I have a 71 dart with a 440 factory electronic igination.when i turn the key on i get spark.i will lose spark when the engine is tryin to start.when i let go of the key i kinda have spark again. Possibly the ignition switch or coil?

Ballast resistor sounds like perhaps..

It sounds like one of two things, loose or dirty connection on the ballast resistor or a wiring problem from the ignition switch to the ballast resistor.
Someone put that motor and ignition in that car, check all the connection closely first and then use a volt meter to check the voltage (check the voltage at the battery first, if it is around 12.6 volts) to both ends of the ballast resistor, you should have almost the same voltage,12.4 to 12.6 V, at the ballast on one end and around 6.0 to 8.5 volts to the other end with the key on and the engine not running Next thing is to put the shifter in some gear position other than neutral or park and have a helper turn the ignition switch to the start position while you check the end of the ballast resistor that had the lower voltage with the key on, it should increase to battery voltage or a tiny bit less in the start position to that end of the ballast resistor to make the coil fire when in the start position
Mopar electrical engineers made it harder on us to trouble shoot problems in their design of theses cars
Let us know what you find and have to do to fix it, that will help others on here later

check the air gap on the distributor also...

5 pin/4 terminal "dual" ballast system or 4 pin/2 terminal "single" ballast system? As said: bad box/open in ballast/too much reluctor gap

Since the ballast is bypassed in start and he's only getting spark in run I seriously doubt it's the ballast resistor.

Since you get a spark at all I doubt it is the coil.

Since the ignition switch is what bypasses the ballast I suspect that or the wiring/connectors.

Crusty wiring at bulkhead etc. Bad ign switch, doubtful.

I had that exact same symptom once & it was either the box or the ballast I can't remember which

Wiring is new

11 out of 10 times it's the ballast resistor...

in that case definitely pay attention to ign1 and ign2.
one is hot when ign is in start, the other hot while ign is in run. they both need to go to...the coil...or the ign box (in my case my msd) or wherever depending on yr ignition setup. look at a schematic.

OK guys..... look at the original post. If I'm reading it correctly, then he has spark at "run", but no spark at "start" position of the key.

This is the OPPOSITE of the typical ballast resistor burning out. If I'm reading that correctly, then there's either a problem with the brown wire coming out of the bulkhead disconnect (the one that bypasses the ballast resistor) or the ignition switch itself.

One way to test this would be to run a jumper wire from the + of the battery to the disconnected ballast resistor plug that has the brown wire. If the car starts & stays running, then you've found the problem. Don't just leave that wire hooked up though since your ignition voltage is supposed to be "stepped down" with the ballast resistor during normal running & this jumper wire isn't going through the ballast resistor.

put it in run and then jump the starter relay.
if it starts, you have a circuit problem.
if it doesn't then the computer probably has an issue.

You mean the ignition box? 71 dart doesnt have a computer.

sure. same thing to me. electronic part.

if the key movement is allowing something to happen in the box even though it isn't working, then you won't be able to start the car by bridging the start relay.

if it does start then you have an issue with the circuit that should power the coil/spark box when in start.

it should be pretty quick to see which.

The ECU doesn't care if the key is in run or start, it is powered the same regardless. What is powered differently is the coil. So, if the ECU makes a spark at all it is working.

sure, but my statement is still good.
if he can start it in run then there is a circuit issue.
if he cannot, then maybe it isn't the ecu, but something died and has to be replaced.
it narrows down this chase.

Rarely a starter can have so much drag or a very poor starter cable or connection can suck all the amps out of a battery, there is nothing left over for the coil to fire off the engine.

However when you release the key, all the battery's pent up energy can be diverted to the coil and either succeed or try to anyway fire off the still rotating engine.

Unlikely, however at times when you hear hoofbeats, it really may be a Zebra and not a Horse.

Run a jumper from battery positive to coil positive, bypassing everything in wiring that is the more likely problem. If starts, work backward toward ignition switch thru all wiring and connectors. My money is on this or ballast.

True, but for any test to be useful you have to know what it is doing.

I'm thinking it might be hard to decipher if it is on "run" or "crank" in that split second when the key is being released from crank to run when there is the one spark. What I would do is pull the yellow wire off of the "ign" terminal on the starter relay to disable the starter (we know it cranks & no need to run the batt down for this testing) and charging the battery up with the alt causes system problems and have a helper hold the key to "crank" & see if the coil positive primary terminal and the blue/yellow ECU terminal is hot (the red/green terminal on an OE 5 pin ECU needs to be hot also) then release the key to "run" & see if everything is still hot. grab your meter.