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Intermittent Brake, Brake warning, ABS, VSC

Discussion in 'Gen II Prius Care, Maintenance and Troubleshooting' started by Goughy, Sep 23, 2011.

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  1. Goughy

    Goughy New Member

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    I know this has been discussed previously, but I'd like to get a second opinion on my particular situation.

    Car is a 2005 Prius with 66000 miles. One hot day (100F) about three weeks ago the brake warning light combination came on (brake, brake warning, ABS, VSC) at 30mph, not triggered by any harsh braking. Braking was normal.

    We took it to the local Toyota dealer who checked for any stored codes but found none. Their diagnosis, from their research, was a possible skid control ECU failure. They reset the warnings and said that if it happened again we should go back to have the ECU replaced (~$2000) but they didn't want us to incur that expense without further proof that something was wrong.

    It has recurred and now seems to follow a pattern. When we set out for the first time everything is OK with no warnings. We drive a few miles and then stop and turn off the engine (e.g. for an errand). When we restart, the warning lights appear and stay on. Next day, same pattern happens again. Braking seems normal when lights are on.

    I'd probably rule out a loose connection as the pattern is repeatable although the intermittent nature of the problem is puzzling. We wondered if the problem might be somehow related to heat.

    Is this possibly a 12V battery issue? The battery has never been changed. After a couple of days without using the car I ran the built-in self test procedure and it was a little low. (12.1V unloaded, 11.8V loaded, 14.4-14.5V charging). The dealer tested the battery when we took the car in and marked it as passed. We haven't noticed any other obvious electrical problems.

    Since ECU failures are rare I'm inclined to change the battery next before replacing the ECU. And I'd hate to replace the ECU and find the problem is still there. Any other suggestions?
  2. caffeinekid

    caffeinekid Duct Tape Extraordinaire

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    You are correct.

    Change the battery. Refer to the Yellow Top threads.
  3. jdenenberg

    jdenenberg EE Professor

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    Yes, tale care of the 12v battery and then see if the symptoms disappear. One thing that worries me is the high charging voltage. My 2004 at 205k miles (8 years) is still using the original 12v battery and it charges at just under 14v and sits at 12.4v under a light load (still over 12v under load).

    JeffD
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  4. seilerts

    seilerts Battery Curmudgeon

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  5. Goughy

    Goughy New Member

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    Thanks for your suggestions, everybody.

    I've ordered the new battery from Optima and hope it'll fix the problem.

    JeffD: I also noticed the highish charging voltage. Any idea what might cause that?
  6. thatguy

    thatguy New Member

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    I had a similar but not identical issue for the second time yesterday.

    It was not raining but the roads were wet and there was some spray from other cars on the highway. Temperatures were in the low 70's. After driving for about 20 minutes I stopped to get gas, turned the car off, and filled the tank.

    When I turned the car back on less than 5 minutes later I was immediately greeted by three yellow warning lights: (!), ABS, and VSC. Unlike the OP the red "Brake" did not come on.

    Torque, my OBDII app, was not able to pull any P codes off of the ECU but it did show me that every time I pressed the brake pedal the friction brakes would engage instead of the regenerative braking. If I took my foot off the gas to glide the car would regenerate the typical 3 or 4 kW but if I pressed on the brake any additional slowing would use the friction brakes.

    The car stayed in this degraded state for the remainder of my drive home. When I got home I turned the car off, waited a minute, and turned the car back on. The warning lights did not come back on and the car drove and regeneratively braked normally. The same thing happened a month or two ago and cleared itself the same way.

    The car is a 2005 with 148,000 miles. No major work aside from a rear bumper replacement due to an accident at 88,000 miles. The 12V battery is original but is still performing well; voltages are in the range I would expect (12.5 in the morning, 13.9 when charging).

    I will probably change the 12V to an optima anyway, but is there a wiring harness I should check out as well? Could spray have gotten into a poorly sealed connector?
    I know Torque can talk to several of the ECUs over the CAN-bus. Is there a way to poll the relevant ECU from the OBDII port using the Torque app?
  7. Goughy

    Goughy New Member

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    I installed the new Optima Prius battery successfully this morning and now the self-test readings are 12.8V unloaded, 12.5V loaded but still 14.4-14.5V charging. Any suggestions for the highish charging voltage?

    More importantly, the original symptoms are still present. Here is what happens:

    Start from cold - all displays normal.

    After driving for about 5 miles at around 25mph, the Brake, ((!)), ABS and VSC all light up. They appear without any brake application - just during normal driving. Subsequent braking seems normal.

    When I stop (without turning engine off or applying emergency brake) all lights go out except for ((!)).

    Moving off again brings up Brake, ABS and VSC again in addition to ((!)).

    Turning the engine off briefly does not clear the warnings, although if I wait long enough it's back to all clear again. (something cooling down perhaps?)

    Before I part with $2000+ to replace the skid control ECU, does anyone have any other suggestions for what might be going on and what I should try next?
  8. Patrick Wong

    Patrick Wong DIY Enthusiast

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    1. Did you have the inverter coolant pump replaced under the recent LSC?

