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MB engine MAC12D ECU fault codes and pinout

The MAC-12 ECU used in the MB-engined ur-quattro and the Audi 200 with the 1B engine can store error codes for faults discovered during operation. If an error code is stored, the yellow "Check Engine" light should blink for one second - but this does not always happen and a periodic check for stored codes even in the absence of symptoms is a good idea. Note that this memory is volatile and the contents are reset when the ignition is switched off.

If this procedure fails to work on an early MB with a code of "211" on top of the fuel pump relay, replace the relay with a new or secondhand one with a code of "215". Audi will automatically ship this relay as a replacement - the earlier version was deleted because the fuse holder contacts were unreliable.

Where multiple codes are stored, it is often worth checking the resistance of the common earth from Pin 18 of the ECU to chassis ground. This is the thick brown wire at one end of the ECU connector - with the ECU in place check for continuity between the back of this pin and the inlet manifold.

"Break in wiring" is often given as a potential cause. In the majority of cases, these breaks are caused by human intervention, most especially the use of Scotchloks during immobiliser installation. Scotchloks in the engine bay can be a particular problem - they break the insulation and allow the conductors to corrode and break inside the wire. Replace them with crimped butt splices, preferably using a ratcheting crimper.

Wiring breaks are also caused by over-tightened cable ties, often where aftermarket equipment has been installed.

MAC12 Error Blink Codes, Components and Diagnostics
(Link to wiring diagram)
Probable cause(s)Tests
(Frame number from microfiche)
1111ECU (J88)
  • Defective ECU
  • Bad chipping
Check ECU
2111RPM sender (G28)
  • Break or short
  • Defective sensor
2112TDC Sender (G4)
  • Break or short
  • Defective sensor
  • Broken flywheel pin
2113Hall Sender (G40)
  • Break or short
  • Defective Hall sender
  • Distributor misaligned
2121Idle Switch (F60)
  • Break or short
  • Defective idle switch
2123Full Load Switch (F81)
  • Break or short
  • Defective WoT switch
2141Knock control
  • Engine knocking
  • Fuel quality
2142Knock Sensor (G61)
  • Break or short
  • Defective knock sensor
2214Rev Limit Exceeded
  • Engine revved past redline
2221Hose to ECU
  • Fallen off, trapped,
    ripped or defective
Check with MityVac
2222ECU Pressure Sensor (J88)
  • ECU pressure sensor
Check ECU
2224Excessive Boost
  • Wastegate defective
  • Boost control hose(s)
Check wastegate
and hoses
2312Coolant Sensor (G62)
  • Break or short
  • Defective sensor
2322Air Temperature Sensor (G42)
  • Break or short
  • Defective sensor
4444No faultsNo action
(no pulses)End of codes

Code 1111 - ECU

The ECU itself has failed one of its own diagnostic tests. The most common cause is a bad PROM checksum resulting from incompetent "chipping" - some companies apparently do not know how to recalculate the checksum after modifiying the boost tables. Remove the cover from the ECU, unscrew the upper board, and check that the original Hitachi PROM (it has a silvered sticker saying "MAC12") is still in place. The stock chip is soldered directly to the board - a modified chip will usually be in a socket. If the chip has been changed, a new chip with a correct checksum should be sourced.

Otherwise, check the ECU in a known good car.

Code 2111 - RPM sender (G28)

Note that it is quite easy to get false 2111 indications, especially if the engine will not start at all.

Code 2112 - TDC sender (G4)

Code 2113 - Hall Sender (G40)

Note: Remove the connector from the coil pack before performing these tests.

Code 2121 - Idle switch (F60)

Note - a throttle body switch that has a seam running around it in the middle of each side should be replaced without testing - if it is not yet broken, it will be. These are generally referred to as "Type 1 WoT switches" and are notorious for the full load side (Code 2123) failing without storing a code in the ECU.

