> So the situation is that I have 3 24 volt 8ish amp fans running under the control of relays (so should not cause a problem even if they have no diode) and a thermostat controls a compressor clutch (that does have a back diode) and 2 8 amp fans also controlled by a relay. When the clutch + 2 fans are switched off via the relay in the thermostat and at the same time another relay is turned off and this turns on a solenoid valve a +/-200V spike is generated on the power in pin of the control box which is not directly connected to anything but the solenoid that has a diode in it.
I would go device by device. Unhook all but one device, turn it on and off, see if you get the spike. Repeat for all devices. If you're lucky, it's only one of them. If you're really lucky, it's just a bad diode, which you can replace and problem solved.
If you're not lucky, well... let's not go there. Test one device at a time in isolation and see what happens.
> Basically the question would be can back EMF propagate up the negative ?
In theory, yes.