First of two issues I need to dive into this week. Hoping to be steered in the most promising direction.

My 1978 XJ12L, 40K miles, original, and stock but for ignition system swap, has developed an issue in which it will not crank when hot. Turning key to on position seems to fire up all systems, I can hear the fuel pump go and pressurize the system but then turning it to the start position doesn't turn the starter. Leave it be to cool down and then it will start fine.

I haven't yet investigated anything. When it won't start, if I have my hand on the shifter, I can lightly feel the click of a switch when I hit the start position, which I assume is the shifter "Park or neutral" switch, right? Should I start there, bypassing it?

Then test the starter solenoid by putting direct power to its lead that hits the starter/ballast relay at the left front of the radiator bulkhead.

Then test the starter/ballast relay to make sure it has power, and see if the signal wire (white red) is getting triggered by turning the key.
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Then potentially the main relay on the right side but probably nothing to do with the fuel relay (or inertia switch) because I am confident there fuel pump is going, right?

Anything else?
Thank you all for being here.
GigPete

 

The trigger/signal wire to the starter relay is probably white/yellow.

The white/red wire should go to the starter. There may be more than one white/red. If so, the heavy gauge white/red will be the one going to the starter.

Next time you have a no-crank event apply power the heavy white/red wire. If the starter does not operate then you have a problem downstream of the relay...most likely a failing starter.

If the starter does crank then you have a problem with the relay or a problem upstream of the relay.

Don't forget to check/clean the battery cable connections and main engine/chassis ground points

Cheers
DD

 

as Doug has mentioned above but an easy test which i've had to do on various makes of cars, you will need to make up a length of hose (garden hose ok) to feed down to the top of the starter solenoid with a little funnel at the top end, now i know this sounds all a bit crazy but it works, get the car hot enough to cause the problem of no start, have a bottle of cold water (from the fridge) & pour it down the tube onto the solenoid, then try start, if it starts then you know it's the solenoid.

 

Thank you, that is helpful, too. While I feel I know a lot of these sorts of tricks, this particular one is new to.me and I will file it away!

Right now it looks like a poor relay trigger connection, but we shall see ...

 

Adding my findings from too many of these beasts,

The White/Red wire from the Starter relay to the solenoid gets loose on the relay spade. Age and heat I reckon.

3 of mine suffered that, and a 10 minute "squeeze" very gently, of the flmale section with some pliers, and problem solved.

I do ALL the wires, one at a time, and the battery disconnected for that Brown sucker.

Only one needed a relay, as the contacts were badly burnt inside.