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Emnyama944's Avatar
 
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Power to the DME Relay

I need some help troubleshooting a no start issue on my 944 turbo, the short question is, should there be 12 volts to the DME when the key is in the on position or do we only get 12 volts when we turn the key to start the car?

The car runs with the DME relay jumped but I've tried three different DME relays and though the car turns over a few times it dies very quickly. I have a VEMS ECU and the car does exactly the same thing, but I found there is only 12 volts to the ECU when I jump the DME relay so I'm wondering if I'm not getting power to the relay.

Any help would be appreciated.

Rudi

Old 02-26-2021, 08:12 AM
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As you can see from this diagram, turning the ignition switch on provides power to the DME... once the DME senses the engine rotation (via the flywheel sensors), it provides a ground for the fuel pump half of the DME relay, turning on the fuel pump.
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Old 02-27-2021, 08:38 AM
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Thanks George, I've been troubleshooting this issue for a while and I was starting to doubt myself about getting 12 volts at pin 86 on the DME relay, the most I get is about 0.5 volts. The question now is, why am I not getting power to pin 86? I've jumped the alarm to make sure it's not causing any issues.
Old 02-27-2021, 12:04 PM
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What year is your car?

The alarm and airbag circuitry come into play on 87+ cars...
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1987 924S 944 SPEC racecar - '88 pistons/DME, short 5th
1987 944S - Restored salvage title, ready to live again
I can fix your late 944 Cruise Control module - PM for details
Old 02-28-2021, 05:07 AM
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Quote:
Originally Posted by Emnyama944 View Post
Thanks George, I've been troubleshooting this issue for a while and I was starting to doubt myself about getting 12 volts at pin 86 on the DME relay, the most I get is about 0.5 volts. The question now is, why am I not getting power to pin 86? I've jumped the alarm to make sure it's not causing any issues.
It's probably a bad connection or wire to pin 86 from the ignition switch. Check your circuit.
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Good luck, George Beuselinck
www.944ecology.com
Porsche Deconstructor (tm)
We buy 944s and sell 944 parts (845-379-1944)
We rebuild torque tubes and instrument clusters for 944/968/928
Old 02-28-2021, 05:33 AM
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Quote:
Originally Posted by kdjones2000 View Post
What year is your car?

The alarm and airbag circuitry come into play on 87+ cars...
Thanks, I've bypassed the alarm and the car has been butchered a little bit over the years so it's possible the wiring is messed up, it's an 87' but doesn't have an airbag.

I spent most of the day going through the wiring diagrams and trying climbing under the dash to trace the possible routs of the wires, I was hoping there might be a common issue but I guess it's time to curl up under the dash.
Old 02-28-2021, 06:13 PM
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This might help you. It's for an '88, but '87 should be similar. The airbag needs to be taken out properly in order for the DME to get voltage.

DME Relay Quiz

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NASA/PCA Instructor: '88 951S - with LBE, Guru chips, 3Bar FPR, 1.3mm shimmed WG, 3120 lbs, 256 RWHP, 15 psig boost
1987 924S 944 SPEC racecar - '88 pistons/DME, short 5th
1987 944S - Restored salvage title, ready to live again
I can fix your late 944 Cruise Control module - PM for details
Old 02-28-2021, 08:37 PM
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