The On Board Diagnostic System

OBDII in Fine Detail

You plug in the OBD lead, connect to the PCM with the Vehicle Explorer Software and check out the CODES page and you find a DTC there waiting for you. Now what do you do?

The DTC (Diagnostic Trouble Code) is a number, and while there are many web sites which detail the text for the numbers, not one I could find details how to troubleshoot a DTC! Since Vehicle Explorer gives the Text of the message as well as the number this is not the problem - the cause of the trouble code is the difficult bit!

I hope that the following pages will be enough to detail the trouble code, how and why it is generated, the probable causes for the message, details of the sensors concerned, if any, and the repair. But first, some theory - how does the OBD actually work? This is the Inside Story of the OBD2, as utilised by Ford in the EEC-V Powertrain Control Module.

OBD2
The On Board Diagnostics version II, almost entirely adopted by the EU as European On Board Diagnostics (EOBD), is a development of the OBD-I used in the EEC-IV. It uses similar components, adds new ones and uses more complex software. Approximately 40% of the software on the EEC-V PCM is dedicated to OBD2. 

OBD2 requires continuous monitoring of the emission-related components for correct operation, the storage of a Trouble Code in the event that a malfunction is detected and a freeze-frame of data from a set list of components to aid diagnosis. It also illuminates a MIL light - but this was not incorporated into the Ford Scorpio because the EOBD was not enacted during the model production.

DRIVE CYCLE (see also here)
A Drive Cycle is any journey in which the engine temperature is raised from cold (below 49° C to normal operating temperature (above 71° C).

An OBD TRIP is a journey during which all OBD tests have been completed.

P1000 - Monitors Incomplete
After clearing DTCs, or a battery disconnect, for a few journeys the DTC P1000 is often found. This is a notification that not all of the Monitors have been completed since they were cleared and is not a fault condition. Once all of the OBD Monitors have been completed the DTC P1000 is cleared from memory.

THE OBD EXECUTIVE
The EEC-V powertrain control module is a computer with a 32-bit architecture that can run up to eleven test programs (Monitors) that can be called upon separately or together, running under the direction of the OBD EXECUTIVE. Some Monitors can run concurrently with others, some need pausing while others run, while others run all the time - and the Executive manages all these.

When each Monitor is run, the Executive stores the result of the test. In most cases the Executive will not light the MIL or store a DTC unless a Monitor fails a test twice in successive OBD trips.

Hardware devices connected to the PCM have their own device drivers, called Smart Drivers. These not only control the device by turning them on/off or controlling their duty cycle (a term for a variable device that opens and closes slowly by controlled degrees) but also tests the voltage and where applicable, the current drawn by the device. In this way connected hardware can be checked by the PCM without the use of secondary testing devices.

The following points are really worth remembering because they have implications for trouble shooting.

 The Executive

1. Arranges the tests so that when a test runs, every input it relies on has already been tested.
2. Controls and coordinates the various tests (Monitors). For the Ford Scorpio these are MISFIRE, Comprehensive Component Monitor (CCM), FUEL, CATALYST,  EGR, HO2S,  EVAP
3. Stores Freeze Frame data
4. Manages storage and erasure of Diagnostic Trouble Codes and MIL illumination (where fitted.)
5. Controls and manages the On-Demand tests and Output Test Mode
6. Manages the transition from one test to another so as to minimise the effect on the vehicle operation.
7. Stores the results of the Monitors. Clearing DTCs also clears the Monitor Results which results in P1000
8. Interfaces between the various Monitor modes so as to provide diagnostic information and responds to special diagnostic requests.
9. Will store a Trouble Code regardless of linked causes. For this reason a single problem with a sensor may produce three, four or even five different DTCs.
10. Cannot run a test it will not store a Code for a failed component. If the reason for failure to run the test is put right, then the Executive will run the test during the next Drive Cycle and a new DTC will result. In this way until it is put right a failed device may mask another problem.
11. If the Conditions for a Monitor to run are not met in an OBD Trip then the test is not run and a problem will not be identified until the next OBD Trip. This explains why a fault condition may show itself several days after it was introduced.

There are two conditions for the OBD Executive outside of which it will not run:

CONTINUOUS MONITORS

Three of the procedures run continuously while the vehicle is being operated. There are:

So these monitors are running all the time that the engine is running. The CCM is constantly checking the components and making sure that they are functioning well enough to provide test data for the other Monitors. The Misfire Monitor is constantly protecting the Catalysts from damage caused by unburnt fuel, while the Fuel Monitor is controlling the fuel mixture, switching it slightly lean and then slightly rich of Lamda (Closed Loop) and commences this 90 seconds after starting from cold.

NOTE: Although the FUEL Monitor is running all the time this does not mean that the fuelling is in closed loop all of the time.

NON-CONTINUOUS MONITORS

The remaining Monitors are run once in an OBD Cycle (a 'Trip'). The information on each monitor shows how long the test is run and what circumstances or criteria the test requires.

THE IMPERATIVE

It is also worth remembering that the entire purpose of the OBD system is the control and reduction of pollutants: the two gases Carbon Monoxide (CO) and Nitrous Oxide (NOx), and unburnt Hydrocarbons (HC). The absolute imperative for the OBD is to control these substances within fine limits and to protect the catalyst(s) from damage. In order to achieve this if necessary it will sacrifice engine performance.

CONTROL LOOMS
Another point to remember is that the OBD and the Comprehensive Components Monitor are only as good as the hardware. If the control looms are faulty then spurious and confusing DTCs can be generated.  When this issue was first raised some owners were put to great expense before the cause was identified. Control Looms in the complete engine bay were Arnitel wired until mid 1996 and some faulty insulation has been identified. Cracking on the thin-wall insulation on a few vehicles with Arnitel has made the Looms very susceptible to damp, which both corrodes the copper conductor through and causes shorts, and causes ghost signals which the PCM can interpret as out of range errors. The cause of the cracking was never found. In mid 1996 wiring was changed to Raychem44 insulation.

Control Loom failure can be suspected in the following cases, when:

If this is experienced by the owner, he should inspect the engine control looms carefully for signs of shorting. Strip back the covering tape and inspect the thin-wall insulation for cracking, verdigris (copper corrosion) and shorting in the following places:

However unlucky for the individual owner, this has proved to be rare, with 7 confirmed cases out of perhaps 800 owners on the List/Message Board.

In addition, experience has shown that the Battery Cover is not a cosmetic item, but is there to protect the Auxiliary fuse box and relays from water ingress. If they become damp they can cause relays to energise at inappropriate times and permit water access to the Loom beneath with serious results.

The Menus on the left detail the Monitors, how they work and their parameters so that the private owner can better understand the reason for the DTC's and work towards a solution to a code that he finds.

This is not the end, but a work in progress. As more information comes to light these pages will be updated.

EricR