Another 84 Won't Start Quandry

[dbp]

Quick Facts:84/ Original 3.2, Manual, 138K, No Alarm
Won't Start, Ran great, no issues, next Saturday - cranks strong but no hint of firing.
Searched through every "Does not Start / DME Issues / threads I could find to help troubleshooting

[B][I][U]Fuel:
Fuel pump runs: jumpered fuse - O.K, put circuit tester on fuse and cranked - light came on.
Can smell the fuel.
I did not test injectors.

Electrical- General:
Battery - full charge
Cleaned Grounds on Engine

Ignition - Coil
With Key to On: (+)11.7v, (-)6.9v: and here is a kicker: voltage across the (+) and (-) terminals shows 4.5 volts.
I noticed that coil wire tower had a big chunk broken out off the top so I R&R'd it. No change to readings. When removed the ohms checks were good.
Checked Spark from distributor using sparkplug grounded to engine - no spark
Checked Spark from Coil using sparkplug grounded to engine - no spark
Checked (-) Terminal to ground when cranking using a digital circuit tester light, Almost wanted to think that there was pulses but maybe not, never having seen a nominal state to compare, very fast very faint if there were any pulses.

DME Relay
Replaced DME Relay with Spare - No change to readings
With Ignition turned to On - could hear and feel relay click.
When turned to Start - could not hear nor feel a second click either from the Relay or DME

Ignition Switch
Switch was replaced a couple of years ago after a mechanical failure, new elect portion, mech part from dismantler
When Switch goes to Start - I swear I can hear what sounds like an electrical pop in the switch? Sound is repeatable
More trouble coming or this is as it should be?

DME
With Key to On: ICV Buzzes
When cranking I get some tach movement - barely off the peg, certainly not a bounce
DME gets extremely hot at forward left hand corner when cranking
I have not done any pin to pin testing.
I have not tested the reference sensors.
I have been looking at solder points on the boards, can't see any obvious breaks - however I have trifocals and a not so great mag-glass.
I do not have access to a scope - nor to another 911 to try my DME in.

Conclusion:
I think I have a DME problem (4.5 volts across the (+)(-) terminals on coil). Thank you ishmitz.
I am concerned about the ignition switch.
I am not really competent with electrical / eclectronic systems so it may be time for "professional" help on this.
I am at the "I don't know what I don't know" point right now - any recommendations?

Thanks - 1st post so I may not have the formatting down.
Doug

[Targa Me]

Hello, Doug.
Some one here on this board once had a similar situation (no spark) and it turned out to be a shorted fuel injector causing the no spark to all six. So before you throw a ton of money trying to diagnose your P-car check each and every fuel injector; it may be far fetched but it has happened.

[Targa Me]

Oh, and by the way, your formatting makes for an easy read, thank you.

[ischmitz]

Hey, this is what I call a descent analysis of a problem. I think you have pretty much narrowed it down to the ignition driver circuit in the DME being short. This is why you measure some voltage over the coil.

I bet you when you connect a spark plug straight to the coil and ground it to the engine it will fire one spark per igntion key cycle. I don't think there is much else you can do short of having a replacement DME or putting your DME in some other car.

At this point I recommend sending in the DME for testing/repair.

Ingo

[Tim L]

One thing you didn't mention, did you check your crank sensors?

[ischmitz]

BTW, not that I want to scare people but this is what can happen.

So at this point with the DME most likely shorting the coil to GND I recommend not leaving the igntion in RUN for too much longer. This will only agrevate the damage by burning up the circuit board inside the DME. It'll make it that much harder to repair. See here for an example. Same exact damage: The ignition final drive shorted and the current shunt got cherry red hot burning a hole into the PCB.

I was able to repair this one but there is a limit as to what is salvagable....

:

[dbp]

I have not checked the sensors yet, I probably should be checking everything going right down the fault tree, with the car being 26 years old, though I have some some concerns about breaking connections with old connectors. Is it possible to check from the back end of the connector without really being intrusive and leaving corrosion paths open?

[S ECU Repair]

Here is how to check the sensors ,this pictures are for the 944 harness but the only difference for the Carrera DME is mounted under the driver seat.




Hope this helps you.

Jose

[kidrock]

Jose, that is awesome....this really simplifies the procedure for me. Thanks!!!!

[ischmitz]

Doug, while you are at it measure the resistance between pin #1 and pin #5 on the DME itself (not the connector). If it is close to 0 Ohm you have confirmed what I described - a shorted ignition stage.

Ingo

[dbp]

After looking at the card Ischmitz posted displaying a "smoke test" I did a visual but could not see anything that looked burnt. I am still concerned about the amount of heat up in that corner of the box, especially now.

Per recommendations of Jose and Ischmitz I did the following pin out checks, box and harness are disconnected.

On the Harness Connector:
RPM Sensor Check: pins #8 & #27 = 1047 ohms
TDC Sensor Check: pins #25 & #26 =1054 ohms

On the box side connector: pins #1 & #5 = 5732 ohms. Is this a nominal state?
Not having a break out box and fast running out of talent, I'll go back and re-read the DME troubleshooting postings.
Unless someone has some shortcuts to the testing I'll get back on it and run through the tables in the Bently Manual. That will have to wait until Sunday as I'm heading up to College Station to watch the CVAR Vintage races.
Thanks for all the help so far.

[ischmitz]

That reading (5.6k) looks good. So the next test needs to be to check the exact voltages at both coil terminals when you switch the ignition to RUN.

Have you veryfied the DC resistance of your coil across the terminals to make sure it is O.K? I didn't see that mentioned in your initial diagnostics.

Your reading confirms that the final stage is not permanentely shorted. But there could still be an issue in the driver circuitry where it switches the coil on permanentely. So unless you have advanced electronic skills and access to an oscilloscope there isn't that much more to test.

Ingo

[dbp]

Thanks ischmitz:
Positive Terminal to Negative Terminal = 4 ohms
Negative Teminal to coil output = 7.29 k ohms
That seems to be o.k.
Voltage is still suspect with key on:
11.7 at positive and 7.01 at negative with 4.5 volts across the two terminals.

As I was doing the pin checkouts for the DME W/H connector: with the ignition on, and testing pin #20 to Ground - I did not hear fuel pump running. Contrary to prior testing..ehhh.
So I jumpered from battery to fuel pump lead (downstream end of fuse) and no sound. Interesting - so I connected up the DME and put a troubleshooting light on the fuse connector and verified that while cranking that the light does come on. I had noticed that the pump while running before was pretty quiet. I will go down and test at the fuel pump terminals in the next day or so and make sure power gets there and that the ground is good.
Sorry but work is getting in the way of this - hate to keep dragging it out.
Doug

[dbp]

O.K. Problem resolved. It was the DME, packed it up and shipped to Ischmitz, a couple of days later it was back. Plugged it in she started instantly.
The clue was in the voltage across the terminals.
Thanks to all.

[Flat Six]

Thanks for the update, Doug -- glad it finally worked out for you.

[Mattcamp]

I gotta know....how much $$$???