A100-2.5tdi-Estate

My UK spec 1993 Audi 100 2.5Tdi has an intermittent fault in the bulb failure warning system which keeps beeping every 20 seconds driving me slowly mad.

Does anyone have detailed circuit digram of the bulb failure warning lamp indicator.
A circuit diagram of the instrument panel would do - including the NEC microprocessor - beeper - and all the other discrete electronics. I can trace the fault out from that and either fix it or at worst disable the beeper.

Edd W

If so, assuming all the bulbs work and the correct wattage. Do you have a towbar fitted or has there been one in the past, most 100's, especially Tdi's have? - if so, check all the wiring round the rear lights and the number plate lamps. It is possible for an earth fault or faulty towbar wiring to cause a fault with the bulb monitoring.

A100-2.5tdi-Estate

Its an intermittent fault - beeps and flashes the yellow symbol, stays on for a random amount of time (2 seconds or 10 mins) then goes off again - then beeps again etc etc over and over. Also it does it even when the Lights are switched OFF so I believe a fault in the monitoring system itself.

There is no towbar, and the external lights are all bright.

Do you know where the current sensing is done? I Guess there must be a sense resistor somewhere that feeds a small voltage to an op-amp in the instrument cluster? I assume there is a loose wire between the current sensor and the monitor unit or even a loose solder joint in the monitor unit. (Often a bump in the road seems to trigger it.)

Without a circuit diagram and not even knowing where the bits are located then its almost impossible to trace.

If I ever manage to trace this I will post the solution on here.

Thanks for your help

Edd W

around the car, then it could be a dry joint on the board.

I would double check, especially the rear number plate light fittings because they are prone to corrosion.

In other words, I would always look at the simple things first especially when they are common faults when the check system itself is usually reliable. I believe it can show faults in the lighting circuit even when the lights are off.

The other lights to check are the brake lights, I would fit new bulbs and clean the fittings before doing any work on the board.

I am not sure where the bulb monitoring unit is located. I do not think it is part of the instrument cluster. BWM's have a module that looks like a double sized relay box so it could be similar and I would guess it is behind the panel in the drivers footwell next to the accelerator?

A100-2.5tdi-Estate

The brake light is covered by a seperate circuit and flashes a different pattern when the brake light bulb is faulty. In fact when you first start the engine by default the monitor circuit diplays the symbol for blown brake light bulb until the first time you press the brakes (putting current through the brakelight bulbs) then it comes up OK.

This is what makes me think the system works by measureing the current through the bulb when the bulb is switched on. It would be a lot more difficult to detect blown bulbs when there are switched off.

I dont think the number plate bulbs are included in the monitoring checks as I have had these blow before and the blown bulb indicator doesnt come on.

If you remove for example a headlamp bulb when the lights are switched off then the detector circuit does not detect it. When you then switch the light on the detector comes up with the blown bulb symbol.

This indicates to me that the circuit works by having an op-amp across a small shunt in line with the lamp wireing. The op-amp is powered up when the lights are switched on. The op-amp works like a voltage detector. If the lamp is good and is taking current then there will be voltage across the shunt. If there is no voltage across the shunt then the op-amp drives a signal to the instrument cluster to indicate lamp failure.

Therefore I am fairly convinced that the unit should not come up with a blown bulb symbol when the lights are switched off, and that the fault must be in the detector circuit.

What I need to a real electronics circuit diagram of the detector system (not a haynes manual mechanical wireing diagram) so I can debug it.

Does anyone know a web-page where I can find this kind of detail ?

ntninja

try http://www.sjmautotechnik.com/

A100-2.5tdi-Estate

Thanks for the link.
Having read through a lot of the info on this site I am even more convinced that what I have is a loose solder joint in the instrument cluster.

I will try tracing it at the weekend

Cheers

Edd W

lights. It is designed to work that way to ensure you press the brake pedal before driving off to make sure the brake system has not lost its fluid or some other fault. It dates back to when Audi used a hydraulicaly assisted servo rather than vacuum as they do now. It had two pressure switches on the master cylinder to check the pressure was sufficient for adequate braking before you drove off. I guess they thought it was a good idea to keep the same system using the brake lights switching on to tell the system you had pressed the pedal.

I am not sure the system does need the lights to be switched on. I have a towbar and if I connect the trailer lighting with the engine already running and the warning display off, ie brakes pressed and no other warnings showing, within about 5 secs the light failure appears. This must be done by the system monitoring the circuit I have connected to. Brakes, side lights, indicators, rear fog and number plate lights.

I do not know if this helps at all but it may tell you more about the way the monitoring works?

A100-2.5tdi-Estate

Thats interesting.

The only way I can figure that you plugging in the towbar makes the warning light come on is that the earth on the trailer plug is the last pin to properly make contact when you plug in the connector.

In that case the brake light supply would momentarily go through the brake light bulb and the rear light bulb and provide a power feed back into the car lighting wire so initialising the auto check mechanism.

So the Autocheck get powered , but as there is not enough current flowing to flip the voltage sensor then it complains that the bulb has blown.

Then as you continue to push the plug in finally the earth makes contact and the trailer is wired properly.

Fixing a good strong earth wire from car to trailer ground before plugging in the trailer plug may stop the problem occuring. It would be interesting to know if that worked ?

Either that or there is something wierd with the way your towbar is wired. :-)

One more thought: Does your trailer wireing loop directly to the back lights in the car or is it via relays?

If its via relays then all I have said above is probably rubbish

Edd W

time when towing so I did not bother fitting the relay pack.

I often connect the trailer with the engine running but the brake light off, parked on the hand-brake. The light will still usually be one by the tiem I am ready to drive off or come on shortly afterwards, but deffinetly before I have pressed the brake pedal.

I think the trailer may earth as soon as the hitch is connected because it is a direct hitch on the trailer, not through a rubber bracket of any kind, so you would think it would sometimes get a ground connection there, if not reliably. I have towed many different trailers and boats with this car and the warning always behaces the same way.