Climate Control channel 22

[pjrose@servtech.com (Phil & Judy Rose)]

Sorry if this has all been covered in past postings; it's new to me.

Mr. Bentley ('89-'91 100, 200) makes a very brief reference to an
interesting Climate Control output function from channel 22: the vehicle
speed. Yesterday, during a boring 150 mile drive, I checked the channel 22
(CC-22) output in my '91 200q and found that it indeed is closely related
to the car's speed. The numbers correspond almost exactly to the speed (in
kilometers per hour) at very low speeds (say 20-40 KPH). But the the
readings obviously start to understate the speedometer speed by an amount
that changes (increases) with vehicle speed. At around 120 KPH on the
speedo, the CC-22 readings are too low by about 12-15 KPH. I don't think
the discrepancy reflects the error in my speedo. My speedo reads too high,
but not by that much--I hope :).

The Bentley manual  doesn't have a chart to convert channel 22 numbers to
actual speed. Does anyone have information about this?

What's the significance of this bit of trivia? Well, a few thoughts:

(1) Isn't it possible that CC-22 may inherently show the vehicle speed with
greater accuracy than the speedo dial? If so, it might give useful
benchmark data if you decide to recalibrate your speedo dial's output by
changing the resistor (archives contain someone's posting on that
procedure).

(2) Speed read via CC-22 could provide a useful emergency backup if the
speedometer dial becomes non-functional.

(3) The digital CC-22 value might be used for a more-precise timing of
acceleration runs (just a guess).

BTW, the Bentley indicated that the channel 22 (speed) output was phased-in
for later models in the '89-'91 series. Do all the later-than-'91 models
have it?

Phil Rose

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