No power to fuel pump (944) help!

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• Reply by Jack D. Russell, Sr.
• Newsgroup: alt.autos.porsche,alt.autos.porsche.944
• Reply to: All; "Elma"
• Date:Tue, 3 Apr 2007 04:29:20 -0400
• Subj: No power to fuel pump (944) help! ===================================================================

E> It's at my mechanics right now for an intermittent starting problem E> I've been having. They didn't hear the fuel pump running so they E> tested to see if the fuel pump was getting power, it wasn't. They E> went backward to see if the DME relay was getting power, it wasn't E> either. Funny enough, They tested it for spark (which it had) and E> when they plugged everything back in, it started right up. I just got E> off the phone with my mechanic and he was stumped, but thought it E> could be the DME or some faulty wiring. We just replaced the fuel E> pump, DME relay and fuel filter last spring. E>

E> When it did this in the past, it seemed like it wouldn't like to E> start in colder temperatures, and when you'd pull it into a garage to E> warm up, it would eventually start; possibly because some bad wiring E> was expanding and contracting? E>

E> Any other ideas why the fuel pump and DME relay wouldn't be getting E> power?? Thanks! E>

Find another mechanic that knows how to work on 944s.

I had a similar problem. Stupidily replace the fuel pump, still didn't work. Turned out the be the fuel pressure regulator. It went bad, allowed too much pressure, computer then shut fuel system down but still allowed spark. Tested fuel pressure by putting gauge on end of fuel rail, replace pressure regulator, started right up. Your regulator may be faulty an allows the pressure to drop while the mechanic takes the time to check other components. Fuel pressure is easy to check. For what it's worth.

Tony

If you figure out the problem, please post the results. I'm sure many are interested. I would mess with the DME, doubtful it's the problem. I've opened up mine and resoldered all connections a few time. Even went so far as to buy a spare DME. Sold the care, still have the DME. They are rugged.\

Tony

If you have a factory alarm, check it. It and the DME relay are the two most frequent culprits.

Kevin Catellus Engineering

Most probably your DME relay, they can create interminent fault which can be sensitive to temperature. This is because the spring loaded contact inside can get weak and the contact points will burn, this will also happen if you try to start the car with weak battery. First think you need to know that the function of the DME relay is to turn on the fuel pump WHEN the engine starts cranking ( on 968 it also turns on the idle stabilized circuit and O2 sensor but that may not apply depending on the age of your 944 ). This is a safety feature, when your engine stops, so does the fuel pump. The way it works is that pin 30 of the relay is always on, connected directly to the battery. The DME computer detects the engine cranking it grounds pin 85 of the relay which then switches power to pins 87 (pump) and

in my case one of the windings (there are 2 relays inside) went open circuit. I know because I unwound it.

Dieter

Martin, you seem to have a good understanding of the DME relay and the control electrics.

So can you help me?

My fuel gauge works when you start the car up but will suddenly drop to zero after about 3 minutes. Longer in cold weather and shorter in hot weather. It may work later in the day if the car is left to stand.

I have eliminated the sender by measuring it when working and when not and it is reading correctly according to the Clarke's Garage figures.

Is there some circuit which turns on or off as the car warms up which could directly or indirectly affect the fuel gauge?

Could the DME relay be implicated?

All other circuits seem OK.

Alec

did you check the connectors to the gauge cluster? that's one likely culprit. also possible is defective ground

Alec, problems with gauges are quite frequent, almost always down to bad ground. There is a common ground point on the chassis for all of the facia equipment. The wiring schema in the workshop manual should tell you approximately where it is located and the wire colours, then you have to dive under the dashboard and trace the wires in between... good luck.

Absolutely nothing to do with the DME relay.

all ground wires on 944, except the one from the battery, are brown

Elma,

Did you resolve this? Contact me direct if you still need help. The BOSCH DME kills the fuel to the engine in an accident [if engine stops]. This is why the DME Relay and fuel pump is powered by the DME. This can fail in a otherwise good DME.

FR Wilk Engineering "Power PROM" Performance Products

Performance Chips for Porsches.

FR Wilk:

Sorry, this isn't quite correct. The DME relay turns on the fuel pump WHEN the engine starts cranking ( on 968 it also turns on the idle stabilized circuit and O2 sensor but that may not apply depending on the age of your 944 ). This is a safety feature, when your engine stops, so does the fuel pump. The way it works is that pin 30 of the relay is always on, connected directly to the battery. The DME COMPUTER detects the engine cranking it GROUNDS pin 85 of the relay which then switches power to pins 87 (pump) and

I've heard that some of the grounds in the interior (OEM stereo, possibly others) aren't ground. No personal knowledge of this. But I believe the above is true for all engine compartment grounds.

snipped-for-privacy@ptn-nntp-reader02.plus.net...

What?

I do have personal knowlege. if someone has cut up the wiring harness doing a butcher job of connecting a stereo, all bets are off, but as shipped from factory, brown = ground. The OEM stereo does have a connection at the back of one connector that I"ve seen idiots cut making one brown wire just float. But the most common stereo installation problem is a failure to connect the antenna amplfier power lead to anything.

I've heard that some of the grounds in the interior (OEM stereo, possibly others) aren't ground. No personal knowledge of this. But I believe the above is true for all engine compartment grounds.