Cruise control switch logic

[Wallace White]

Huw Powell wrote:
>> I still don't get why this circuit is in series with the brake light
>> switch...
>
> As far as I can tell, that is one more element in the multiply-redudant
> system to turn off the cruise control.  Much as the vacuum breaking and
> electrical circuit interrupting switches on the pedals act to turn it
> off if you tap pedal, the brake light switch is yet another way to
> instantly kill it given any evidence of a desire to slow down.
>
> Whoops, I had another theory, too... that the cruise won't work if your
> brake light(s) are out.  Now, did I ever decide which one it was...?

I think it could be both. Let's update that schematic:

     o  +12V
     +
    |
  ==|   Brake light switch
    |                               brake light
     +-------------------------------\/\/\/---+
     |                                        |
   ==|  CC stalk coast switch                 |
     |                                      -----
     +                                       ---
     |                                        -
   ==|  CC brake switch
     |
     +
     |
   ==|  CC clutch switch
     |
     +
     o  CC module, pin 3

The controller's logic must be that it needs a ground path on this pin
to allow cruise control.

In normal driving, the CC brake, clutch, and coast switches are closed,
connecting pin 3 to the inactive tail light bulbs. The brake light
switch is open. So current flows out of pin 3 to ground and the
controller says it's okay to cruise.

Then you hit the brake or clutch or the coast switch. Current flow is
interrupted. If the brake light switch closes, pin 3 is now connected to
12V (current still doesn't flow out of pin 3).

(The brake light switch isn't really connected to the brake light but
instead to the "rear lamp control unit," so maybe it's a path to ground
through the collector of a transistor rather than a resistor. In that
case, the pin 3 current must be small enough to not turn on the brake
lights....?)

If this explanation is correct, I don't know why they chose this ground
path. It could've had a pull-down resistor interally. I guess it is for
the redundancy: even if the CC brake switch doesn't open, the closure of
the brake light switch prevents current flow out pin 3 and interrupts
cruise.

> PS!  I reveiwed the Bentley procedure in my Coupe manual, for measuring
> various reistances and voltages from place to place to troubleshoot the
> system, and there was at least one error.

Interesting... remember what it was?

I've found some wiring diagram errors: the '87~88 5kcs shows another
wire in parallel with the CC clutch switch, bypassing it, but that's
only on the automatics. (The other years don't show it.)

> While I commend trying to follow the proper procedure to track down
> problems, in this relatively simle system, it also helps to understand
> why a given resistance or voltage should be present at any given time or
> place.
>
> It's not too hard to unplug the control module and make measurements,
> and inject voltages here and there to test the circuits, once you figure
> out what those R's and V's actually do.

Right--now that I know about that coast switch!

Thanks,
Wallace
'87 5kcstq 203k