Cold starting

[Barry Townsend]

On my RN I've always had cold starting using the starter motor a bit hit and miss.
Whilst attending the recent Scottish Rally it absolutely refused to cooperate after several attempts one morning despite it turning over freely.

As it is used to being in a nice warm garage most of the time I assumed it was protesting at being outside all night!

In sheer frustration I gave the starting handle a swing and it burst immediately into life and ran like a little gold watch.

Using my declining powers of deduction I got to wondering what the voltage at the coil might be whilst using the starter and after the installation of a volt meter I found that the voltage dropped to about 4 volts whilst cranking on the starter when it was cold resulting in a very weak spark. When the engine is warm the voltage drop is much less and it fires up ok.
Having taken so long (nearly three years) to fathom out what the problem was I now start it from cold using the handle and after tickling the carb it bursts immediately into life every time.

As usual the bl...y obvious doesn't always come to mind and I'm sure the diehards will all be telling me that it's the norm for 6 volt sevens.

[Ruairidh Dunford]

I run a proper earth cable directly from the battery to the gearbox bolts to overcome this, normally.

["Slack Alice" Simon]

Ditto, but using extra thick cable as stocked by my local car spares shop.   Expensive, but it made a big difference to the speed the starter spins at.

[Rick F]

In my experience, such as it is, if the starter motor turns over the problem usually comes back to a dodgy battery and, as Ruairidh says, poor earthing.

[AustinWood]

We had the same symptoms. It was bad earth from battery to frame. The head of the bolt got very hot which was the give-away.

[John Cornforth]

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.

[Dave Mann]

Just been through this problem with a friends 1934 tourer, with a fully charged battery we had 3.5 volts at the coil when cranking, the battery was shot, a replacement gave 5.5 volts when cranking.

[Barry Townsend]

I eventually found the time to install a dedicated earth cable from the starter to the battery and am amazed at the difference, even with just 6 volts available
The cranking speed is way up and the engine starts from cold easily now!
Thanks Rury for the tip,