Introduction | 3B Description | ABY Changes | Schematics | Boost Control | Diagnostics

Bosch Motronic Info

Highlights

• The ABY uses a completely distributorless ignition system. A pair of ignition packs are used to generate ignition trigger voltages which in turn generate a high tension spark by means of tiny coil packs which are located just above each spark plug. This is the main reason that the rocker covers on the 3B and ABY engines look so different. This distributorless system was first introduced on the AAN engine (UrS4) and is also used on the ADU engine (RS2). Similar technology has followed in more recent Audi vehicles from entry level 4-cylinder A3 models to top of the range S8 vehicles and beyond.

• The lack of a distributor on the ABY engine dictates that the hall effect sensor (G40) resides elsewhere on the ABY engine. The location of G40 on the ABY is at the timing end of the camshaft where it provides the same function as on the 3B engine i.e Allowing the ECU to determine the position of the camshaft to establish a firing point for cylinder-1. For this reason, the workshop manuals on the AAN engine give G40 the name of camshaft position sensor (CMP). As on the 3B engine, a faulty (or disconnected) G40 device will prevent the engine from starting - it will crank forever but the ECU will not generate any sparks.

• The ABY engine uses slightly higher fuel pressure and higher flowing fuel injectors than the 3B. The ABY runs at 4.0 bar above manifold pressure whereas the 3B runs at 3.0bar above manifold pressure. Reduce those figures for ~0.5bar for the guage pressure you should see idling due to vacuum in the inlet manifold.

• The ABY injectors utilise a 4-hole pattern for improved vapourisation in comparison to the 1-hole injectors on the 3B. Even higher rated injectors are used on the RS2 (ADU) engine in conjunction with a slightly higher rated fuel pump. As far as I know, the 3B, ABY and AAN engines all use the same fuel pump.

• The pinout of the ECUs on the 3B and ABY engines is COMPLETELY different. They cannot be interchanged without major surgery on account of the different ignition systems on the two engines. The internal software workings of the ABY (AAN and ADU) ECU are somewhat advanced in comparison to the 3B as they provide much more diagnostic info on the engine's operation. The 3B ECU only provides visibility of basic settings via the OBD port, but the ABY has much more intelligence to assist diagnostics. More details in the VAG-COM section.

• The ECU of the ABY engine has one other fundamental difference compared to the 3B in that it has a special 'overboost' mode which allows boost pressure up to a maximum of 1.15bar in short bursts to allow increased torque to be generated when needed e.g overtaking. Outside of the 'overboost' mode the ABY generates the same 0.83bar of boost as the 3B in normal circumstances. The overboost mode is limited by the factory ECU to bursts of no more than fifteen seconds.

• Another intelligent feature of the ABY ECU is that in the event of failure of the mass air flow meter (MAF) it uses the throttle potentiometer as a substitite input for determining air flow into the engine.
  

• One significant (though hidden) mechanical difference between the ABY (AAN / ADU) is the use of a different dual mass flywheel from the 3B engine. On account of this the 3B and ABY engines use different clutch components, but I have no details on the reasons why the 3B flywheel was changed for the ABY, AAN and ADU engines. The latter flywheel is approx three times the price of a 3B item, so the reasons must have been compelling for someone in Ingolstadt to make such a change.

• Other mechanical differences on the ABY include a revised oil pump pickup and increased baffling in the sump. The ABY also has a slightly larger intake manifold than the 3B engine. Differences also apply to the crankcase ventilation system. Finally the ABY engine has a slightly different mechanism for automatic tensioning of the timing belt.