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Re: [T3] Dead Injectors Puzzler

Recap for those who still have the stomach for this: Intermittent and progressively worsening missing/stall/no-start problems plagued me most of December and into last week.  69 Fastback, FI.  Tried all of the List's suggestions to help a FI novice track the problem down.  Became quite educated in the process.  

Last weekend I swapped the brain (Bosch rebuilt) with a spare I had of the same generation (also with a sticker indicating Bosch rebuild).  Changed nothing else.  Problem transformed into intermittent loss of signal (and I know for sure it was power loss) to two injectors: 1&4.  2&3 kept the engine running.  So it seems that with the old brain the engine shut down, but the spare brain kept the motor running, but on just 2 cyls.

Now, tonight I have confirmed the hunch of several who guessed the wiring to the 3/4 injectors was switched.  It was indeed.  Traced the wiring with the VOM to the big plug-in connector to the brain.  Also confirmed the continuity and correct wiring of the FI trigger points from the dist to the brain.  Same with the temp sensors, the injectors and the grounds.  The wiring all checked OK.  Started the car up with the injector wiring fixed and with the spare brain, let it run for a while.  Drove around the block and let it idle for a bit.  When at temp, the idle dropped.  Sure enough, injectors for 1&3 were not firing anymore.  

Shut car off.  Following the advice of many, I suspected the trigger points.  So I cleaned up the prettiest of the two spares I have with electrical contact cleaner to make it sparkle.  Let it dry completely (took my time) and I changed the trigger poins out.  The set that was in there didn't look any worse than the replacement, but I changed them anyway.  

Put everything back, started the car (started right up on all 4).  Within about 10 minutes it happened again.  I lost 1&3.  Keep in mind I have ruled out the injectors themselves because when I corrected the wiring, the problem followed the wiring and did not stay with a particular injector.  And the problem only occurs with pairs of injectors, not just one.

I remembered the dwell problems reported by Steve and Jim.  The boot on my trigger connector at the dist is gone, so I was able to jam my VOM probes into the top of the connectors to make contact with the wires while the engine ran.  With the car still running on just 2&4 (1&3 not firing) I measured voltage in turn between the outer connectons and common (the middle one).  For both sides I measured a pulsing signal that varied between 2 and 3 volts.  Does this tell me that my dwell for both trigger contacts is similar?  Can I rule out trigger points?  Keith, you mentioned something about resistance across a rivet in the trigger points.  Like from top to bottom?  Please elaborate just a little for me here..

Now, I am thinking I need to try a third brain (mine's beginning to hurt).  I think I threw in another variable when I changed out brains.  First brain shuts engine down, second one allows engine to keep running but on just 2 cyls.  Even after changing out the trigger points.  I do still have one more spare and I think access to a few more I could try.  Greg, if I can't come up with other avenues to check, I may be in touch soon to borrow your brain.  Now THAT sounds like a plan!  Make Greg's brain solve it... and save my own!

-Mark Fuhriman
 '69 Fastback 
(I'm almost missing that old '70 I drove with the dual Solexes...  But nah, those carbs came with their own problems)

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