The kilma is a VERY complicated relay. I am not 100% sure whether it monitors the rpm of the engine. I know for sure it monitors the shaft of the compressor to determine whether it is running or is stuck. This is to prevent the engine from chewing up the drive belt. It is entirely possible that there is nothing wrong with your kilma relay.

If you want A/C then my relay method will give you A/C control and disregard with rpm of the engine, you can then sort out the tach later.

W124 1986 Tach schema
 

Here's the W124 schema for the tach for the 1986 year. Maybe the 126 SDL is similar. However this does not help much. This is all i can find in the whole FSM on tachometer. Does anyone know where any more info is in FSM? . From this i can't tell where the tachometer gets its signal from unless it is the EDS control Unit - N39. But i can't find N39 in relay control modules location pdf in the FSM. Also I don't see the OVP relay in here at all. It also looks like it is just touching the Klima relay at pin 2 and Klima relay has no effect on connection to the EDS. It may be i just can't read the diagram. Thanks for any help.

Well, you could try a new klima. But I'd bet your tach problem is the instrument cluster. You could either get a different one from a recycler, or send yours to programa (google them) to get them to rebuild it.
http://catalog.peachparts.com/ShopByVehicle.epc?q=1987-Mercedes--Benz-300sdl-Climate--Control&yearid=1987%40%401987&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6213%3AMBC|1512%3AED|10000 100%40%40300SDL&catid=242213%40%40Climate+Control&subcatid=242276@@A%2FC+Relay&mode=PA

Or you could just send your klima relay to programa and get them to do it as well. I heard that their rebuilds are better than the off the shelf ones.

Simple Kilma
 

This is how I wired up the relay in place of the Kilma. This setup will monitor the CCU signal to call for A/C. The CCU monitors the freon pressure, not the job of the Kilma. The signal for the compressor to come on is thru PIN 10 on the Kilma, it goes low ( 0V ) to call for A/C.

What I did is I get a Kilma from a junk MBZ. Any model will do as they have the same pin out. I remove the gut and wire it as follow.

1) Get a 12 miniature relay that fits inside the unit.
2) PIN 5 (12v) and PIN 10 goes to the coil connection. ( 5 to 85 and 10 to 86 of relay )
3) PIN 7 connected to 87 of relay.
4) Tie 30 and 85 together, i.e. tie one end of the normally opened contact to 12v.

When the CCU demands A/C, PIN 10 goes low, and the coil will pull the relay causing PIN 5 and 7 to make to start the compressor.

Put the miniature relay into the Kilma and bingo. You have a Kilma relay and save yourself $100+. This circuit will NOT monitor compressor shaft rotation or cut the compressor during kick down for passing.

300 SDL tach issue
 

I had a tach and A.C. that was not working my M.B. mechanic told me I needed a klima Relay to make the A.C. work after I purchased a used one for about $80 found out it did not solve my problem the A.C. was'nt kicking on because the tach was not working a $10 OVP relay solved the tach + A.C. problem on my 85 300SD.

Thanks everyone for the responses. I have been out of the loop for a few days. I still have not solved the tach issue. I don't think it is the klima, so I'll replace the OVP relay and see what that does. Thanks for the wiring for the home made klima ah-kay. I'll try that if needs be, but I'm more inclined to just replace the relay for $115 if that is the culprit.

Keep us informed on how you resolve the tach issue as any of us will come across it one of these days with the SDL. It could be the tach itself as it is electronics and mechanical.

Just got off the phone with PO's mechanic who has serviced the car for years. He actually diagnosed the problem and said all the relays are good and the signal stops at the instrument cluster and he's almost sure it is just a matter of replacing the tach. It's really nice having the service history on these older MB's.

Any leads on one for an86 SDL would be appreciated. I was at the local j yard this am and couldn't find one. There was one from an 86 420sel gasser, but I'm pretty sure that one wouldn't work. Thanks again for the help and I'll keep you posted.

Send the instrument cluster to Programa. They will rebuild the instrument cluster so you still have the original one. that way on a carfax, it doesn't look like someone attempted to rollback the odometer.
You can send them an inquiry form and see what it will cost.

Well, i don't have the Klima relay. Someone nabbed it or something. I'm going to try jumping pins to see if i can get tach to work. My Klima is same as 1984 190D from that other thread. They posted a schema of the pins.