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post #1 of 15 (permalink) Old 10-15-2008, 01:37 PM Thread Starter
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Default Master Warning Light

Last night as I was making the 30-mile drive home from work, I had gone about one mile on surface streets, was slowing down to a red light before getting on the freeway, and the Master Warning Light (red triangle around "!") came on briefly, about 1/4 second, and then went back off. There was NO "check engine" or other warning, nor was there any message on the MFD, which is what the manual says should occur when the MWL comes on. There was no noticeable change in performance.

So I thought it must have been a fluke.

I drove about 22 miles on the freeway, all normal, and then after I got off the freeway, and was accelerating on a surface street, the MWL again came on briefly. No other warning lights, no "check engine," etc., no MFD message, no change in performance.

The fact that this happened twice concerns me.

Any thoughts?

Doug

Escondido, California (San Diego County)
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post #2 of 15 (permalink) Old 10-15-2008, 04:00 PM
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Default Re: Master Warning Light

What's the year of the Prius? How many miles?
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post #3 of 15 (permalink) Old 10-15-2008, 06:46 PM
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Default Re: Master Warning Light

And of course with a Prius any warning light noticed a couple times warrants a trip to the dealer. It just might be a minor leak of hydraulic brake fluid ect.
We read so many posts about poor service centers as far as the Prius is concerned that we are getting to think of them as "dentists."
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post #4 of 15 (permalink) Old 10-15-2008, 09:24 PM Thread Starter
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Default Re: Master Warning Light

The Prius is 2004 with 120,000 miles on it.

It happened again today driving TO work, on surface streets about 4 miles from home and 2 miles before getting on the freeway.
Going DOWN a downgrade about 50 miles per hour with foot on neither brake nor accelerator.
Master Warning Light flashed on briefly with no message on the MFD, no other warning lights and no change in performance. I drove the rest of the way including about 22 miles of freeway with no further incident.
Before this trip, I had driven several local errands today without incident.

The only other times I have ever seen the Master Warning Light flash is a couple of times when (at very slow speeds) I made the mistake of opening the driver door, but when that happened it was accompanied by an MFD message, which is what the manual says should happen, and I could associate it with a specific cause.

Doug

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post #5 of 15 (permalink) Old 10-16-2008, 10:03 AM
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Default Re: Master Warning Light

Could there be something that is obstructing your door from latching completely?

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Maple Grove, Minnesota
2004 Tideland Pearl now belongs to my daughter.
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post #6 of 15 (permalink) Old 10-16-2008, 01:36 PM Thread Starter
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Default Re: Master Warning Light

I suppose a door issue could possibly be a factor, however when the door is ajar or open, the Master Warning Light stays on and there is an MFD message.

Right now, the light just flashes on and back off quite fast.
In fact, there may have even been an MFD message but it was on and off so briefly that maybe I didn't see it???

I called my dealer to make arrangements to have it looked at -- he said with that symptom it seems most probable that it is a bit low on oil and, since it is about due for a routine oil change, I'll bring it in and have that done. But I have always been extremely religious about regular service, there is no indiciation of oil leakage, and this has never happened before, so we'll see.

Doug

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post #7 of 15 (permalink) Old 02-03-2010, 01:56 PM
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Default Re: Master Warning Light

was there a resolution to this issue?
I'm having a similar problem with my 2004 (80k). The warning light comes on randomly (twice per 20 mile trip), mostly when slowing down. The display simply says 'Problem' and it lasts less than a second.

thanks
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post #8 of 15 (permalink) Old 02-03-2010, 02:07 PM
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Default Re: Master Warning Light

Just my guess and that's you might be developing a slight leak in your hydraulic brake accumulator. As it starts to lose pressure you would get the light which will go off as soon as the electric hydraulic pump "kicks in."
Not to worry because a complete failure of either the accumulator or electric pump will give you a "steady" light. "Book" then says, "pull over" to side of road and call Toyota.
There are "Two" warning lights in the system. One for the pump failure, and the other for the accumulator, so with only one, you could drive safely home or to the dealer.
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post #9 of 15 (permalink) Old 02-03-2010, 02:16 PM
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Default Re: Master Warning Light

this has been happening for a week...Your theory appears sound, is there any way to verify?

Would the brake fluid level be low? I haven't noticed any drips on my garage floor.
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post #10 of 15 (permalink) Old 02-03-2010, 04:58 PM
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Default Re: Master Warning Light

No, this one would have no indication of a fluid loss. The brakes are mainly pressurized by the accumulator. (Picture a ball with a membrane dividing the center.) On one side of the membrane is hydraulic brake fluid and the other an inert gas. The pressure is maintained by the membrane forcing the hydraulic pressure when the brakes are applied. There is enough pressure stored in the "ball" by the movement of the membrane for many brake applications. At a certain setting the electric hydraulic pump comes on to re-establish full system pressure. That's the noise you hear when the electric pump comes on. The brakes can be powered by either system and you get a separate warning light when the pressure is too low in either. The leak in the accumulator is usually in the "gas" side of the membrane. Your pump then runs more often. Then one or the other warning light illuminates. Until you get any one steady light you really have no problem. Same as just about any system in the Prius. They have always told you to ignore all unusual sounds and drive normally until you get a steady "idiot" light. The occassional one should be overlooked as worthless as even the Toyota dealer will be confused.
My guess is that sometime in the future you will have to have the accumulator replaced. It could be next month or next year. Good luck!
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