    2. While the Prius is READY, look at the inverter coolant reservoir to see if you can detect fluid turbulence which shows the inverter coolant pump is running.

    3. After the warning lights appear, check 12V battery voltage while the Prius is READY and post your measurements.
  9. seilerts

    seilerts Battery Curmudgeon

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    Check the resistance between the battery negative post (i.e. the lead part) and the fender well. Should be less than 1 Ohm.
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  10. Goughy

    Goughy New Member

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    1. Yes. This was replaced in March 2011 under LSC A0N.

    2. Yes. Pump is running.

    3. 14.2-14.3V

    It's becoming hard to see any real pattern with the problem. I drove about 20 miles without any warnings yesterday and after leaving the car for about 15 minutes, they appeared immediately I started up again.
  11. jdenenberg

    jdenenberg EE Professor

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    A high charging voltage is usually due to a Prius detecting that the 12v battery has a low charge and it ups the charging rate. Since the charging voltage still reads high with the Optima, get to Radio shack and buy a digital voltmeter to read the battery voltage directly at the battery under charge. If that voltage reads lower than 14 volts, Seilerts is right about a poor connection.

    The new Optima comes fully charged. Mine read 12.7 volts out of the box.

    JeffD
  12. Patrick Wong

    Patrick Wong DIY Enthusiast

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    If you see the ABS/VSC and brake warning lights, then the skid control ECU has logged DTC. I suggest you visit another Toyota dealer and ask for the DTC to be read, as well as their diagnosis.



    Make sure that both the positive and negative cable connections are tight, at the battery as well as the negative cable connection to the body. It is a good idea to verify the battery voltage and cable resistance with a digital multimeter, as suggested elsewhere.

    Obtain a second opinion as I suggested above. The skid control ECU may be bad but some other less-expensive brake system components may be at fault depending upon the logged DTC.
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  13. seilerts

    seilerts Battery Curmudgeon

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    Faulty computers are either extraordinarily unusual, or commonplace (Saturn Body Computer, RAV4 transmission control). The skid ECU on the Prius is not known for having problems, though the early version rather twitchy and overbearing in preventing any sort of wheel spin. If you were to try replacing the ECU, just get one from a salvage yard. There is no reason whatsoever to spend $2000 for a new one.
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  14. Goughy

    Goughy New Member

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    Thanks everyone.

    I'll check the resistance and charging voltage directly.

    Depending on what I find, I'll probably drive the 130 miles to the next nearest Toyota dealer in Vegas to get a second opinion (brakes willing!).

    To help others with a similar problem, I'll update the thread when I have more information.
  15. Goughy

    Goughy New Member

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    I measured the charging battery voltage directly across the terminals at 14.01V.

    The resistance between the negative terminal and the connection to the fender measured 1.9 ohms.

    Sounds like the resistance is high. Could this cause problems with the ECU or is it a different problem?

    I'll get a second opinion on the warning lights next week.
  16. Patrick Wong

    Patrick Wong DIY Enthusiast

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    That is quite high. Before you accept that measurement, connect your digital multimeter test leads together and see what resistance measurement is produced. Consider that to be the baseline, and subtract that value from 1.9.

    If in fact you have 1.9 ohms resistance, then a modest 2A current flow from the battery will result in a 3.8V drop across the negative terminal cable (according to Ohm's Law.) No wonder that the inverter will think that the battery needs to be charged. This may cause problems with the ECUs when the car is IG-OFF since the battery will not be able to keep the 12V bus voltage at the correct level under conditions of heavy current draw (for example, when the brake pressure accumulator pump runs as the driver's door is opened.)

    Remove the negative terminal cable and clean both ends, removing any corrosion or dirt that is present. Do the same as needed to the battery and the body where the cable attaches.
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  17. edthefox5

    edthefox5 Senior Member

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    Hey Pat mine has always charged at that level since new using the mfd check.
    I checked it again tonight and the steps are 12.6/12.3/14.4

    Car runs fine. Never a dtc.

    Have not checked it with my fluke though. I'll do it tomorrow.
    I think the charge is higher than a 13.8 you would expect.
  18. 2009Prius

    2009Prius A Wimpy DIYer

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    Charging voltage for my 2009 Prius as reported by the ECU:
    [​IMG]
  19. Goughy

    Goughy New Member

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    It looks like my brand new multimeter is not reliable. I checked the resistance again with a friends meter and got a reading of 0.7 ohms which seems more believable.

    The negative terminal cable looks in good shape with no obvious signs of corrosion.

    I'll post the DTC information when I get it.
  20. Patrick Wong

    Patrick Wong DIY Enthusiast

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    0.7 ohms is still not good. 0.1 ohms is what you would like to see.

    You need to measure the resistance of the multimeter's test leads shorted together, and subtract that from your reading. If you still get 0.7 ohms then you'll need to figure out why, and correct the problem.
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