Occasionally the vehicle will not start. Checking for spark and it is very weak. Then with no changes I try it again and the spark is hot and it fires right up. No pattern, sometimes when hot, sometimes when cold. When this happens the spark directly from the coil is weak. Any ideas?
first off.. lets look at this image...with a test light like this. hooked up with the CLIP on the positive coil terminal.. please stick the probe into something grounded and turn the key on.. in the RUN position you should have a medium bright test light.crank the engine and it should go BRIGHTER.. release the keyand it should return to a medium bright light... this proves at the current time that you are getting proper voltage on the positive side to the coil positive.. ..please... also invest 6 bucks.. and pickup one of these from the parts store..mount it to one of the end of the cylinder head holes and to the mounting bolts on the ignition module... you won't believe how much longer your ignition module lasts... i will explain in another reply below.. why.. just do it.. trust me.almost every parts store in the country will have these in stock.. this image is printable.. please perform the 4 part test and post results by number..~~~~~~~~~~~~~~~~~~~~results 1.___2.____3.____4.____also.. run another test while the engine is running and the headlights on..alternator output to the battery positive.. the factory service manual says 0.7 volts is expected here.. yes 7/10ths of a volt.. thats a LOT.. but its a LONG way for the alternator output to go.. post that result also..a last check.. the engine block to the voltage regulator body.. should be less than 0.02 volts DC.. if its more.. pick up another ground strap and go from the engine to that also.. there is a fairly good chance your coil may be failing..but if you are loosing voltage to the blue wire sometimes.. you could have a bad ignition switch.. a bad ballast resistor.. a bad connection in the bulk head connector... the test light to the coil positive is just one of the important tests.. post what you find please.. i fix these cars all the time.. some friends came by last week .. i showed them how to perform most of the these tests on the cuda.. and solved a lot of issues..
OH... and WHY the braided ground strap between the engine block and the mounting bolt on the ignition module..its the HIGH voltage spark return path on the negative side... what?????the distributor contains the pickup coil and reluctor.. as the reluctor poles pass the pickup coil.. it creates an AC voltage. just over 1.0 volts AC..as the ac voltage positive signal is going up above ZERO. when it reaches 3/10ths of a volt above ZERO.. it turns on the power transistor in the module and that connects the Negative side of the coil to ground.. at that point electrons rush into the coil primary winding creating a magnetic field.. as the reluctor pole piece lines up perfectly with the pick up coil the voltage drops back to ZERO.. this turns off the power transistor and the magnetic field in the coil collapses.. this collapsing field does so thru the secondary windings creating a HIGH Voltage spark out the top of the coil.. this goes thru the cap, rotor, cap, wires, spark plugs. and into the heads.. but thats NOT a complete circuit. it needs to get back to the ignition module ground connection to be a complete circuit.. if there is NOT a braided ground strap.. to get to the ignition module grounded case.. it has to go up the negative battery cable to the battery terminal.. then down the smaller negative cable to the inner fender.. then around to the firewall and then to the module mounting bolts and the module.. let it take a short cut with a braided ground strap... and why the braided ground strap instead of a piece of wire.. because electrons travel along the surface of the wire..not thru the core..shorten the path of electrons.. your ignition module will work so much better and so much longer.. do you really want the HIGH voltage electons on the negative side.. looking for a way to get back to the module.. thru the 14 volt electronics.. nope.. or thru the 2 volt electronics.. NOPE..this ground strap.. actually is important on ALL CARS even brand new ones... learn the 4 part voltage drop test.. it will save you hundreds if not thousands of dollars in car repairs.. on another forum.. 2005/2006 caravan owners spent over $2,000 bucks in parts to not fix a surging voltage problem.. new batteries, new alternators, new PCMs, New fuse boxes, new harnesses.. when it actually was the crimp at the positive battery cable that looked good and was not.. a proper voltage drop test narrowed the possible causes in 5 minutes.. does this help.?????you can look at the voltage to the blue wire with a digital volt meter also.. slowly rock the key from off to on to cranking.. back to run.. .. oh.. and what is that brown dashed line??? some cars have that from the ignition switch.. to supply full voltage to the coil while cranking.. for a hotter spark.. some cars get this resistor bypass power from the BAL connector on the starter relay..see it on the upper left in this image..that relay is also used on fuel injected cars.. with electric fuel pumps.. why.. because the fuel pump is powered by the outer 2 wires of the 3 wire oil pressure switch.. when cranking.. the BAL connection supplies power to the fuel pump circuit to allow fuel pressure to build as you begin cranking not needing to wait for the oil pressure to come up..