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MAC01 WR ECU General Description
(Much of this information also applies to the MAC02 used in the North American,
Swedish and Swiss WX cars, although the emission control logic is not dealt with.)

The official factory fault-finding table for the WR engine will be found in the Workshop Manual.
For the moment, most of the electrical tests are better documented here. Links have been added to each test pointing back to the Workshop Manual pages - for the most part, these links are not yet functional.

The electronic ignition system fitted to the early Audi ur-quattro is known as a "Hartig" system after its inventor because it uses a tooth-sensing sender (U.S. Patent 4,121,112 - October 1978). The MAC01 Electronic Control Unit (ECU) uses six inputs:

There are three outputs:

The MAC01's only way of influencing engine performance is by varying the engine's timing. There is no provision for ECU control of the cold start valve, mixture, boost level, airflow or deceleration cut-off. The MAC01 controls idle stabilization between 600 RPM and 940 RPM using advance/retard only. This means conventional timing analyzers, strobe lights, etc., cannot be used.

Because there is no physical compensation in the distributor - no vacuum-controlled platform or centrifugal weights - it is imperative that the rotor arm has a wide enough tip to accommodate spark transfer over a significant advance/retard angle. The stock rotor arm is specific to the WR application - using a generic part with a narrower tip can cause performance problems at high boost and RPM levels. The rotor arm has a tendency to crack near the tip and should have a resistance of 1k Ohm

Ignition coil primary current is regulated to 7.4 amperes. With the ignition on and the engine not rotating, this current is interrupted after 1 second to prevent the coil overheating. Only original Audi ignition coils should be used - the primary resistance and the 5.5 mm safety valve opening are unique to the WR/WX ur-quattro. Nominal coil resistances (with all plugs disconnected):

Fundamentals of Operation

There are more exhaustive texts elsewhere, but the basics of starting and operating are as follows:

Audi I5 Turbo Ignition Timing Diagram

MAC01 WR ECU Diagnostics and Tests

Before all tests - especially in no-start conditions - the mechanical timing of the engine should be checked:

The harmonic damper (crankshaft pulley) has no useful marks. The scribed line on the flywheel is for use on other engines.

The WR's ECU is mounted in the plastic trim underneath the right hand side of the dashboard. On LHD cars, it is necessary to remove the glove box. On RHD cars, the trim underneath the steering wheel is secured by three screws at the front going up into the dashboard and a single screw at the very back of the driver's stowage compartment - a torch or flashlight is essential.

The most common ECU problem so far met is inlet boost pressure sensor failure caused by fuel contamination; this causes poor starting, uneven running and erratic behaviour during acceleration - although even this is rare. The primary cause seems to be the last section of rubber hose before the ECU - after the fuel trap - going porous. It is worthwhile replacing this hose during any test procedure. There is an official test procedure for the pressure sensor - a VAG 1367 is attached to measure the timing point - it should advance from around 6° (before VIN 85-B-901 297) or 12° (85-B-901 297 or after) to over 20° if the hose is pulled off the ECU with the engine idling. Electronic failure is extremely rare - only one case of a cracked PCB board (already the subject of a repair at some time in the past) has so far been seen.

As above, it is important to distinguish between 'no spark' and 'maverick spark'. For this purpose, either a Gunson spark cell can be inserted in #1 plug lead or #1 plug can be removed and tie-wrapped to the lifting eye above the hydraulic pump. If no spark at all is ever seen, classic electrical failure in the ignition circuit may be suspected. If an occasional maverick spark is seen, the ECU is receiving either no or incorrect information from one of its sensors. If all of the documented tests are passed, it may be necessary to try the ECU in a known good car. This is preferable to putting a known good ECU in a car that may have destroyed one already. There are no specific ECU-related output diagnostic tests.

If start testing is prolonged, be aware that the cold start injector will spray a small amount of fuel into the engine at each start attempt. This can result, over time, in a significant amount of fuel contaminating the oil requiring an oil change during testing. Monitor the oil level throughout these tests.

MAC01 WR ECU Pin-out and Cable Colours

With the ECU removed from the car and the connector held with the engaging hook on the left and the locking tang on the right, the connector pins are numbered 1 - 13 from right to left on the bottom row, and 14 - 25 from right to left on the upper row. In-car measurements with the ECU in place require the cover to be removed from the connector and pushed back up the harness - the pin can then be identified easily via its wire colour as per the table above.

