Thank you very much P.E.H for going through that extra effort! (A good example of how nice & helpful some people can be here on this forum.) I wrote this little summary right before logging on & seeing your reply:


I feel that is always good to finish off a thread so that people searching the archives in the future may get some use out of it.

According to the somewhat butchered German to English translation & the wiring diagrams in the service CD, the following is supposed to be the proper glow plug relay function:

The glow plugs are supposed to turn on in the pre-glow stage (duh). They also remain on for a short while after the dashboard indicator light goes out (again, duh). The length of time for each stage is temperature dependant, determined via either a temperature sensor located on the engine (for earlier cars) or, on the later cars, via a temperature sensor located internally in the relay. This function is activated by voltage to terminal 15 when the key is in position 2.

The glow plug relay is supposed to continue supplying voltage to the plugs *while* the engine is cranking. This function is activated by voltage supplied by terminal 50 when the key is in position 3 (cranking).

Once the engine fires & the key is released back to position 2, there is no longer any voltage supplied to the glow plugs. They do not remain on to help keep the engine firing smoothly, as some people have assumed/posted.

So… if you're not getting voltage at terminal 50 (or the relay is not responding to the voltage at terminal 50) while trying to crank the car, the glow plugs can cool off a bit if you need to crank the engine for more than a couple seconds.

Hope this helps someone in the future.

RTH