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Re: flakey ABS Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Thu, 16 Aug 2001 09:20:41
In Reply to: flakey ABS, johnf, Thu, 16 Aug 2001 06:37:01 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The ABS system does a few self-tests that can turn on the ABS light. The ABS reads pulses out of the sensors, where pulse speed equals engine speed. If the ABS sees no pulses or much slower pulses out of one wheel than the others, and the brakes aren't on, it flags that wheel.
In addition to the speed of pulses, the ABS looks at the voltage level, for testing purposes only. If the voltage level of one wheel is lower than the others by a large enough margin, it flags that wheel sensor as bad. Now, this can be tricky at low speed, as the signal levels drop at low speed.
So how is does this help you fix the problem? Well, you could have a few, if not simultaneous failures, at least concurrent ones. A common problem in the wheel sensors is the cable - with flexing, the insulation breaks down and the frequency response of the cable changes. We're talking about some pretty small signal levels here. So it's possible that you've got flaky cables on more than one sensor. That would explain the suspension movement issue - that would flex the cable more. And if two or more cables were bad, that could make the ABS give you faults for good sensors - after all, the ABS compares the sensor outputs, and the 'bad' sensors could be outvoting the good ones.
I would do the following - first, make sure the GROUND connection for the ABS box is clean and good. Follow the black wire out of the connector and make sure the grounding point is good.
Next, carefully inspect the cables from each sensor, especially as they leave the hub and move up into the body. The most common place for a cable to fail is where part of it is secured. If a cable looks bad or feels bad - CAREFULLY bend it a little, looking for internal breaks - replace it. The bad part is that the cable/sensor is one piece, so you need to replace the whole thing.
Lastly, check the reluctance wheels on the hubs. These are the toothed rings on the hubs that trigger the ABS sensors. If they get really plugged up with stuff, they can affect the sensor signal. I've got no proof, but it is possible that if you use metallic or semi-metallic brake pads, that dust can lodge in the gaps between the teeth and mess the signals up.
Good luck!
posted by 140.157....
, Thu, 16 Aug 2001 09:20:41 <-- Viewing This Message, Thu, 16 Aug 2001 11:20:56
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