John L Posted March 18, 2023 Report Share Posted March 18, 2023 What should the secondary coil resistance be for a 3ohm coil? Will a high energy coil like a Flamethrower or a Bosch blue one, have a higher secondary reading than say a std Lucus DLB 101? I have several 3ohm coils with slightly different secondary values, and wondered which value would give the better spark? John Quote Link to post Share on other sites
RobH Posted March 18, 2023 Report Share Posted March 18, 2023 The resistance of the HT winding will depend on the number of turns used and the gauge of the wire and those will vary from maker to maker. There probably isn't any simple direct correlation between resistance value and spark quality as there are lots of other factors too which will influence that. Typical values for the HT winding are around 9 to 10 thousand Ohms. Quote Link to post Share on other sites
John L Posted March 18, 2023 Author Report Share Posted March 18, 2023 Thanks Rob, The Lucas new one I have 9.12 on both +&-, the new Bosch Blue one 7.68 on both, Bosch one on the car is 8.90 on - and 8.86 on the +, does this mean on this one its getting a bit weak? John Quote Link to post Share on other sites
RobH Posted March 18, 2023 Report Share Posted March 18, 2023 3 minutes ago, John L said: does this mean on this one its getting a bit weak? That is a surprising difference John. The coil is an auto-transformer so one way round the primary resistance is included and the other it is not but since the primary is only around 3 Ohms it should hardly register on the meter when reading thousands of Ohms, and certainly not show 40 Ohms difference like that. Strange - it might just be the meter I suppose but it read the others OK........ Quote Link to post Share on other sites
John L Posted March 18, 2023 Author Report Share Posted March 18, 2023 Those last readings are for the secondary readings, I suppose they should be the same on the + to the centre as to the - to the centre? John Quote Link to post Share on other sites
RobH Posted March 18, 2023 Report Share Posted March 18, 2023 The coils are connected like this: So when you measure from HT to the - terminal it is just the secondary coil resistance . When you measure from HT to + it is both coils in series. The difference should just be the resistance of the primary. Quote Link to post Share on other sites
KiwiTR6 Posted March 19, 2023 Report Share Posted March 19, 2023 From memory my Flamethrower measured around 12K when I checked it the other day. Quote Link to post Share on other sites
J R Martin Posted March 19, 2023 Report Share Posted March 19, 2023 Anyone know where I can buy a Bosch Blue coil, they seem to be like hen's teeth at present and I need one for a spare? Thanks John Quote Link to post Share on other sites
Mike C Posted March 19, 2023 Report Share Posted March 19, 2023 23 minutes ago, J R Martin said: Anyone know where I can buy a Bosch Blue coil, they seem to be like hen's teeth at present and I need one for a spare? Thanks John This one? https://motociclo.com.au/product/ignition-coil-12-v-bosch-blue-guzzi-universal/ Quote Link to post Share on other sites
John L Posted March 19, 2023 Author Report Share Posted March 19, 2023 (edited) Martin, Not knowing where you are based from your profile, here is the reference and a supplier in UK, there may be many, if that helps, https://www.merlinmotorsport.co.uk/p/bosch-blue-high-energy-coil-0-221-119-027-sport-coil This is a 3 ohm coil. John Edited March 19, 2023 by John L Quote Link to post Share on other sites
Waldi Posted March 19, 2023 Report Share Posted March 19, 2023 (edited) Martin, make sure you select a coil with the correct resistance: early TR6 (Cp): 3 Ohm, later (CR): 1.5 Ohm; later cars have a resistance in the “feed wire”, which is by-passed during cranking (starting). If you are not sure which is needed, measure the voltage with the contact switched on (but NOT cranking). The early cars should have the same as “battery voltage” (say 12.5-13V), the later cars will have around 9 V (or so) measured at the coil. Waldi Edit: I’m sure Rob will explain it better, I’m a bit cryptic I think. Edited March 19, 2023 by Waldi Quote Link to post Share on other sites
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