From:    (Adam Lotz)
Subject:          [z-car] [zx]	Cruise Control
To:               z-car@CS.Princeton.EDU
Date sent:        Thu, 1 Jun 1995 00:56:05 -0400 (EDT)

Here are ALL my notes on how the cruise control (Automatic Speed Control
Device) works in my 280ZX.  Er...doesn't work in my 280ZX.

>my cruise is acting up too. With mine it shuts off when I hit any largish
>bump. I think the brake switch (that shuts it off when you hit the
>brakes) is set wrong and is too sensitive, you might see if that is
>waht's wrong with yours. It may just be on all the time making the cruise
>think you have your foot on the brakes.

Hey Mark,

Went home this weekend, and found out that a local junkyard was having
their $30 for all you can carry day... got everything that looked
cruise-related off a burnt 280ZX, and brought it home to compare to mine..
Here are my notes on the cruise control, in general, in case any one else
is having problems:

1) It's a REALLY simple system.  None of the manuals that I've seen will
tell you how to work on it, presumably for insurance/safety reasons, but
for people who change their own brakes, it's not too bad to look at.
Okay, first, there is a switch module in the dash.  Turn that on, and a relay
under the passenger seat audibly clicks to the on position. 

This relay supplies power to the cruise brain, which is the largest (4"
square) metal box under the seat.  

The brake pedal and clutch pedal each have plastic-tab operated switches,
which are wired in series:  This means that if either the brake or clutch
pedal is pushed in (or both) the circuit is broken, and the cruise will

The above switches can be checked by running a voltmeter to various points
on the cruise brain wiring harness -- you will see 12v on 2 of them, then
when you push in the clutch, 0v... push in brake, 0v... etc.  As you
mentioned, it's a very good idea to make sure there isn't a lot of extra
play in the pedal so that it doesn't bounce off and break the circuit

The set switch on the turn signal lever can be tested WITHOUT taking apart
the steering column area.  Again, a voltmeter and some poking around in
the wiring harness will find a closed circuit when the button is pushed
in, an open circuit normally.

That's all there is to the electronics.  

UNDER the hood, there are 3 main components.  

First, there is the big round thing next to the battery, which has a cable
going to the throttle linkage.  With the car off, you should be able to
manually press the throttle to the floor, and then wiggle this cable in
and out (it takes a BIT of effort.)  Making sure this cable isn't frozen
up would probably be a good idea.

The big rubber 'nipple' on the round thing (which I'll try to call vacuum
actuator from now on) is held on with a bit of adhesive which seems only
moderately important.  If you're adventurous, feel free to push in on it,
to seperate it from the metal, then pull out gently and slide it an inch
or two down the throttle cable.  Looking into the big round thing, you'll
see that the cable is attached to a strange rubber bag, which lo and
behold can be filled with air.  (the inflation/deflation tube is on the
bottom)  If you try to push on this bladder now, it should NOT budge, if
all of your hoses are in good shape.  If it moves easily, you probably
have a leak somewhere.  

Now onto the other 2 big components of the ASCD under the hood -- two
electric vacuum switches.  These are hooked up via thin rubber hose to
either your vacuum pump or the intake manifold (if your car doesn't have
the pump) As I experimented, I gathered that these switches can be
actuated with somewhere between 3 and 12v, and will open according to the
amount of voltage they get. IE open a little at 3v, a lot at 12v)  That
MIGHT be wrong, I don't know.  All I know is, I took a battery charger
(14v) to the contacts and they made a nice buzzing sound, which I took as
a sign that they were WORKING.  Again, if you supply 12v to them, they'll
make a loud doing-something noise -- if yours don't make any noise,
something is probably wrong.  I would NOT recommend supplying them with
more than 10v for any extended length of time, as I have no idea what
their normal operating voltage is.  

Basically, what I gathered from experimentation/reading was this:

When you turn the "ON" switch on, the relay under the seat flops and the
system gets 12v.  This illuminates the "ON" light.

When you hit the button on the end of the turn signal stalk, SOME MAGIC
THING INSIDE THE DASH (this is the one part I really don't know ANYTHING
about) determines your speed (from the speedometer) and stores it in some
form of memory.  

