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ABS problem, locks rears. Help needed.

 
Old 06-28-2019, 08:59 PM
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Default ABS problem, locks rears. Help needed.

A friend of mine has a '97. 1.8, pretty much bone stock with ABS.

Anything other than a very light touch on the brake pedal causes the rears to lock.
And,that is not the whole story.

I drove the car to see what is happening...

You hit the brakes, the rears lock and stay locked.
Meanwhile, the brake pedal keeps going down - on it's own. Like, your foot is stationary, and the pedal goes down, away from your foot, and a pretty big gap appears between your foot and the pedal.
The rear brakes finally release, and the pedal comes back up to meet your foot.

If the car is driving along on a straight line when this happens, it comes to a screeching stop.
If you happen to be turning at the time, well, you can imagine the carnage.

I searched as best as I could and have not been able to find a similar case.

What do you think?
Have you ever seen/heard of anything like this on a Miata?

Any help/input is greatly appreciated..
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Old 06-28-2019, 10:21 PM
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Does pulling the ABS fuse fix it?

You could check the wheel speed sensors using an oscilloscope to look at the waveforms, but I suspect this is just something busted inside the ABS box (or its control computer) itself.

--Ian
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Old 06-29-2019, 09:35 AM
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Originally Posted by codrus View Post
Does pulling the ABS fuse fix it?

You could check the wheel speed sensors using an oscilloscope to look at the waveforms, but I suspect this is just something busted inside the ABS box (or its control computer) itself.

--Ian
Yes, pull the ABS fuse, and the rears do not lock up.
I am suspecting a wonky solenoid in the ABS module.
I wanted to present this problem to the collective Miata experience here before doing anything radical.
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Old 06-29-2019, 10:33 AM
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I've never heard anything like that but it must be terrifying and extraordinarily dangerous.
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Old 06-29-2019, 07:13 PM
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Following this thread, as my second 1998 NB MX-5 1.8 with ABS does this too. Whenever you brake hard, it locks up the rears pretty bad. The ABS does release the rear brakes after like 0.5s~1.0s. But at that time you can be pretty sideways and the engine stalls as well: not nice.
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Old 06-29-2019, 07:26 PM
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Originally Posted by DaWaN View Post
Following this thread, as my second 1998 NB MX-5 1.8 with ABS does this too. Whenever you brake hard, it locks up the rears pretty bad. The ABS does release the rear brakes after like 0.5s~1.0s. But at that time you can be pretty sideways and the engine stalls as well: not nice.
Was any work done on the brakes before it started locking up?
Specifically, could any of the brake lines may have been installed incorrectly, or crossed?

I am asking this, because a knowledgeable mechanic suggested this sort of unpredictable stuff happens if the "lines are crossed" somehow...
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Old 06-29-2019, 07:28 PM
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Originally Posted by sixshooter View Post
I've never heard anything like that but it must be terrifying and extraordinarily dangerous.
Absolutely.

I mean, I was prepared for what was to come, and it was still pretty shocking, because you have no control over the car while the brakes are locked.
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Old 06-30-2019, 03:14 PM
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Originally Posted by Godless Commie View Post
Was any work done on the brakes before it started locking up?
Specifically, could any of the brake lines may have been installed incorrectly, or crossed?
I do not know much about the history of this car. I am pretty sure it received 4 new brake calipers in the last 5 years.
Last year, I was driving on pretty crappy tyres and the front pads might have been glazed.
I installed a new set of AD-08Rs on the car yesterday and did some heavy braking today. The balance seemed to be fine.
I did not yet man up to get ABS working, so my ABS might still be wonky.
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Old 07-02-2019, 05:59 AM
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I presume the NA has the separate ABS computer, the 98 as well? I'd just throw another ABS unit in there and see if that cured it, but here all we have are the ADM integrated units, you might have trouble finding one without the internal computer?

My JDM 2000 racecar came with one of those, we tossed it for the integrated unit (because that was what we knew). May have been a mistake as the ABS refused to work, until in desperation we tried a new (secondhand) unit - just plugged in the connector, it self-checked OK, and worked perfectly ever after. Point being that in spite of the conventional wisdom (here, at least) that 'these units never play up, must be your wiring/sensors/something else', they do go **** up, and if you can beg borrow or steal another it is a cheap diagnostic check.
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