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ON BOARD DIAGNOSTICS BASICS.    

On Board Diagnostics and our European On Board Diagnostics (EOBD OR OBDII) originated in the USA as this quote from their Environmental protection Agency shows.

"On-Board Diagnostics, or "OBD," is a computer-based system built into all 1996 and later light-duty vehicles and trucks, as required by the Clean Air Act Amendments of 1990. OBD systems are designed to monitor the performance of some of an engine's major components including those responsible for controlling emissions"

OBD is accessed via a 16pin connector - the DLC (Data Link Connector) aka diagnostic socket.

This has to be sited close to the drivers footwell, so sometimes it is behind a lift up flap low down on the dash, under the bottom of the dash trim below the steering wheel or in the case of the 45, it is at the front of the centre console on the drivers side near the clutch pedal. On Freelanders it is in the passenger footwell near the heater outlet.

All petrol vehicles sold in the USA from 1996 had to have this socket and fully implemented OBD. This means those never destined for the USA may be, may partially be, or may not be compliant at all. In Europe petrol vehicles from 2001 and diesel vehicles from 2004 had to be compliant.

Please note, pin 16 is permanently at battery volts and only a few of the positions will be fitted with a contact.

Faults which affect the emissions must result in a fault code - a DTC (diagnostic trouble code) being logged by the ecu, which can then be read out to determine the problem and possible cause. Other faults may result in a code being logged as well. 

Serious faults also turn on the check engine light/engine warning light, more correctly known as the MIL (Malfunction Indicator Light) to indicate that you should take the vehicle to a dealer for attention and may cause the engine to stop. Sometimes the MIL  will remain on even if the fault has been fixed but usually it will either switch off next time the ignition is turned on or after several engine starts (upto 50 in some cases)

Very serious faults affecting safety can also cause a big reduction in power - so called limp home mode. For example a faulty throttle position sensor is guaranteed to invoke limp home mode and in the case of Rovers & MGs this can mean the rpm being held at 1300rpm with no response at all to the accelerator. The car is driveable and you can go up through the gears because the ecu will increase the rpm back to 1300 (about 35mph in 5th) after changing gear. My 45 even climbed 20% inclines in first gear with no problem.

Some faults of a transient nature do not cause a fault code to be logged but instead the ecu adopts a limited operating strategy (LOS) mode in which power is reduced by 30-40%, maximum rpm may be reduced and the heater plug light flashes. Switching the ignition off will reset the condition. Ford diesels can experience this if one or more of the following occur: excessive boost pressure, cylinder head temp too high, fuel pressure not reaching expected value etc. Its a protection strategy against short term, occasional, & mild out of range events.

HOW IT WORKS:

The code reader and vehicle communicate by using different protocols depending upon the manufacturer, but the commands & responses are the same. Usually the K line is used and this operates at a slow 10.4Kbaud for the ISO9141-2 protocol which is the most common. Ford use  PWM and others use KWP.

The process starts by the code reader establishing communication and then it sends a Mode 01 command. Multi protocol readers will try the different protocols in turn until they get a response. The reply received tells the tool which parameters are supported. Then the following modes may be available depending upon the compliance of the vehicle to the OBDII standard. (See SAE J2190 for a more complete list)

01    SHOW CURRENT DATA including OBD compatibility

02    SHOW FREEZE FRAME DATA  - sensor data at the time a DTC was logged.

03    SHOW STORED FAULT CODES

04    TURN OFF THE MIL & CLEAR FAULT CODES AND STORED VALUES - also may reset learned values.

05     SHOW TEST RESULTS - OXYGEN SENSORS - CONTINUOUSLY MONITORED

06    SHOW TEST RESULTS - NON CONTINUOUSLY MONITORED 

07    SHOW PENDING TROUBLE CODES - faults that have been detected but have not recurred enough times to be a 'current' DTC.

08    SPECIAL CONTROL MODE - often used to test components like idle steppers, egr valve, a/c clutch, cooling fan, glow plug relay etc. 

