A crash course in the basics

OBD1 and 11 – On Board Diagnostics is the automotive industry standard that has been around since the early nineties, governing the control of electronic vehicle operations and emission controls instigated by the American Government. Regarding our Jaguars, this system has been available on the late XJ40’s on which were initially the OBD1 system which is fairly basic and only covers the simpler systems.

The introduction of the OBD11 system came with the 1996 XJ8 models and subsequently introduced later in the S-Types, diesels and X-types after 2003 with ensuing updates right up to current models. To describe many of the sensors you’ll find under the bonnet of an OBDII vehicle, and what they do. All the built-in diagnostics of OBDII really won’t help you fix anything unless you have some basic knowledge of these sensors to do some common-sense type troubleshooting. Let’s dive right in.

Oxygen Sensors (or Lambda sensor)

This provides information about the fuel mixture. The PCM (Power train Control Module) uses this feedback to constantly re-adjust and fine tune the air/fuel ratio. This keeps emissions and fuel consumption to a minimum. A bad O2 sensor will typically make an engine run rich, use more fuel and pollute. O2 sensors deteriorate with age and may be contaminated if the engine burns oil or develops a coolant leak. The normal operating life for these is between 120,000-180,000klm and should be replaced as a period service item. OBDII vehicles have several oxygen sensors. You’ll typically find one in each exhaust manifold (so on a “V” engine you’ll often find two). In the case of most Jaguars sold in this country since the late nineties, there are one or two (whether it be a six or eight cylinders and including V12’s) ‘Upstream’ O2 sensors just before the catalytic converter/s and are normally electrically heated to improve their response. There are one or two more ‘Downstream’ sensors just aft of the cats. A similar system exists for Jaguars V6 engines.

Coolant Sensor

The coolant sensor monitors engine temperature. The PCM uses this information to regulate a wide variety of ignition, fuel and emission control functions. This sensor is separate from the coolant sensor used for the temp. gauge. When the engine is cold, for example, the fuel mixture needs to be richer to improve drivability. Once the engine reaches a certain temperature, the PCM starts using the signal from the O2 sensor to vary the fuel mixture. This is called “closed loop” operation, and it is necessary to keep emissions to a minimum.

Throttle Position Sensor (TPS)

This keeps the PCM informed about throttle position. The PCM uses this input to change spark timing and the fuel mixture as engine load changes. A problem here can cause a flat spot during acceleration (like a bad accelerator pump in a carburetor) as well as other drivability complaints.

Airflow Sensor

The airflow sensor, of which there are several types, tells the PCM how much air the engine is drawing in as it runs. The PCM uses this to further vary the fuel mixture as needed. There are several types of airflow sensors, older flap-style vane airflow sensors which were fitted to early Bosch based EFI systems up to the late eighties and the hot wire mass airflow sensors which are universally used now. All are very expensive to replace.

Crankshaft Position Sensor

This one does two things: It monitors engine rpm and helps the computer determine relative position of the crankshaft so the PCM can control spark timing and fuel delivery in the proper sequence. The PCM also uses the crank sensor’s input to regulate idle speed, which it does by sending a signal to an idle speed control motor or idle air bypass motor. On some engines, an additional camshaft position sensor is used to provide additional input to the PCM about valve timing. This is pertinent to 4.0 litre AJ26/27 V8’s that use the VVT (Variable Valve Timing) units on the inlet camshafts operated through an electro-hydraulic solenoid.

Manifold Absolute Pressure (MAP) Sensor

The MAP measures intake vacuum, which the PCM also uses to determine engine load. The MAP sensor’s input affects ignition timing primarily, but also fuel delivery. Knock Sensors are used to detect vibrations produced by detonation. When the PCM receives a signal from the knock sensor, it momentarily retards timing while the engine is under load to protect the engine against spark knock.

EGR Position Sensor

This one tells the PCM when the exhaust gas recirculation (EGR) valve opens (and how much). This allows the PCM to detect problems with the EGR system that would increase pollution. Thankfully, this troublesome system was not fitted to Australian market cars.

Vehicle Speed Sensor (VSS)

The VSS keeps the PCM informed about how fast the vehicle is traveling. This is needed to control other functions such as torque converter lockup. The VSS signal is also used by other control modules, including the antilock brake system (ABS) and Traction Control (TRAC) systems.

How’s that for a crash course in OBDII sensors? While there can be a few more sensors under the bonnet, these are the ones that really effect how well your car runs. Keep this info in mind the next time your car stumbles, hiccups or sets a trouble code. A little bit of information goes a long way towards knowing what to fix.

Happy Repairs, Mike Fahrion www.autotap.com

Copyright 2006 B&B Electronics