[DML] RPM Relay - Problem Solved!
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[DML] RPM Relay - Problem Solved!



Problem: RPM Relay never shuts off after the car starts  even when the engine 
dies.
 
Tom Niemczewski wrote me off  list and suggested I check out a few items in 
the RPM Relay schematic.  I  went through a few of them with no luck and tried 
experimenting to try to get to  the bottom of this seemingly simple problem.  
After I tried and tried  unsuccessfully, so I measured all the voltages at all 
the pins of the 555  timer.  Then I sent an email of to Tom with the info.  
As I was  composing that email I reviewed the data that I collected and one 
piece seemed  to stand out.  That was the fact that there was a constant 4 volts 
on  the 6/7 pins of the 555.  Since it was below the 2/3 of the 10 volt power  
supply it meant that the timer would be on. But this is a simple RC circuit  
and thus the cap voltage should rise until it reaches the 10 volt  supply.  
Something was keeping it low and thus keeping the timer on.  I scrutinized the 
circuit some more and realized that either the  transistor across the capacitor 
that is connected to pins 6/7 was leaky or the  cap itself was leaky.  Since 
I had already replace the transistor, i  replace the cap and viola!  (Tom did 
mention the cap in his original reply  to me but I couldn't believe it could 
go bad.  Now I believe  it.)
 
I'm sorry if all this is too technical or boring for  some of you but is just 
supports the fact that this group is a great bunch of  people that spend a 
lot of their valuable time helping each  other.
 
So thanks Tom for your help on this one.  (I still  have that annoying tach 
flicker but hopefully it will be solved too some  day.)
 
D² and 6530
Dave Delman
 
BTW Tom was not the only one to lend a hand on this  one, Elvis was in there 
too.


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