Weird electrical Gremlin

[gruffa]

Wondering if any clever people can help me out here..
I have a strange, intermittent problem with the ignition circuit for my diesel engine. (Perkins 4-108)
So what happens sometimes is, after I switch on ignition, all lights, instruments and fuel pump come on, then when I press the start button everything switches off.
I'll usually switch off ignition, the on again, Some of the time it will start second go. Sometimes it takes a few attempts switching off and on. Other times it will start first go days on end.
I have changed the ignition switch once. Cleaned connections behind the panel.
Plenty of charge in the start battery as it starts easily when everything behaves normally.

It's got me stumped why it should switch everything off when the start button is pressed..

[jimbunyard]

Common problem, I'm sure there're dozens of threads on this forum alone about it.

There's probably a large ampacity fault somewhere; the gauges and panel lights don't require much juice, when you hit the start circuit, which does require a lot, it pulls what's available to try and turn the starter. The fault then does or does not return to adequate capacity, which explains it's intermittent nature.

I always start at the primary, high amp side, beginning with the battery, and check every connection for corrosion and tightness, and every cable for uniform flexibility (uneven flexibility means internal corrosion). Sometimes connections can appear good, but if they've been in place for a long time, can fail to pass full current. Best to remove, clean and reassemble everything possible, and use some sort of antioxidant 'grease' when reassembling...

[smac999]

stick a volt meter on power input to the key. and see what happens when you push start. does it stay at 12v or drop to 0?

then measure on the output side of the key. or towards the battery. follow untill you find where the voltage drop happens.

either the whole battery is dropping voltage or the ignition circuit is.

[gruffa]

Thanks for your help Jim and Smac.
Problem with checking the voltage is that when it happens it is usually when you are in the middle of anchoring, or leaving an anchorage, etc, so not always ready to dig in to it. But I get your idea Smac about checking voltage drop.
I went over the connections again yesterday. I did find that the ignition wire terminal on the battery side had a crack in it and looked a bit fragile, so I have changed that out.
Only started a couple of times since and went ok.
We shall see if it holds. Otherwise I shall keep digging as suggested.
Thanks again.

[Manateeman]

The first two replies are absolutely correct and ought to solve your issue.
Mark