Audi TT MK1 Electrical Fault Diagnosis

Audi TT MK1 (8N) Fault Diagnosis

Guide, hint's and tips to diagnose and correct electrical faults.

Introduction

If you’re looking for help finding electrical or electronics faults with your Audi TT MK1 instrument cluster then this article is for you. Presently this page only covers instrument cluster faults but we'll be expanding coverage in the near future. Feel free to contact us with additions or corrections.

We’ll cover all the common instrument cluster faults, how to diagnose them and, where possible, how to repair them. Links are included to additional supporting documents that may be downloaded.

Caution

This article is a guidance only. As such we take no responsibility for damage or injury relating to the information or instructions we’ve provided. If in doubt, seek assistance from an automotive electrician.

While most electrical systems in your TT, aside from the ignition coils, won’t electrocute you there is plenty of scope to cause a fire, burn yourself or cause expensive damage to your car.

Some diagnostics require the car’s electrical systems are powered on. Where this is not required we advise disconnecting the battery negative terminal. Take care not to cause short circuits and not to apply excessive strain on the wiring loom and electrical connectors. Do not disconnect any connectors with the ignition on unless specifically advised to do so.

Shortcuts Menu:

o Tools

o Fuel gauge faults

o Coolant temperature gauge faults

o Speedo and tacho (rev counter) gauge faults

o Centre LCD faults

o Backlighting faults

o Cluster power up failure

o Cluster erratic behaviour

o Odometer stuck at 999999

Tools

If you're comfortable getting 'hands-on' with your TT, here are the tools you'll need:

Basic

Selection of screw drivers including Torx drivers
Metric socket set
A digital multi-meter (DMM), $40 / £30 on Amazon
Insulation piercing needle probes for the DMM, $10 / £8 on Amazon
A test light, $10 / £8 on Amazon

Advanced

Good OBD2 scan tool with ability to show live data (we recommend VagCom VCDS)
A set of small picks, helpful to unlatch connectors

Cluster Faults by Symptom

Fuel Gauge Faults

Fuel gauge is showing incorrect fuel level

There are 4 possible causes:

One or both of the fuel tank level sensors are failing
There is a wiring fault between the fuel tank and the instrument cluster
The instrument cluster gauge needle motor is failing
There are failing solder joints in the instrument cluster

The first step is to determine which of these 4 faults are present in your car. The best approach is cut the problem area in half by testing mid-point in the circuit. You need to verify that the signal coming from the fuel tank sensors is good. Audi reversed the direction of the tank sensor output during TT production, we think around 2002. In both cases though a ½ tank is equivalent to a sensor resistance of around 180ohms.

The most effective place to measure the fuel level sensor signal is at the rear of the instrument cluster. This requires the cluster be removed from the dashboard. We won’t cover that here as a quick search on YouTube will find many how-to videos.

The fuel level signal comes into the instrument cluster on pin 5 of the blue connector (T32). Pin 7 of the same connector is signal ground. More detail on this connector and its other connections can be found the TT MK1 (8N) Electrical Equipment document, page 308.

With the ignition off, unlatch the connector and measure the resistance using a multi-meter between pins 5 & 7. The resistance will vary dependant on fuel level, with ½ tank being around 180ohms. Further information and typical resistance values for LATER model TTs can be found in TT MK1 (8N) Electrical Equipment document, page 316 – 317.

If the resistance values measured are not correct then the fault is most likely with one or both of the fuel tank level sensors. Often this can be corrected by simply cleaning the level sensor contact services with 1000 grit wet-dry paper, taking great care to bend the ‘wiper’ that runs over a circuit track.

If the tank level sensors look OK, when measured at the instrument cluster, then the fault will be inside the cluster. If your cluster has a trip computer, check the range to empty display. If it’s approximately 180miles (290km) for ½ tank, 320miles (515km) for a full tank the gauge input circuit in the cluster is good but the gauge motor is likely failing. If the ‘range to empty’ is incorrect based on the volume of fuel in the fuel tank, then there is a circuit fault in the cluster’s fuel gauge input circuit.

