1987 911 Cab won't start

[gomezoneill]

Solution:
Thanks to @mysocal911, there are really three critical circuits to look at. Open the ECU, and take the back metal plate off. Check the three solders in these pictures. Upon close examination, I could barely make out little hairline cracks. I reflowed some solder over them, reinstalled the ECU, and problem solved.

To save everyone some time in troubleshooting, my takeaway is this - DME relay first, if not resolved 80% of the time it's the DME ECU, check those three solder joints. If you're unsuccessful, well then you still have another 10% ref/rpm sensors, 10% misc (wiring, ignition switch, head temp sensor, etc) chance that you can track down.


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[david3299]

Charged the new battery for read 12.7vdc. I checked the ECU transistor tracks and found no problem. Still unsuccessful in starting the car. I think the ECU is dead. This might be the second ECU in 4 yrs.

[david3299]

Swapped my 87 911 3.2 ECU with a friends' and the car started. Checked the new battery voltage on the car while idling. It read 15.6 VDC and I slowly applied gas and the voltage across the battery showed 17.9 VDC. Basically, the alternator voltage regulator is defective. This over-voltage damaged my ECU.

THANKS FOR ALL YOUR HELP..

[mysocal911]

Originally Posted by david3299

Swapped my 87 911 3.2 ECU with a friends' and the car started. Checked the new battery voltage on the car while idling. It read 15.6 VDC and I slowly applied gas and the voltage across the battery showed 17.9 VDC. Basically, the alternator voltage regulator is defective. This over-voltage damaged my ECU.

THANKS FOR ALL YOUR HELP..

Fairly common, but less common than an intermittent spark.

[david3299]

I had my ECU repaired. Car is now running. However, my battery light is ON when the car in off and it's OFF when the car is running. I could have a defect Porsche Alternator Diode. Does anyone know the location so I can check the forward and reverse bias on the diode?

[mysocal911]

Originally Posted by david3299

I had my ECU repaired. Car is now running. However, my battery light is ON when the car in off and it's OFF when the car is running. I could have a defect Porsche Alternator Diode. Does anyone know the location so I can check the forward and reverse bias on the diode?

You need to remove the alternator and use a VOM to check for shorted diodes.

Next time you have a DME ECU problem use this Pelican Parts post;

http://forums.pelicanparts.com/porsche-911-technical-forum/621262-911-3-2-no-start-troubleshooting.html

[david3299]

Originally Posted by mysocal911

You need to remove the alternator and use a VOM to check for shorted diodes.

Next time you have a DME ECU problem use this Pelican Parts post;

http://forums.pelicanparts.com/porsche-911-technical-forum/621262-911-3-2-no-start-troubleshooting.html

Do you know where to find the part number?

Thanks

[pmax]

Originally Posted by david3299

I had my ECU repaired. Car is now running.

Any evidence of a burnt out component in the circuit board ?

[david3299]

Originally Posted by pmax

Any evidence of a burnt out component in the circuit board ?

The bottom circuit board needed replacement.