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Re:Cruise Ctrl and Horn prob on '85 4KCSQ

 # >When the outside temp is cold...the cruise control...slowly drops after 
 # > settting the speed...Once the car warms up and/or the outside temp
 # > warms, it works fine.
 # It sounds like the vacuum is bleeding off when it's cold.  Maybe the
 # pie-pan diaphragm that opens the throttle is getting tired.  Take off the
 # hose at the back when the rubber is cold, push the diaphragm in, plug the
 # port and and see if it will hold its position against the throttle return
 # springs.  If it's holding there, it's leaking someplace else: the vacuum lines,
 # possibly the brake/clutch vacuum dump switches, or the pump itself.  
... that was my initial suspicion as well, but if you think about it the 
actuator cannot be at fault (unless the leak is SO bad that the vacuum 
motor can't possibly draw enough vacuum to hold it in).  The cruise control
system is trying to maintain speed, so if it sees the speed decrease it will
apply more vacuum to the actuator.  I don't know how the guts of the CC are
designed, but to explain the behavior let's assume that the control circuitry
inside the control unit use "analog" circitry (as opposed to digital).  If 
there were some marginal component it might not be functioning properly at 
low temperatures (an electrolytic capacitor comes to mind).  

I know that the speed input to the CC comes in digitally (typically a hall 
sender that is attached to the speedo), so this would not obviously explain 
the symptoms.

It certainly couldn't hurt to do the tests you state (as long as you don't 
rip off chunks of flesh after touching the cold surfaces ... :), but I'm 
skeptical ...  I'd recommend swapping the CC controller from a car that 
doesn't have the droops at cold temps.  I've noticed that most all of the 
cars use the same part for the CC controller.  An '84 4kQ is the same as 
the '87 5kQ which is the same as the '83 urQ ... it shouldn't cost too much 
to replace the thing.  I suppose another quick test would be to take the CC 
controller out at night to make sure its nice and warm when you test it the 
next day ...


Steve Buchholz
San Jose, CA (USA)