300SDL Tach Issue
 

Hi,

I just bought a 1986 300SDL in great shape. The PO said his mechanic told him he needed to get the tach to work for the AC compressor to kick in. I looked for a tach amp and didn't see one on the fender well. Is it true that the AC comp is connected to the tach in some way? Is there a sensor or relay somewhere I need to look into? Any advice would be welcome. Thanks.

Check the Klima relay near the fusebox...

Thanks for the quick response. I see the Klima relay. How would I check it and would this have anything to do with the tach not working? The PO told me the AC works perfectly, it just needs the tach working for the system to function.

I'm not sure if the Klima relay runs the tach. If your tach isn't working, it might be easiest to grab the instrument cluster out of another wrecked 126 body car and hook it up. If the tach works, it's ok, if not, it could be another problem...
My ac problem on my car was fixed by simply putting in a new klima relay. Testing mine was rather simple. I opened it up, and there was a place where the circuit board circuitry was melted. It was a big piece melted out too... From what I understand, the klima relay is usually the problem on these cars...
However, it could just be a sensor problem, or a wire broken or unhooked... That I dont' know how to check.

Thanks Amosfella. I did some snooping and found that the OVP relay next to the Klima might be the issue. Can anyone chime in and suggest a route to take with this issue. I'd like to get the tach working first and then work from there. Thanks.

What's a Kilma relay and OVP relay and where are they? searching now. :)

Where the Klima relay is.
 

Klima says "Klima" on it plain as day probably. It is known as "N6 -- AC compressor control unit" in the electrical troubleshooting manual from the FSM. Its in different places on different cars. For the W126 its by the fuse box. For the W124 TDT its on right fender by battery. Check the FSM.

http://www.peachparts.com/shopforum/showthread.php?t=228264 this post about 190D gives this info: "I got my Tachometer working by jumping pins marked with X ........


Car's Firewall
--------------------------
O X O O O O
X O O O O O
------------------
Radiator Tank

Again, thanks for the info. Since both the Klima and OVP seem to control the tach and AC, is there a method to figuring which of these is bad other than buying new ones and plugging them in? Are there any other components that would make the tach not work other than faulty wiring?

klima is right next to the fuse box on the driver's side on the SDL, but I believe the rest of the info is correct.
The only other thing I can think of causing the tach not to work is the cluster itself. You may find it will have to be replaced or rebuilt. I hear that Programa does a really good job rebuilding the instrument clusters. They have aprox a 3 day turnaround as I have heard.

Where and what is OVP?
 

Corrected above . Thanks. Now what about OVP -- what does that stand for and where is it?

Over Voltage protection relay. Right next to the klima I believe....

Okay - an update - I saw a thread that Brian Carlton recommended a jumper between the 5 & 7 pins on the Klima to see if the compressor kicks on, so I did the mod and lo and behold the compressor engages now and she blows nice and cold. The tach still doesn't function. Will I harm something if I leave the jumper on those pins. A new Klima is $115. I don't mind spending that if it will fix this problem and especially if it solves the tach issue as well. Thanks for the help.

Jumpering 5 & 7 it will leave the compressor to be on ALL the time. You cannot control it from inside the car nor it is monitoring freon level. If you do not want to spend the money then you can put a relay in which is triggered by a control line from the CCU. I have the schematic at home and I can post it tonight. This will give the basic function as the Kilma relay except kickdown during passing or monitoring the engine rpm etc. There may be postings to wire the relay somewhere here.

The kilma relay may be monitoring the tach for rpm as one of its input paramters but I do not think it will fix the tach.

Thanks ah-kay, yes I would love to see that schematic. I obviously don't want to keep it jumpered if the compressor stays on all the time and it's not picking up other system critical signals.

My question is whether or not I just proved that my Klima was actually bad or not. If the AC compressor needs a signal from the tach and the tach isn't working is it possible that if I replace the Klima the compressor still won't kick in?

I'm a bit confused about how the signal from the tach gets to the Klima- is there a sensor somewhere else I should be looking at?

Thanks. I searched but all i found was OVP relay. Everyone seemed to know where it is but me.