[V8] More about cruise control

urq urq at pacbell.net
Sat Aug 20 16:17:34 EDT 2005

... I must say ... from my point of view it is difficult to figure out what 
is and isn't happening based upon the tale ... it was indeed an interesting 
tale, just one I found difficult to extract information from ... and I'm 
sure it is partly my fault ...

I too consider myself to be a good problem solver ... but if there is ever 
any available documentation I seek it out for the quest ... here again, I 
know that there are those who consider the challenge to be more daunting 
when you have to figure things out from general principles ...

Did you know that on automatic transmission cars the cruise control receives 
power via the transmission controller? ... at least it is supposed to.  I 
don't know what you did to resolve the loss of power in your case.  If you 
have all the lights on the "PRNDL" display illuminated you should expect the 
cruise control to be inoperative.

It seems to me that you may believe the CC uses engine vacuum to control the 
throttle?  There's a pump under the left fender (behind the hydraulic 
reservoir) which draws the vacuum and vents when it is necessary to decrease 
the throttle.  These pumps can fail.  On the pump there's a small vacuum 
line that goes from the pump toi the vent valve ... when troubleshooting the 
CC on one of my cars I found this hose had broken off the nipple on the 
pump.  The vacuum circuit is very simple ... the pump goes to a "T" fitting 
... one of the other legs of the "T" goes to the vent valve on the brake 
pedal and the other to the vacuum motor.

You have focused on the power getting to the CC controller ... interestingly 
enough, the Audi CCs have a circuitous *ground* path that can be problematic 
too.  Believe it or not the CC controller gets grounded via the brake light 
filaments ... an odd fail safe mechanism ... if there's blown brake lights 
or a brake switch that is always on the cruis control will be inop ...

Search the recent archives and the Audifans KB for more information on this 

It is also odd to me to hear that people will continue to try to operate a 
vehicle with a stuck throttle ... for me the first thing to do is to find a 
safe place and time to shut the ignition down!  This also brings up an 
interesting item ... apparently some cars engage the steering lock when the 
ignition key is turned to the off position, but others will not lock the 
column until the key is removed from the cylinder.  I'd recommend testing 
how your car works in this regard so that you will know how it works should 
you find that you need to shut the engine off while the car is moving.  Also 
remember that the steering will get *much* more difficult when the engine 

Steve B
San Jose, CA (USA)

> When I got this car, the cruise wasn't working. Being the avid
> 'trouble-hunter' that I am, I got right into it, without benefit of
> either parts book or repair manual.
> First I checked the fuse panel, and there was no power to the cruze fuze.
> I figured that one out and ended up getting power where it belonged.
> Still nothing.
> I also had problems with the wipers not working. I had to pull the
> steering wheel to get at the column switches. The wiper switch was
> shorted and I repaired that, then discovered that 5 of the 6 wires for
> the cruise were broken at the base of the handle. I resoldered them and
> had continuity all the way.
> Still no cruise. Also have an airbag light on that won't go out, but
> that's another story.
> So I got into it to find out why the throttle stuck. Turns out it was the
> cruise. The little plastic collar that holds the rod into the bracket on
> the vacuum motor had disappeared (although I didn't know there WAS one,
> I figured thaere was SOMthing missing there), and when I floored it, the
> rod from the throttle body to the vacuum motor fell out of the motor
> bracket, onto the engine blocked and wedged there, keeping the
> accelerator at full throttle.
> So I replaced the little plastic collar and lo and behold, I have cruise.
> Sweet.
> For about 2 weeks.
> Driving along in my automobile, the cruise just lets go while I'm at the
> wheel.
> Nothing. Nada. Zip.
> So back to the hunt. Fuse and power: good. Switch: good. Pull vacuum on
> the cruise motor: good. Check the vacuum lines from the motor to--hello,
> what's this? The control? Vacuum lines all connected as far as I can see.
> Except, start the car, no vacuum at the tee in the line to the motor.
> More hunting; here's what I find: hose from the motor to a tee fitting;
> one side from the tee to the brake pedal switch (hose and connections are
> good), other side of the tee to the control unit (hose and connections
> are good). But, I can't find an 'In' hose to the control. Search all
> around and find no sign of any hose bringing vacuum from the engine to
> the control unit. Now c'mon, it worked before, right? Even if briefly, it
> DID work. And there SHOULD be an 'In' hose, right? I see what looks like
> a connection on the control where vacuum would go in, and drawing vacuum
> on that port pulls air back into the port where the line to the vacuum
> motor connects, which is what I think it should do.
> No hose. Start another search. The diagram on the front panel is useless.
> I've got the air cleaner top and box at the rear of the engine off. At
> the back of engine there's a port with a hose that goes to a tee on the
> cowl for the air conditioning controls and the charcoal canistor; they
> both have vacuum and the a/c  controls function properly. But I find no
> other hose, or connection at any point on the engine for a hose, to go to
> the cruise control unit.
> Also, while looking around back there, I find a hose that looks like it
> went to a vent port on top of the transmission, up on the cowl, and then
> to---nowhere. I figure the tranny shop cut the hose to get the trans out,
> then didn't reconnect it, but the other end, at the cowl, is connected to
> nothing.
> So, I'm looking for help to get vacuum to the cruise control unit, and
> connect this trans line to something.
> Anyone offer me any assistance in this adventure?

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