Re: cruise contrl oscillations, uphill in '91 200tq

[Robert Myers <rmyers@inetone.net>]

I think that this "searching" behavior is "normal" with a higher boost
engine.  The design of the CC regulator assumes a certain engine output and
feathers the throttle in anticipation of that much available power.  The
higher boost provides an excessive amount of power (as far as the CC is
concerned) and the CC overshoots the needed power to maintain the set
speed.  So, it backs off the throttle a bit, the car slows a bit and we
begin the process all over again.  It's a bit annoying - until you watch
the other cars who have their CCs set at the same speed as yours (on the
level) disappear toward the rear as you simply cruise on up the hill and
leave them behind.

At least this is what happens in my modified '89 200tq.  I suspect that the
CC electronics have not been modified between your car and mine.  Mine
worked perfectly when boosts rose only to 1.3 or 1.4 bar.  Now that 1.9 or
2.0 bar under cruise control are common and this "searching" behavior results.

All right, now, all you cruise control experts, go ahead - shoot me down.  :-)

At 11:31 AM 6/17/97 -0600, you wrote:
>i've noticed some strange cruise control behavior,
>with my '91 200tq wagon:
>
>picture going up a long slope, at 2800 rpm, with a boost pressure
>of about .9 and in cruise control.
>i would have expected that this would feel the same as if
>a driver's foot was simply on the gas pedal.
>instead, i get a second and a half of very slight acceleration
>followed by a second and a half of very slight deceleration
>(and during the deceleration an almost inaudible sound as of
>a vacuum leak or an air leak), and this recurs regularly,
>on off on off on off...
>
>my mechanic says that that is normal for these cars.
>a virtually identical car behaved that way recently,
>he reports, going across kansas, where there were gentle
>slopes up or down for 30-40 seconds at a time.
>(i'm inclined to accept that, but i can also easily imagine
>that there is a characteristic problem that develops with age
>in these cars. i wonder.)
>
>so, i would really appreciate knowing others' experiences
>with this kind of thing.
>
>thanks,
>gideon
>(gideon weisz, boulder co)
>
>
>
>
>
___
   Bob
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