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Post by dave1800 on Jul 5, 2018 14:43:53 GMT
I'm helping my friend sort out his MGB engine (1798)and we've found the resistance of the carbon/graphite brush in the distributor cap is consistently measuring around 18000 ohms. Previously I have measured these at close to zero ohms but have found references to the higher value on some websites. The various symptoms include possible insufficient ignition power - still at the exploring / analysis stage.
Is this a change in the manufacturing or should it still be zero ohms which makes more sense other than when considering interference?
David
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Post by Penguin45 on Jul 5, 2018 23:15:23 GMT
I'll have a look at some caps on Friday David and see what the meter says.
Chris.
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Post by dave1800 on Jul 6, 2018 6:39:51 GMT
Thanks Chris that's very helpful I'm beginning to suspect the electronic ignition unit but after a garage clean up no sign of the original points and plate to substitute. David I'll have a look at some caps on Friday David and see what the meter says. Chris. |
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Post by Penguin45 on Jul 6, 2018 17:21:56 GMT
45D Cap.
1/ 2.25Ω
2/ 2.5Ω
3/ 2.25Ω
4/ 2.25Ω
Centre post with brush - 19Ω
25D Cap.
1/ 2.75Ω
2/ 3Ω
3/ 3Ω
4/ 3Ω
Centre post with brush - 6.5KΩ
No idea of the significance of the variance with the centre post readings.
HTH,
Chris.
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Post by dave1800 on Jul 6, 2018 22:12:22 GMT
Thanks Chris. All well below the 18K we found. The higher the resistance between the coil and the plug the lower the electrical interference unless it creates a leak to ground via another path. However, this raises the LT voltage spike which is clamped by the electronic ignition unit at around 400 volts so in theory may weaken the output transistor (IGBT) over time. We have a new cap on order and also leads.
David
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Post by dave1800 on Jul 30, 2018 10:09:36 GMT
Just curious. For those who have experienced rotor arm failures I wonder if there is any correlation with the use of high electrical resistance in the carbon / graphite brush? The brushes used to be close to zero ohms. David Thanks Chris. All well below the 18K we found. The higher the resistance between the coil and the plug the lower the electrical interference unless it creates a leak to ground via another path. However, this raises the LT voltage spike which is clamped by the electronic ignition unit at around 400 volts so in theory may weaken the output transistor (IGBT) over time. We have a new cap on order and also leads. David |
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