Daytime running lights modification possible on all MY 90 and younger cars...


This topic has been discussed on Rennlist previously. Somebody on PFF-Online in Germany modified his 928 to daytime running lights. The modification is easy and done in a couple of minutes. Here is my write-up and some pictures.

Relay III on the main circuit board is empty and can be fitted with a standard relay. Porsche uses a relay with an extra diode to prevent running the fog lights when not intended (thanks Allan for the clarification). I took a standard relay and soldered a diode between Pin 85 and the coil.

Here is how it works: As soon as the LH-ECU energizes the fuel pump relay the fog lights come on as daytime running lights. Relay III receives power from the fuel pump relay output.

The ground power supply for the relay is provided by the position of the headlights. As soon as the headlights are out ground power to relay III is interrupted and the fog/daytime running lights are turned off.

Bottom line: The daytime running lights are an excellent indicator if your fuel pump is running or not

Here is the Porsche part number: 431 951 253 G Relay 14,11 and it is also used in Audi

You can also use a standard 53 relay but this has minor side effects.

Modification should be possible on any MY 90 or younger.

Location of relay III on the CEB

Daytime running lights on, fuel pump running

Headlights on... ground power supply to relay III interrupted, DRL (Daytime Running Lights) is off since main lights are on now.

(some additions by Theo)
This is how it basically works. The headlight connection is to the headlights on (high or low beam) connection inside the relay and in the CEB connected on pin 56. This pin is wired since MY1990 but exists in the relay starting 1987. There will be an effective ground for the coil via the headlight filaments (either one since one is always connected to this point) as long as headlights are off. This will only work if at least one the filaments selected are good - two burned out low beams or completely missing HL bulbs means the fogs won't work with the headlights off. As soon as the headlights turn on (must be up) the fogs will go off - since there is no voltage difference across the coil and therefore the Daylight Driving relay is powered off.

The diode in the Porsche DRL relay is to avoid the fogs coming on when not intended - the fuel pumps will never run due to this since the relay coil in series limits the current to ~150 mA.  The relay coil takes its power from the output of the Fuel Pump relay.

When using a standard relay: the fuel pump is off and the headlights are on the coil will energize supplied in reverse by the headlights and grounded through the fuel pump(s). So if you turn the ignition on (engine not running) with headlights on the DRL fogs will come on.. this does not happen with the stock DRL relay.

Probably a rather minor issue - but that is what the diode is for. Adding a diode on the back of the CE panel in series with the coil connection would allow a regular Bosch SPST relay (...'53 type) to work exactly like the stock DRL relay does.

Alan - Theo


I modified a type 53 relay. Small mod to pickup the tiny coil wire and implement a diode in it. The cathode of the Diode is on pin 85 and since there was no room there due to the mechanics of the relay, I added it to pin 86 using the anode of the diode to connect to #86. To secure it I used some hot-melt glue which avoids any mechanical movement to. If you use a 53 relay, remove the resistor between 85 and 86 as well. Here are a few pics.

As you can see on the pics from schocki, the outer (Fog) lights in the  front go on.

It works like this: The relay III engages when the fuel pump runs (basically: when the engine runs) and (!) when the lights are off. The 86 side of relay III is connected to the filament power-side of the headlight bulbs. When the headlight bulb is off, the filament pulls down the relay to Gnd. When the headlight is switched on, it gets 12v on the filament and now both sides of the relay coil carry 12v, so the relay de-activates turning the DRL lights off.


1992 928GTS midnight blue

The Netherlands