duanem1 Posted October 6, 2009 Share Posted October 6, 2009 I need some help understanding (and perhaps correcting) a problem with the resistor on the ignition system of my stock 240Z. I am on the 2nd new one in a month and the same condition exists. I understand that running the car without the resistor will burn the points quickly, however with the resistor hooked up I am often getting over 12 volts coming out of the resistor. It seems to flip back and forth. At times it is only slightly over 6 volts, then for no apparent reason it reads over 12. (This is without the car running but with the ig. switch in the run position.) When the car is running it reads over 12 volts coming out of the resistor. How does this resistor work? Why will it cut the current to about 6v and then go back up to 12+? Have I missed something in the wiring of this? Is there some fuse in the resistor that is not apparent? This is the ceramic resistor with a coiled wire in it. Doesn't seem to be a place for any fuse. Any answers? Quote Link to comment Share on other sites More sharing options...
Zmanco Posted October 6, 2009 Share Posted October 6, 2009 How are you measuring the voltage? Are you putting the probes on both ends of the resistor? And is the engine running or stopped? I very much doubt that there is anything wrong with the resistor - it's hard to see how it could fail, much less 2 would fail. What it is the problem you are trying to solve? You didn't say why you suspect the resistor. Quote Link to comment Share on other sites More sharing options...
duanem1 Posted October 6, 2009 Author Share Posted October 6, 2009 Checking voltage from end of resistor going to coil and ground. Checking it both when running and when stopped. I agree with you that it is hard to see how it could fail, but when at one point it reads just over 6 volts and a few minutes later reads over 12, (engine stopped) something is going on. Trying to avoid point burning with 12+ volts going to coil. It would seem to me that the resistor should either impede the flow of elec. or not carry current at all due to a burned wire. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted October 7, 2009 Share Posted October 7, 2009 You still haven't said if there is a problem you are trying to solve. Measuring the voltage across the coil (what you are doing) with the engine running really doesn't tell you anything. The actual voltage is changing constantly as the points open and close, but a voltmeter cannot respond that fast so it just averages it. The actual voltage you measure will depend on a variety of factors including the setting (dwell) of the points themselves. When you measure with the engine stopped but ignition on, you will get different readings depending if the engine stopped with the points open or closed. Unless you are experiencing a problem driving it, you haven't described anything wrong with your car. Just drive it and enjoy. Quote Link to comment Share on other sites More sharing options...
duanem1 Posted October 7, 2009 Author Share Posted October 7, 2009 That is encouraging. Thanks for your input. The change in the readings when the engine is off occur with no change in the point position. In other words, I measure the output end and get a 12+ reading, and shortly, with no change in the engine position, a 6+ reading. That has concerned me. The reading you describe with the engine running makes sense. But, as you say, I will drive and enjoy and see what happens. Just getting it back up and running after restoration. Thanks again. Quote Link to comment Share on other sites More sharing options...
FricFrac Posted October 7, 2009 Share Posted October 7, 2009 Bad connection? Typically with those power resistors if it fails it will fail open. Unlikely to be intermitantly shorted. Did the other resistor do the same thing? The resistor is acting as a voltage divider so if you have 1.5 ohms resistance on the coil in series with the 1.5 ohms on the resistor from the ground through the resistor and the coil you should see 12 volts. Across either the coil or the resistor you would see 6 volts. (50% of the resistance is in the resistor and thus 50% of the voltage = 6 volts). The voltage on the coil is switching off and on rapidly (producing the back EMF to give you spark) so if you have a digital multimeter (DMM) you might just be seeing a peak reading. Try an analog meter and watch as the needle bounces back and forth because of the pulsing. Quote Link to comment Share on other sites More sharing options...
zcarnut Posted October 7, 2009 Share Posted October 7, 2009 ...The resistor is acting as a voltage divider so if you have 1.5 ohms resistance on the coil in series with the 1.5 ohms on the resistor from the ground through the resistor and the coil you should see 12 volts. Across either the coil or the resistor you would see 6 volts. (50% of the resistance is in the resistor and thus 50% of the voltage = 6 volts)... That is only true when the distributor is not rotating and the points are in their closed position and the resistor and coil current is pure dc. The switching of the points transforms this current into a pulsating (ac) current. When the engine is running the (average) voltage drop across the resistor is determined by the average current through the coil primary. The average coil primary current is mainly a function of the coil primary inductance, the engine speed (rpm) and to a lesser extent the primary resistance. The typical voltage drop across the resistor is from 1V to 2.5V. If you do not trust your readings then try another meter. Some digital meters will produce incorrect readings when measuring pulse waveforms. Quote Link to comment Share on other sites More sharing options...
duanem1 Posted October 8, 2009 Author Share Posted October 8, 2009 Thanks for the help on this. It helps to understand the kind of readings I should expect to get from this part of the ignition system. I will continue to drive and see what happens. Have just gotten the car up and running after a long process of restoration. Still sorting things out but this helps with this part. Thanks again. duanem1 Post Falls, Idaho 1972 240Z Quote Link to comment Share on other sites More sharing options...
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