Posted in the discussion section as it is more a brain teaser for us older techies than a serious problem.
Having owned and maintained my Rover P4 90 for over 30 years it doesn't hold many surprises any more. However today when retrieving it from its winter hibernation and pressing the starter button just got a click from the solenoid, no problem, obviously flattish battery, so to check this I switched on the headlamps and tried again.
To my amazement the starter then whirred into life. To double check I repeated the whole process, again the starter would only turn when the headlamps were on.
Now that the car has been run up and is out in the sunshine everything is working normally, but what happened originally? The starting circuit is ridiculously simple; an ignition controlled supply feeds the separate solenoid ( unfortunately not the type with a rubber push button ) via the push type starter button. No doubt it was a poor connection somewhere but why should switching on the headlamps correct this?
It is more a puzzle than a problem, I can always revert to the starting handle, but I don't like electrical mysteries as it might be the prelude to a more serious problem, maybe it was a Freudian sniff but I got a whiff of ozone during the process..
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One mans junk is another mans treasure
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This sounds like a battery connection problem and It even be a plate connection problem inside the battery. the low current of the solenoid 2 amps ish was not enough current to warm up the bad joint and temporarily make a better contact. This may come back to get you. Remove and clean the battery terminals and the earth straps. Regards Peter
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I not only remember separate starting solenoids, I remember pre-solenoid days!
Just a socking great pull-switch somewhere under the dashboard, taking the full starter current.
Also similar, but the switch in the engine bay, operated by a wire a bit like a choke button.
Also no switch at all, just a handle on the front of the engine.
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It always worth remembering that a surprising number of circuits come together at the voltage regulator; the lights, typically being connected via terminal A or A1. Any poor connections to the regulator can produce weird faults.
Number_Cruncher
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Well I had a cup of tea, read the very helpful replies, then went out with my multimeter. Thank heavens, in the interval the fault had reproduced itself.
On this car the battery lives under the back seat and is directly connected to one terminal of the solenoid from where all the other feeds are taken. There was a low/varying voltage on the teminal post of the solenoid, so as Peter surmised the problem was due to a dry/dirty connection here (battery, earth strap, and connectors are relatively new) and switching on the lights pulled enough current through to warm it up temporarily.
Thanks very much for your help, great result, fault to cure in less time than it took to write this thank you post, isn't it great to work on simple cars with no can bus's or ECU's.
Cliff, I think I do remember the cable starters, I'm sure my Ford E93A Prefect had one
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One mans junk is another mans treasure
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"Cliff, I think I do remember the cable starters, I'm sure my Ford E93A Prefect had one "
Sure did.. At least mine had :-)
madf
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many motorcycles still have a separate starter solenoid - and they can still cause the described problems.
I still remember the 'joys' of kick starting a sick Velocette 500.
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