Carter Serve Over Switch

Chaps, little poser for you.

Worked on a 12ft Carter serve over, all working now no problem but got a query about a switch in the electrics box.

Unit has a pull out electrics box with an RDM controller and a Danfoss tandem compressor and the bit that puzzles me is in the middle of said box is a rocker switch to select 'Self Serve' or 'Serve Over' and next to it a rotary knob which goes to a component underneath on a heatsink.

Question is what is this switch and rotary jobbie supposed to do?

Had a bit of a fiddle and on pull down from room temp (30*C - hot shop!) it activated the second compressor when switched to serve over and once setpoint was a degree off the same compressor cycled on/off for a bit until setpoint was a bit lower when just one compressor operated.
So this switch somehow selects the second compressor (and the rotary switch some type of delay function maybe, but how exactly? Perhaps wired to the controller as an input and a subsequent setting for it?

No wiring diagram and looks like the electrics were designed by NASA so not obvious what goes to where.

Anyone come across this before?

Thanks,
Andy.

Edit:

Just had a look at the controller guide. Mercury 6-5 M/E. Makes a Carel look simple, jeez. There are settings for comp lag delay and diff so looks like the switch does something else I guess.:confused:

Self serve counter would have front glass removed so bigger cooling duty, is solid state yoke just a delay to stop both comps kicking in?
Have one site with the RDM, painful to use when you've not doing them frequently.

Hi Al, thanks for the reply.

Second comp came in when switch moved to serve over which made me begin to question why it went that way round as load is now less. Well, assuming switch is wired like the sticker next to it!
RDM has a setting for controlling 2 comps but I don't know if it is set up for use, only saw the setting when sat at home having a look at the manual in a bit of peace and quiet.

Yes, bit painful to get back into. Messed about a fair bit when we did the Co-Op's but that was a while back so a bit rusty. Glad to have found air on and off probes wired opposite way round on this model amongst other faults though.

Had the privilege of going back to it today, condenser fan out, but in a shop at 30*C sure I'll be back...

Anyway, got a couple of photo's of the controller.
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