I know these things are pretty voltage sensitive to begin with and low voltage creates all kinds of problems with either chattering or not pulling in at all, but we have a controller that has good voltage (129-130 vdc) and we get intermittent relays that don't want to pull in, usually after the car has sat for a bit. This new job to us just had a pump and pump motor failure (pump broke and locked up) so we installed a new power unit. When we fired it up on automatic after setting the valve up the car would intermittently sit there and go down then up, down then up. Found the DLV relay not pulling in, but if you helped it with your finger it would run fine for 20 trips and then do it again. Changed out relay and it seemed to fix problem, but now the S relay is doing it too. Trust me, all contacts have been cleaned, voltage is good, and it is very intermittent. I believe the DLV relay that wasn't pulling in is what caused the pump failure due to the car sitting in one spot and going up and down for probably hours. Time for a controller mod if you ask me but you already know what the customer is going to say after we just installed a new power unit.

Air gap between coil and armature is pretty important on slimline relays. An old timer told me that everyone used to carry film negatives to use as a feeler gauge. I just use a business card.

Also there are two types of cores, do not mix them up. The standard as NewMech talked about, the armature does not touch the core. On the set-Reset the core must barely touch to latch.

The core for a set/reset relay will have a groove around one end of it. Removing the armature, cleaning it, setting the air gap, and proper spring tension will help the relays pick and drop better. After reassembly and setting the air gap, you may need to adjust the contacts with a contact bender. Otis used to supply us with relay tool bags that included a contact bender, but those were the good ole days.

If this good advice doesn't take care of your problem check voltage again but put your meter on AC. When a rectifier starts breaking down it will start passing more and more AC causing relays to not pull in or chatter. On newer equipment it can cause all your inputs or outputs to latch on. I have fixed a lot of elevators over the years by checking for this problem. Good Luck

So far so good installing a new coil, adjusting gaps and spring tension.I still have one of those Otis contact benders, great little tool. It's a must have when setting the air gap correctly because the n/c contacts need to be bent to remain closed.