Advanced

If you have a good scan tool such as VagCom VCDS, connect to the engine ECU and access real time data (advanced measurement value in VCDS). Check the fuel level in litres, is this in-line with what you believe is in the fuel tank and does the fuel gauge reflect the litres reported by VCDS? If the reported level is correct but the gauge is incorrect, this is a cluster fault. Note that VCDS will report an incorrect fuel level if there is a fault with the in-tank level sensor(s), wiring or instrument cluster input circuit. In this case you should measure the sensor resistance at the blue cluster connector as previously described.

Temp Gauge Faults

Engine coolant temperature gauge is showing incorrect temperature

There are 4 possible causes:

The engine coolant temperature sensor is failing
There is a wiring fault between the temperature sensor and the instrument cluster
There is fault in the engine cooling system - thermostat, water pump, radiator or cooling fans
The instrument cluster gauge needle motor is failing
There are failing solder joints in the instrument cluster

The first step is to determine which of these 5 faults are present in your TT.

If the temperature gauges moves away from expected reading slowly (2 to 10 minutes), for example if it drops below or rises above mid point while driving, this usually indicates a problem with cooling system as mentioned in (3) above. Using a good OBDII scan tool, such as VCDS or OBD Eleven it is possible to read the measured coolant temperate from both the engine ECU and the Instrument Cluster. Note that that the temperature sensor contains 2 separate circuits, one going to each of these modules. More on that a bit later.

The best approach to finding a temperature gauge electrical fault is to cut the problem area in half by testing mid-point in the circuit. You need to verify that the signal coming from the coolant temperature sensor is good. The best place to make this measurement is the rear of the instrument cluster.

The temperature signal enters the cluster on pin 8 of the blue connector (T32). More detail on this connector and its other connections can be found the Audi TT MK1 (8N) Electrical Equipment document, page 308. Remove the blue connector from the cluster and measure the resistance at the cable end of the connector between pin 8 & pin 7 (signal ground). With coolant temperature at 90°C resistance should be approx. 110 ohms, and at 120°C should be approx. 50 ohms.

Be aware that the temperature sensor actually contains TWO sensor circuits. One of these connects to the engine ECU and the other the instrument cluster. It’s most common for only one of the two circuits to fail first which can be misleading. Please see Audi TT MK1 (8N) Electrical Equipment document, page 325 - 327 for more information including a pinout of the sensor.

If the resistance values measured at the instrument cluster are as expected then there is a fault in the instrument cluster. If the values are not as expected, remove the connector from the sensor (located in a coolant pipe on the transmission end of the cylinder head) and measure the sensor resistance on its pins. See Audi TT MK1 (8N) Electrical Equipment document, page 327 for the pinout. If the resistance here is incorrect then the sensor has failed. If the resistance is as expected then there is a wiring fault between the sensor and the instrument cluster.

Advanced

If you have a good scan tool such as VagCom VCDS, connect to the engine ECU and access real time data (advanced measurement value in VCDS). Check the coolant temperature. A fully warmed up engine should be around 85 to 100 Celsius (185 to 212 Fahrenheit). Does the temperature gauge reflect the temperature reported by VCDS? If the reported temperature is correct but the gauge is incorrect, this is a cluster fault. Note that VCDS will report an incorrect temperature if there is a fault with the sensor, wiring or instrument cluster input circuit. In this case you should measure the sensor resistance at the blue cluster connector as previously mentioned.

Speedometer or Tachometer (rev counter) gauge needles are showing incorrect values / are outside the normal range

Speedo and Tacho Gauge Faults

The tachometer and the speedometer needles are prone to hairline cracks where they press onto the gauge motor spindles. This will eventually cause them to slip out of position (they will typically read too low and rest below zero). It is a relatively simple DIY task to reposition the needles. However, needle fit needs to be tightened to prevent a repeating problem. DO NOT glue the needles onto the motors as they must be removed to make any future repairs. Often the needles or the motors are damaged while removing glued needles, increasing the cost of repair.

Advanced

The speed sensor mounted on the transmission can also cause the speedometer to read incorrectly. To test for this connect a scan tool with live data capability. In VCDS this is called Advanced Measurement Values. Select the Engine ECU, go to live data and select vehicle speed. If the reading is correct then the transmission vehicle speed sensor is OK. Most likely fault will be an instrument cluster failure.

