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1974 charger wont start?

moparvinson

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bosque farms N.M.
My son's 74 charger won't start so I am gonna go over tomorrow to see if I can help trouble shoot it. We had it running great a fee weeks ago and now he said it just cranks but won't fire? He said he tried starter fluid and still nothing? I am thinking no spark but not sure until I get over there. I plan on taking a spare spark plug to check that. I know not a lot of info here but any other checks I can do since its electronic ignition? Thanks for any help.
 
Without more information it's really hard to say, but with that said, break out the multi meter and start tracing power. First turn ignition on and check at the main ignition feed to the ballast resistor, should be 12V. Next, check the other side of the resistor for power, usually around 8v. (single ballast resistor) Check power at positive side of coil, if you have power here check for spark at a plug-should be nice blue spark. If no spark check ground of ECU. This is a pretty simple system, and fairly easy to find the problem.

Most common failure of these cars is the ballast resistor, so may want to bring a spare with you.

Mark
 
12v at ballast resistor. Good ground at edu. 12v at pos. Side of coil. No spark from plugs? Bad coil?
 
12v at ballast resistor. Good ground at edu. 12v at pos. Side of coil. No spark from plugs? Bad coil?
Did you check the ballast resistor output side? If you have around 8 volts there the ECU should be getting power. That does not rule out a defective ECU though. Would be nice to have access to a loaner ECU and coil for a few minutes.
 
Ok, to test the coil set the multimeter to ohms and touch the the pos and neg of the coil. If it's a factory style (non blaster) you should get a 1.5 - 2.0 ohm reading, 0 ohms is a bad primary side.
Next touch the pos side and the center terminal, reading will be between 6000 and 12000 ohms, a reading outside of this range would be a bad secondary.
Probably the easiest way to test is with a KNOWN good part (not always a new part)

Mark
 
We just got a new ecm and a new ballast resistor since they were both original 1974 parts and it started with first turn of the key. Thanks again for tips.
 
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