Demystifying the K Series Starter Solenoid Circuit

[revilla]

OK 8.5V at the solenoid is struggling. You need to work backwards from there back towards the battery looking to see where the voltage drops. That's not going to be easy as you will have to test it with the key turned to start, so ideally you need a helper, one person to operate the key in short bursts while the other checks the voltage. Probably worth a quick check again of the voltage directly across the battery when trying to start ... if that drops to anywhere close to the 8.5V then you've found the problem, if that holds up around 12V and you've only got 8.5V volts at the solenoid then you know it will be dropping somewhere in between and you can work your way around the circuit I showed in the diagram to find where it is. Because of the currents involved, be prepared to find lots of little drops at different points, but hopefully there will be one big one that will give away the culprit.

[john milner]

I am experiencing some cold start problems. I need to apply the throttle when starting and after coming off the engine is a bit lumpy then dies. If I hold the throttle open for 30 seconds to a minute then shut down it restarts perfectly. My feeling is that I have the early signs of a failing battery. I also have a cheap fag lighter voltmeter and have noticed that when starting the voltage very briefly drops to 8-9V. As the fag lighter is directly connected to the battery it at least partially removes HT leads, solenoids, relays etc. from the equation.

I am curious to know if 8-9V for no more than a second is normal or can be used a quick way of identifying that the battery is most likely on the way out.

[SM25T]

Sounds like battery failing to me.

[revilla]

8-9V for anything like a second is low. Low enough to upset things. As multimeters usually only refresh one or two times a second it's hard to see exactly what it's doing. If you're measuring at the battery as you say, anything further down the line drawing load is only going to see an even lower voltage. The battery doesn't sound too healthy to me.

[john milner]

Thanks I will be getting a new battery I just wasn't sure how valid the short drop was as a test as I have never looked closely when starting with a good battery. I had been using the voltmeter infrequently just to keep an eye on things as part of general maintenance but I'll probably use it all the time now as it is more useful than I realised.