DME question

[mike5876]

I have a 1984 Carrera that won start..it started perfectly an then i tried to clean up some wires in the trunk and disconnected some wires from the fuse box i shouldn't have.....the fuel pump won't come on unless i put power to it then it still won't start...only cranks

in a working situation how does a dme work? is there a terminal in the fuse box that powers the dome which in turn powers up the fuel pump?

[Flat6pac]

The fuel pump is suposed to run in the cranking position, not before. You might remove the fuel pump fuse and jump from battery to the bottom terminal of removed fuse to run the pump.
Bruce

[steely]

Originally Posted by mike5876

in a working situation how does a dme work? is there a terminal in the fuse box that powers the dome which in turn powers up the fuel pump?

This is a quick answer, but the DME relay is really 2 relays in one, 1 stage powers the DME, and as Bruce mentions, if the DME (ECU) sees the engine turning (via Reference sensor), it will latch the second relay - which provides pwr to the FP. And yes, there are fuses involved but I'd have to look them up to see which ones.

[86 911 Targa]

^^^^^
This is for our M/Y 86, but should be close to yours M/Y.

Best,

Gerry

[mike5876]

Originally Posted by steely

This is a quick answer, but the DME relay is really 2 relays in one, 1 stage powers the DME, and as Bruce mentions, if the DME (ECU) sees the engine turning (via Reference sensor), it will latch the second relay - which provides pwr to the FP. And yes, there are fuses involved but I'd have to look them up to see which ones.

i would appreciate that!

also thank you 86targa for schematic i have like 6 wires....the small ones.... disconnected and am having a hard time telling what color they are and on the schematic which wires are the small ones

[86 911 Targa]

^^^^^
Fuse panel with headlight relays.

Also, the wire gauge/color code is printed on the previous post schematic.

Example: Fuse #3 (6th from the left), Fuel pump 2.5 (mm) RT/GN Red/Green (NR 173 is the harness number).

Gerry

[86 911 Targa]

^^^^^
Wire color codes.

Gerry

[steely]

Someone confirm
- the FP fuse is a 25A, Blue, and 6th from the left in Gerry's picture (Thanks Gerry).

Mike - look for a chart on the inside of your fuse panel cover - or search here for one.
I am a retired EE but I find the porsche electrics docs/numbering unnecessarily goofy.
For example and Please note - the pic on this chart is upside down relative to the fuse panel pic that Gerry provided, and the numbers are not continuous.

[ischmitz]

Check that with the key in RUN the idle control valve on top of the engine vibrates. That tells you the DME gets power from the ignition. Also, when you turn the key to RUN you should hear the DME relay switching. This is the first stage of the DME relay turning on. If that doesn't work you need to check a wire from T15 (hot in RUN and START, top of Fuse 7) to the DME relay was disconnected at the fuse box.

Then, once you crank the DME relay stage 2 should put power onto Fuse 3 (fuel pump).

[mike5876]

Originally Posted by ischmitz

Check that with the key in RUN the idle control valve on top of the engine vibrates. That tells you the DME gets power from the ignition. Also, when you turn the key to RUN you should hear the DME relay switching. This is the first stage of the DME relay turning on. If that doesn't work you need to check a wire from T15 (hot in RUN and START, top of Fuse 7) to the DME relay was disconnected at the fuse box.

Then, once you crank the DME relay stage 2 should put power onto Fuse 3 (fuel pump).

thanks everyone going to try and get his today

there are two fuses called #7 on chart or do you mean 7th from left in the pic above?

[86 911 Targa]

pm me as needed.

Gerry