Heads-up re. LED brake lamps causing TDI limp mode

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syncroandy
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Heads-up re. LED brake lamps causing TDI limp mode

Post by syncroandy »

Just a quick heads-up to anyone running a VAG drive-by-wire engine including any 1993-2005 TDI (and probably later), or petrol engine from ~2000 onward, with a harness NOT from Syncrosport.

The standard wiring for these engines includes two ECU inputs from the brake switch, which are a safety feature to allow the driver to cut off fuel, in the event the accelerator sender goes haywire. These inputs are the brake lamp circuit itself (known as 'F'), and the same signal but inverted (eg. when one is 'off' the other must be 'on') known as 'F47'.

The ECU has a pull-up resistor inside, on the F input, so it will be 'high' or 'on' by default. The circuit relies on the low resistance of the brake lamp bulb to pull the F input down toward 0V/Earth, indicating 'off'.

So, if the lamps are swapped out for ones with a higher effective resistance, such as an LED (which draws far less current than a regular filament), the ECU F input will not reliably transistion OFF/ON as it should, and will be stuck in the ON state.

As the ECU software varied over time and between engines, the symptoms of this may not be consistent. They may appear to be random or intermittent, but typically will involve sudden power loss and the engine refusing to rev fully. The ECU may or may not put on the GP lamp to indicate the fault. Generally, there should be a brake system fault code logged. However sometimes I have seen some fault codes suppress the display of other codes, so ideally all other fault codes should be fixed and cleared during diagnosis.

The definitive check for this problem, is to use VCDS or similar to check the relevant ECU measuring blocks. Commonly this is displayed in a group of three values for the two brake switch inputs and the clutch switch input (F,F47,F36). With the ignition on and no pedals pressed, these should all read '0'. F & F47 should change to '1' at the same time, when the brake pedal is pressed.

It should be possible to work around this issue if LED bulbs are fitted (so long as they are E marked otherwise they're not technically legal for fitment) by adding a suitable value resistor in parallel with the LED bulb.

Syncrosport harnesses use a 5-pin relay instead of the four-pin pedal switch to provide the F & F47 ECU inputs. This avoids the LED bulb problem, as the relay trigger coil resistance (~70 ohm) provides the required 'pull-down'. The relay also ensures the inputs switch reliably at the same moment. For anyone wanting to add this feature to an existing harness, the wiring is as follows:

Image

'F' -> brake lamp circuit and the ECU 'F' input
'F47' -> ECU 'F47' input
31 -> Earth
'Switched 12v' -> Ignition
'Sender earth' -> ECU Sender earth input (not chassis earth) - commonly Brown/Blue

'Early' - cars originating from before ~1997 eg. Golf3, Passat 3A (also Sharan 68-pin I think but please check)
'Late' -cars originating after ~1997, eg. Golf4, Passat 3B, anything with 80 or 121-pin ECU really.
Syncrosport
Volkswagen Transporter, reloaded.

252 GC5 EJ25 AAN L90D
246 097 AFN AVL+ L90D
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