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Is My DME Dead??

My 1983 euro 944 will not start, It was running 4 days ago when i put it in my garage, have gone to start it now and it turns over but will not start. I have been through the following from Clarks Garage

1. No spark at plugs when plug removed and engine cranked - have replaced distributor cap, rotor and all HT Leads, needed doing anyway

2. Carried out Coil test as per clarks garage, spark when coil shorted out. although not overly strong.

3. Voltage to primary side of coil, 12v at each of the two contacts when iginition on

4. Restistace of coil checked and all within limits

5. Checked resitance at speed and crank sensors, all within limits

6. DME relay replaced

7. Voltage to pins 18 and 35 on DME

8. Checked Tacho for bouncing when trying to start, Nothing seen

9. DME taken apart and checked for dry joints.

The ony other i can think of is that the diagram on Clarks Garage page shows a connector near the DME plug for the refernce sensors, i cant find this, is it near the DME plug or does it join the wiring loom much earlier on?

Can anyone help on anything further i can check, i have been in the cold all day and dont really want to but a new DME ( if i can find one) unless i really have to.

Many Thanks

Shaun
Old 01-28-2006, 08:24 AM
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Reference sensor. Check the plugs at the back of the engine. Take them apart and replug them a few times. Look for broken leads into the plugs and into the tops of the sensors. Sometimes you can tie the cables up a bit and make them stay connected or pull tie the connectors together.

While Clarks is a good reference it seems to bypass critical checks that are part of the Porsche DME Testing Procedures.

Basic Testing:

With the key on check for voltage at the coil and at one of the injector plugs.

No voltage at the coil means the ignition switch is suspect/bad. Could be the connector at the back of the ignition switch too.

No voltage at the injector plug (pull one to check) but at the coil means the DME relay is suspect/bad.

Voltage at both means the reference sensor is suspect. Test for spark by pulling the number one spark plug wire, inserting a spare plug, lay it on the intake to ground it in a possition you can see when you crank the car. If you don't have spark check the sensor connectors as described earlier. Could also be an out of adjustment sensor.

No spark can also be the HV wire from the coil to the dist cap. Can also be a broken or loose rotor. Can be a broken timing belt that will not turn the rotor. The ignition switch could be making the circuit to the coil when in the on possition but not in the start posstion. Use a test light with aligator clips on the power side of the coil to see that voltage is there during cranking.

Since you checked for voltage at the DME connector it is to be assumed that the first set of contacts in the DME relay are working. These power the injectors and the DME.

The fuel pump runs only when the engine is cranking. You can hear it if you lean out and down of the car while cranking. It will run for a half second or so after you stop cranking.

You can bypass the DME/fuel pump relay with a simple aligator tip jumper between aux fuse #2 and #3. This will run the fuel pump. Used to check fuel pressure and flow at the rail with out running the engine.

In most no start cases it's not the coil or the wires or the dist cap or the rotor. Usually the DME relay, ignition switch, reference sensor(s), broken timing belt then fuel pump. The DME is way down the list.
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Old 01-28-2006, 08:50 AM
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Thanks SoCal, i have checked the voltage at the injectors, All ok, I have have checked the fuel , disconnected the starter and moved the ignition swith to the start position, fuel pump runs ok. Timing belt is ok ( Replaced 5k miles ago), Rotor turns when the distributor cap is off. I checked the cables from the sensors and re connected them a few times. I have checked voltage at the positive side of the coil while cranking and get about 9v, 12v when not cranking. Any other ideas. The only thing i can think is a broken reference sensor, is there any other way to eliminate these as being faulty without replacing them?

Thanks again
Old 01-28-2006, 09:27 AM
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Check the resistance across the three pins on the reference sensor. From the center to one side should be open circuit -- no resistance. The other side should show 0.6 to 1.6 K Ohms.

The resistance across the coil is Term 1 to 15 = 0.4 to 0.6 Ohms. Term 1 to 4 = 5 to 7,2 K ohms.

The reference sensor should show a 2 volt pulse when cranking, the speed sensor should show a constant 2 to 2.5 volts when cranking.

Jumper between the coil and the battery and between aux fuse 2 and 3 and see if the engine will run.

P.S. Make sure the sensors are plug back in to their respective cables. Try changing these if the tach does not bounce when cranking.
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
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Old 01-28-2006, 09:50 AM
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Thanks again SoCal. I have the right resistance on the reference sensors and the coil, Where would i check for 2v? at the sensor themselves when cranked, or at the DME plug. I am trying to borrow and oscilliscope to help with this at the moment, but i am currently stuck with a multimeter. if i jumper between the coil ( Pin 15 and earth I Guess) and accross aux fuse 2 and 3 what would this indicate?


Many thanks again
Old 01-28-2006, 10:30 AM
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You can send the DME to me and I will test it for you.

gb
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Old 01-28-2006, 01:15 PM
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remove the dist cap and make sure the rotor is turning when trying to crank it



EDIT: disregard, I see that you already checked it. Where is the car?
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Last edited by earlr85944; 01-28-2006 at 02:09 PM..
Old 01-28-2006, 02:01 PM
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The car is in Berkshire UK, Think i should be able to get an oscilliscope on monday, i Belive that this should help with testing the reference sensor, any other ideas what i can check in the meantime?
Old 01-28-2006, 02:37 PM
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Jump the coil to the battery is like turning on the key. Jumpering across aux fuse 2 to 3 is running the fuel pump. Crank it and see if it starts.
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Hugh - So Cal 83 944 Driver Person
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
New Users please add your car's year and model to your signature line!
Never break more than you fix!
Old 01-28-2006, 03:01 PM
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Thanks SoCal, I managed to try this earlier, still not started. Cannot see any spark at the plugs at all. The more i test the more it seems to be that the secondary winding on the coil is not outputting, Any further thoughts are much appreciated

Thanks
Old 01-28-2006, 03:06 PM
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Coil failures happen but are rare. Take the number one spark plug cable and plug it directly into the top of the coil. Use a spare spark plug and lay it on the strut top. Jump pin #15 to the battery. Ground out pin #1. When you remove the wire to pin 1 you should see a spark across the plug.
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Hugh - So Cal 83 944 Driver Person
NOT a 'real' Porsche -- Its Better!!!!
When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
New Users please add your car's year and model to your signature line!
Never break more than you fix!
Old 01-28-2006, 03:43 PM
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