Porsche Enthusiasts Club Forum

Hi, can anyone remind me.... does the DME turn the fuel pump on/off briefly when you turn the ignition on before cranking like on a lot of 1980s cars, or does it wait for the cranking/ 200+ rpm signals before doing anything. Pump runs fine with relay bypass, and relay appears to be good.
No fuel or spark, relay is sending power to DME, but I don't think the DME is 'waking up'

Pump should run continuosly when the ignition is on as far as I'm aware.

Have you tried another DME relay? the solder inside breaks down and causes all sorts of gremlins like your issue.

I don't think it's the relay in this case, because I can do the 3 wire bypass, and still no spark or injectors when cranking. I'm going to do a wire by wire check on the harness from the DME connector at the weekend to make sure it's all intact, as it's all a bit of a mess under the dash. The DME was chucked in the boot when I got it, so I'm not even sure if it came from this car

Check the crank sensors are plugged in and the resistance, should be about 900 ohms I think

whats the part number of the DME you have

Will do. going to check all the wiring and components at the weekend. Checked the DME part number. It's correct for the car. whether it's any good remains to be seen

do you have manuals and wiring diagrams,

the DME is fed power from the ignition lock, just read which ones in the 968 manual but i don't think that's relevant other than the information is in there.

manuals can be found on a drop down list here...

http://www.pdlibrary.co.uk/

Have some info, but thanks, these are handy. Will get stuck in at the weekend.

Could be crank sensors, DME does not get signal to fire it if these are down (2 on the turbo) let me know and I can let you know where to get them for £40 not £200 that Pork want, or £175 from euro,genuine Bosch, not long since I changed both on my Turbo due to no spark or fuel.
The lead is slightly longer but the same animal fitted to 80 / 90's BMW six cylinder engines as a cam position sensor.

Thanks. Will check them out while I'm looking at the wiring.

bit late in answering this but as far as I know on all 944 models the initial ignition switch-on briefly powers the fuel pump to pressurize the system then switches off .

When turning the engine over on the starter the fuel pump is powered continuously (provided the ECU sees 200+ rpm cranking speed from the speed sensor.)

I'll check this with the Bosch manual later and amend if I am incorrect on this for pre 85.5 cars

Thanks for the tips. all inputs and outputs appear correct, and fuel and ignition system components check out OK. Suspect the DME is toast.

My early 944 has sat a long time before I got it, the DME had obvious corrosion so I replaced it, still wouldn’t start then discovered the fuel pump had seized up, still wouldn’t start, 3 of the injectors were seized up( freed off by tapping with a screwdriver while cranking over) it then started no problem. Might not help but it’s just what I found.

had any luck resolving this yet ?

Tell us a bit more about what led up to this . Did it suddenly cut out ? had you been doing any work on the car or washed under the bonnet ? has the car been left unused for a while ?
Do you have tach bounce when cranking ?

Car has been off the road for years. Fuel pump runs with 3 wire bypass, Tacho bounces when flicking ignition switch on/off but not while cranking. Have fitted a pair of known good reference sensors. DME not energising relay even on initial switch on. Am going to lock myself in the garage at the weekend and do a pin by pin check on the wiring again.

If I can't find anything, would anyone have a running '85ish car who would be willing to plug my DME in to it and see if it runs?

if the fuel pump runs and the tach shows signs of movement, then the ECU is definitely receiving power on ignition switch on.

Looking at the DME relay ........pins 85 & 86 energise the first relay coil when the ignition is switched and supply power to the ECU , injectors and various sensors via the ECU, so the first checks should be on the DME relay base with the DME relay removed and ignition off.


1. Check that pin 85 ( pin 1) is permanently connected to ground using a multimeter switched to continuity(.If it whistles its good) if it doesn't then check and clean the ground points (brown wires)

2. Check pin 86 (pin 3) has no current with ignition off

3. Switch ignition on and check pin 86 now has 12v+ (battery voltage)

4 Confirm that terminal 30 (pin 2) has 12v battery voltage with ignition off and with ignition on.

If this all checks out then the issue lies with the second relay coil not being energised by the ECU .

Reasons could be many but it is very unlikely to be the ECU at fault .

The ECU will only function when all the various sensors send a satisfactory signal to it.

First and most critical is the engine speed sensor. If you are seeing a tach bounce then the sensor must be receiving power from the ECU
If the tach needle doesn't bounce when cranking the engine then either the engine is cranking too slowly (needs to be 200rpm min ) or the sensor must be too far away from the flywheel starter ring or there is a fault with the sensor , the wiring or the connectors between the sensor and the ECU

The speed sensor should be your next check before looking at the ECU ........follow a logical sequence based on testing and don't change or adjust things at random