Digiswing/Ecomatic Control Unit J327
Digiswing is the Ecomatic Control Unit and is an Engine and Clutch Management System. By definition it is a Powertrain Control Module (PCM) which controls the clutch operation and engine switch-off and start-up routine based on operating status and sensor input. It is located under the raintray, where the ECU normally is on other Mk3 models.
Battery voltage measurement - disables Ecomatic operation if battery voltage is too low, green light (engine running control lamp K111) illuminates.
Fault recognition - recognises faults based on sensor input both by plausibility and electrically, disables Ecomatic and/or engine switch-off operation, all 3 warning lamps flash for 10 seconds, green and/or orange light then stays lit depending on fault.
Fault tolerance -Various emergency running programmes and substitution of sensor data based on predetermined values and from other sensors in case of sensor/mechanical failure.
Self-diagnostics - To access: Clutch electronics (12).
Warning lamp suppression - suppresses warning lamps K3 (oil pressure) and K2 (alternator) during engine off phase.
Gear change notification - illuminates orange light (gear display control lamp K48) [k48] when an upshift should take place, flashes orange light when a downshift should take place. This is a non-linear calculation based upon current gear selected and load signal potentiometer position.
Self-learning capability - Adapts to change in components through age/adjustment.
Clutch operation strategy
The clutch operation strategy and how it is accomplished is somewhat interesting because of the semi-automatic gearbox used in the design. Because the accelerator pedal and gearstick is operated by the driver, the clutch system has to adapt quickly and appropriately according to what the driver is doing. Various sensors are employed in the design so that the control unit has information about the current driving situation. This is accomplished very well for a car of it's time and is dynamic in it's principle of operation. For example, slamming the accelerator pedal down will cause the clutch to close quicker, to avoid excessive clutch wear, whereas light throttle will allow clutch slippage, just like a driver would do in a conventional manual gearbox. It is likely that it contains a default mapping of clutch operation and that other sensors contain a weighting to adjust the operation from the default values.
Engine start procedure
Aside from the obvious logic used to restart the engine, the control unit uses several routines/algorithms to ensure a reliable and smooth starting procedure which may go unnoticed by the average driver.
- If an engine start is called* immediately after the engine has just entered the overrun/stopped phase, two things may happen depending on the operational conditions.
- If an engine startup call occurs, and the engine has only just entered the stopping phase (RPM dropping towards 0 from idle) (i.e., within less than a second of the ECU deciding to stop the engine) the ECU de-energises the fuel cutoff solenoid allowing fuel into the engine initially to prevent the engine from completely stopping, in some instances, the engine will return to idle without the use of the starter motor. This has the effect of immediately bringing the engine back to life.
- If the engine speed is too low to start from the procedure above, an extremely small delay occurs (less than a 1s) before energizing the starter relay, this would prevent, for example, the starter motor trying to engage into the flywheel when it is still spinning.
- When output to the starter relay (J53) is activated to restart the engine, the engine speed is monitored, if the engine remains stopped after approx. 3-4 seconds (for example, the starter motor has not engaged properly into the flywheel), the power to the relay is interrupted for approximately 1.5 seconds and then another attempt is made to start the engine. This process appears to continue indefinitely until the ECU receives a RPM signal greater than the predefined** amount indicating the engine has successfully started.
I have found that the accelerator pedal position threshold to restart the engine is higher (in percentage terms) in 2nd gear and above compared to 1st gear. In other words, you need to depress the accelerator pedal down less in 1st gear for the engine to restart, than you do in other gears. In certain circumstances (if the load signal potentiometer is out of adjustment, or basic setting is required) it is actually possible that the engine will switch off correctly in gears 2nd-5th, however, the engine will not switch off at all in 1st gear, and if 1st gear is selected, the engine will restart. Presumably, this is intended so that in higher gears, if a driver is "dithering", the engine will not cut out, or in, inappropriately. In addition, this is because the engine speed sender information can be used to determine what is going on while driving along.
The Ecomatic should not behave like an automatic, if the vehicle creeps along in 1st gear, something is out of adjustment!
1H0 927 303 / 1H0927303 - Original ECU fitted to early Ecomatics.
1H0 927 303 A / 1H0927303A (this modified control unit now moves the clutch lever up or down slightly (approx. 3mm) every 10 seconds when the engine is switched off and ignition is on, this changes the diagnostic procedure required to find out whether there is a leak in the system, or if the clutch final control element is faulty. The noise emitted by the clutch control element can sound somewhat "scratchy" and is not a cause for concern. It is not known what other changes were made. It is possible that the clutch control algorithms were changed slightly to allow trailer towing as early instruction manuals did not permit trailer towing, however later did. One could also speculate that other improvements to the engine and clutch operation were made from driver feedback once the car was "out in the field".)
1H0 927 303 B / 1H0927303B (unknown what revisions were made, this part was *never* available on parts catalogues. If it was, it was most likely for a very short period around 1995.)
1H0 927 303 E / 1H0927303E (discontinued 2010 - no replacement available).
It is not known what (if any) changes were made to B and E revisions, nor what happened to C and D.
Effects of failure
Vehicle is not operational.
- Engine start call describes situation where engine has gone into the overrun phase and is called to restart, such as the accelerator pedal being depressed, engine-brake button pressed, gear stick movement, reverse selected, vacuum pressure too low.
- The actual RPM threshold used to indicate that the engine has started internally to the control unit and thus to de-energise the starter relay is not known, but would suspect it is in the region of 300-400RPM, accompanied by a very brief static timing element to ensure the engine has started properly.