The sleeve is nickel plated brass.
It worked perfectly in temp mode, no changes in resistance with spring or with sleeve, the difference between the spring and sleeve would have be measured with voltage under load to see if there's any loss with the spring but there is no doubt a large one, I just haven't measured it with a meter, it hits way harder with the sleeve.
Note: in the first part of the post where I discuss the sleeve I was using regular resistance wire for the coil. Either way there was never a problem with it going in and out of temp mode.
I'll try to give an example of how this can not be detected with an ohm reading, just an example not real numbers of anything.
The spring is able to provide a good solid connection to the parts on each end up to 3 amps, so if you measure resistance on these parts all looks good, you then apply 4 volts with a 2 amp load and measure voltage at the load end and get almost 4 volts, again all is good.
Next repeat, take ohm reading all is good, this time we have the same 4 volt supply but have a 5 amp load and we measure the voltage and get only 3 volts at the load end.
This example is figured with the idea that if the 5 amp load was placed on the 4 volt supply that the supply voltage would only drop to 3.8 volts.
So what happened to the .8 volts, resistance under load, in this case the spring's ability to not conduct more than 3 amps without a loss.
EDIT: I did have a problem with it going into "Temperature Protected" would display on the screen with the spring installed but not going between voltage and Temp. No problems at all when using the sleeve though.