My Road Runner is acting like it's bad (tries to start, then won't do anything once the ignition is selected to 'on'). How do I test one to see if it's bad;i.e. how much resistance should I be reading with a meter? And the local auto parts store only sells a 4-terminal piece & mine's the 2-terminal type that came in the Mopar Performance electronic conversion kit. Can I interchange the two?

If it starts and when you release the key it dies that is the sign of a bad ballast resistor , don't remember the ohms value off the top of my head but it is very small , like 10 ohms , and lastly yes you can use the 4 prong unit in a pinch , the values are different though so measure them and use the right side.

Hopefully someone will chime in with correct value.

I dont know the only thing I can tell you i always did was carry a good spare one around in the glove box and in the tool box..if it started acting "bad" as you say the first thing I did was switch out the possibly "bad" one for a new one...easy to tell ...now if the one you switched was bad also your out of luck lol. But I always tested them on running cars to be sure they worked fine B4 putting them into "reserve" service.

Turn the key to the on position, The side with the blue wire will read the batts positive voltage. Once you measure 12+ volts on that side, measure the other terminals volts. If its 0, then its a blown ballast. It should read around 7 + volts or so, resisted down from the full 12+

If it dosent have resistance its bad.The piece that runs in the back cracks and goes open.Easy test get a test light turn Ign on and you should have power on both sides.One side is 12 volts the other about 9 volts.Cranking bypasses the ballast thats why they usually start and then die when you quit cranking.Rocky

If you wanted to test them IIRC, they could be from 1.25 ohms down to .8. I always use one(or the side of the 4 prong) that has the least resistance ie; .8 .

Thanks for the help, guys; good info!

As said check for fire at the downstream (brown wire) end of the ballast in ign1 (run) to see if it's open & if good work back toward the ign sw (start w the bulkhead). I just checked a bunch today & got 1.3-1.6 ohms on a handful of 2 prong ballasts and 1.6/6 on a 4 prong unit & w it in parallel reduced it to 1.1 which w work excellent. EDIT if you want to mix/match/experiment you can wire in the 4 prong connector (2 dual connectors) & the width is just right so you can use 2 single ballasts side by side. Wire em in parallel & ohm it