What are the symptoms of a DME relay failure?

[poprock]

I'm just curious "¦

If your DME fails, what exactly are the usual symptoms?

Am I right i thinking that because the DME should control the fuel pump, so turning the key in the ignition would lead to the starter motor running but the engine not firing?

I've never had the misfortune, and I was idly wondering exactly how I would know that the glove box spare was finally needed.

 

[TTM]

Your thinking seems right.

The DME relay on my S2 once "half-failed", as applying some finger pressure on it allowed for starting the car, but as soon as I got home I replaced it with a new unit.

 

[peanut]

the DME relay is a double relay . A 12v+ live feed is fed to terminal 30 on the DME relay constantly even with the ignition off.
When you switch the ignition on a 12v+ feed is fed to DME terminal 86 which earths via terminal 85 this activates one half of the relay which makes a contact between terminal 30 (12v+) and terminal 87b which completes the circuit to the fuel pump.
At the same time a connection is also made in the DME between terminal 30 and terminal 85b which activates the other half of the DME relay
. This half connects the 12v+ from terminal 30 to terminal 87 which feeds 12v+ to the injectors and the ECU .
The injectors are then grounded within the ECU by a variable timer which pulses the injectors.
All this is effected within the ECU by various sensors .

Its not commonly understood that the 944 motronic system requires the engine to be cranked at a minimum rate of 200 times per minute. Any less and the ECU disconnects the feed to the injectors.This is why the crank sensor is so important and a very good battery.

Easier to follow the circuit diagram than describe its function really.

So to answer your question if either half of the DME relay fails to activate then the 12v+ supply to various components of the fuel injection system fails

 

[Eldavo]

When my dme relay went it would start absolutely fine and off you'd trot. Within 2-5 mins when the car started warming up it would cut out and not restart. If you pulled the relay out and waved it/blew on it to cool it down then put it back in it would start on the button. With cold weather such as now I would imagine that it would cool down pretty quickly on it's own. Before I realised it was the dme on mine I once had to wait 30 mins before it was cool enough to restart.