
E I ect r ica I 
15.1 SUPPLEMENTAL RESTRAINT  SYSTEM 
15.1.1 System Description 
The Supplemental  Restraint System (SRS) installation  comprises: 
0 electronic Diagnostic  Module (DM) 
0 driver and passenger-side air  bag  modules (including  firing mechanisms) 
0 two front impact  sensors (left and  right) 
0 one safing  sensor 
0 dedicated wiring harness 
0 two cable reel cassettes  (integral part of the  steering  column harness) 
0 Malfunction  Indicator Lamp (MIL) and driver information  message 
The  system  is designed  to provide  protection  for both  driver  and front  seat passenger  by automatically  deploying air 
bags  in the  event  of a collision  during forward  travel. The driver
-side air bag  is located  in the  centre  of the  steering 
wheel  assembly  and the passenger-side  air bag  in the  fascia panel. 
In  the  event  of a collision  the impact 
/ safing sensors  operate,  completing the  electrical firing circuit  and causing  the 
air  bags  to inflate  within 32 milliseconds.  At least two of the  three  sensors (at least  one impact, and  the safing  sensor) 
.must  be activated  to initiate firing.  System operation is  dependent upon battery voltage supplied  directly, and via the 
ignition  switch, to the DM  and the  correct installation and operation  of all system  components,  including the wiring 
harness.  Faults  in system components,  installation or wiring will be indicated  by the MIL  Lamp, located  on the instru
- ment  panel,  which will illuminate 'SRS AIR  BAG'.  Indication  is also  given  by the  driver information message'AIR BAG' 
displayed  on the  LCD  panel  below the  speedometer. 
15.1.2.1  Diagnostic Module (Fig. I) 
The Diagnostic  Module (DM), mounted below  the passenger-side  air bag  module,  behind the console  fascia panel,  is 
the  electronic  microprocessor  unit which monitors  the whole  SRS system.  The state of  the three  system  sensors,  two 
air bag modules and  the wiring  harness  is monitored  constantly to detect  activation criteria  and component  faults. 
15.1.2 COMPONENT  DESCRIPTIONS 
The unit also  confirms  correct supply conditions  by compar- ing  a direct battery voltage  input with an input,  via  the igni- tion switch,  of the  same  voltage  value.  Detection  of system 
faults  will be relayed  by the  DM  to the instrument  panel and 
the  air bag  MIL Lamp illuminated. 
The  DM contains a  reserve power supply  unit, enabling the 
air  bagsto  fire even  if supply voltage  is lost during  an impact 
situation.  The reserve  power charge  will be retained  for ap
- proximately  one minute  after the positive  supply voltage  is 
disconnected.  An auxiliary  internal  circuit, known as the 
'dwell  enhancer',  provides a temporary  ground to compen
- sate  for damaged  primary crash sensors. If either primary 
crash  sensor  operates  for 5  milliseconds  the 'dwell  en- hancer'  circuit will turn  on, completing the  firing circuit  to 
ground  for 90 milliseconds.  The purpose  of this  is to  allow 
air  bag deployment  even if the  operated  primary crash sen- sor circuit  is defective  or opens. 
15.1.2.2 Wiring  Harness 
Fig. 1 
A dedicated  wiring harness,  covered with yellow  sheathing,  independent  of any  other  vehicle system,  is used  to electri- cally  connect all the component items.  To allow movement  of the  steering mechanism,  two cable reel cassettes are 
incorporated  into the driver-side  air bag  module  circuitry,  as a means  of compensating  for steering wheel  rotation is 
required  to prevent  harness damage  or disconnection.  The  two cable  reel cassettes  form an interface  between the 
steering  column and air bag  module  and, due to their  coiled  construction,  are able  to contract  or expand  as required. 
15.1.2.3 Air Bag  Modules 
The  two  air  bag  modules,  driver and  passenger  side, are 
each activated  when either  front impact  sensor (Fig. 2) and 
the  safing  sensor  operate  simultaneously.  Both modules 
contain  a charge  of sodium 
azide/copper oxide  which,  when 
ignited  by an  electrical  impulse,  generate  a volume  of  ni
- trogen gassufficientto  inflatetheair bag. Theamountof  gas 
generated  is greater  in the  passenger-side  air bag due  to its 
larger  size. Both modules,  including surrounding  trim pan- els, are  non-serviceable  and once  activated  must be re- 
newed  as a complete  assembly. 
Fig. 2 
X300 EDM 15.1 - 1 Issue 1 August 1994  

E I ect r ical IT 
SPEED CONTROL 
System  Description 
0 15.3 15.3.1 
The speed  control  system  provides  an automatic  cruise facility,  allowing a constant speed  to be  maintained without 
accelerator  pedal operation.  The system comprises  the following  components: 
0 Electronic  control unit 
0 Vacuum dump valve 
0 Control valve 
0 Vacuum  pump 
0 Mechanical  throttle linkage and bellows 
0 Selection  switch 
0 Brake and clutch  pedal switches 
On  selection of  speed control,  via the  switches  mounted on the  driver's side fascia,  the vacuum  dump valve,  pump and 
control  valve will energize,  creating 
a partial vacuum  in the  bellows  assembly.  The bellows  assembly  acts upon the 
throttle  linkage to adjust  throttle  position, maintaining  vehicle speed at the value when  control is selected. 
Speed  control is determined by the  Speed Control Control Module 
(SC CM) which monitors a speed signal delivered 
from  the instrument  pack (initially  from the ABS  rear LH wheel  speed sensor)  to determine  vehicle speed. 
Speedcontrol isdisabled, returning control  tothedriver, 
byoperationofthevacuumdumpvalve,ifanyofthefollowing situations occur: 0 
0 Braking 
0 Clutch operation  (3.2 / 4.0 litre  manual  transmission  vehicles only) 
o Drive de-select ie P, N or R (automatic transmissions  only, both electronic and  hydraulic) 
0 Traction control system  (where fitted)  in operation 
0 Rapid speed  reduction without  braking input, ie double brake switch failure 
0 Actual  speed drop to 3/4  of cruise  setting (eg when  towing  excessive  loads under  adverse conditions) 
0 Vehicle speeds  below w 25 mph 
0 Ignition OFF 
0 De-selection  at switch controls 
15.3.2  COMPONENT DESCRlPTlO NS 
15.3.2.1  Electronic Control Module 
The Speed  Control Control Module (SC CM), located  behind the  fascia, adjacent  to the  steering column,  provides sys- tem control  and an interface  to external  input signals  from the instrument  pack, gear selector module,  traction control 
system,  control switches,  and foot  pedal  switches.  The set speed  value is retained  in SC CM memory  and continuously 
compared  to actual vehicle  speed. Adjustments  are made,  as necessary,  to maintain  set speed  by control of the  vac- uum pump  and control  valve.  Connections  to the SC CM are  listed  in the table on  page 3. 
X300 EDM 15.3 - 1 Issue 1 August 1994