Re: ABS question on 89 100

[johnc@together.net]

Mike:

i take it the missing word was 'new' to go with the 'nearly'.  lots of  tire doc
will tell you that diff mfg will have slightly diff actual size for the same
published size even when new, then there's the new vs. worn thing too.

i would be pretty sure the ABS knows about this and isn't too happy.  look at this
example with a 1/4 inch treadwear or tire size difference:

a tire with a 12" radius will be turning at approx. 210 RPM at 15 MPH
a tire with 11.75" radius will turn at approx. 215 RPM at 15 MPH
-- or another way to look at it --
if the ABS thinks the 12" tire is going 15 MPH, it will perceive the smaller tire to
be going 15.3 MPH

my guess is that the ABS sees this as a pretty big difference. (maybe someone on the
list knows the sensitivity of this particular system?).

of course if you turn off the ABS the car will be fine until you straighten out your
tire situation.

btw, did this start when you put the tires on the back?  that would be a giveaway
too...

JC

Mike Larosa wrote:

> Looks like I've been christened to the wonderfull world of ABS troubleshooting
> :-)
>
> Scenario:
>
> Wife's 89 100 Avant,  she starts to hear/feel the ABS kickin on moderate braking
> intermittently.  Say's "honey my car's acting up again...."
>
> HHHmmmmmm:
>
> We swap car's and I notice that under moderate braking the ABS consistently
> kicks in.
>
> Next day, the ABS light comes on saying the ABS is off.  It did this without
> me hitting the brakes....  hhmmmm thinking maybe I bumped the switch putting
> on the radio...
>
> Day three,  Know I notice ABS turns itself off (Light comes on)
> when I hit 15 or so MPH.  Hit brakes increasingly harder and don't feel
> ABS.  ok, ABS is really off.
>
> Start looking through the bentley, doesn't there is much troubleshooting
> I can do without a 725psi pressure gauge or a Bosch ABS 2 led tester. :-(
>
> Possible causes ?
>
> I have fairly worn d60a2's on the front and nearly BFG touring t/a's out back.
>
> I plan to replace the d60's with t/a's in the next month.
>
> Could the tire circumference diff be enough to disable the ABS ?
>
> Does anybody how much a diff will cause the ABS to turn itself off ?
>
> I'm not opposed to measuring the difference to check...
>
> Am I at the mercy of the dealer..... :-(  gulp.
>
> Mike L.
> 90 V8
> 89 100 Avant
>
>

--

__________________________________________________________________

John J Cunningham III           Project Leader / Technical Analyst
                                             Country Home Products
johnc@together.net                            (802) 877-1201 X1242
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