Much of this information has been sourced from Dave Evans' page, at www.lotuscarlton.demon.co.uk. Please visit his page for a more detailed look at the electrics.
The Cruise Control system is one of the most mega-complicated systems in 80's car history. In total, there are 4 main elements to the system. The actuator, relay, brake switch linkage and CC unit. All can fail quite easily, and it is often hard to diagnose the system.
The actuator is basically a belt-driven motor which is connected via a cable to the throttle. When the motor is activated, the cable pulls, thus engaging the throttle. The engine is engaged and controlled by the amount of 'pull' from the actuator.
The relay lies behind the fuse box, and controls the "on-off" function of the system. It's main job is to turn the system off when you press the brake or clutch, or put the car into neutral. It is the thing that "clicks" when you do one of the above.
The brake switch is a small white plastic tab which lies at the top of the brake pedal. It serves to send electricity to the brake lights, but it also sends a positive feed to the cruise system via the relay. If there is no "click" when you put the brakes on, and no feed to the CC unit, then look here.
However most often it will be the CC unit that has failed - they aren't built to last, and the capacitors on them can lose their effectiveness. You can find these in scrap yards (they are the same as Carltons and Calibras of the same age), but finding a good one is hit and miss. You can also try your hand at replacing the dodgy electrics, but that is beyond most of us.
The CC unit is a microcontroller which feeds to the actuator, and is in turn feed by other parts of the car.
The CC unit lies behind the glove box, the actuator on the passenger side of the engine bay, at the top near to the suspension turret and starter motor.
The good thing about this arrangement is that the CC unit is easily accesible for testing, and because of the in-out nature of the controller, you can test some functions of the actuatar, CC relay and other sensors too.
If any of these tests fail, it will be either the component you are testing, or the CC unit itself. Often replacement is the only cure, as these items weren't made to last more than about 2000 hours of use, which is nothing to a 8 to 14 year old car.
|
Control
Unit
|
Wire
|
Source/Actuator
link
|
||
|
1 |
Motor position sense input |
Black/White | Potentiometer central terminal | 7 |
|
2 |
GND /position sensor GND |
Brown | Potentiometer outer terminal (also ground circuit) | 3 |
|
3 |
Motor drive |
Black/Yellow | Motor drive | 5 |
|
4 |
Motor feed |
Black/Green | Motor feed (also CC relay) | 1 |
|
5 |
Resume CC |
Lt. Blue | 'R' button on Stalk | |
|
7 |
Road speed |
Lt. Blue/Black | Speedo feed | |
|
9 |
Position sensor smoothed supply voltage |
Black/White | Potentiometer outer terminal | 4 |
|
10 |
Actuator motor engage solenoid feed |
Lt. Blue/White | Motor engage solenoid | 2 |
|
11 |
Actuator motor engage solenoid return |
Green/White | Motor engage solenoid | 6 |
|
12 |
12v supply to drivers and cruise cancel |
Grey | '0' button on Stalk | |
|
13 |
12v...electronics supply voltage |
Red | 12v +ve | |
|
14 |
Initiate/Accelerate |
Green | 'I' button on Stalk | |
 |
|
9
10 11 12 13 14 15
1 2 3 4 5 6 7 8 |
 |
|
Testing
for circuit between 13 and 14
|
 |
|
Internals of Actuator: a:
Solenoid The theory is that when the solenoid (a) is engaged, it pushes the motor drive pivot (b) to engage the belt drive. The 'b' part was loose when I checked it out to take this photograph. I also couldn't find the position sensor. |
Tests to check working functionality of CC system:
|
From
|
To
|
Test
Type
|
Outcome
|
Check
|
What
does this test?
|
|
13
|
14
|
Continous
Circuit
|
+
|
when
'I' button pressed
|
Function of CC Stalk |
|
13
|
5
|
Continous
Circuit
|
+
|
when
'R' button pressed
|
Function of CC Stalk |
|
13
|
12
|
Continous
Circuit
|
-
|
when
'0' button pressed
|
Function of CC Stalk |
|
7
|
2
|
Voltage
|
changes
with speed
|
speed
being fed to CC unit
|
Link btw speedo circuit and CC circuit |
|
10
|
11
|
Voltage
|
12-15v
|
when
actuator solenoid is engaged (at above 40KPH with 'I' button
pressed)
|
Actuator solenoid is functioning at correct voltage |
|
10
|
11
|
Continous
Circuit
|
+
|
when
actuator solenoid is engaged (at above 40KPH with 'I' button
pressed)
|
Actuator solenoid is functioning at right time |
|
3
|
4
|
Continous
Circuit
|
+
|
when
motor drive is engaged (should be cancelled if brakes engaged
or car put into neutral)
|
CC relay and motor drive engagement |
Untried but theoretical tests:
|
From
|
To
|
Test
Type
|
Outcome
|
Check
|
What
does this test?
|
|
2
|
1
|
Resistance
|
3K
Ohm
|
at
rest
|
Connection/wiring of potentiometer |
|
2
|
1
|
Resistance
|
Var
with usage
|
when
motor is engaged
|
Motor sensor function |
If these tests 'check out', then I would suggest that it is a physical problem i.e. loose cogs in the actuator or slack cables. I can't see the wiring being a cause, as bad wiring would show up in these tests.
There's a chap here with similar, but more comprehensive instructions which should be of use to you - cruise.rtf