As you *RELEASE* the button, the computer does some quick calculations and
tries to decide how this compared to your last preset speed.  If you
didn't have a previous speed, it sets it to the new one.  If you HAD a
previous speed, it resets it to the new one. (this redundant logic becomes
important when they add things like the accel/decelerate button in later
years)  So now the magic speed computer knows how fast you were going.  As
soon as it figures this out, it sends some electrical signal (again, don't
know what) to the two vacuum relays (as I understand it, one is an
emergency cutoff) that allows a specific amount of vacuum to be sucked
into the bladder.  As the bladder gets smaller, it will start to pull on
the accelerator linkage, eventually matching speed and then going a LITTLE
BIT FASTER. **THEORY --> I think at this point, the computer is still
watching the 'current speed' indicator, and when it sees the cruise has
sped up it thinks to itself "gosh, that must be just a LITTLE bit too much
vacuum" and it adjusts itself to get you your preset speed.  The computer
then monitors the current speed x times per minutes (probably quite a few
for safety reasons/hills/etc) and tries to compensate for any speed
differences by opening/closing one of the vacuum valves slightly. 

So, here's a brief list of the parts I know of, and my own (not
necessarily recommended) testing procedures to see if it's good:

1) Switch in dash - lights come on? probably good
                    otherwise, swap from a parts car

2) Relay under seat - makes "ping" noise when you hit ON button - good
                      also, 12v is being supplied to ASCD brain
                      else replace/parts car

3) Brain box - check to make sure power is going to it. 
               If in serious doubt, the top can be carefully pried
               off with a screwdriver - there is a gasket, but can be
               re-sealed with permatex.  Metal case is VERY durable, even
               if it looks rusted on outside may be perfect inside.  If
               you decide to get inside, look for traces (smell?) of 
               Best bet is to swap from a parts car / grab a friend's.
(Incidentally, when I say swap from a parts car, I typically mean go to
the local u-pull-it junkyard and grab one.  While against the law, many
people find it acceptable to pocket small electronic parts, and/or explain
to the clerk that you think the mystery metal box in your hand is part of
the intermittent-wiper and/or hazard light system.  If they come back with
"What?! Heck no, that's a 280ZX cruise control module!!" the correct
response is "Oh my gosh!  I needed one of THOSE, too! I'll take it!"  --
either way, most electrical parts seem to run US $7.99-$29.99 around here
at junk yards)

4) Pushbutton on turn stalk - check with voltmeter at brain box
                              possibly check on stalk with steering 
                              cover removed
                              if it doesn't work, good luck to ya.
                              (real easy to hotwire one with a momentary
                               contact 12v switch from radio shack)

5) The two vacuum solenoids under the hood -- *danger - haphazard idea *
                                              disconnect the wires to them,
                                              then use a 9-14v dc power 
                                              supply to test.  DO IT QUICKLY.
                                              My 2 made slightly different 
                                              noises, but both hummed and
                                              vibrated slightly = prob OK

6) The vacuum bladder -- have an assistant hold the throttle down (car OFF)
                         which will get some slack in the cable.  disconnect
                         the vacuum line that runs out of the bottom of
                         the bladder from the solenoid (at the solenoid end)
                         which will give you a few inches of vacuum tube
                         to work with.  Suck on it.  (or get attractive 
                         volunteer to do it)  If the cable tightens, the
                         bladder is in ok shape.  Now lick your finger and
                         close the end of the vacuum line with it.  This
                         should MAINTAIN the vacuum in there, and keep the 
                         cable at it's present place. (have assistant stop
                         pushing down on throttle)  If this fails to keep
                         a good vacuum, the bladder could be punctured, or
                         just as likely, your finger isn't a good seal.  If
                         the bladder is punctured, again, find a parts car.
                         I got a replacement by just snipping the throttle
                         cable with wire cutters (in a hurry)  The cable
                         can be reattached at either the point inside the
                         bladder, or on the driver's side. (driver's side
                         is preferable, but either will work.)

**** NEVER, NEVER use manifold vacuum to test the bladder.  In a
split-second of stupidity, I tried to save time by hooking up a vacuum
line to the bladder.  Immediately, the bladder started sucking in, which
made the engine idle faster, which made the bladder suck in farther, which
made the engine hit 5000 rpm before I knew what was happeneing.  EXTREMELY
important to remember that you *are* dealing with an extention of the
throttle cable when fooling around with the ASCD ****

7) Rotten mystery part -- somewhere, attached/combined with the
speedometer, is a speed sensor.  As I've taken my dash completely off once
before (in no mood to to it again) I haven't had time to fool with this. 
If ANYONE can shed some light as to what this is or where it hooks up, I'd
appreciate it!

Other than that ^^^ though, the ASCD was a remarkably simple system.  The
mostly on/off controls made diagnosing the brain easy, and the rest of it
is a simple vacuum system with only 3 major mechanical components that
could fail.

Ah yes -- one last tip -- ALWAYS make sure the vacuum lines are clean by
disconnecting one at a time and blowing through it... something could be
lodged in one, and checking in advance will save you a lot of time.