Hi TwoTired
I wondered if there was a way to tell which coil was which. I compared the coils on my 400F1, 550, & 750F1. The marking on the coils is, 400F1 FL703-12V57 F, 550 ( not sure which 550 ) FL703-12V 68L & FL703-12V 67L, 750F1 FL703-12V G. Tomorrow I will check the resistance of these coils.
TomC in Ohio
Which I did but forgot which thread I was on. All of the above coils measured 4.5 +/- 0.1 ohms. The 750 and 500 coils may have been different when the manuals were written. But by 1976 all the SOHC4 seem to be using the same coil, except for wire length.
TomC in Ohio
I have not done extensive testing across all models of Honda coils. The only coils I have off the bike right now are from my 74 Cb550. Their numbers are FL703-12V48 C (2-3 coil) and FL703-12v48 B (1-4 Coil).
The way the numbers are cast into the coils, show distinct variation in the size and shape of the 48 in the number. This leads me to believe this is a batch number or more likely a date code of some type. Perhaps batch 8 in 1974?
Anyway, the number is clearly cast from a mold which does not have to change in order to encapsulate whatever coil winding arrangement was placed into the machine. Remember these coils were made to Honda specifications by TEC. I've no idea what their convention was fro marking the coils or if "batch numbers" were used to identify internal characteristics or make up. Anyway, TEC's final product would be delivered to Honda where they would assign the Honda part number for distribution to the appropriate assembly line.
I don't know that WAS done this way. I just observe that it COULD have been done this way.
I guess the point is, that I don't know that the numbers on the coils have a known correspondence to the Honda part numbers, outside of Honda record keeping.
But, I measured the coils anyway. I used two DMMs.
Bear in mind that thee coils were replaced because the there is an intermittent wire on the 2-3 coil. I have to wiggle it to get a reading. And, it's accuracy is suspect.
With the Fluke 0804 I measured the 1-4 coil to have a 14.95K secondary and a 4.8 ohm primary. The meter leads account for .3 ohms of that reading. So, the primary measures 4.5 ohms.
With the Fluke 0804 I measured the 2-3 coil to have a 16.90K secondary and a 4.8 ohm primary. The meter leads account for .3 ohms of that reading. So, the primary measures 4.5 ohms.
With the Fluke 73 I measured the 1-4 coil to have a 15.02K secondary and a 4.6 ohm primary. The meter leads account for .2 ohms of that reading. So, the primary measures 4.4 ohms.
With the Fluke 73 I measured the 2-3 coil to have a 17.60K secondary and a 4.6 ohm primary. The meter leads account for .2 ohms of that reading. So, the primary measures 4.4 ohms.
I'm not sure about the magnitude difference would be between an 380 turn primary and a 420 turn primary. If I knew the length and gauge of the wire, I could make a pretty close estimate.
But, I am assuming it would be pretty small and possibly difficult to measure, without a very sensitive meter. Probably better to infer it, by applying a known precision voltage and measuring the Vdrop across it, while noting the current being drawn. Just a calculation, then.
Seems likely there would be a more significant difference in the secondary resistance. This would be smaller gauge wire than the primary, offering more resistance per length of wire, and 2000 turns difference ought to show a significantly different resistance.
I'll try to remember to measure and collect data on my other coils, next time I have the tank off for access. Maybe Honda did change to a universal coil. Wouldn't be the first mistake I've found in the books. I'm just not quite ready to make that conclusion.
Cheers,
