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HELP! 996 won't start. Need Advice FAST

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Ok Experts, this is my situation. I took my '99 996 off the trailer, disconnected the battery and stripped a little more of the interior out (nothing more than carpet). After about a week, I connected the battery back up (plenty of charge) and attempted the start the car back up. All the lights on the dash lit up but the engine would not fire or crank. I tried a couple of times but no luck.

After having a couple of local Porsche mechanics come over to test everything, this is what we found;

All fuses are fine. No problems with the Battery & Clutch Switch. We tested the Starter and it seemed to be fine. Then we tested the Ignition Switch and there was plenty of power coming in and going out of the Ignition Switch. Then we tested the Ignition Switch Relay and as is turns out, there is plenty of power going into the relay but no power going from the Ignition Swich Relay back to the Starter. When we replaced the old relay (which looked perfectly fine) with a new one, the car would still not start. For some reason, the contact within the relay was not triggering on either the new relay or the old one. When we manually engaged the relay, the car fired right up. Now, we are stumped as to why the Ingition Switch Relay is not engaging. We printed out a all the Ground spots on the car and double-checked them as well, still nothing!. Does it have to do with one of the four wires that goes into the relay or is it a simpler or harder problem than that? Any help would be greatly appreciated. I have a race this weekend that I would like to compete in but I need my car to start first. After that, everything is ready to go!




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  • 1 year later...

+1 on the ignition switch. I am on number 4 now and will likely change the whole assembly next go around. The switch life is totally unpredictable. Switch is easy to change (yeah I have lots of practice). I just slide the vent duct out of the way to change the switch. You may want to pick up a Picquic Teeny Turner.

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  • 2 weeks later...

Here's what the garage has said:

Diagnostics does not pick up DME (ECU not responding)

Check voltage at starter - 12V

Crank key - No voltage at trigger on starter

Checked wiring diagram - System has a starter lock and MFI relays

Change relays - No difference

Connected terminal 30 & 87 on starter lock relay - This allowed engine to crank, but not fire. This side of circuit ok.

Thoughts currently - Either relay trigger circuit faulty or ECU

Power to relays, but not trigger signals. Wiring seems ok.

Check continuity of wires - All ok

Check pin for trigger switch (Pin 54 of ECU = UB switched) - This controls Air mass, MFI and DI relay, Injection relay, Oxygen sensor, Secondary airpump relay, Fuel pump (through a connecting relay)

This item is not switching - Controlled directly by ECU

Check pin 26 (Referred to as terminal 30 on wiring diagram - feeds MFI and DI relay also) - Has the required 5V switching voltage

In conclusion the ECU control from Pin 54 is causing the problem. This is the direct control for the majority of engine fuelling systems. Therefore an internal ECU fault.

Still waiting to find out the results...

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  • 6 months later...
  • 1 year later...

I have this same issue with my 2001 996 C4. After replacing my aftermarket radio with a newer model, I experience this situation where all lights come on, as well as I hear a vibration in the front driver side fender area, as well as a click (sort of an igniting sound) but the vehicle will not start. Any ideas?

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