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Old 08-31-2007, 09:51 AM
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71 ply scamp cranks but dies when key is released

Gee guys, I am stumped. These friends of mine have a slant 6, 71 scamp. They bought it new, its really straight and solid. Their son had been driving it, he's a soccer player. The car has about 124K miles on it. They said it tries to crank but won't keep running. So I went over to check on it, I turned the key but I held the key from going all the way to the ignition position and it continued to run as long as I held it about 1/8th before the rotation of the key. After that, it would die.
Thinking the weak link would be the electrical part of the switch, I replaced it, and it still does the same thing. I got to thinking that the key lock might be the problem. The electrical switch is held in the column with 3 screws, has a shaft coming out towards the key lock, with a lever and a short peg. The peg engages a wheel with a gap for the peg. The wheel is on a shaft going into the key lock. I suspect, the soccer player may have torqed the key and slightly twisted the lock shaft.
Other people said it is the ballast resistor, it has the two terminal white block on the windshield wiper motor. I put a jump wire and it did not change the problem.
Help anybody? I ordered a new lock assembly from az parts, its' 12 bucks. What do you think?

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Old 08-31-2007, 09:57 AM
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I'm no mopar expert, but did you check the fuses? If the ign fuse is blown it will act just like that. The ign wire to the ignition is cold during crank, so it gets power from the crank terminal of the starter. It sounds like you're getting the voltage from the cranking wire, but the actual ign circuit is dead. Verify that with a meter. test for 12v at the wire while cranking, and my guess is that as soon as you stop cranking the ign wire isn't supplying anything.

All you're doing by holding the key partially on is engaging the crank-to-ignition post on the back of the switch without actually engaging the start circuit.

Sounds pretty simple to me... test for voltage at the coil/dist with the key in the ON position. If you don't have it, trace that circuit back until you find what's wrong.
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Old 08-31-2007, 10:21 AM
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Curtis73,
I was told something like that by a friend, but he says the cranking mode would provide ignition power at 12V, and when you have it running, and release the switch it goes to 8-9V. I may have that backwards. But he said it might be the ballast block. He is mopar nut. I will have to check that ignition fuse, if I cannot get it going, I can run a wire from the battery to the coil and just drive it over to his shop. It would be awkward to have to drive it holding the switch for 10 miles. I am not a electrical engineer, but you seem to have more smarts than I on that subject, thanks Curtis.
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Old 08-31-2007, 01:20 PM
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Most likely your ballast resistor is bad, it's very easy to change if indeed that is the problem.
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Old 08-31-2007, 01:21 PM
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Your friend is right. The key switch provides 12v to the ign circuit and the resistor drops that to 8-9v. When you have the key in the start/cranking position, the switch is NOT providing juice to the system, instead, the "start" post on the switch provides a full 12v for starting.

Since you have power to the ignition system during cranking, that part seems to be working correctly. Since it dies after you let off the starter, my guess is that it is NOT providing juice like its supposed to.

Here is how I would test things in this order:
- check the ign fuse (easy stuff first)
- turn the switch on and verify that you have power after the ballast. It should be somewhere between 7 and 12v. If you don't have any power there, go back to just before the ballast where there should be 12v. Keep going backwards until you find where there IS 12v and you can isolate the problem part.
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Old 08-31-2007, 01:27 PM
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I have had a few old chryslers and all ways carried a spare ballist resistor in the glove box.
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Old 08-31-2007, 02:04 PM
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O.K., you all are great, I am not sure where the ignition fuse on the 71 plymouth scamp, got any clues? I also think I need a ohnmeter before I get back on it. Thanks a bunch.
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Old 08-31-2007, 07:10 PM
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Fuse should be in the fuse panel maked IGN.
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Old 08-31-2007, 07:42 PM
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ballast resistor, it's porcelin, and with Mopars, always keep an extra 1 in the glovebox. ign fuse could be blown also, but, most likely, the ballast resistor.
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Old 08-31-2007, 09:08 PM
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The ballast resistor for the ignition is not part of the wiper circuit.
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Old 09-09-2007, 07:55 AM
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71 plymouth resolved

Yesterday, I replaced the key with ignition switch, (separate from the electrical ignition switch to the left). The mechanical switch had a broken lever, the new switch had the lever and I thought how to slide it into the hole in the column, I pulled the key out about half way, and the lever laid down so I could push the key lock into the column. After that, I tried to crank it, and it still would not continue to run. So, I then went under the hood. I checked the ballast (located on the wiper motor), and it seemed fine, I looked around in the engine bay, and when I moved some pine straw near the drivers' side fender, I saw another ballast, with several wires on each end. I put a jumper across the terminals, and guess what, it cranked up and continued to run without holding that key. Went to AutoZ and picked one up for a mere 4.99. I had put 85. electrical switch, 13. key assy, and finally a 5. ballast. I should have known better, my dad had to replace those ballast on his 72 ply valiant and our 75 dodge dart SE too.
I just wanted to thank all of your support, I am glad to be a part of the hotrod forum!
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