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Thread: No Brake Light=No Brake Pressure?

  1. #1
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    Question No Brake Light=No Brake Pressure?

    Quick background,//When installing my master cylinder/ABS assembly in my 90' SC, I think I hooked up the switch on the pedal incorrectly as I have no brake lights. So question is, does that switch send a signal to the ABS pump to kick on the pressure? I can't seem to get the M/C to put out any pressure. I hear the pump kick on upon key on posistion, but never does kick back on when bleeding the brakes, thus no pressure. I bled the M/C, Accumulator, 3 brake Lines coming out of the M/C, etc...All electricals are plug in correctly (Triple checked) ,, No Dice...

    Anyhow, There are two switches under the dash on the brake pedal.
    1. Way up top and is mounted to the pedal bracket. (I thought that was the switch for brake lights)
    2. Another switch mounted ON the stud that sticks out of the brake pedal about 1/3 the way down..and it is also wrapped around the master cylinder pushrod that connects to that stud.
    Any ideas? Merry Xmas to all..

  2. #2
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    If you can Find Duffy Floyds SC article, I believe you can find your answer. I do not remember from reading it anymore, also can't find it.

    From what I think I remember others finding, this switch may be part of a complete circuit and thus could cause loss of pump operation. Sometimes they simply break.

    All I can think of. If you can locate Duffy, he most likely knows.

    Sorry I can't be more help..

  3. #3
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    Actually that does help Digital.
    I'll try another search on that switch again. Thank You..

  4. #4
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    OK, I've bled these stupid brakes 15 times!
    It seems as the rear brakes are SUCKING in rather than spitting fluid out. I turn the key on and hear a "Clunk" that souinds like it comes from the pump, but when I put my hand on it while someone turns key on and I DO NOT feel pump move or vibrate, which it probably should right? So as electrical and EVERYTHING else has been rechecked 15 times, obviously the pump is NOT kicking on. I put an ohmeter (Checking Voltage) on the big square fuse (#B) in the fuse block next to the battery and get NO VOLTS when key is on. Is this correct? If I check one side to fuse and one to NEG battery, then I get 11.8 volts. ONLY thing I changed was the Pressure Switch off a Lincoln. But if this switch was bad or incorrect, the pump would just stay running, right? I took old Lincoln ABS Pump Motor and plugged in and turned key on and it didn't turn on...... Electrical????
    Help!!!

  5. #5
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    Re: No Brake Light=No Brake Pressure?

    Originally posted by BobGPz
    So question is, does that switch send a signal to the ABS pump to kick on the pressure?
    No. Nothing to do with the ABS. However, if you have no brakes lites, then you also won't be able to start the car.
    I can't seem to get the M/C to put out any pressure. I hear the pump kick on upon key on posistion, but never does kick back on when bleeding the brakes, thus no pressure. I bled the M/C, Accumulator, 3 brake Lines coming out of the M/C, etc...All electricals are plug in correctly (Triple checked) ,, No Dice...

    Anyhow, There are two switches under the dash on the brake pedal.
    1. Way up top and is mounted to the pedal bracket. (I thought that was the switch for brake lights)
    2. Another switch mounted ON the stud that sticks out of the brake pedal about 1/3 the way down..and it is also wrapped around the master cylinder pushrod that connects to that stud.
    Any ideas? Merry Xmas to all..
    The upper switch looking thing should have a yellow tip. It isn't a switch, it is a vaccuum cut off valve which shuts off the cruise control. BTW, when your cruise control stops working, you'll need to adjust this back-n-forth a couple of clicks at a time to get it working again.
    The only other thing which would NORMALLY be on your brake pedal lever is the brake light switch. It fits around the stud which fits throught the rod which drives the M/C. One thing to note is when its hooked up properly, it makes almost a 90 deg angle to the lever and fits with its flat side flush against the flat side of the M/C rod. Then there is a single connector with a couple of wires which hook it up.

    Neither of these items will affect your brake hydraulics in any way.

    When you go to bleed the rear brakes, start at the RR, and take a piece of clear plastic tubing over the bleeder valve and feed it into a jar of can with an inch or so of fluid in the bottom. Simplest way to do this is to take the wheel off. Pump the brake pedal 10 or more times, then turn the key on, listen for the pump running, and watch the brake & anti lock lights go out. If they stay lit or never come on to begin with, there is some other problem to deal with.
    Now, with an assisant, open the bleeder valve slightly and have someone softly push on the pedal; have them inform you when it reaches the floor, but make sure they understand not to let up on it until you tell them to. Turn the bleeder screw in to trap the air. Then have them relase the pedal, and repeat the process until the fluid runs clean with no air bubbles. Its the normal bleeding process, except you need the key turned on. You will hear the pump kick in every few pedal presses while doing this.
    The clear tubing allows you to see when all of the air is out. The can of fluid with the tubing immersed is to keep air from getting back in when the pedal is released but the bleeder screw isn't entirely closed.

