Volvo: still wont start, exuast, fuel pumps

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1981 240 w/B21F engine won't start. turns over but will not
catch. Checked the fuel pumps using a wire across the # 5 and #7
fuses, they sound like they are working. Replaced the relay at $40.
Still won't start. Replaced the plugs, wires, distributor cap and
rotor, fuel injectors, fuel filter. Still will not start.  
What  else can I try?
Answer -
Do you get spark at the plugs when cranking?  Does fuel squirt
from the injectors while cranking?  You can check for these by
disconnecting them and turning the engine over.  If both are
present, check your knock sensor and catalytic converter.  
Disconnect the exuast at the manifold to make sure it isnt a clog
in your catalytic converter.  Also check to make sure your
frequency valve is functioning.  Good luck.

The wires,distributor,rotor and plugs are new, but there isn't any
spark
Answer -
Check the obvious, like making sure the distributor is installed correctly, and the rotor can spin without obstruction.  Also check the coil, specifically the 12V contacts around it's perimeter.  They can get corroded and short the circuit.  Running sandpaper over the contacts helps.  Check to see if you have voltage to the center large wire of the coil as well.

I am new to the mechanical working of a car, so how do I check the voltage to the center large wire? Is it the same as with a spark plug, hold it close to the engine  and see if a spark jumps across?

Answer
For the center wire, yes, hold it close to the chasis and crank the engine, looking for a spark to arc to the metal.  As for the smaller 12 volt wires going to the coil, around the center , you'll need a voltmeter to make sure you're getting 12 while the engine is cranking.