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Old 04-24-2007, 06:37 PM
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overheating coil

I have a 74 260z and it has electronic ignition. I changed over to a Crane ZR700 system and an Accell performance coil. I check the voltage at the coil from the resister block and was only getting 6.5 volts so I bypassed the resister and connected to the 13.5 volts source. If I run the car for a time it will get really hot. When I hold it I seem to be able to feel it pulsing. After awhile it is to hot to hold. The car runs fine. I can't put it on the road to test until I fix the clutch. With the 6 volts my platinum plugs gapped at .30 were black. I changed to NGK at .45.

Is this a bad coil? or should I be looking at something else? I will try it again with the 6.5 volts but I really want the full voltage to the plugs. I think the low voltage has something to do with it not passing emmisions.

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Old 04-24-2007, 07:54 PM
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Iam Don't Know About Your Ignition But You Only Need Full Voltage At Start Up Then Reduced Voltage By The Resister Line Voltage Will Fry The Coil If Not Reduced.
Rambler Said That
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Old 04-24-2007, 11:31 PM
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I tried a different coil and am getting 10 volts at idle and 12 when I rev it. It is running cool now so I guess I will leave it as is.

Thanks for the help
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Old 04-25-2007, 06:16 PM
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Doc here,

First, to measure the voltage of a coil, you Can Not do it while it is running..It will only give you Peak average Voltage.

To see if your ballast is working, open the Distributer cap, Insure the POINTS are dead closed (low lobe OR run a jumper from the points wire to ground). Turn on the key, measure the Voltage from the Coil + to ground on your DVOM, DCV Scale, R X 20 or higher, It should read 6 to 9 volts to be correct.

NEXT, Open the points, or put an insulator between them, and perform the test again. It should read 12 VDC.

The difference is "Loaded and Unloaded " Coil..You could put 1000 Ballast resistors in line with the coil + with the coil not consuming current, (unloaded) It will read 12 volts..It's not producing useful work.

With the points CLOSED, the coil now is producing useful work, due to the internal resistance of the primary winding to ground, the coil is now consuming current..The Higher the current, the lower the Voltage.

For a visual reference, use a headlamp on a small 12 volt battery, like a bike battery, measure the voltage with the lamp running at the headlamp, it will probably read around 11.95 Volts..

Next Lift the load from the battery on the ground side of the lamp and measure the voltage at the headlamp power wire, It will probably read about 13.95 volts BECAUSE the lamp is not consuming current to ground (The filament being the value of resistance to ground) YET..It still has power to the lamp. It is producing no useful work.

If your old coil felt like it was Vibrating, My guess would be it had Loose laminations on the windings, Every time the points opened and closed the resulting surge of current would vibrate the windings of the coil. This can change the internal resistance of the coil, and may cause the insulating paint to break down causing "Arc Over " between the windings..poor performance and heating would be the result..

You can OHM the old coil out, With a DVOM,set for OHMS scale, R X 1, Calibrated "000", The primary will read LESS than an ohm but more than 000 to be good.

The secondary will read , on the R X 20 K scale, between 10,000 and 11,000 to be good.

This will ONLY test static readings of a coil. To test for winding break down usually requires an Oscilloscope, unless the insulating paint is so far gone it reads a short without power.

Doc
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