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An update in this:
My friend and her '94 has been through a lot to try to fix this. The ignition module and fuel pump has been changed, and the alternator belt was suspected to be broken. Our local Classic Porsche center had the car for a week without being able to provoke the fault to appear. However, when the owner is driving the car, it stalls. Yesterday, I was able to join her for a ride, and voilá, it stalled! I had brought my PST-2 diagostic tester, and while the engine would not start, (while the fault was present) I was able to connect to it with the PST-2. This rules out any serious hiccup in the Motronic or power supply failure.
This also made me think one more time on this, and I spent an hour or two on the schematics today. The '94 immobilizer disables the power supply to the ignition module, fuel injectors as well as the starter relay. I am almost certain that this must be where the fault appears. I called my friend, and we opened up the immobilizer and discovered a couple of suspected solder faults in the relay solderings.
This may be the culprit in this case, only time will tell. I'll keep my fingers crossed.
Cheers,
Tore
An update in this:
My friend and her '94 has been through a lot to try to fix this. The ignition module and fuel pump has been changed, and the alternator belt was suspected to be broken. Our local Classic Porsche center had the car for a week without being able to provoke the fault to appear. However, when the owner is driving the car, it stalls. Yesterday, I was able to join her for a ride, and voilá, it stalled! I had brought my PST-2 diagostic tester, and while the engine would not start, (while the fault was present) I was able to connect to it with the PST-2. This rules out any serious hiccup in the Motronic or power supply failure.
This also made me think one more time on this, and I spent an hour or two on the schematics today. The '94 immobilizer disables the power supply to the ignition module, fuel injectors as well as the starter relay. I am almost certain that this must be where the fault appears. I called my friend, and we opened up the immobilizer and discovered a couple of suspected solder faults in the relay solderings.
This may be the culprit in this case, only time will tell. I'll keep my fingers crossed.
Cheers,
Tore
Hi Tore,
Sorry for resurrecting an old post, but....
What was the eventual outcome of this issue? I am experiencing a non-start scenario with my 95 and suspect the immobilizer circuit as well.
DCD. thanks for pointing out the missing conclusion in this. The cause for these problems was indeed failed solderings in the immobilizer control unit. This caused internittent power supply to the ingnition module and/or fuel injectors.
The problem was solved by resoldering the relay connection pins on the control unit PCB.
Cheers,
Tore
Tore,
I'm glad the issue was eventually sorted. What are your thoughts about these types of solder joint failures - incorrect soldering by the manufacturer, or stresses from 20+ years of vibrations & temperature cycles?
Solder failures on heavy components such as these relays is a common issue. This is due to vibration, and modern automotive electronics secure the components with glue or potting compound to avoid stress on the solderings.
Cheers,
Tore
You identified the immobilizer unit in a 94 built 993. Where can I find info about that unit and the unit itself? Whenever I try and research, i am being pointed toward the lack of this unit in early 993s such as mine.
I would like to open mine up and rule these soldering issues out, too.
The 993 immobilizer control unit is placed under the driver's seat, adjacent to the Motronic engine control unit. Here's a picture of the early version, which is the one in question in this thread.
By the way, the later and more common immobilizer version is placed in the same location, with a metal cover over the two control units.
Cheers,
Tore
Thanks, ToreB, I think I need to dig a little further. I am getting stalling issues / no-starting issues both when hot and cold, wet and dry, slow and fast. I had my car at a shop for 3 days, no one could replicate it and today it stalled again on numerous occasions. It didn't start for 10 minutes or so, I replaced the DME relay and it started right up, stalled again. Could not replicate DME relay fix. I later found that I had an easier time starting it when I turned the ignition on, turned radio, AC on and then started it. However, on its way home, it stalled on the highway with the CE-light and I tried to force it to continue running to a short pull-out, which caused a burned and fuel smell. 10 minutes later, it worked again, with violent stalling occurring some 5 minutes later. Eventually made it home. I also get bad fuel milage. When it runs, it runs like a kitten.
I highly suspect some electrical fault (we had a lot of rain) and I don't know where to start. Is there a way I can check the immobiliser for the fault you mentioned? I would need some more guidance as I am certainly not well-versed when it comes to electronics (I was able to solder your LED brake light and interior button lights, but that is about where my experience ends). What part in that unit do I need to check?
Thanks, ToreB, I think I need to dig a little further. I am getting stalling issues / no-starting issues both when hot and cold, wet and dry, slow and fast. I had my car at a shop for 3 days, no one could replicate it and today it stalled again on numerous occasions. It didn't start for 10 minutes or so, I replaced the DME relay and it started right up, stalled again. Could not replicate DME relay fix. I later found that I had an easier time starting it when I turned the ignition on, turned radio, AC on and then started it. However, on its way home, it stalled on the highway with the CE-light and I tried to force it to continue running to a short pull-out, which caused a burned and fuel smell. 10 minutes later, it worked again, with violent stalling occurring some 5 minutes later. Eventually made it home. I also get bad fuel milage. When it runs, it runs like a kitten.
I highly suspect some electrical fault (we had a lot of rain) and I don't know where to start. Is there a way I can check the immobiliser for the fault you mentioned? I would need some more guidance as I am certainly not well-versed when it comes to electronics (I was able to solder your LED brake light and interior button lights, but that is about where my experience ends). What part in that unit do I need to check?
You need to provide more info about your vehicle, e.g. actual production date, U.S./Euro., stock/mods, etc.
An interesting read, I wish the mechanic calling himself a 993 specialist had taken a look at this sentence "The key point about an intermittent problem is that tests are only valid when then intermittent problem occurs" before changing the head temp sensor at a time where the fault did not appear, "hoping" it would not return, ultimately leaving me stranded one day later.
"Luckily", this time the car gave out altogether and I was unable to turn it back on for the last 24 hours, perhaps giving me a chance to identify the problem for as long as it persists.
94' 993, US model, coupe, Steve Weiner Chip, otherwise stock.
Update and Solution: Now that the fault has evolved from intermittent to constant, we were able to diagnose more effectively. ToreB had suggested a faulty connection between aftermarket chip and DME. We swapped the chip for the OEM one and the fault disappeared. After more than $1k and weeks of unreliable stalling, the car seems to be back to normal. Thank you, ToreB, for saving me from sending my DME in for repair.
03-14-2017, 04:03 AM
