| Dustbowl | -- 06-25-2019 @ 1:59 PM |
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I sometimes - not always - have some problems with slow starting after long cranking so I measured voltage at inlet to coil with ignition on/engine not running. It was 2.7 volts soon after switching ignition on and later went down to 1.9 volts, presumably as resistor heated up. Are these voltages too low and should they be above 3.0 volts as some old postings have suggested? (I also measure voltage in and out of resistor and find - not surprisingly - that that is where voltage drop occurs)
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| cliftford | -- 06-25-2019 @ 3:58 PM |
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Is that reading with the points open or closed?
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| JayChicago | -- 06-26-2019 @ 6:34 AM |
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Those readings are with points closed. With points open there should be full battery voltage at the coil. (with points open, 0 current flow = 0 voltage drop thru the resistor) Yes, those old postings saying 3-4 volts at the coil failed to mention that is with engine running! With engine running the points are open 20% of the time, full battery voltage to the coil 20% of the time, which raises the averaged reading. With engine stopped, continuous current flow maximizes the voltage drop thru the resistor, and the reading at the coil will be considerably less. Based on my limited experience, I think 2.5-2.8 volts can be considered normal with engine stopped. So I think your 2.7 volt reading is good. And yes, the resistance in this type of resistor does increase as it gets hot, so the reading at the coil would go down some with time. But all the way down to 1.9 volts? That seems excessive to me. Maybe that's normal if left on for a long time, I'm not sure. But it could also indicate excessive current flow, resulting in excessive voltage drop. If the coil has an internal short (a common problem with these coils), it could allow excessive current flow, draining the potential (voltage) at the coil input. And a shorted coil would be consistent with your symptoms, producing a weak spark and hard starting. You might want to call Skip Haney to discuss this. He is considered an expert at rebuilding coils, has a standing ad running in the V8 Times. This message was edited by JayChicago on 6-26-19 @ 8:18 AM
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| Dustbowl | -- 06-26-2019 @ 12:21 PM |
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thanks jay; I was just going to respond to Cliftford's question when I see your first sentence correctly answers his question. I took further readings on voltage at coil inlet this morning, all with engine not running. After each reading, I rotated engine a little . Results were: 1. 2.1 volts 2. 2.0 3. 6.2 4. 2.0 5. 2.0 Reading 3 must have been when distributor circuit was open and no current flowing, as you said. All the reading were when the resistor was not hot, so resistance was minimum I now repeat the basic question of whether 2.0 volts is too low. If so , it must mean resistor is bad or there is a poor connection somewhere. Please let me know what you think. Incidentally, I tried calling Skip Haney (who rebuilt my coil a few years ago) but he does not answer.
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| 40cpe | -- 06-26-2019 @ 5:00 PM |
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deleted This message was edited by 40cpe on 6-26-19 @ 5:03 PM
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| JayChicago | -- 06-26-2019 @ 5:35 PM |
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2.0 volts at the coil input is too low, I think the coil will produce a weak spark. Your readings indicate a problem somewhere. But where??? Even though you have a Skip Haney rewound coil, it could have gone bad. I think he would tell you they don't last forever, and a few don't last long. I still think it could be a short in the coil, which would let the current rush to ground too fast, reducing the voltage (think pressure) at the coil input post. But since this is a Skip Haney coil, makes me want to also look elsewhere. We know you are getting 6.2 volts fed to the circuit because you read that at the coil with points open. But there could be parasite resistance upstream of the coil, and resistance does not show itself without current flowing. With circuit on/current flowing, check voltage at the output side of the resistor which should be same as at the coil. Then check voltage at input side of resistor, which should be close to your battery voltage. Assuming these check as expected, we are still left with: is it the resistor or the coil? And I don't know how to check either one, short of replacing it with another one. Maybe someone else will know how to check these things, or have some other thoughts on trouble-shooting this problem. I have an extra known-good resistor on hand that I can send you if you get to wanting to replace the resistor. But from what I understand, resistors rarely go bad, are expected to last forever. The coils are known to be fragile, do go bad often.
