17-05-2022, 07:59 PM
Hi Barry
It's unavoidable to have some voltage drop in the battery, due to its internal resistance. However, much less than 5 volts at the battery terminals when cranking (unless the weather is really cold) and it's probably time for a new battery.
Voltage drop in the starter motor wiring should be not much more than 0.1 volt each for the flow and return cables. In the standard arrangement, the main cable goes straight to the switch on the starter, and the return goes from a lug under one starter motor/crankcase bolt straight back to the battery negative (assumes negative ground). A suitable modern cable would be 37/0.9 or the more flexible 196/0.4 which both have about 25 square mm cross section.
Taking these two things together, the voltage from starter switch to crankcase will be 4.8 volts minimum when cranking. The coil will get a little less, due to maybe 0.2 volts drop in the wiring to and from the PLC2 switch and the switch itself, and maybe another 0.1 volts dropped in the closed contact breaker. That makes 4.5 volts at the coil. PLC2s are well made and chunky inside, but they are all getting on in years. Worth checking coil voltage when not cranking and getting someone to jiggle the key/spade to make sure the switch isn't iffy.
I feel there should be just enough margin built into the ignition system to give a spark good enough for starting even at 4.5 coil volts, provided everything is in good condition and the plug gaps are OK. New plugs might help, as the centre electrode will have sharp corners which needs less voltage to fire. Checking my notes on previous ignition bench tests I found that even at 4.1 volts I was getting about 15 kV which is enough to jump a 5 mm gap (not under pressure).
As you can see, there are plenty of opportunities for a little bit of voltage drop or inefficiency. Each one on their own wouldn't be a show stopper, but when you tot them all up you can have a reluctant starter.
If the engine is rubber mounted, it's good practice to also have an earth link from the body to battery negative (or from body to the crankcase/engine ) but this link should only be called upon to handle the return current from stuff like lights, petrol gauge, trafficators etc. It shouldn't be handling starter or ignition current if the main starter cables are arranged as described above.
It's unavoidable to have some voltage drop in the battery, due to its internal resistance. However, much less than 5 volts at the battery terminals when cranking (unless the weather is really cold) and it's probably time for a new battery.
Voltage drop in the starter motor wiring should be not much more than 0.1 volt each for the flow and return cables. In the standard arrangement, the main cable goes straight to the switch on the starter, and the return goes from a lug under one starter motor/crankcase bolt straight back to the battery negative (assumes negative ground). A suitable modern cable would be 37/0.9 or the more flexible 196/0.4 which both have about 25 square mm cross section.
Taking these two things together, the voltage from starter switch to crankcase will be 4.8 volts minimum when cranking. The coil will get a little less, due to maybe 0.2 volts drop in the wiring to and from the PLC2 switch and the switch itself, and maybe another 0.1 volts dropped in the closed contact breaker. That makes 4.5 volts at the coil. PLC2s are well made and chunky inside, but they are all getting on in years. Worth checking coil voltage when not cranking and getting someone to jiggle the key/spade to make sure the switch isn't iffy.
I feel there should be just enough margin built into the ignition system to give a spark good enough for starting even at 4.5 coil volts, provided everything is in good condition and the plug gaps are OK. New plugs might help, as the centre electrode will have sharp corners which needs less voltage to fire. Checking my notes on previous ignition bench tests I found that even at 4.1 volts I was getting about 15 kV which is enough to jump a 5 mm gap (not under pressure).
As you can see, there are plenty of opportunities for a little bit of voltage drop or inefficiency. Each one on their own wouldn't be a show stopper, but when you tot them all up you can have a reluctant starter.
If the engine is rubber mounted, it's good practice to also have an earth link from the body to battery negative (or from body to the crankcase/engine ) but this link should only be called upon to handle the return current from stuff like lights, petrol gauge, trafficators etc. It shouldn't be handling starter or ignition current if the main starter cables are arranged as described above.