Brake Light Switch Defective?

[jaustinmd]

I've been on a crusade to resurrect the cruise control on my '82 911SC. I initially ran the Bentley Manual diagnostics on the cruise module socket and most everything checked out OK or was OK after some minor adjustments such as to the clutch switch. There was one check I wasn't sure about, however. When checking the brake light switch, Bentley says to put an ohmmeter across terminal 6 and ground and the meter should read "infinite ohms." It does not specify the scale, but on the lowest ohms scale on my meter it did read "OL" or infinite. On a higher scale it read 1.4Kohms.

I had Kris Rose repair the cruise module and installed it but the cruise still didn't work (wouldn't engage at all). Today I pulled the module and re-ran the Bentley diagnostics. This time, when checking the brake light switch between terminal 6 and ground, I got "0.6 ohms" on the low scale and not out of limits or infinite as before! Bentley says if the meter reads "0 ohms," the brake light switch is faulty.

Well, 0.6 ohms is darn near zero, so I understand that. However, my brake lights DO WORK, but they don't come on until the pedal is pressed firmly (and yes, they're incandescent 1157 bulbs - I know about the problems with LED bulbs and the cruise).

What makes this even more difficult for me to understand is that the Bentley Manual does not call for this check to be done with the engine running. I would expect that the brakes might take a firmer push since there's no boost with the engine off, but you can definitely feel the pressure building prior to the brake lights coming on. With the engine running, it takes less effort (the brakes are boosted), but it appears to me the pedal travel is about the same to the point where the brake lights come on.

So, the brake lights work - but, is one or both of the switches on the master cylinder actually faulty? Should they be coming on with the slightest pedal movement or should it take a firm push?

I really would like for them to come on a little quicker and if they're indeed defective, maybe that'll finally let the cruise work! Normally, would new switches be more sensitive and activate the brakes earlier/less pedal pressure?

[r-mm]

Lotsa threads on general dissatisfaction with the pressure actuated brake light switches (note to general public: make sure yours are working, mine were not for a spell and I didn't realize it).

You'll find people discussing "prefilling" new (or, I suppose, old) sensors with fluid, various methods to get any bubbles out etc. Think of it as the worlds most annoying hydraulic bleeding problem. My new ones now work pretty well after trying the various techniques discussed. Rx times are not like a new toyota, but they're reasonable. You might start with two new switches to rule out the problem.

Sorry no comment on the CC part of you question, I ditched mine.

[Rodsrsr]

I had a similar problem in my 76. Brake lights would just stop working. Replaced the manual switch on the pedal assembly with a OEM and it worked for a while, but the problem would return. It was to the point that I always checked my brake lights before driving. Installed a new cheap version switch and its been working for 2 years now. Its almost like there was poor contact or something inside the switch. I adjusted the switch to go off with slight pressure BTW.

[jaustinmd]

Originally Posted by Rodsrsr

I had a similar problem in my 76. Brake lights would just stop working. Replaced the manual switch on the pedal assembly with a OEM and it worked for a while, but the problem would return. It was to the point that I always checked my brake lights before driving. Installed a new cheap version switch and its been working for 2 years now. Its almost like there was poor contact or something inside the switch. I adjusted the switch to go off with slight pressure BTW.

Just to be clear, the '82 911SC has dual pressure switches on the master cylinder and not on the pedal assembly.

[uwanna]

I posted the info below on another thread about a GLARING error in the Bentley CC
troubleshooting chart. It concerns the test for the clutch switch. Read it and you will get the idea. As well as the test I suggested to check the clutch switch, here's a quick
definitive test: turn on the ign sw and have someone press and release the clutch while you listen for the "tempostat" relay to click. (4th relay from front of the car) If the switch is operating correctly it will click on and off. By the way, if the relay is "picked"/energized the CC will not engage.
Can't blame the Bentley folks for the trouble chart error, I looked it up in the factory manual and it has the SAME error. Bentley probably just copied it error and all! :

As far as the brake switches are concerned, pin 6 should read open/infinity on ohms scale and then with ign on 12v on DC scale with brake depressed (see circuit chart)
__________________________________________________ _______________

Cruise Control trouble chart has a huge ERROR on checking the clutch switch!!
The cruise control troubleshooting chart has a GLARING ERROR. The test that states
"Measure ohms from pin 3 to ground, depress clutch pedal, and the meter should read 10 ohms." is dead wrong! If you look at the wiring diagram below with the control unit unplugged, you will see there is NO WAY to get a 10ohm reading between ground and pin #3. I troubleshot my CC using the chart and could not get the 10ohms, so I traced the circuit.

The clutch switch contacts are "normally open" when the clutch is at rest. When you press the clutch, the switch contacts close and provide ground to the "Tempostat" relay on pin #85. This "picks" the relay and opens the relay contact path from pin #3 to the tempostat motor, deactivating CC.
The clutch switch is hooked to ground and ONLY goes to the relay coil. There is NO PATH to pin #3. Ergo you can't ever get 10ohms from ground to pin #3.


The absolute way to check if your clutch switch is operating is
1. pull the Tempostat (cruise control) relay
2. connect ohmmeter one lead to ground, the other to pin #85 of the tempostat relay plug. Should have no continuity, then depress the clutch and you should read 0 ohms (continuity) That is an ABSOLUTE TEST of the switch!
I don't know how many folks have been led down the "garden path" by using the trouble shooting chart in the Bentley, and thinking they have a bad or maladjusted clutch switch. But I'll bet it's quite a few!
I'm not the first guy to discover this error, see the quote below from a couple of years ago!













Quote:
Originally Posted by dlshady View Post
Guys,

I know this has been discussed to death here but I've read through EVERY single thread that contains anything related to the cruise system and haven't found anything close to what I'm experiencing. Hopefully there's somebody out there that has been down this road before that can shed a little light on the problem for me.

Here's what I got: 83SC with a completely inoperative cruise system. I've been through this system from front to rear several times now, and this is what I come up with:

The schematic and the troubleshooting chart (Table C) in the Bentley manual don't jive. The second step in the chart shows that depressing the clutch pedal should result in a reading of 10 ohms between pin 3 and ground, but the schematic shows that depressing the clutch should merely ground the tempostat relay and interrupt the circuit on pin 3 between the cruise amp and the servo. In checking pin 3, I do NOT have 10 ohms to ground when pressing the clutch pedal, but I DO have an interruption in the circuit back to the servo. When checking resistance between pins 3 and 7 I have @15 ohms (this is the resistance of the servo and the wiring back to it), and it goes to open when the clutch is depressed. So by my logic, checking the circuit this way verifies that 1) the servo has the correct resistance, 2) the wiring back to it is indeed good, 3) the clutch switch and tempostat relay are working as they should.

So:

The clutch switch is good
The brake light switch applies @11.8 volts to pin 6 when the brakes are applied


I'm at the end of my rope with this car and I'm literally about ready to push it outside with the keys in it, hoping that some poor sucker will steal it.

[Rodsrsr]

Originally Posted by jaustinmd

Just to be clear, the '82 911SC has dual pressure switches on the master cylinder and not on the pedal assembly.

My mistake, I wasn't sure but thought I'd throw out my experience just in case.