1992-96 Audi S4/S6 Ignition System

Details and Troubleshooting

Ignition Misfire on 1992-96 S4 AAN engine with coil over ignition?

The common misfire problems under load when the boost is increasing are usually caused by either a problem with the spark plug(s), one or more of the ignition coil connector leads, or from one or more defective ignition coil(s).

In less often cases, the Power Output Stage can also cause an intermittent misfire under load, so don't rule it out.

I also recommend checking and/or replacing your fuel filter, as a plugged filter can often feel like an engine misfire under high boost levels.

Ignition misfires that happen all the time are most often caused by a defective Power Output Stage (POS) that has one or more defective drivers used for a particular cylinder. Swapping the two POS units can sometimes locate the defective POS if the location of the misfire changes after the swap.

Also keep in mind that there have been cases where the ignition coil(s) fail all the time, i.e. at engine idle, cruise, and under load.

Keep in mind that it is also possible for the Power Output Stage (POS) to have an intermittent failure for one or more cylinders that only happens during driving which can make it difficult to locate by the swap method.

This may only happen when the POS gets warm/hot during normal operation or under extreme cold conditions.

Poor heat sink conduction from the back of the POS to the metal bracket may cause thermal shutdown problems when the POS gets warm/hot.

The plastic cover on the firewall area, may also help keep radiated heat from the turbo and downpipe off the POS units.

The S4/S6's that are running higher boost levels seem to be more prone to ignition system problems as they are trying to provide a spark under very high cylinder pressures.

Trouble-shooting these ignition misfires can be tough as the problem may only occur under load and/or high boost conditions.

It can be difficult to locate which cylinder spark plug, coil connector lead, ignition coil or the particular intermittent POS that is causing the misfire problem under load.

In some cases you can use a special spark plug with an adjustable electrode to stress test the ignition coils when cranking over the engine. See below for details.

One suggested method is to remove one fuel injector connector at a time and see if the misfire behavior changes when driven under load.

Ignition System Description:

The Ignition coils are mounted individually in the top section of the valve cover.

Ignition Coil Power Output Stages (POS)

The System use two ignition coil Power Output Stages (POS). The Engine Control Unit (ECU) sends 5 separate control signals (one for each cylinder) to these 2 Power Output Stages which are used to supply the higher current needed to accurately turn on and then off the Ignition Coil to produce the high voltage ignition spark.

The Left POS is used for Cylinders 4 and 5, the Right POS is used for Cylinders 3, 2 and 1

Ignition Coil Power Output Stages (POS)

If the misfire is happening all the time, even at idle, consider yourself lucky. You can remove the fuel injector electrical connectors one at a time with the engine idling and see if one cylinder has no change in idle speed or idle quality when the fuel injector is unplugged.

If you locate a misfiring cylinder this way, you can then swap the two Power Output Stages and see if the misfiring cylinder location changes which will tell you that one of the POS units is causing the problem.

The modules have Phillips screws with a nut on the back, holding them to the bracket on the firewall. The entire bracket with the two POS can also be removed, by removing the two 10mm nuts inside the plenum area under the plastic cover.

You may want to get some Heat Sink compound from your local electronics supply store to apply to the back of the POS module to increase the heat transfer between the POS and the metal bracket for improved cooling.

After swapping the two POS's, restart the engine and remove the fuel injector connectors one at a time to locate which cylinder is now missing.

If the misfire goes away completely,

then you most likely had a misfire on cylinder #3 in the original Right side POS, and now the unused channel (Terminal 3) from the Left POS is being used to drive Cylinder #3 with the Left POS being moved to the Right POS position.

If the misfire moves to another cylinder by 3 locations.

If the misfire moves from Cylinder 5 to 2, then the Left Side POS driver for Cylinder 5 was defective.

If the misfire moves from Cylinder 4 to 1, the Left Side POS driver for Cylinder 4 was defective.

If the misfire moves from Cylinder 2 to 5, then the Right Side POS for Cylinder 2 was defective.

If the misfire moves from Cylinder 1 to 4, then the Right Side POS driver for Cylinder 1 was defective.

If the misfire moved to cylinders 3, 2, or 1, then you should return the modules to their original positions and this indicates the left POS was the defective one and you may be able to rewire the input and output connections to utilize the one unused channel in this left POS.

If the misfire moved to cylinders 4 or 5, then your right POS was defective and you may be able to rewire the two connectors on the Left POS to utilize the one unused coil driver on Terminal 3 from the original Right side POS.

