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'73 won't start. No spark while cranking. Help!

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'73 won't start. No spark while cranking. Help!

 
Old 06-11-2019, 12:52 PM
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blu73
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Default '73 won't start. No spark while cranking. Help!

It's been a couple of weeks since all this began. It was running so poorly, I could barely get the engine to rev to three grand. It didn't seem to want to take gas. Had a lot of missing and a few loud bangs from the exhaust when excess gas would light off in there. Now it won't start at all. I have checked for power at the positive side of the coil and see 6 volts with the key in the run position. I pulled the high voltage lead from the coil tower and rested it close to the contact and get nothing at all while cranking the engine. As I recall, this test should show about a half inch of spark jumping that gap. I have not tested for voltage at the positive coil connection while cranking yet. Should I get something like 10 or 12 volts there while trying to start the engine? If that should be the case, and I'm getting nothing, where would be a good starting point to find out why there is nothing at that terminal?
The engine is a small block if that matters and most of the wiring is pretty much original.
Thanks for any and all help.
Russ
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Old 06-11-2019, 01:21 PM
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If the coil doesn't get a steady 12v, it ain't gonna run. You've got 6 volts there, so good job on checking that. Now you've got to find out, why the coil isn't getting the full 12V. Your spark, totally depends on that voltage build up and collapse to create the spark.
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Old 06-11-2019, 04:11 PM
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F22,
That brings up a question I've asked myself a few times. Do the ignition systems on our cars operate at a reduced voltage like others do? I can relate to older Chrysler products that have a ballast resistor as part of the ignition system. Supposedly, during cranking, the starter is drawing so much power, the resistor was bypassed to let the coil see as much voltage as possible to create plenty of spark for starting. Once the engine fired up and the key was returned to the run position, the resistor was put back into the circuit to cut the voltage to the ignition to help keep the points from excessive arcing. I was not too surprised to see only 6 volts at the coil with the key in the run position. Do these cars from around '73 have a feature to do something similar, or do these things actually run at a full 12 volts all the time?
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Old 06-11-2019, 06:28 PM
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That's a good question and it's been too long, since I worked on a stock ignition system. I've got the FSM (Factory Service Manual) and I'll peruse that, if nobody else chimes in.
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Old 06-11-2019, 07:15 PM
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F22,
As a bit of a test, I jumped a full 12 volts to the plus side of the coil while I had the high tension lead resting just above the contact. By tapping the jumper on the 12 volt source, I got a nice big spark to jump to the contact like I thought it should in free air. As another test, I guess I could leave the jumper in place as a good 12 volt source and see if the engine will fire up. If it does, I could then disconnect the jumper and see if the engine continues to run or not. Do you see any problems with these tests?
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Old 06-11-2019, 07:39 PM
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Originally Posted by blu73 View Post
F22,
As a bit of a test, I jumped a full 12 volts to the plus side of the coil while I had the high tension lead resting just above the contact. By tapping the jumper on the 12 volt source, I got a nice big spark to jump to the contact like I thought it should in free air. As another test, I guess I could leave the jumper in place as a good 12 volt source and see if the engine will fire up. If it does, I could then disconnect the jumper and see if the engine continues to run or not. Do you see any problems with these tests?
Not that I can tell. I think it's a good start. Still working on getting to the garage to get the FSM.
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Old 06-11-2019, 08:04 PM
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Factory Service Manual Description In this order.






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Old 06-11-2019, 10:35 PM
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F22,
Thanks for your help in regard to what voltage should be seen at the coil while cranking versus running. I'll be back under the hood tomorrow and see what it is getting and when. I'll update with any info, but I have a feeling the old wiring is giving up.
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Old 06-12-2019, 08:39 PM
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Okay, small update here. Didn't have a lot of time to work on this problem, because we do as much yard work around here as possible between rain storms.

Got a reading of just over 9 volts at the distributor terminal while cranking the engine. I then pulled the high tension lead and rested it near the coil contact but saw no spark jumping the gap like I did when manually making and breaking the jumper wire connection I tried yesterday. I guess that takes me back into the distributor for more testing. I know the points open and close because I have already tested for that. Could be a bad condenser in there leaking a little too much power to ground, or extremely bad dwell.

Is it possible to get any kind of dwell reading for the points while cranking the engine? I imagine the meter would show a lot of needle bounce due to the low speed of the points opening and closing. All this is really testing my memory, as I haven't been a full time auto mechanic for over 40 years. Yikes!
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Old 06-13-2019, 11:22 AM
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Another update. I'm was able to spend some time on this problem, as it's raining here (again). Checked again, and the voltage readings are as they should be at the coil terminal during cranking and while the key in the run position. Nine volts and six volts. I pulled the condenser out of the distributor - still no spark. I'm now assuming that it isn't shorted, so I'll put it back in. I did manage to get a dwell reading while cranking the engine and it showed something very close to 30 degrees. Points seem to be okay. Next, I disconnected the capacitor connected to the positive terminal on the coil. I believe it's there to reduce radio interference. Still no spark jumping the gap at the coil high tension contact.

Now I'm thinking of shorting the distributor end of the high tension lead directly to ground to bypass the cap and rotor.

A thought I just had. I've been doing all these tests with the ground jumper connected to the ground terminal on the back of the alternator. Maybe I should be checking from a ground on the engine block itself. If the block and alternator are somewhat isolated electrically as far as a ground path back to the battery, this could be a poor choice for the tests I've run. It has been some years since I put this engine into the car, but I have a nagging feeling there was a fair sized ground wire from the block to the chassis. More research needed on that. I'll look into it while the battery is recharging.

Last edited by blu73; 06-13-2019 at 01:59 PM.
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Old Yesterday, 03:19 PM
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Another update. I went and got an ignition spark tester and after making up a jumper so I could use it for testing the coil, I found that there is an intermittent spark coming from the coil. I should have seen a steady series of sparks jumping the gap in the tester. Not so. Totally random. I'm now thinking the points and/or condenser may not be as good as I thought. Right now, it looks like I'll just replace both while I'm messing around inside the distributor. On second thought, that won't tell me exactly which of the two was the defective part. I know it'll be a little more work, but I guess the wise thing to to is change one, put it all back together and test the results. That's pretty much what I've been doing up to this point. One change at a time.

Like Sonny and Cher said. "The beat goes on."
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