    Hope this helps.

  6. #6
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    Originally posted by BobGPz
    OK, I've bled these stupid brakes 15 times!
    It seems as the rear brakes are SUCKING in rather than spitting fluid out. I turn the key on and hear a "Clunk" that souinds like it comes from the pump, but when I put my hand on it while someone turns key on and I DO NOT feel pump move or vibrate, which it probably should right? So as electrical and EVERYTHING else has been rechecked 15 times, obviously the pump is NOT kicking on. I put an ohmeter (Checking Voltage) on the big square fuse (#B) in the fuse block next to the battery and get NO VOLTS when key is on. Is this correct? If I check one side to fuse and one to NEG battery, then I get 11.8 volts. ONLY thing I changed was the Pressure Switch off a Lincoln. But if this switch was bad or incorrect, the pump would just stay running, right? I took old Lincoln ABS Pump Motor and plugged in and turned key on and it didn't turn on...... Electrical????
    Help!!!
    Before you try this again, key off, pump the brake pedal at least 20 times. Then key on to the run position, but don't start the car. You should hear the pump running, the brake and anti-lock lights will come on and then turn off within about 1 minute.

    And on your other question, if you measure the voltage from one side of the fuse to the other, it should read 0. From either side of the fuse to the negative battery post or any metal part of the frame, should read battery voltage which is around 12V.

  7. #7
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    Quote..."No. Nothing to do with the ABS. However, if you have no brakes lites, then you also won't be able to start the car."

    I have no brake lights, but I CAN start and RUN the car.
    You may have missed my last post while posting, but I think the problem is that the pump isn't kicking on AT ALL, and I know everthing is hooked right.
    Thanks on the info TBirdSCFan, as it explains the 2 switches on the Brake Pedal. I'll check and see if I hooked up the one on the stud correctly.

    PS. BOTH ABS Brake Lights are on when key is on and they do not go off at all...

  8. #8
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    Originally posted by TbirdSCFan
    ..........if you measure the voltage from one side of the fuse to the other, it should read 0. From either side of the fuse to the negative battery post or any metal part of the frame, should read battery voltage which is around 12V.
    Yep, that is correct. That is what I got so I can rule that out...it's like no power to the ABS pump...

  9. #9
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    Will a faulty ABS relay (Passenger firewall) cause the pump not to turn ON.
    I have heard of it going bad and causing the pump to constantly run and not shut OFF, but would it also make the pump NOT turn on?
    Also got code 21 and 38 from the ABS Test connector..(Not the EEC connector)

  10. #10
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    Originally posted by BobGPz
    Will a faulty ABS relay (Passenger firewall) cause the pump not to turn ON.
    I have heard of it going bad and causing the pump to constantly run and not shut OFF, but would it also make the pump NOT turn on?
    Also got code 21 and 38 from the ABS Test connector..(Not the EEC connector)
    Absolutely. Your expensive pressure switch does 3 things: lites the brake light bulb on the dash (low pressure), sends a signal to the ABS computer in the trunk which may subsequently lite the anti-lock light on the dash (low system pressure), and triggers the ABS pump motor relay on the firewall. That relay is what provides the high current juice to the pump motor.
    So, assuming that the pressure switch is good since its new, that would point to the relay itself. Did you look into replacing it before the pressure switch? Its a low cost part and they do tend to fail.
    There is a way to easily check the pressure switch by doing a continuity check. Do you want to know what pins to test on it?

    There is a possibility that the pump motor itself is bad, or that theres a wiring problem. A real quick check on the pump motor is to disonnect it at the HCU (has 4 wires into 2 pins) and check the voltage (key on) at the wiring harness end. The pin with the gre/red wire should read 12V. If it does, and the motor still isn't running, then the motor's the problem. If not 12V, then it could be the relay, wiring, fuses, fusible links or the pressure switch.

  11. #11
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    Originally posted by TbirdSCFan
    So, assuming that the pressure switch is good since its new....................There is a possibility that the pump motor itself is bad, or that theres a wiring problem.
    Well that "New" pressure switch, actually I pulled a used one out of a wrecked car as even the dealer didn't have one. The used one doesn't leak like the old one though
    When I pulled out my Master Cylinder/ABS Pump as a unit, it WAS in working order, so I am doubting the pump is bad. I am however not ruling it out, but before I yank it out again, I will check the volts/ohms as you specified above and on the other post...
    I did pickup a NEW ABS Relay from the part store @ $18.00. I will leave it in the box until I get done with volts/ohms readings.
    Thanks TBirdSCFan!

