It is not absolutely necessary to disconnect the battery before doing this job as all the relevant connectors have female plugs, but it is better to be safe than sorry with all electrical tasks and the negative lead at the battery should be disconnected if you are in any doubt about proceeding further.

What is the MIL circuit ?

It is a little known fact that Audi 80 & Coupes exported to the USA had to comply with Californian legislation which dictated that the driver had to be informed of certain (emission related) engine fault conditions by means of a warning lamp. This is known as the Malfunction Indicator Lamp - or MIL for short.

The MIL circuit is not fully wired up on Audi 80s & Coupes that were not destined for California, but I have discovered how to activate this function on the S2 after many hours of 'reverse engineering' the instrument cluster and probing round the wiring loom.

Why should I be interested in a MIL circuit ?

The MIL circuit is connected to the blue coloured OBD plug that is fitted to the car which is ultimately controlled by the ECU. This same blue (MIL) plug can be used to retrieve fault / blink codes from the ECU.

All of this means that, by adding a MIL circuit onto the S2 cluster, there would be no need for messing about with LEDs and test lamps to pull fault codes . But more importantly, the MIL circuit will inform the S2 driver of many active fault conditions that the ECU can detect whilst the engine is running. It does this by illuminating the MIL until the fault clears. Any faults are stored in the ECU and can then be blinked out on the MIL as soon as it is convenient to do so.

The cost of adding the MIL circuit is very low indeed - needing only a single instrument bulb, a short section of narrow guage cable and a connector or two.


NOTE - The MIL circuitry does not flash out the blink codes until the blink code sequence is activated as normal. This prevents driver distraction.

How do I wire up the MIL circuit in my S2 ?

PART 1 - Preparing the Loom

1. Firstly, remove the instrument cluster as described here , and set it to one side in a safe place to prevent damage until Part 2 below.
2. Locate the yellow coloured 26-pin connector (T26) behind the instrument cluster and snip off the tie-wrap which secures the wiring to the connector shell. Be extra careful to only snip the tie wrap !
3. Using a miniature screwdriver (or a fine tipped probe), tease apart the hood from this yellow connector and slide it off to expose the electrical wiring and connection pins as shown in this photo .
4. Locate pin 24 (twenty four) on the yellow connector. The connector moulding is numbered to assist with this task - it is two from the end as shown in this photo . Pin 24 is an unused connection point, so there should be no wire attached to this pin - until these tasks are complete. If your car is an MY95 vehicle, fitted with the factory immobiliser, then this connection point must NOT be used. Click here for more details and an alternate lamp location.
5. Again using the miniature screwdriver (or probe), GENTLY disengage the locking tab for pin 24 and remove it from the yellow connector, as shown here .
6. A special crimping tool is specified for these pins, which none of us will likely have access to, so I recommend that a soldering iron is used to connect a 1m length of multicore (0.5mm ² diameter) cable onto this pin. If you choose cable which is too thick then this will be difficult and the pin may not fit back into the connector.
7. Carefully return the locking tab of the connector pin (with new wire attached) to its original position and slide it back into position 24 (twenty four) of the yellow cluster connector. It will lock itself into place.
8. Reassemble the yellow connector hood, taking care not to snag any cables and attach a new tie wrap onto the connector hood when finished. With this new length of wire dangling from the yellow cluster connector the fiddliest task is complete.

PART 2 - Preparing the Cluster

9. Now the attention turns to the instrument cluster... With the cluster lying face down on a soft surface, remove the sticky cover for the unused 'CAT' warning lamp as shown in this photo and fit the green 2W lamp in this location. At this point it is worth noting that the early cars have an unused yellow warning symbol that resembles a steering wheel in deference to the 'CAT' legend on later cars. Dunno why - but I'm 100% sure that this is the correct lamp location for the MIL bulb on Californian cars. This link lists the different warning lamp legends applicable to the S2. Please note that if you have an MY95 S2 (S chassis) with the factory fitted immobiliser - you will have to work with the unised lamp location above the 'CAT' lamp. More details here.
10. Reconnect all of the electrical plugs back into the instrument cluster as normal.
11. At this point, you may wish to test the newly installed cable and CAT lamp. Do this by turning on the ignition and then touching the other end of the new wire to the chassis (0V). The red CAT lamp will only be lit whilst this circuit to 0V/earth is closed.
12. Temporarily loop or tuck the new section of wire in behind the instrument cluster which can then be refitted as normal.
    