This is the lower connector on the Left Side POS.

The BLK/Y wire should be for Cylinder 4

The BLK/White wire should be for driving Cylinder 5.

The BRN/White stripe wire is the ground for the POS.

Depending on which channel is defective, either one of these wire terminals can be inserted into the blank terminal opening after disassembling the connector housing to utilize the 3rd channel that wasn't being used previously.

The upper Connector on the POS that goes to the Cylinder 4 and 5 Ignition Coils also needs to have the appropriate terminal for cylinder 4 or 5 moved into the unused terminal location depending on which cylinder is being moved to the unused 3rd channel.

After the offending POS channel is identified you will end up with one 3-channel POS and one 2-channel POS. The 3- channel one goes on the right (looking at the firewall) for cylinders 1, 2, & 3, with the top & bottom harnesses plugged in normally. The 2-channel one goes on the left for cylinders 4 & 5. Swap the wires/terminals (if necessary) in the top & bottom harnesses for the left one to make sure the good 2 channels are now used; the dead one will be blank.

Looking Inside the POS

Ignition Coil Connector Lead

This system uses a ignition coil connector lead between the ignition coil and the spark plug which normally should have approximately 5000 ohm resistance when measured across each end with an Digital Multi-Meter (DMM) in the Ohms setting.

Unfortunately, this resistance test will only help locate a lead with excessively high or low resistance, and won't find the ones that are arcing across the rubber boot to the cylinder head under high loads due to deterioration and breakdown of the rubber boot.

One suggested method is to remove one fuel injector connector at a time and see if the misfire behavior changes when driven under load.

You can also swap in a complete set of 5 coil packs from another S4/S6. You can also install one new coil connector lead at each cylinder location, one at a time, and then test drive the vehicle after each swap, to try and locate which cylinder coil lead is causing the problem.

Other Ignition Coil Tests:

The Bentley manual describes a test for the ignition coils. You remove the coil pack cover from the valve cover, and lay the coil pack cover on its side with spark plugs inserted into the coil leads. You have an assistant crank over the engine while you watch for spark at the plugs.

Note; Disconnect the 5 fuel injector electrical connectors first before doing the crank test.

Unfortunately, the spark plugs don't have a wide enough gap to stress test the coils to locate the defective one that has a weak output and will only work to show which coil has no output, possible due to a POS failure.

To put higher stress on the coil and force it to output the max voltage, you need to use one of the special coil testers below. Insert the coil tester into one of the coil leads to stress test each coil while cranking the engine over with regular spark plugs inserted into the other 4 coil leads.

Test each coil one at at time using the coil tester until you find which one of the coils has poor output or no output.

If your coil leads are fairly new, they will usually hold the spark plugs in place to make contact with the coil lead internal connection. If your coil leads are older, you may have to use a conventional Audi spark plug lead to connect the coil output to each spark plug or to each coil test fixture. You want to lay the coil pack so that the spark plugs are grounded to the engine valve cover, as well as the cover being grounded.

You can also make up a sheet metal plate with holes drilled in it for each one of the spark plug ends to hold them in place and ground them all with a wire going to the coil cover ground.

IF YOU ARE ONLY TESTING ONE COIL AT A TIME WHEN CRANKING THE ENGINE OVER, YOU MUST GROUND THE OTHER COIL LEAD ENDS TO AVOID DAMAGING OTHER THE COILS DURING THE TEST.

YOU DO NOT WANT TO LEAVE THE COIL HIGH VOLTAGE OUTPUT OPEN CIRCUIT AS THIS CAN CAUSE INTERNAL ARCING INSIDE THE COIL SECONDARY WIRING.

Note: Disconnect all 5 of the fuel injector electrical connectors when doing this test to avoid flooding the engine.

If you suspect that you may have an defective ignition coil that only fails under high boost conditions, but are not sure if you have an ignition misfire or fuel related misfire, you may want to find a repair shop which has a portable exhaust analyzer.

If you drive the car and monitor the exhaust gas levels, you will normally see a very high Hydrocarbon (HC) reading when the ignition system is failing and you have a misfire. Normally the HC reading will only be 50-150 ppm under load and high boost. With a misfire this reading will skyrocket often to over 1000ppm HC.

Another suggested method is to remove one fuel injector connector at a time and see if the misfire behavior changes when driven under load.

If you are unable to locate the defective coil using the above methods, you may have to swap in a known good ignition coil into each coil position one at a time and then test drive the car under load/boost until you locate the defective coil.

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