Code 2123 - Full load (WoT) switch (F81)

Note - a throttle body switch that has a seam running around it in the middle of each side should be replaced without testing - if it is not yet broken, it will be. These are generally referred to as "Type 1 WoT switches" and are notorious for the full load side failing without storing a code in the ECU - the wrong solder was used during manufacture. In general it is not worth proceeding with the following on a Type 1 switch, since these tests will not detect an intermittent fault - the solder failure leads to the upper leaf of a switch rattling. It may make good contact with the vehicle stationary but open intermittently when in motion.

Code 2141 - Engine knock control

This occurs when the ECU is unable to suppress knocking (pinking) using the map for the lowest octane it supports - in the MB's case, 95 RON. There are many possible causes, the most common being poor fuel quality. Also check compression, the fuel injection system and the wastegate.

Code 2142 - Knock sensor

Code 2222 - ECU pressure sensor (MAP sensor)

The pressure sensor used in the MAC12D ECU is proprietary to Hitachi and Audi recommend swapping the ECU as the only fix. One test is not documented:

Code 2224 - Overboost

Code 2312 - Coolant temperature sender (G62)

Code 2322 - Inlet air temperature sender (G42)

Note: If there is no "Check Engine" light when the ignition is switched on, either the bulb has failed or it has been removed to conceal a fault on the car. It should be replaced.

If the light then comes on permanently, proceed as for Code 1111.

If there is no time to replace the bulb in the "Check Engine" light, a diode light can be used in the same fuseholder used to trigger the diagnostics. This procedure is a little tricky, as the codes are displayed as dark periods - the diode lights when the "Check Engine" light would be off, and is off when the light would be on.

MAC12 ECU pin assignments

The MAC12D that controls the ur-quattro's MB engine was derived from the MAC11 used to control North American Audi 5000s. The major change involved the deletion of the 5000's lambda probe and the replacement of the code with a table-based calculation to provide a duty cycle output for the fuel frequency valve. In the table below, the MAC11 assignments are included for completeness.

Viewing the connector with the prongs on the right and the side locating lugs at the top, the upper left pin is Pin 1, with numbering proceeding along the connector to Pin 18 at the top right. The bottom row is also numbered from left to right, from 19 to 35. If in doubt, check the wire colours.

1Blue/WhiteCold Start Valve (N17) - power switched by fuel pump relay, grounded by ECU
2Brown/YellowInlet manifold ground - ignition
3 (Carbon canister in MAC11)
4 (Decel shutoff valve in MAC11)
5Brown/YellowInlet manifold
6 (Code wire in MAC11 - grounded for auto transmission)
7MauveEngine rpm shared with Idle Stabiliser (J142), Oil Pressure Warning Controller (J150) from dashboard pin 4
8Blue/BlackFuel Frequency Valve (N7)
9Brown/RedChassis ground
10BlueCoolant Temperature (G62 - side of block)
11BlueEngine RPM (G28)
12MauveIgnition Timing Sender (G4) grounded through Pin 18
13RedIgnition Timing Sender (G4)
14Blue/WhiteElectronic Thermoswitch (F76)
15BrownKnock (G61)
16WhiteKnock (G61) grounded through pin 18
Brown(shield for above)
17(MAC11 boost gauge - unused on ur-quattro)
18Brown/YellowECU chassis ground - grounds pins 12, 16, 23, 28, 29
19Green/YellowWastegate Control (N75)
20GreyIdle Switch (F60), Idle Stabiliser (J142)
21Brown/RedFuel Pump Relay (J17)
22Green/WhiteElectronic Ignition Power Stage (N70)
23BrownInlet Air Temperature (G42) - grounded through Pin 18
Brown/RedHall Sensor (G40) - grounded through pin 18
24BlueHall Sensor (G40)
25Red/BlueHall Sensor (G40)
26WhiteFull Load Switch (F81)
27GreenHall Sensor (G40)
28BrownShielding for 11, 12, 13, 29 - grounded through pin 18
29GreyEngine RPM (G28)
30 OXS sensor signal on MAC11 - unused on MAC12
31Blue/WhiteFuel pump relay fuseholder and 'Check Engine' light.
32 (Brake light input in MAC11)
35Blue/RedPower via Fuse (S24, 10A)

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.

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