  12. #12
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    I put new ABS relay on last night. Then did a little measuring. I measured Ohms on the 7 pin connections for the M/C Valve:
    Going TO it from main wire harness I got 7.27 ohms on all seven wires. Going INTO the valve itself, I got (Ground on left wire measuring over to the right side):
    Grnd/8.9 Ohms/8.9 Ohms/11 Ohms/8.9 Ohms/11 Ohms/5.6 Ohms.

    Now where it gets strange is measuring the volts from the 4-pin connector that goes TO the ABS Pump in clockwise direction with the KEY ON:
    22.8 volts / 9.6 volts / 22.8 volts / 9.6 volts.
    How can a 12 volt system produce 23 volts?
    I measured the ohms of the pump at said 4-pin connection going TO the pump itself: Got 2.3 ohms all the way around.
    I still have more measuring to do, but thought I'd post before I lose piece of paper.....
    Last edited by BobGPz; 12-27-2003 at 11:20 AM.

  13. #13
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    OK. Just to be sure we're in sync with measuring stuff. When you measure voltage, the black lead is always either on the negative battery post/clamp, or the frame of the car, and the red lead is on the point where you want to measure. When you measure resistance, the black and red leads are on the 2 points where you are measure between.
    As far as the voltages to the pump, what you should see is this:
    On the connector at the HCU with the 4 wires and 2 pins, it should have a grey and grey/red wire. When you turn the ignition on to the run position, with that connector unhooked, on the wire harness side, you should have 12Volts on the pin where the grey/red wire is connected to. That is the power to the pump motor itself. If it reads 12V, but when you plug it back into the motor, the motor doesn't run, then the motor is bad.
    If it doesn't read 12V, then you need to back up to the pump motor relay and see if it is 1. engaging, and 2. passing power.
    Pop the relay out, and probe for voltage in the holes (key on again). There should be at least 2 pins which read 12V.

  14. #14
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    Lightbulb

    Originally posted by BobGPz
    I put new ABS relay on last night. Then did a little measuring. I measured Ohms on the 7 pin connections for the M/C Valve:
    Going TO it from main wire harness I got 7.27 ohms on all seven wires. Going INTO the valve itself, I got (Ground on left wire measuring over to the right side):
    Grnd/8.9 Ohms/8.9 Ohms/11 Ohms/8.9 Ohms/11 Ohms/5.6 Ohms.
    Aha! I just noticed.. this 7 pin connector goes to the solenoid valves, not the pressure switch.. The resistances you measured won't mean anything for the problem you're having.

    The pressure switch has a 5 pin connector with Grey, Green, Brown/White, Pink/Lite blue, and Lite Brown/Red wires on the harness side. I can't see mine from the top and its too cold out to crawl under the car, but I think its on the bottom of the HCU somewhere.

  15. #15
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    Originally posted by TbirdSCFan
    OK. Just to be sure we're in sync with measuring stuff. When you measure voltage, the black lead is always either on the negative battery post/clamp
    Yip, That is correct TBirdFan. Black probe on Negative battery terminal, and red wire on 5pin pump motor wire harness. Got roughly a total of 60 volts going to the pump! Didn't know you can get more than 12 from a 12 volt system. Hmmm. Very odd.
    Anyways I re-read Duffy Floyd's ABS Article on the 35th Website for the 3rd time,
    It seems that I grounded the #4 pin of the ABS Pressure Switch Wire Harness. According to Duffy's article if that #4 is grounded, then the pump should turn on as the circuit to the relay is closed. The pump DOES turn on. Yahoo! The old pressure switch was pulled for bad interns causing a HUGE leak. Pulled a Teves Mark II ABS System from a wrecked Lincoln and took switch from it and installed into mine. Ever since then the pump didn't turn on. Either switch is bad (But is not leaking), or the lincoln switches are incompatible. There is a "M" stamped on mine and some other letter stamped on the junkyard switch. Regardless it sounds like the junkyard switch ain't going to work on my car. What do you think TBirdSCFan? Also, there is a posting of a guy that says a GM switch is the "Same part" for the Ford ABS. It is $124 from Bill Copps Buick/Pontiac Dealer. If lincoln is incompatible then I would assume that the GM may also not work. Maybe I can test the junkyard switch when I RE-Pull it out of the car. Obviously the ground trick worked...so junkyard switch for some reason not working....I should have tested it before installing, but too excited about saving $200 from Ford and not having it leak 32 ounces fluid in 30 seconds...

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