1985 N9t Turbo only starts and runs when :

[Paul]

In order for this engine to start and run : 

• disconnect  trigger wire from starter solenoid. Connect this wire to battery +12V
• Turn ignition switch to run.
• Connect jumper wire to starter solenoid and touch to +12V battery momentary for start.
• This puts +12V on Pin 4 the ignition  ECU.If I remove the +12V from this disconnected trigger wire, engine quits. 

This has nothing to do with the tachometric relay as ll the engine has the same problem with the tachometric relay bypassed.

I am beginning to think that the injection ECU has a problem, because looking at the fuel injector signal, it is not proper.

I think that applying +12V to pin 4 of the Injection ECU is normally only done with ign. SW in start position, for longer pulse width to add fuel during start up.

I am in the process of checking all inputs to both Injection and ignition ECU s.

I would appreciate any ideas on this problem.   Thank You    .....Paul

N9t_1985_fuel_injection_connection_pictorialF0.023.pdf

[Bean]

Welcome to the forums Paul - That's some fantastic diagnostics for a first post too!

85's have been known to have had faulty ECU's with most folks finding a fix with a replacement ECU, but this is literally the first time I've heard of or read of your trick to get it to start!  I had an 85' that was a no start, and I assumed it was the ECU but never had the chance to really test it out.  With this new info however - Fixing the ECU shouldn't be that hard.

I would wager that the ECU board has a cold solder joint or some other broken power path that is preventing pin 4 from getting power via the proper internal circuit.  You'll likely need to open up the ECU to inspect for cold solder joints (cracked), or possibly a scorched trace or component on the PCB itself.

These are such lovely cars and the series 1 1985 505 Turbo is such a neat car.  Lots of cool one-off stuff on them like a bigger oil cooled turbo with a 5 bolt exhaust flange, and sodium cooled exhaust valves.

Rabin