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DME Fuel Pump Relay Narrative

The 1987 924S has been unexpectedly and intermittently shutting off (like four times in as many weeks) while on the road. It *felt* like a fuel issue, so replaced the fuel filter. Immediately the problem got worse! The car would not run 5 minutes. Problem turns out to likely be the DME relay. Why?

My reasoning: The fuel filter was fairly gunked up with blackish stuff mixed with gas when we shook it out. With the new filter installed and increased capacity for fuel flow the injection system (which may have been a bit starved for fuel previously) is now asking DME relay to function more often than just staying constantly pegged in the fuel pump "on" position, which is taxing the relay.

In essence, a dirty fuel filter may have prematurely taken out the DME relay.

I'm suspecting the DME relay now as I bypassed/jumpered the DME and the car ran fine.

Holiday today, so no car parts...

Any thoughts???

Old 07-04-2013, 10:58 PM
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Wait for a DME relay or resolder the one you have? The DME relay does not cycle to manage fuel pressure. I do not know if there can be a real connection between cleaning the filter and the relay getting worse. IF the DME relay you have does not have the updated part number 993.615.227.00 I would not be interested in trying to diagnose around it.

-Joel.
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Old 07-05-2013, 06:01 AM
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Buy the Stribel DME/fuel pump relay. Not the local auto parts store cheap substitute.
Porsche 924S/944/968 Fuel Injection - Page 4

I doubt if an old fuel pump filter caused a relay failure. It is an electrical part. Bad grounds, weak, old battery--4 years old or older, overheating--bad connections or as mentioned dirty grounds. Now an old fuel pump that buzzes like crazy may be a cause.

J_AZ
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1988 924S, 80K ..+ 1987 924S, 150K DD (+15K est. bad odo)
Old 07-05-2013, 11:13 AM
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Thanks guys for the response! Apparently, I was wrong on several counts. First, on the operation of the system, and also on the DME being the problem.

I bought a new DME, and unless it is bad out the the box, the problem is probably not there. After replacement with the new DME (which cost far too much anyway!), the car started acting up again. I had to re-install the jumpers to get home, and it ran great again.

Going to be testing the sensors and the DME computer next. I suppose something on the low voltage side is causing the DME to shut off.
Old 07-05-2013, 01:52 PM
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I'm assuming this video is relevant to the sensor testing.

How To Test RPM & TDC Sensors in 944 and 911 3.2 Carrera Porsches - YouTube
Old 07-05-2013, 01:56 PM
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Let me ask, could problems with either Reference Mark Sensor be causing the DME relay to shut off the fuel pump?
Old 07-05-2013, 03:36 PM
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Or without removing the main computer cable:
Speed and Reference Sensors - Checking, Replacement, and Adjustment
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Old 07-05-2013, 03:37 PM
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Thanks John!

From the link: "Another way to check the operation of the speed and reference sensors it to crank the vehicle and watch the response of the tachometer. If the tachometer jumps during cranking, the speed and reference sensors are probably good."

Any idea 1) how much the tach will jump, and 2) if the tach will jump with the DME relay removed?

I plan on testing them later with the MM, but need to get a new battery for it first.

The sensors look like a pain to remove.
Old 07-05-2013, 03:44 PM
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Test #1-
When you crank the engine by initially turning the key--------------at that moment when the starter engages the flywheel ring gear------do you see the tach needle bounce 1/16th of an inch??

If no bounce, do the test you found or measure the sensors with a meter per Clarks.

The 924S also may have a cracked igniton switch and is failing to make contact.
Pelican Parts - Product Information: 111-905-865-L-M293

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Old 07-05-2013, 03:46 PM
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Removing the reference sensors:

Put a piece of tape over the TDC bell housing hole--if you drop a tool or bolt in the hole it is a lot of work to fix.

The sensors have an "O" ring in the middle that rots, gets very hard and makes the removal impossible---usually you damage the sensor or break it in half. Again a lot of work to fix.
Gently wiggle-turn- lift carefully and it may come out easy.

Replacement reference sensors are cheap if you use the BMW interchange instead of the OEM. The cable is a little longer.

J_AZ
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Old 07-05-2013, 03:53 PM
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Thanks so much for all the help, John!


There is bounce on the tach when I turn the key. Does this mean the sensors are likely OK, and I should focus my attention on the ignition?

I'm not really in a position to disassemble the steering column, having no replacement part in hand. Is there a decent test for the ignition switch crack without a complete tear down? I found the Clark's instructions replacing the ignition switch. ( http://www.clarks-garage.com/shop-manual/elect-25.htm ) While the car is not very reliable on it's own, it is somewhat drive-able (with the DME jumpers), and I'm not looking to offline it until I have parts, and lots of confidence I've found the problem/solution.

Last edited by tmk04747; 07-05-2013 at 06:24 PM.. Reason: Add more content
Old 07-05-2013, 04:07 PM
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After more time spent diagnosing the problem... when the vehicle stalled, smacking the DME computer helped. So I ended up removing the DME computer and resoldering the joints near the capacitor. So far, no problems. It may be fixed.
Old 07-10-2013, 03:33 PM
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Nice.

I am impressed you went directly to the DME Computer capacitor to resolder. Nice work on searching for a solution and doing the resolder.

The DME computer on the '87 924S comes up on ebay or Pelican parts for sale occasionally. 944 618 121 04.
I got a spare for about $100-$125.

J_AZ
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Old 07-10-2013, 03:45 PM
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Thanks John!

I just saw one today on Ebay near that price range. Thanks again for your help!

85 5 86 87 88 Porsche 944 924s DME ECU ECM Engine Computer 0 261 200 077 | eBay

Old 07-10-2013, 04:43 PM
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