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What causes brakes to occasionally grab after a start?

1245 Views 2 Replies 2 Participants Last post by  Adrenochrome66
I could really use someone's insight on this.

For several months, our 2004 Sienna has had a weird problem: When starting from a stop, the rear brakes hold on for half a second before letting go. This happens maybe once every fifth time we start moving. It's usually so slight that it's barely noticeable, but every now and then it's very noticeable. When this happens, the traction control light often blinks.

I've also gotten it to happen when moving along at less than 1mph, sometimes dramatically. There's a sort of hum that seems to come from behind the dashboard when this happens. More rarely, sometimes just as the van stops, there's a tiny pulse as if the ABS is tripped.

Our mechanic couldn't figure it out. We took it to a dealer who said they couldn't make it happen (ridiculous), but they said our brakes and speed sensors were all functioning properly.

Anyone have any ideas?
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I believe, traction control means you have 4-wheel disk brakes, but I could be wrong. So, I'm going under that assumption. If that's wrong, I have slightly different advice.

If the traction control light blinks, and the brakes grab, that means the system thinks there's slippage happening. I'd start with the free/easy action. Make sure the tires are properly inflated and then re-calibrate the tire pressure system on flat, level ground. I don't know if the Sienna's traction control takes that into account, but I know some vehicles do. Assuming that doesn't fix it, I will say that it's likely a wheel speed sensor that's bad. It's possible that there's a partial short in one of the wires or some corrosion on a connector. Unfortunately, I don't know if there's any test procedure for an intermittent issue which occurs in semi-arbitrary, inconsistent situations. If throwing parts at the problem is something you're interested in doing, the sensors look like they aren't super expensive. That said, I would probably consider a few general items like flushing/bleeding the brakes and lubricating the caliper slide pins. Usually, if a caliper is grabbing, you can tell which one because the brake pad will be slightly more worn on that side. If it's not the sensors and it's not a sticking caliper and it's not fixed by brake fluid, it's probably something central to the ABS system. In that case, I would actually probably advise taking it to a dealer to do a full diagnostic on the car.
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Thanks, BillG.

I've been working under the assumption that it's an intermittent speed sensor issue triggering the traction control, but I have no way to figure out if that's the case or which sensor might be bad. The dealership says the sensors checked out fine (though they may not be able to detect an intermittent problem), and the brakes and brake lines are definitely fine - both the dealer and my usual mechanic checked them over.

It's definitely not the tires because this was happening with the snow tires as well.

I've found a way to disable the traction control in the AWD system, so I'm going to try that today to confirm that it's what's causing the problem.
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