    

PART 3 - Completing the MIL Circuit

13. Now working in the driver side footwell, use an 8mm socket to remove the four bolts which secure the parcel shelf to the dashboard. Three bolts are on the top edge, and the fourth is down near the bonnet release.
14. Slide the parcel tray carefully out of the plastic clip at the pedals. If your car has diagnostic plugs in the drivers parcel shelf (early 3B only) then you may wish to unclip them, so that the tray can be moved out of the way for the next tasks. Try not to panic when you see the mad rampant mess of spaghetti cabling which is lurking under the dash. Fortunately, the access you need is very easy.
15. Locate the new loop of cable which connects to the yellow cluster plug from Part 2. Unloop it, then tie-wrap and/or tape it along the existing loom from the cluster such that it makes a sensible route to the black 5-pin connector (known as T5) shown in this photo . This is where it will connect to the 'blink output' driver from the ECU. Leave enough slack cable to work with for the new connection point.
16. Looking at the T5 connector, you will see a gray/brown (gr/br) coloured wire on the pedal side of the connector which is unterminated. On the ABY engine this corresponds to pin 4 (four) on the T5 plug, but it is pin 3 (three) on the 3B engine (thanks to Steve Henson for confirming that). This is the wire needed to drive the MIL function. As a double-check, you can perform a continuity test with a multimeter from this wire to the BLUE diagnostic plug on your car. The reading should be a short circuit (0 Ω) confirming that you have located the correct wire !
17. Trim and connect the new wire from the cluster to this gray/brown wire on T5. An ideal connection would be with a crimped pin that inserts into the empty receptacle of T5 but such connector pins and crimp tools are very hard to find. Other ways to make the connection are with insulated male & female spade terminals or with a 'scotchlok' style connector. The latter has the advantage of not needing to cut into the gray/brown wire and fiddle about with wire strippers and crimpers etc. Either way, make a secure connection and ensure all the wiring is neatly and safely taped or tied into the factory loom.

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Now turn on the ignition and look with awe and amazement at the newly illuminated 'CAT' lamp which serves as a wonderful Malfunction Indicator Lamp (MIL).

• When the engine is started the CAT lamp will extinguish.

• It will only illuminate with the engine running when there is an active fault detected by the ECU.

• Additionally, the MIL circuit can be used to view blink codes whenever required. Simply follow the directions in this page but forget fiddling with the LED or test lamp at the blue connector.

• Finally, when the ECU is communicating with VAG-COM (or VAG1551) in basic settings mode the MIL circuit is activated. This will undoubtably raise an eyebrow at your local VAG dealership ;-)
  

Warning lamp legends

There are several subtle differences in the legends used for the warning lamps as outlined in Part 2. Each of these lamp legends can be ordered individually from Audi and can easily replace the existing one by simple disassembly of the instrument cluster. The table below captures each of the fourteen variants. One of these must have been used in the Californian cars with a proper 'check engine' symbol, but ETKA does not give any clues. Best guess would be 893-919-249-H. Comments & corrections welcome as always... Images to follow as and when.

Thanks to Jim Slevin for spotting the fly in the ointment with the MIL circuit on cars fitted with the factory immobilser. These vehicles, produced in MY95, use the 'CAT' location for a lamp that is activated by the immobiliser. ETKA only shows this anomolie on the RS2 but it is applicable to the latest S2 Coupes. An alternate location for a warning lamp on these cars that can be used for the MIL circuit is available at the yellow cluster connector on pin 26 (twenty six). The legend for this lamp is actually for roof operation on the 80 cabriolet so the sense is a bit odd, but it will work fine.

Part Number	Application	Region	Notes
893-919-249-M	S2	-	Up to chassis No. P-003 000 (mid MY93). Believe this is variant with yellow steering wheel legend.
8A1-919-249-A	S2	-	From chassis No. P-003 001 (mid MY93). This is the variant with red CAT legend.
8A1-919-249-H	S2, RS2	-	From chassis No. S-000 001 (MY95). This variant is for immobiliser equipped vehicles. An alternate warning lamp location is needed on these cars for the MIL circuit.
893-919-249-N	S2	Italy	Up to chassis No. P-003 000 (mid MY93). Guess this is variant with yellow steering wheel legend. Italian variant has additional 'lights-on' warning lamp in deference to trailer turning indicator elsewhere.
8A1-919-249-C	S2	Italy	From chassis No. P-003 001 (mid MY93). Guess this is variant with red CAT legend. Italian variant has additional 'lights-on' warning lamp in deference to trailer turning indicator elsewhere.
8A1-919-249-K	S2, RS2	Italy	From chassis No. S-000 001 (MY95). Guess this variant is for immobiliser equipped vehicles originally destined for Italy. An alternate warning lamp location is needed on these cars for the MIL circuit.
893-919-249-H	80/90/Coupe	-	Up to chassis No. M-040 000 (end MY91)
8A1-919-249	80/90/Coupe	-	From chassis No. N-000 001 (start MY92)
893-919-249-J	80/90/Coupe	Italy	Up to chassis No. M-040 000 (end MY91)
8A1-919-249-B	80/90/Coupe	Italy	From chassis No. N-000 001 (start MY92)
893-919-249-K	80/90/Coupe	Japan	Up to chassis No. M-040 000 (end MY91)
8A1-919-249-D	80/90/Coupe	Japan	From chassis No. N-000 001 (start MY92)
893-919-249-G	80/90/Coupe	KSA	Up to chassis No. M-040 000 (end MY91)
8A1-919-249-F	80/90/Coupe	KSA	From chassis No. N-000 001 (start MY92)

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