88 325 - ABS dash light on

ABS dash light used to blink occasionally when I would hit a nasty bump. It started staying on for longer periods after hitting a deep pothole. Now it stays on for quite a while- like half the time I'm
driving. I did replace the battery in the dash last year. Any ideas? Thanks as always.
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in article snipped-for-privacy@4ax.com, cosmo at snipped-for-privacy@blah.com wrote on 2/1/09 12:29 AM:

check your brake system ...
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I'm confused by your description. If the ABS system was having a failure -- speed sensor going out, valve body sticking, those sorts of things -- then the lighth should come on and remain on through the completion of that particular drive cycle. On the next drive cycle, the light might not come on, but if it does then it ought to remain on until the key is switched OFF.
The system undergoes a self-test when you start the car, and the light should come on for a few seconds during this test, then go out and remain out. Any failure of the system renders the ABS unreliable, and the light comes on to tell you that the anti-lock feature has been disabled. Since there are no further self test cycles until the next time the ignition turns on, the ABS light should never turn off (once it comes on during any given drive cycle) until you park the car and shut it down.
Given the fact that you have an '88 model car (20+ years old), I suspect you are having a grounding problem where the light can come on intermittantly without regard to the actual operatioinal condition of the ABS system.
Are you sure the ABS light is the one that is on? There are other braking system lights, one of them tells you the fluid level is low. Have you looked into the master cylinder to be sure the fulid level is up?
When the ABS light is on, the only affect you will notice is that the tires can lock in an emergency stop -- you can skid if you are not careful. If you can live with this, then forget about the light. Your brakes work fine, assuming they work fine anyway. There is no danger that you will lose brakes altogether.
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I don't believe the original ABS systems were like this. The modern ones are, but I believe that in 1988 there was still just a combinational logic box, rather than a computer-controlled system with a test mode. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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wrote:

In any case, all that happens is that anti-lock feature won't provide the services one might expect during an emergency stop. Barring a diagnostic procedure that helps sort this out, one need only assume his brakes will lock if he mashes the brake pedal hard enough, and one can go a lifetime without ever invoking the services of his ABS system.
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cosmo <and,.. blah blah> wrote:

The bad news is that there isn't really much in the way of diagnostic hardware for these things.
The good news is that things that blink when you hit a bump are invariably loose connections.
I would start by inspecting all the cabling from the sensors to the ABS box for damage. Then I'd measure the static resistance of each of the sensors and see if one of them is different.
You can't just plug a scanner into the ABS unit on these, and see plots of the sensor inputs, the way you can on the newer cars. That's a minor annoyance. Remember, 1988 is only the second year they sold ABS, at least here in the US. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Scott,
Just an FYI. I had a 1986 325es and it had ABS. I still miss that car...
On 1 Feb 2009 15:45:33 -0500, snipped-for-privacy@panix.com (Scott Dorsey) wrote:

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Sorry, I did the math wrong. I think 1985 was the first year. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Thanks, I haven't had time to look at this. I thought I'd mention that since it warmed up 10 degrees I haven't seen the problem. It's a minor annoyance but I thought it might be a common aging problem with a known solution. Thanks again.

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