2Brown/whiteIgnition output stage (1980-2)
Dashboard boost gauge (1983-7)
3Brown/blueIdle switch28-35
4Brown/redHall sender ground return28-41
7PurpleRPM to oil pressure controller & dashboard
11YellowRPM Sensor signal28-20
12YellowTDC Sensor signal28-16
13BlackTDC Sensor signal28-16
14BlackTerminal 15 Ignition +12V
15Brown/yellowIdle Switch28-35
16Green/purpleFuel Pump Relay control
17Green/whiteIgnition output stage28-51
18RedIntake air temp sensor28-25
19RedIntake air temp sensor28-25
20Red/blackHall Sender + 12V28-41
22GreenHall Sender timing signal28-41
23 RPM sensor shield ground28-20
24BlackRPM Sensor signal28-20
25 TDC Sensor Shield ground28-16

Spare pins for the connector are available from Audi as part number 000 979 225A - this is actually a wire with a new pin connected to each end as few garages possess the correct crimper. The repair is thus a butt splice to one of these wires cut in half. Old pins can be released with a dressmaker's pin pushed down outside each flat of the pin from the face of the connector to release the tangs.

MAC01 WR ECU Diagnostic Tests

28-16 TDC (G4) Sender Test

G4 is located half-way down the engine/bell housing union on the left, below the brake accumulator. The connector is identical to that for G28 - after performing the following resistance test, trace the wire back from G4, detach the connector and repeat the test to check for open circuit.

28-20 RPM (G28) Sender Test

28-25 Inlet Air Temperature Sender Test

The following is valid up to 85-B-901140. From 85-B-901141 onwards the connections on the inlet air temperature sender are soldered. Up to 85-B-901344 the connector was retained, but moved to the wheel housing. Later vehicles have no connectors, but the tests are the same.

The connections to the sensor itself are soldered and are notoriously unreliable. It is worth replacing the soldered connections with crimped ones:

28-35 Idle Switch Test

Consolidates changes from Technical Bulletin D 000.5147.01.00 (TSB)


Note that there is no specific full load switch test defined because it is not connected to the ECU in the WR ur-quattro. With the ignition on, +12V should appear at the black/blue wire on the throttle body switch. The black/green wire should have continuity to the solenoid valve on the bulkhead, and the other wire from that connector should go to chassis ground. Opening the throttle by 50° to 55° should open the full load switch and cause the solenoid valve mounted on the bulkhead to de-energise with an audible click. This operation is best verified by attaching a MityVac and applying vacuum before operating the switch - see the discussion of the WR's full load enrichment in the mechanical pages. The thermopneumatic valve (left rear edge of the head) plays an important role in full load enrichment - it should be open up to 58°C +/-3°C and closed at higher temperatures.

28-41 Hall (G40) Sender Test

28-51 Ignition Output Stage (TSZ) Test

MAC01 WR ECU "Chipping"

The easiest way to increase WR performance is to hot-wire (or "jumper") the fuel pump so that it always runs with the ignition on, and then replace the spring in the wastegate with a stiffer one.

This is DANGEROUS for two reasons:

The MAC01 ECU is sometimes modified by "chip tuners" to permit higher boost levels than stock while retaining the "dead engine" cutout feature. The most common modification is via a resistor/Zener diode bridge on the manifold pressure sensor output. A 4.3v Zener diode is soldered across pins 1 and 5 of the manifold pressure sensor daughter board - marked end to pin 1 - and the connection between pin 1 and the motherboard is replaced by a 470 ohm resistor. Unscrupulous tuners have been known to charge seven or eight hundred pounds for doing this, while referring to the process as "chipping" and even telling their customers that the ECU has indeed been remapped - although this would be pointless because the MAC01 does not control either boost or fuel.

The effect of the Zener diode modification is to clamp the maximum boost level seen by the ECU to around 1.35 bar - the fuel pump cut-out function is disabled. Severe engine problems can be expected if the wastegate diaphragm tears, if the wastegate control hose splits, or if the fueling is inadequate for the boost levels being run. Note that the dashboard boost gauge - already an inaccurate device - will reflect the boost as seen by the ECU and not the actual boost level. This provides the best method for detecting a modified ECU:

Robert Houk's pioneering work

RDH - as he is normally known - gave up messing with quattros many years ago. But his famous ur-quattro tuning page is still online.

MAC05 Replacement

The MAC05 fitted to early Type 44s is very similar electrically to the MAC01/MAC02 and uses the same connector. Adapting the loom is fairly easy. The benefit is that the MAC05 stores fault codes - a MAC05 conversion gives a WR ur-quattro similar diagnostic capabilities to the MB.

Back to ur-quattro Wiring Index

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