09    REQUEST VEHICLE INFO - eg VIN etc

For none OBD compliant vehicles such as pre 2004 diesels only mode 01 is recognised and not many generic tools support all 9 modes even on compliant vehicles.

There are many more modes but these require a manufacturer specific tool. For example mode 14 is actually used on pre 2004 diesels to access fault codes, mode 30 is used to download data blocks and mode 32 to upload - eg. remapping. More on this below.

One thing to be aware of:

If you try to read fault codes on a pre 2004 diesel it may tell you there are none. However there well may be, its just that the tool cannot read them. Similarly if the MIL is on and you query this, the tool will tell you its off! Only if your vehicle is reported as being OBD COMPLIANT, should you believe exactly what you see! For example, when the throttle position sensor failed on my previous 45 TDi, the software said there were no DTCs, that the MIL was off  when it was on and there were in fact 4 DTCs logged!

WHAT'S DIFFERENT ABOUT MANUFACTURER SPECIFIC EQUIPMENT?

DIY & generic tools don't support modes higher than 9, but pro ones can send a mode 27 command. This a request by the tool for security access. The ecu then sends a seed to the tool. Using an algorithm contained within this and the ecu, both devices calculate the key from this seed. If the keys match, the ecu allows access. Clearly, the tool must have the same algorithm as the ecu or the keys won't match and this is not in the public domain. So companies like Sykes Pickavant, Rovacom, Snap On, and remapping product manufacturers will have either purchased it from the manufacturer or reverse engineered it by monitoring the data stream and collecting seed/key pairs in the hope of deciphering the algorithm that generates one from the other.

Once access has been granted, the tool can use the other modes as required. For example: by using a Scantool interface in monitoring mode, I could see that my ACR4 was using mode 21 to request sensor data, mode 30 to test various actuators such as air con clutch, and mode 14 to clear fault codes. Modes 34, 35,36, 3B, 3C are data transfer modes used when reading and writing to the ecu (eg when remapping). 

Bedtime reading: SAE documents: J1979 PRN ASSIGNMENTS. J2190 DIAGNOSTIC TEST MODE ASSIGNMENTS, 

J1850, J2178 PARTS ,1,2,3,4 

You need an ISO interface for a Rover and some software. (There are different versions as different manufacturers use different protocols.)

The most popular DIY interface is a Scantool ISO Interface available from Gendan.co.uk. This is supplied with all necessary cables & connects to the serial (com) port on a laptop or PC and to the car's 16pin diagnostic socket. Later ones can connect via the USB port.

BUT NOTE, a mod may be required since MGR missed a pin out of the connector! SEE HERE 

This interface uses a single chip microprocessor from ELM Electronics and there are several programs which are compatible with this chip. See below for a list. The ELM323 versions don't support the Can Bus, but the later ELM327 ones do.

There are others interfaces however.

A dumb type is available from SK Pang which, for non VAG vehicles, only works with Vehicle Explorer version VE116CT software from OBD-2.com - this not freeware and costs about $60.

But it will also work with many VAG vehicles using the freeware version of VAGCOM from Rosstech.

Recently, the July/August issue of Elektor featured a kit of parts for £54 that uses the Ozen Electronics chip and works with a special version of Scanmaster. These interfaces go by the name of MyOBDic.

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There are several programs that will display the data & some that will log it for later examination. All the software that works with Scantool (or any other ELM based interface) has the same capabilities - its mainly the user interface and display format which differs.

Here are some screen shots:

Various shots of Scantool, Digimoto & Scanmaster. 

Here are 3 screen shots thanks to Craig L, from his 2003 ZS180 using my Scantool Iso and Scanmaster.

OBDlogger reads the data in sequence, so the more parameters you log, the slower it is. Usually I just log 3 at a time when I'm doing performance testing. Digimoto also has a logging feature. 

Logging the data in .csv format means you can import the data straight into Excel as I did for this test and then plot it.

Dashboard screen shot from Scanmaster - you can choose what to display. Its early days, but there's definite potential for experimentation with a 'carputer' here.

I prefer Scanmaster and Wobd.

SCANTOOL COMPATIBLE SOFTWARE LIST.