. I know the brake light switch is for sure not on the MC, like it is in later cars.

I have a 1970 T and the same thing happened to me. The wire to the switch in the emergency brake handle was grounding out. The switch is just rivited together and it kinda just fell apart (after 38 years) and the wire and connector were able to go to ground.

Have you tried just unplugging the handbrake light switch? If so, does the light go off? If it does not go off, you might have a problem, like the one, that I helped a friend with. The wire that connects to the brake light switch, got pinched under the plate that houses the handbrake lever and heat control levers. He had removed the mounting plate with the handbrake lever and heat levers, to repaint the handles. When he reinstalled the plate, the brake light wire, moved out of the little indentation in the mounting plate, where the wire runs underneath it to the switch, and it actually smashed it so much, that it was getting grounded by the mounting plate to the top of the tunnel. The switch just completes the circuit for the light to be actuated. Hope this might help. Good luck!! Tony.

Jaybird840 is right.

I just rebuilt the entire brake system - new master cylendar, rebuilt all calipers, new SS brake lines, and all new fluid and flushed. But, the switch on the new master cylendar came with only a single spade wire connector versus the old master cylendar switch and car wiring that had two spade wire connectors. So, I had to pull my old swicth from the old master cylendar and use it in the new one - sounds like that might be it in that some of you are suggesting this switch fails alot.

As for unplugging the emergency brake switch to test that - where does it unplug at?? Mine seems to have wires running out from the brake handle mount bracket that runs into the center tunnel housing and into a cable harness - no connector to unplug - thus I assume the brake mechanism must be pulled up and the swicth diconnected/bypassed under there??

Jass,
If Jaybird840's suggestion doesn't work, consider that your old master cylinder was a dual "fail-safe" system - in case one of the brake circuits failed, the other would still work. I wonder if putting the old switch on the new, single? MC is causing the problem. What if you used the new single switch and connect both of the old connectors to it?

Hand Brake Switch
 

Hi, the wire connects to that little switch that you said you tried to adjust, it is there., If there is no wire on the switch, it may be laying in the tunnel, and grounding, causing the brake light to be on. The switch has a little set screw, and a little button or bump on its side that moves up or down in the adjustment slot. take a look at the parts diagram in the Pelican link below. Hopefully you will see what part I am referring to. It is part #3 Good luck!! Tony. SmileWavy




http://www.pelicanparts.com/PartsLookup/HTML/911_USA_70_73_KATALOG/701-20-Size4.htm#

I have not taken the emergency brake assembly apart at all, but it sounds like the wire connections to the switch are all under that assembly - I suppose it's possible that the wires are either disconnected from that switch or perhaps that switch has failed.

As for the master lylendar switch, not sure how I would try what C4 PAZZO is suggesting of connecting two wires to the single wire switch that came with the new master cylendar - seems like that could short out the wiring??? Anyone know about that risk?

To access the e-brake switch remove the 3 bolts that hold the handle to the tunnel and lift the assembly up (the brake has to be released to lift it up). The switch, which has only one wire connected to it, is located under the back of the handle. Check for damage to the wire insulation...37 years of vibration may have rubbed a bare spot in it. If that wire shorts to ground the light will stay on.

OK, so I pulled up the emergency brake housing and the wire to the switch and the switch all seem fine - no shorts or wear to the wire. Tried grounding it to see if the dash lite went off, but no luck. I then had the ignition switch on accessory as suggested and disconnected BOTH batteries, shut off the ignition switch, reconnected the batteries and tried again, but the lite remains on.
So, I'm thinking it's the master cylendar switch, but would like to test it to verify before I go and replace since replacing I think will mean re-bleeding the whole system. I do not have a complete wiring diagram for this car so I'm not even positive that the master cylendar switch even is connected to this dash lite - verification of that from someone too would be nice.

I was thinking to test the master cylendar switch I could disconnect the two wire leads and jumper them together as maybe that would complete that circuit, but I'm unsure if that would do the trick. Does anyone know if that would damage anything if I were to do that, or if indeed that would test the system and isolate the issue to that switch???

Well, Being impatient, I went ahead and tried connecting the two leads from the master cylendar switch together. Turns out that connecting them causes the lite to illuminate, disconnecting them turns the lite out. Thus, my master cylendar switch must be failed and always be on from what I can assess. When the lite was out, I also tested the emergency brake switch which when grounded would illuminate the lite, and the lite would go out once the wire was pulled from the ground. Darn, I guess I'll have to purchase that $35 switch after all! I was dissapointed when I purchased a new master cylendar from our host that is correct for a 1970 911T and it came with the incorrect switch (single post vs dual post) installed which they tell me later is the case and that I have to separately purchase the correct switch - frustrating!
Anyway, I think the mystery is solved.

Mixed Feelings??
 

jass, I am glad to hear that you found the handbrake switch wire still intact.:)!! But that really blows about the new master cylinder and switch problem:mad: !! I hope you can get it sorted out with the correct parts.SmileWavy Good luck!! Tony.

Jass,

Out of curiosity, did you try my recommendation? It should be the quickest test you could try.

Thanks.

CAB83-750
Yes, I did try your suggestion (see my previous post), but that didn't solve it.

If my memory serves me correctly, you can use the single contact switch on the new master cylinder and hook up one of the two wires. I don't recall which wire, but whatever one makes the light go off.

Hi,

I had a similar problem with my handbrake/master cylinder light. After studyinghte wirign diagram, I realized tha thte wiring ran from the battery, to the light, to the switch on the master cyliner and then to the handbrake. When the switch onthe master cylinder or hand brake is activated, the circuit is grounded and hte light comes on. I pulled the connector from the master cylinder and the light went out. My switch does NOT have a reset button but you could see corrosion on the switch so I suspect I have a bad switch.