If I jumper DME relays pins 30-87b, should I see 12v at the FP? - Rennlist - Porsche Discussion Forums

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If I jumper DME relays pins 30-87b, should I see 12v at the FP?

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If I jumper DME relays pins 30-87b, should I see 12v at the FP?

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Old 06-11-2018, 09:56 PM
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Jeff N.
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Default If I jumper DME relays pins 30-87b, should I see 12v at the FP?

Working a no start issue.

If I jumper dme relay pins 30-87b, key in off position, I would expect to see 12v at fuel pump harness connector in aft section of the car. Is this correct?

I can confirm the following:
- 12v at DME relay pin 30
- Jumper to pin 87b, 12v at 87b
- Fuse #34 OK
- No 12v at the fuel pump harness plug when jumpered
- No FP tester available so can't test directly FP at rail.
- When I remove the fuel rail nut, i just get a drip of fuel...no gush like the rail pressurized
- When I crank the car with a noid light in the injector harness, I get flashes.
- Plugs confirmed firing

Currently thinking I have some sort of odd problem where I have lost power to the fuel pump. Car died suddenly while driving.

Next step is to find way to power FP directly with 12v. Guessing I will get FP and car will start.

Any suggestions on where to look for problems?

TIA!

Last edited by Jeff N.; 06-11-2018 at 10:14 PM.
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Old 06-11-2018, 10:07 PM
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odonnell
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You must have a wiring break, or a problem in the fuse panel. Jumping 30 to 87b is sending battery power right though the harness to the pump.
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Old 06-11-2018, 11:01 PM
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Right. Diagram here on page 7 shows the same - http://www.arnnworx.com/download/DME_KLR_Test_Plan.pdf

Anyone have any idea where that part of the harness runs? Under the car or ????
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Old 06-12-2018, 12:41 AM
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Check that fuse #34 and the socket its' pins fit into have good connections. It's not unusual for them to get loose and cause this problem.
If you can, check to see if you have 12 volts at one or both pins of the fuse, with the DME jumpered.

FWIW, you don't state what model year you have, but I discovered a while back that the O2 sensor heater is also connected to this circuit.
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Old 06-12-2018, 01:34 AM
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Thanks Paul. It's an 86.

I do have 12v to both fuse pins but I have been unable to verify continuity from the panel to the 12v lead at the fuel pump. Still working on that.

Will try to look closely at the panel bottom to see if I can see any problem. I undid the screws but it was really unhappy to lift up so I didn't force it.

Regarding the 02 heater..yeah...good reminder. I did have the problem some time ago. Melted the O2 lead and it killed the FP. I don't seem to have any symptoms of a short at the moment. More an open circuit problem. :/

Last edited by Jeff N.; 06-12-2018 at 02:07 AM. Reason: typo
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Old 06-12-2018, 12:35 PM
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Originally Posted by Jeff N. View Post
Regarding the 02 heater..yeah...good reminder. I did have the problem some time ago. Melted the O2 lead and it killed the FP. I don't seem to have any symptoms of a short at the moment. More an open circuit problem. :/
What was the problem you had? And, how did you fix it?
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Old 06-12-2018, 12:48 PM
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Originally Posted by Dan Martinic View Post
What was the problem you had? And, how did you fix it?
Oh..it was years ago. The car died and it turned out that the O2 sensor wire had melted and shorted. This in turn killed fuse 34 which also runs the fuel pump. Solution was to put in a new O2 sensor and improve the routing / heat shielding of the wiring. It's a bit of an 'undocumented feature' that the O2 sensor runs off the same fuse as the fuel pump.
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Old 06-12-2018, 01:02 PM
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Interesting.. thanks

Learn something new every time!
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Old 06-12-2018, 01:02 PM
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Hmm. I just reviewed some documentation and it turns out that I'm not testing the right place for 12v at the FP. DOH!

Backstory: Was testing at the fuel level sender in the aft area...assumed the pump was an in tank pump like my BMW. Looking at Clarks, I now see that the pump is under the car. DOUBLE DOH.
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Old 06-12-2018, 01:05 PM
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I bet you find your 12v and it's just your fuel pump died. Pretty much standard whenever a modern car dies while driving / running fine
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Old 06-13-2018, 10:55 AM
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12v confirmed at the pump. Pump removed and confirmed that it operates when 12v is applied. Tank strainer removed and confirmed clean.

Next steps are to run fuel pump flow test, replace fuel filter and blow out fuel lines.

Car has been been parked for 18 months. Possible that the check valve was stuck as mentioned by Clarks.
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Old 06-13-2018, 02:19 PM
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Curious to find out; haven't had this particular issue yet
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Old 06-18-2018, 04:05 PM
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Update / closeout post. Car now runs. In summary:

Symptoms:
- Occasional no-start problems followed by the car dying on road and failing to start period
- Car would not run correctly under boost

Diagnosis
- Spark OK via plug tester
- Injectors firing OK during crank via noid light test
- No fuel at fuel, - was failing this test - remove fuel rail cap and ball, jump DME and expect to see steam of fuel

Resolution:
- Took apart rear fuel pump assembly and R&Red the following
- Replaced FP check valve ($20)
- Replaced fuel filter ($25)
- Drained tank, confirmed that fuel strainer was clean, OK
- Removed fuel pressure damper, blew out fuel line back to tank
- Verified FP ran when connected to 12v and with DME relay jumped.
- painted the rear fuel pump metal housing covers (sure this fixed everything)

When re-assembled, fuel now gushed from fuel rail when DME relay was jumped. Car promptly started. Guessing maybe had an obstructed/flakey check valve but that's just a guess.
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Old 06-18-2018, 07:54 PM
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Thanks for the resolution post. Good to know!
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Old 06-19-2018, 06:41 PM
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Good news! Glad it's back on the road and not waiting to be parted out!
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