Engine speed (RPM) is sent over the CAN bus to the cluster. If the cluster tachometer is reading incorrectly and it's not reporting any communications faults via your scan tool then the issue will most likely be an instrument cluster fault, or a slipped needle as mentioned above..

Central LCD display has dark ‘missing’ pixels or is blank

Central LCD Faults

The infamous ‘missing pixels’, ‘dead pixels’, ‘dark pixels’ pixel repair! By far the most common failure with the MK1 TT cluster. This is caused by a weakness in the OEM LCD display panel so the only fix is to replace the panel. Be aware that a lot of the cheap aftermarket LCD display panels are poor quality, often over-bright with poor contrast. This repair requires excellent fine-soldering skills, a good soldering iron with 0.5mm tip, a hot air desoldering tool (or skilled use of gas soldering iron) and a magnifier. To see what is needed, see the video of us making this repair on the Parrot Brothers YouTube channel.

Backlight not working or flickering

Backlighting Faults

This is another common TT cluster fault. Any area of the cluster can be a problem but most commonly the fuel and temp gauge needle illumination, the centre LCD display backlight and tachometer dial illumination. The faults are often failing solder joints but in some cases are caused by component failure.

Instrument cluster fails to power-up when ignition is switched on (often results in engine cutting out after starting)

Cluster Power Up Faults

If the engine cuts approximately 1 second after starting this indicates an issue with TT’s immobiliser. This is incorporated in the instrument cluster. The engine ECU will ask for ‘authorisation to run’ from the cluster immediately after its started. It’s not possible to simply swap in another cluster into a TT. Advanced coding is needed to adapt the replacement cluster to the engine ECU. The ‘SKC’ or ‘Security Key Code’, sometimes called the PIN code is needed for this adaptation. The SKC is a 4 or 5 digit code stored in both the engine ECU and the cluster. There are a variety of tools on the market to read the SKC.

The immobiliser can also be disabled, ‘coded out’, in the engine ECU. The engine will no longer have an immobiliser and the immobiliser warning light will continually flash on the instrument cluster. However, you can now swap in any cluster that is a part number match with your original.

Fuses 11 & 15 in the fuse box driver’s end of the dashboard should be checked to ensure they are not damaged and that the connections are clean & free of corrosion. These fuses supply battery live and ignition live to the cluster.

If the cluster appears to not start up when the ignition is switched on (cluster remains inactive aside from a few waring lights) this means that the cluster’s microprocessors have not started up. In turn, the immobiliser will stop the engine after trying to start. The causes can be bad solder joints in the cluster or a failed microprocessor chip. Often this fault is intermittent and first, some times worse in hot or cold weather.

Advanced

If you have a good scan tool such as VagCom VCDS, connect to the engine ECU. Check for fault codes, specifically for codes relating to no communication with the instrument cluster. The scan tool should also indicate if this fault if ‘static’ (permanent) or ‘sporadic’ (intermittent).

Instrument cluster behaves erratically while driving

Cluster Erratic Behaviour

If the gauge needles flick around and/or drop to zero, sometimes accompanied with displays going dark, beeping and random warning symbols showing this is a fault in the cluster’s microprocessors circuit. It’s advisable to avoid using your TT if you have this fault. The microprocessors can (and often do) overwrite the odometer data causing ‘999999’ to show on the odometer. The data is corrupted and must be corrected. Typical OBD2 scan tools cannot correct this fault. Repair options do exist but this will add to the cost of repair.

Advanced

Odometer Stuck at 999999

Odometer shows 999,999 miles / kilometres

This indicates that the odometer data in the cluster’s internal memory has become corrupted. This can be caused by the erratic cluster behaviour as previously described but it can also result from jump-starting. Always ensure the ignition in your TT (and the other car if applicable) is OFF with key out BEFORE attempting to connect jump leads or a jump pack and then ensure the ignition is OFF before disconnecting the jump leads or jump pack.

This odometer issue can be corrected with specialist tools but not through a regular scan tool.

Reference Documents

Note that Audi made changes to model specification based on geographic location, date and selected vehicle options. There may be discrepancies between your TT and these documents.

Audi TT MK1 (8N) Electrical Equipment

Audi TT MK1 (8N) Wiring Diagram

Audi TT MK1 (8N) Electrical Component Locator