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Desertt5
Join Date: Jul 2008
Location: ABQ, NM
Posts: 391
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DME relay questions
 I thought my DME relay died on me, car runs great with a jumper. Got a new relay in today and no start, no fuel pump. Put jumper back on and starts fine. Do have tach bounce whene not starting, just no fuel pressure. (fuel guage mounted on rail: worth every cent at times like this) Was looking at the control lines to the relay. I have the 12vdc on pin 86. Should the ground for pin 85 be there once key is in the run position? or start? Trying to figure out what is missing to find out why. I believe that the ground for pin 85b is only when started? Just got the electrical PDF and started looking.
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07-12-2010, 05:34 PM
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Desertt5
Join Date: Jul 2008
Location: ABQ, NM
Posts: 391
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Looks like I am missing the ground to pin 85b.
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07-12-2010, 06:15 PM
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Old 928 Guy
Join Date: Oct 2005
Location: Maryland
Posts: 408
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it's strange that it runs with the jumper then..because you usually aren't jumping the ground connections, only the relay contacts
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1990 C2 Cabriolet Guards Red  '81 928 Zinnmetallic Project Car (sold)  )'87 944na Maraschino Red Metallic (sold) )
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07-26-2010, 06:22 PM
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Registered
Join Date: Jan 2003
Posts: 377
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The reason it runs with the jumpers is because, instead of the relay closing the power wire connections via 85/85b electromagnets, you are manually jumpering the power connections to the fuel pump and injectors.. Terminal 85b is still a voltage/current path, rather than a ground connection. It allows current to flow from terminal 30, into the DME, where it would then be shorted to ground inside the DME, after the DME get's a "good/run" signal from the speed/reference sensors.
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"porsche it's an institution" |
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07-28-2010, 12:47 AM
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Desertt5
Join Date: Jul 2008
Location: ABQ, NM
Posts: 391
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Just to follow up - I swapped the DME and she started right up. Put the old one back and no fuel pressure. New DME and running fine after 2 weeks.
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07-30-2010, 07:46 AM
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