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DME Relay

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barks944

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I've been looking at the DME relay tonight to help work out possible solutions to some starting issues my dads car has been having and I now am puzzled enough to want the answers to a few questions about it. Is the reason it is setup with two coils is so that the second coil can short out the output of the relay to ground and hence shutting off the fuel pump as a safety feature. It might be so you don't have to show that the DME is safety critical or something. If that is the case what is it that drives the second coil, my Haynes manual tells me that for a late 944 its the A/C compressor which suggests it isn't part of a safety feature!? For my early model car I couldn't work out what the signal came from.

Cheers

Tom
 
It does shut off the fuel pump if the engine stops turning (or at least: it did when Porsche built it, but many are 'modified' when alarms and immobilisers are fitted), but its a double relay inside one case: one relay operates the DME and the other the fuel pump. Simon
 
Are you sure it controls the DME? From what I can see its a relay to control the fuel pump with another relay to provide a backup shut-down of the fuel pump. One of the coils seems to be operated by the DME and switches the live feed to the fuel pump but its weird because one of the terminals of the coil has two connections to the DME. Perhaps this is some kind of shut-down so it can ground its own live feed to the coil. The other coil is operated by another signal (not sure what) but it seems to short out the live feed that supplies the fuel pump to one of the terminals of the other coil. I can only presume that the terminal is the ground terminal so is effectively shorting out the live feed to the fuel pump. Its a little confusing without knowing what its actually meant for and what each of the terminals connected to the DME is doing.
 
Now youve got me thinking, because this is something that Ive 'known' for almost 20 years, if you know what I mean? Its certainly known as a 'fuel pump relay' in series one cars..... I'll read up in the morning.
 
Im relatively sure yes. Without intending to sound facetious, it is called 'DME relay', which certainly supports the argument that the relay provides power to the Bosch DME (Digital Motor Electronics / ECU). In earlier models it was referred to as the fuel pump relay, although the part is common and I believe that the function is the same (one internal relay for the DME and one for the fuel pump).
 
You are probably right. The way its wired is just a bit weird. I think then that the First coil provides the positive to the DME and the second coil. This means that the DME grounds the second coil to power the fuel pump. Here is a well drawn schematic of the DME relay. http://picasaweb.google.co.uk/tomspics1234/Pictures02#5317083618327607954 The only thing I cant quite explain is why the DME has 3 connections to the relay.
 
Could be a fuel shut off system in case of a crash? I don't know what the sensor would be, Try putting your car on its roof[:D] Mike[:'(]
 
One coil powers the DME and coil. This is switched by the ignition switch. The other powers the fuel pump. The fuel pump coil is switched by the DME based on whether or not it sees engine rotation from the crank sensor (this is the 'crash sensor': if the engine stops so does the fuel pump) Not that I spend all my time linking to Clarks garage, but: [link=http://www.clarks-garage.com/shop-manual/fuel-05.htm]http://www.clarks-garage.com/shop-manual/fuel-05.htm[/link] I once spent three days trying to get the ^%$£ thing started after doing something fairly simple, turned out I had a duff DME relay AND I'd disturbed the crank sensor so even when I'd managed to get power to the DME, it still didn't think the engine was turning hence no fuel [:mad:]
 
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