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Brake Grab (for 5 minutes)

Discussion in 'Gen 2 Prius Main Forum' started by daniel, Sep 1, 2004.

  1. daniel

    daniel Cat Lovers Against the Bomb

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    Today as I left the gym I had an odd experience. I have read reports of "grabby" brakes, but had never experienced it before. In fact, I have been utterly unable to detect the transition from regen to friction braking.

    So this morning I drove to the gym, and all was normal. I was there for a couple of hours. But on leaving the gym, when I came to the first stop sign I slowed down gradually, but just as I got close to the stop sign, I felt a strong grab, as though I had stomped hard on the brake (which I had not). There are a number of stop signs on that trip, and my brakes grabbed hard at every one. It might have been at the regen-to-friction transition, though I didn't look at the speedometer - I was too surprised.

    The drive was, as I said, about 5 minutes. Again the car was parked for maybe an hour and a half, and when I came back out, the brake grab had disappeared. For the entire drive home the braking was flawless and the transition undetectable.

    So I just thought I'd post this as an "odd experience."

    Conditions: Cool in the morning, mild later. Humid but no fog or rain. Around 4,600 miles on the odometer, maybe closer to 4,700. I had not used the parking brake at either place. My driving style is just drive it. No special babying of brakes or ICE, but no extreme driving either. I have had to slam on the brakes only 2 or 3 times, but not since winter.
     
    #1 daniel, Sep 1, 2004
  2. HTMLSpinnr

    HTMLSpinnr Super Moderator
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    Completely different odd experience for me - I accelerated this morning (probably S3 or S4 mode), the engine spun up, but sounded like it was bogging briefly, then suddenly the electric assist kicked in 3-4 seconds later and it felt "normal".

    Haven't had the grabby brakes though - or at least it didn't seem brake related in my case.
     
    #2 HTMLSpinnr, Sep 1, 2004
  3. efusco

    efusco Moderator Emeritus
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    dew on the pads?
     
    #3 efusco, Sep 1, 2004
  4. Frank Hudon

    Frank Hudon Senior Member

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    Daniel just a thought, if it does it again, and if safe, pull over and reboot it and see if that does anything. If that stops the grabbing, chalk it up to a comp clitch, if not, a trip to the dealer might be in order. It's possible a residual check valve is sticking or some other problem in the hydraulic portion of the brake system.
     
    #4 Frank Hudon, Sep 1, 2004
  5. hdrygas

    hdrygas New Member

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    I had a bit of strangeness last week. We had our first rain in weeks and it was a good one. That meant water washing oil out on the road. I was going to work stopped on a moderate hill at a light. Remember I have only had the car for a week and a half. I took off and the ICE and the car did the Cha Cha. Felt like, and sounded like ICE on ICE off, and no I had no Karate master in the car directing me. Then it settled down and went up the hill. I figured it was the traction or skid control but it was quite a stutter. Thoughts?
     
    #5 hdrygas, Sep 1, 2004
  6. Frank Hudon

    Frank Hudon Senior Member

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    it was the Traction Control. I find it way to aggressive for the wet coast. Painted cross walk lines and man hole covers in the rain will cause this to happen with very light pedal pressure. Try and quickly accelerate and it'll fall flat on its face. I would like a defeat switch on the traction control just for rainy day's here.
     
    #6 Frank Hudon, Sep 1, 2004
  7. daniel

    daniel Cat Lovers Against the Bomb

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    That sounds reasonable. It was in my garage overnight, but at the gym it was out in the open, humid and still cool weather. If 2 hours is enough time for the pads to cool enough for dew to form on them, and if dew can cause grabbing. On the other hand, there was no dew on the outside of the car.
     
    #7 daniel, Sep 2, 2004
  8. HTMLSpinnr

    HTMLSpinnr Super Moderator
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    No such luck on TRAC override. Should wheel-spin suddenly stop, that extra momentum is going to be transferred right into MG1, potentially causing damage to MG1 and/or the PSD itself. The repair manual states it as:

    If the drive wheels rotate without traction, the HV control ECU performs the motor traction control that provides a restraint on a rotation of MG2, in order to protect the planetary gear unit and prevent MG1 from generating excessive electricity.

    I'd take falling on my face on a slick manhole cover vs. thousands of dollars to repair the HSD/THS-II under my hood. Granted, the '04's TRAC also implementes wheel braking which should help a bit over the rudementary "Classic" traction control which is only intended to protect the drivetrain.

    The only way to override in the '04 is with a test-tool, placing the car in INSPECTION MOD1 or INSPECTION MOD2 modes. Both will allow the front wheels to spin regardless of rear wheel speed - the former keeping the engine on continuously in P mode for testing as well. No user disable for now.
     
    #8 HTMLSpinnr, Sep 2, 2004
  9. Frank Hudon

    Frank Hudon Senior Member

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    I'll take the rudementary one on the Classic any day of the week here on the wet coast. The brake application on the 2k4 will get some one killed here. The car virtually stops. This will cause rear enders and T-bones for sure. I can see it coming, just a matter of time.
     
    #9 Frank Hudon, Sep 2, 2004
  10. daniel

    daniel Cat Lovers Against the Bomb

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    I only had the brake grab happen that one time, and it only happened after I had slowed down almost to a stop. I suspect it was at the regen-to-friction transition. Definitely not a safety issue, since I was already nearly stopped.

    If I had braked hard enough to get friction braking at speed, it would have been for an emergency stop, and I'd have wanted to stop quickly as possible. And the ABS and VSC would have made a loss of control unlikely.
     
    #10 daniel, Sep 3, 2004
  11. kam

    kam New Member

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    I get this behaviour consistently when gliding to a stop with my foot lightly on the brake pedal. I always assumed, like you, that it's just the transition from regen-to-friction. I found it disconcerting at first since at the transition the car seems to coast momentarily and you suddenly think your going to end up past your projected stop point. As a result I'd press the brake harder and then the pads really dig in and you stop abruptly. I find now though that I can predict when it's going to happen and account for it.

    This is only coming to gradual stops though. When braking just a little more aggressively it seems totally linear right to the end.
     
    #11 kam, Sep 3, 2004
  12. seanwachob

    seanwachob New Member

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    I get the opposite of grab when the friction breaking cuts in on my '04 Prius. I'll be slowing down at a light and around 10 MPH it feels like the brakes fade for a second and then back to normal.
     
    #12 seanwachob, Sep 3, 2004
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