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| juergen | -- 06-27-2019 @ 5:41 AM |
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Check the voltages along the circuit. Check before and after the resistor. If the voltage before the resistor is less than battery voltage (about 6.2 by your measure) check before and after the ignition switch. None of these will have a drop when the points are open. Drops over 1 % are excessive.
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| JayChicago | -- 06-27-2019 @ 8:09 AM |
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Dustbowl I thought of another couple checks you can do: Using an ohmmeter, check resistance across the the resistor. (with switch off; an ohmmeter would be confused if the circuit was live. An ohmmeter applies its own micro-voltage to measure resistance) The resistor should measure 0.6 ohm. Check resistance thru the coil primary winding: Connect the ohmmeter leads to the coil input post and to the base of the coil where the condenser is attached. The book says the coil primary should be 0.47-0.51 ohm. But there is a weakness to these resistance "bench tests". It can identify an obvious problem, but a good reading does not necessarily indicate all is good. Sometimes an electrical component can test good, but then fail immediately when it is carrying a real-world load, or when it gets hot.
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| TomO | -- 06-27-2019 @ 8:41 AM |
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Dustbowl, before spending a lot of time and money on chasing the "low" voltage at the input to the coil, you need to determine if your starting problem is ignition related or fuel related. Check your spark, with the engine idling at normal operating temperature you should have 1/2" blue colored spark when a wire is held close to a head nut. When cranking the engine with the starter, it will be much shorter but still blue in color. If the spark is OK, check the fuel delivery. Too much fuel Caused by carburetor leaks or mal adjusted float level. You may see some signs of a leak by stains around the base of the carburetor. I like to set the float fuel level 1/32" to 1/16" lower than specifications when using fuel with 10% Ethanol in it. I use an extra gasket on the throat body when I adjust the float to accomplish this. It will not affect high speed or turn performance. Leaks past the power valve gasket will cause a flooding condition. Using the correct power valve and making sure that the gasket is sealing are important. The condition of the seat is also important. Too little fuel Caused by carburetor leaks Caused by fuel pump issues Determined by looking down the throat of the carburetor and moving the accelerator linkage. You should have 2 strong steams of fuel coming from the nozzle. Do a fuel delivery test to determine the cause. Good luck and come on back with the results Tom
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| supereal | -- 06-27-2019 @ 8:58 AM |
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Measuring coil voltage with the engine running will not provide an accurate result. This is because the circuit is pulsing as it opens and closes. Only a meter that can measure RMS (root mean square) is accurate. If the battery is fully charged, the ignition switch is making proper contact, the resistor connections are secure, and both connections to the coil are in place, there is one more factor to be. considered. The starter draws high current when engaged, robbing the coil of sufficient power. Years ago we took note of how this was addressed. During starting, a connection to the ignition switch or starter directly to the coil to compensates for the voltage drop. We added a circuit from the starter side of the solenoid to the coil through a diode. It s a simple, effective way of enhancing starting, warm or cold. I posted a page of this, and will do so again if interested.
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| 37RAGTOPMAN | -- 06-27-2019 @ 9:04 AM |
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What year is this car or truck, how old is the wiring ? is battery charged up ? terminals clean, you say you have a rebuilt coil, is the distributor also rebuilt, ? dirty point will cause hard starting, starting at the battery checking the volts there then to, starter switch and work my way to the ignition switch terminals voltage should be the same, with no lose of voltage, if you want try using a jumper wire from the battery to the resister, making sure it has the same voltage reading as the battery you can also jump the resister directly and if it starts better, I would replace the resister, or clean the contacts on the resister terminals using a jumper wire this removes a lot of thing that could be the problem, corroded terminals will add lose of voltage in the system, hope this helps 37 RAGTOPMAN My question is why does it crank a long time ? and not start ? maybe needs a good tuneup ? if you have it running, use a small sprayer and spray a mist of water on the ignition wires,and see if they arc or motor misses or shut off
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| JayChicago | -- 06-27-2019 @ 10:23 AM |
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Dustbowl I agree with Juergen. You should probably do the tests he suggest first. We started this thread with a good 2.7 volts at the coil, so I have been thinking you are getting good voltage to the resistor. But then other times you see lower readings at the coil. Explanation could be, at some times there is a voltage drop ahead of the resistor, like at the ignition switch.
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