Ballast Resistor Not Resisting?

[Hardway]

Yesterday as I drove my 240z home from the upholstery shop the tachometer took on a mind of its own and the car seemed down on power and running rough at idle.  Once I got home I looked under the hood and immediately saw my coil was leaking oil.  Worse yet, it looks like it has been doing this for awhile, how did I miss this?  Anyway, I have a new coil on the way but also wanted to test the ballast resistor.  With the car running, if I ground the negative lead of my digital multi-tester against the body and place the positive lead on the resistor terminal on the right I get 13.65V.  If I place it on the left terminal with the + sign I get 11.74V.    From what I have read the voltage going out of the resistor should be closer to 6V.  With the engine turned off and the resistor disconnected I get 2.1 ohms across the terminals and the audio signal from my multi-meter that there is continuity between the two terminals.  Again, the engine runs but not that well and I sure the coil oil leak is a big contributor to that.  Do I need to get a new resistor?  Is it possible it is broken or damaged even though the car run and it has continuity?

[Zed Head]

Test the coil resistance, even though it's toasted now.  I'm not sure that voltage out is supposed to equal 6 volts, the thought comes from people calling it a 6 volt system.

Your resistor resistance is actually higher than spec. but it also rises with current and temperature.  Might get a lower number when it cools down.

Seems more like your coil just crapped out.  What's the brand?  You can also overheat ignition parts by leaving the key at Run, if the points are closed.  Maybe somebody was playing the radio at the upholstery shop, using Run instead of Acc?

[beermanpete]

Yes, it is possible the ballast resistor has changed value over time. The '72 service manual indicates the correct value should be 1.6 ohms. This is at ambient temperature (68 degrees F). If you just ran the car the resistor will still warm and will measure too high. The voltage measured at the positive terminal of the coil will vary with engine speed, points dwell angle, and other variables. As I recall my car ('73) was closer to 9 volts at idle. Before chasing ghosts or your tail, replace the coil (you already know it is bad) and check all the basic stuff: points condition, dwell angle, primary wiring, etc. If the car still is not running right look at the less likely stuff.

[Hardway]

So I went and picked up my coil at the auto parts store and installed it.  While everything was out of the way I cleaned up all the oil, cleaned the wiring, and I had to replace a spade connector as the original broke upon removal.  It says on the coil "12V Use With Primary Resistance Wire Or External Resistor."  They also had a coil that said 6V "No External Resistor Required"  My coil also has a condenser connected to the positive side of the coil.  Not sure why it is is there as the distributor has one too but I put it back in place.  Was this as they came from the factory?  The car starts and runs fine, tach seems to be back to normal too.  I will check everything in the morning when the car has sat over night and is completely cool.  When I take my readings I will get the temp of items using my IR temp gun.  A good drive with some high RPMs will help determine if any real change has been accomplished.

The upholstery shop did not do anything to the car.  The tach was acting wonky when I left to go drop it off in the morning.  

Edited July 9, 2017 by Hardway

[Zed Head]

What are the specs of the new coil?  Is there a brand?  Matching the parts specs is the key.  You haven't provided much info.

Are you sure it didn't say "external resistor" not "external regulator"?

[Hardway]

Good questions and good catch Zed.  I corrected my post as it was supposed to say "resistor" and not "regulator"

The oil coil had no markings of any kind on it, I still have it.  The new coil is a MasterPro 2-5195.  The website just says 12V.  It does not provide any other specs.  I checked the resistance of the resistor on the car, it was 80 degrees (its been a hot day) and I got 1.7 Ohms.  I picked up a new resistor and it was 1.8 Ohms so this leads me to believe the resistor is good.  I could not get a Ohm reading on the old coil with the leads on + and - terminals.  My digital tester would constant hunt, sometimes as high as 30 ohms, sometimes as low as 2 ohms.  My new coil reads 1.5 ohms across the + and - terminals.  Should I consider going back and getting the 6V coil?

Should I check the voltage of what is being sent out of the coil to the distributor?  Looks like I could slide the boot back and stick a piece of wire in to it, start the car, and check it with my digital multi-tester.

Edited July 9, 2017 by Hardway

[Patcon]

Will your meter handle in the 1000's of volts? I am not sure mine will...

[Zed Head]

44 minutes ago, Hardway said:

 The new coil is a MasterPro 2-5195.  

.  My new coil reads 1.5 ohms across the + and - terminals.  Should I consider going back and getting the 6V coil?

Everything looks right.  Sounds like your old coil just went bad.  Your MasterPro coil is right on spec., should work fine and last long.  246Blue won't like it, but it's the world we live in.  Google says lots of good things about that coil.

Here's the 1972 specs. which should be common across all of the points ignition systems.

 

[Hardway]

Right on!  I will just drive it and see what happens.  I saw the same reviews of it so hopefully it will give years of trouble free service.

[Hardway]

Good news.  I spent the afternoon re-syncing and re-adjusting the carbs.  As I had noticed the idle getting rougher I had made adjustments to the carbs on the fly.  I now realize i was making these adjustments to compensate for the dying coil.  With everything back within factory spec and a little tweaking, the car idles like a purring kitten at 900 rpm.  I took it on a spirited test drive, winding the motor out to 6500 rpm on several pulls.  She still had more in her but I do not like to push it as the car is a cruiser and has its original engine.  Power band felt really good so I think I am all set.  I am going to drive it again tomorrow evening on a cold start and see how it does.

As a final question, once I got back I looking everything over and noticed by accidental touch the coil was hot.  Using my IR temp gun it red 121 degrees F.  Is this normal or am I some how slowly killing this coil too?