Strange dual battery problem

Sorry, I should have said.
The voltages taken across the solenoid are with the engine running.

That's ok, see Athol's post above, no more than 0.2V if the solenoid switch is engaged (running engine). But your's has 2.6V across it, so it's either not receiving the signal from the ignition switch, or it's stuffed.

cheers, Peter

Or the little earth line might need a tweak. Look for the simple things first. No earth no solenoid.

Sorry but that is TOTALLY incorrect.

A voltage drop as little as 0.02v is unacceptable and a voltage drop of 0.2v would be untenable and if any heavy current load was applied, with in a minute or two, the isolator would literally glow in the dark.

Hello drivesafe,


that's an interesting observation.
A set of contacts like the ones in solenoid switches or relays do actually have a small electrical resistance in the low current region.
The contact material is found to be in the solid state here.
But once the contact current ramps up, localised heating at the point of contact causes some contact material to melt until a new current/resistance equilibrium is formed. During the melting process the contact resistance decreases (negative region of dE/dI). So yes, you're right at high currents the initial voltage drop above a couple of hundred millivolts causes contact point melting, and a new, lower level of voltage drop establishes after a few seconds during the liquid to solid transition.
That's why solenoids have strong spring loading on the contacts to break open the sometimes welded shut contacts when the coil is de-energised.

But since the OP reports longer lasting voltage drop exceeding 2V, there's another underlying problem.
Either the coil isn't being energised, or the contacts are burned out (or dirty if that's a possibility).

cheers, Peter

OP FWIW I'll add my vote for a stuffed solinoid.

SDG What I have is a "smart solinoid" between the cranking and deep cycle batteries and a manual isolator so that if the solinoid dies out bush it is just a matter of putting the key into the manual isolator and turning it on.
The only downside is to remember to isolate the batteries when parked up or you may flatten both cranker and DS.


Cheers
Pop

I've got a couple of cheats ways to charge your aux battery when your solenoid fails SDG.
If you have bolt on terminals on your solenoid just join the 2 batteries by putting both wires together on one pole of your solenoid.
Or if that is out of your league try connecting the 2 positive terminals of your batteries with a jumper lead.
Or if that doesn't suit you, swap your batteries around & put your aux battery in where your start battery goes & jump start the car off your normal start battery.

Derek.
I just bought one of your solenoids off ebay. Arrived today. Put in already. Easy to do and works well. Should have looked here first.