[DML] Scoping out the dwell and RTFM

["drdhdmd" <doctorDHD@xxxxxxx>]

It was a beautiful almost "Spring" day so I couldn't resist.  I 
connected my scope to the diagnostic port to "see" what was going 
on.  According to my calculations the frequency of the signal was 70 
Hz which I believe is correct.  The problem is the duty cycle ON was 
only 38.8% with the WOT switch closed.  The manual says 50 - 60.  
Something did not seem right so I went back to the DeLorean Workshop 
Manual and checked all the "Pulse Ratios" on page D:04:15.  
Everything seemed to be inverted, that is, the lower part of the 
square wave pulse signal got bigger when the WOT switch was closed.  
I tried all the test configurations and the all were right except 
they were inverted.  Attaching a known voltage and polarity to the 
scope proved that + was indeed up.  I was confused. It was getting 
dark and I had to close up shop.  Now sitting here something I read 
popped into my head. It talked about connecting the dwell meter to 
the pinS on the diagnostic plug.  A quick check of the schematic 
revealed the problem.  I had connected my dwell meter and scope to 
one pin on the diagnostic plug and ground. (They way we used to check 
dwell on a car with points.)  It seems that the other pin of the 
diagnostic plug is not ground but rather positive 12 volts.  Thus all 
my measurements were off and I guess I adjusted the CO to around 60 
instead of 40.  Now I have a project for next weekend!

I believe this has been address in the past but none of the answers I 
have found seem to make sense...to me.  When using a DWELL meter to 
measure these "ratios"  What scales and setting do you use?  Since we 
are talking about something that in theory can go from 0 to 100 
percent, it doesn't seem correct to use even the 4 cylinder range 
because that maximum is only 90%.  Does anyone have an answer with an 
explaination?  (I know I am asking alot but remember what the old 
wise man said about giving a fish vs teaching someone how to fish...)

When all else fails Read The Manual.  Fortunately when I was 
flying Pipers I ALWAYS used my checklists and went by the book 
carefully.

Thank you,

Dave and 6530



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