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ABS and brake warning lights

Discussion in 'Repairs, Maintenance, Help' started by NYBo, Oct 23, 2017.

  1. AllanC

    AllanC Well-Known Member

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    I wholeheartedly agree with the comment made by I C. No manufacturer would only equip a vehicle with L F antilock yet no antilock on R F wheel. That would be more dangerous than having NO antilock. There should be remnants of the sensor near the R F wheel hub. If it is missing or broken then that needs to be addressed. As suggested you need to get the ABS codes retrieved to get a firm direction for solving the problem.
     
  2. bguy

    bguy Well-Known Member

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    The LF sensor would a reference signal to allow the computer to read the speed differential between the front and rear wheel.
     
  3. ImperialCrown

    Level III Supporter

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    But it must have both left front and right front ABS wheel speed sensors.
    I am now reading conflicting information
    about a rear differential sensor or individual rear wss. One would need to get underneath and look.

    From the 2007 service manual:

    DESCRIPTION


    The hydraulic system is a three channel design. The front wheel brakes are controlled individually and the rear wheel brakes in tandem. The ABS electrical system is separate from other electrical circuits in the vehicle. A specially programmed controller antilock brake unit operates the system components.

    ABS system major components include:
    1. Antilock Brake Module (ABM)
    2. Hydraulic Control Unit (HCU)
    3. Wheel Speed Sensors (WSS)
    4. ABS Warning Light
    Apparently a differential sensor was mentioned for the 2500/3500 RWAL, but not the 1500. In another Description, it does mention the rear differential RWAL sensor, but doesn't state which models.

    I have had road debris (like tree branches, etc) tear ABS and transmission wiring out from underneath a vehicle. This may make it appear that it doesn't have a sensor when the harness has been torn off.
     
  4. NYBo

    Level III Supporter

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    You'll have to trust me on this. It is rear-only, with just the two sensors, just like I had on my '99 Dakota. I think the rear sensor on the rear diff may even be the same part number. I have a new one coming as it was only $20 with shipping and sales tax. The wiring to it appears to be intact, same for the front sensor. Next, I'll be buying a new code reader with ABS capability.
     
  5. NYBo

    Level III Supporter

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    Okay, I finally got my new code reader today (Innova 3030, $62). One ABS code came up: C1009 - low brake fluid level. The fluid is definitely not low. Would this be triggered by the sensor on the brake fluid reservoir, or is there a sensor inside the HCU?
     
  6. AllanC

    AllanC Well-Known Member

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    There is a 2 wire sensor for fluid level on the side of the master cylinder reservoir. At rest it may appear that the brake fluid level is not low. But as disc brake pads on the front wear, the piston extends more to keep the pads in close proximity to the rotor. The extra fluid retained in the caliper pistons drops the fluid level in the master cylinder reservoir. Then when you hit a bump or make a wide, sweeping turn the float level sensor can drop below its limit momentarily and illuminate the low brake fluid lamp / set low fluid level code. Also it is possible that there is a mechanical problem with the float level sensor in the master cylinder. There is no brake fluid level sensor in the ABS hydraulic control unit. It is full of fluid at all times.
     
  7. NYBo

    Level III Supporter

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    At rest, the fluid is right to the top of the reservoir. The pads and rotors have fewer than 5,000 miles on them. The lights also don't come on in a predictable manner like in a turn or when hitting a bump. Therefore, I think the sensor is bad. I've read about a hack to bypass the sensor with a 1000 ohm resistor; I may try that for diagnosis since just measuring the resistance of the old sensor may not proven anything.
     
  8. AllanC

    AllanC Well-Known Member

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    I am thinking that the float mechanism in the master cylinder reservoir is just a simple OPEN / CLOSE switch. There is no need to have a resistor in the circuit. Just unplug the 2 wire sensor at the master cylinder reservoir. If the low brake fluid lamp turns off, then you know that the float sensor has failed or is stuck in the down position.
     
  9. AC TC

    AC TC Well-Known Member

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    There might be some thruth in the 1000 ohm resistor thing.
    - having a known load on a "open" or "closed" circuit makes it possible to monitor Circuit/ sensor problems.
    I dont know if its so in this case thou, but i wouldnt be surprised, most ABS speedsensors have this fex.
     
  10. ImperialCrown

    Level III Supporter

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    There may be a switch-open fault as well as a switch-shorted fault. The ABS module looks for a switch value that is real and believable.
     
  11. NYBo

    Level III Supporter

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    I discovered today that wiggling the connector for the sensor will turn the light off so maybe a little cleaning and tightening the connections will do the trick.
     
    ImperialCrown likes this.
  12. AC TC

    AC TC Well-Known Member

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