Need help on a coil rebuild

[Don ellis]

What’s wrong with this coil? What can I do to repair it? Everything checks but that.

[jab35]

Try prying up the back of the top bridge mount to lower the current value. IIRC the Primary coil inductance should be ~ 3 mH, so the value
given seems reasonable. jb

[MKossor]

This is a great educational example of what can be encountered given the vast population of Model T coils and their age. The nominal Model T coil Primary Inductance value is 3.3mH as Bob points out. The test results shown measured the primary inductance 2.56mH which is lower than the nominal range (3.3mH +/- 20%) so was flagged as Fail. Note also that the coil firing current (7.24A) is also higher than the nominal 6A. That means this coil May make it more difficult to hand crank start the car because of the higher required firing current. However, note that the firing time has been adjusted to occur right at the nominal time (2ms, center scale 0 degrees timing error) AND the coil firing consistency shown in the Multi-Spark test result indicates Excellent firing consistency (>89% of firings at 0 degrees timing error). That means Engine ignition timing will be excellent when this coil is operated with 3 other coils adjusted for the same firing time and firing consistency.

Now, consider what would happen if this coil was adjusted using average coil current as an indirect approximation for coil firing time and all 4 coils were adjusted for the Exact same 1.300000A of average coil current. This coil would ramp up to fire spark significantly faster than the other coils with nominal 3.3mH coil primary inductance. That guarantees this coil will fire spark sooner than the other 3 coils leaving one pondering why the heck does the engine not operate more smoothly and have more power.

[Don ellis]

So is it repairable?

[MKossor]

The question really should be: is it Useable? The answer is YES. You compensated for the lower primary inductance by adjusting the points to fire at the correct time. Same time as the other coils you use in the car and can expect excellent engine ignition performance. There really is nothing to "repair".

What makes this coil have a lower primary winding inductance? The coil primary inductance primarily depends upon the number of turns of wire to wind the primary winding and the coil diameter and also a function of the secondary winding characteristics. This coil most likely has fewer turns of wire either by design or due to a prior repair where some of the windings have been removed. Another possibility would be a short in the primary coil winding where a section of winding is effectively bypassed. Can the coil winding be restored to the nominal 3.3mH value? Yes but that would require a lot of work to disassemble the coil, separate the primary and secondary windings to rewind the primary coil with the correct number of turns of wire. Completely unnecessary and not worth the time and risk in doing so. You have adjusted the coil for the nominal firing time and Excellent firing consistency. You can expect excellent engine ignition performance when this coil is used with 3 other coils adjusted for the same nominal firing time and firing consistency. The only potential issue I can envision is greater difficulty hand crank starting the car using this coil but have no data to support that. It would be a good experiment to collect 4 coils with lower than nominal primary inductance, adjust them all for the nominal firing time and drop them into a hand crank start car to see if there is any validity them being more difficult to hand crank start.

[Don ellis]

Thanks Mike

[MKossor]

Thanks for sharing your coil test anomaly Don. Please let us know if/when you have a chance to verify engine performance results are excellent using this low inductance (2.56mH) coil with 3 other coils with nominal coil primary inductance (3.3mH +/-20%) that are all properly adjusted (for the same nominal firing time and excellent firing consistency).

[Darren J Wallace]

Question for Mike: how much forgiveness would your methods, software/ hardware, and equipment have for early coils like Heinze for example. Provided they can be used with an adapter that would fit into your unit and then fit the Heinze coil into the adapter. I have made such an adapter to use in my HCCT, and a Strobospark unit. Are the early coils too problematic to use with your system?

[MKossor]

Great question Darren, several folks have successfully built early coil adapters for Heinz, Kingston and early trembler coils with good reported results.

All that is necessary is to build a physical coil adapter that contacts the coil power terminal and timer terminal. These 2 wires are then connected to the coil tester using a hollowed out 1913-1927 Model T coil inserted into the coil tester. The wires from the Model T insert run to the same terminal on the coil adapter. The coil adapter must also have a 1/4" spark gap between the High Voltage (spark plug) terminal and the Timer (ground) then the coil tester is operated as usual (you do NOT have to port the high voltage terminal from coil tester to coil adapter). Testing the coil is easy. Adjusting all coils for equal and consistent firing time is the challenging part. The coil points lack the cushion spring that prevents immediate contact opening; allowing the coil current to build up to a suitable level prior to opening very rapidly when the moving cushion spring contact hits the limit rivet. That means you have to get the point gap and spring tension of the moving element just right to ensure the points break cleanly and rapidly without the benefit of the limit rivet.

Perhaps someone can post photos of their early coil adapters and performance they have achieved.