Well i found that i have a bad ground at the gauge cluster which was causing the temp gauge to read above 120c. That good news but the gauge still reads just over 100c. When replaced the thermostat I got it from a local NAPA store instead of a MB supplier. I checked it before installing it and it started opening at a water temp just above 80c and didn't fully open until about 93c. Does this sound about right? Also i read there should be an arrow which when installed should point up. Neither the stat i took out or the one i put in had such an arrow. Should I pull this stat and get one elsewhere, if i can use this stat without the arrow does the position of it matter, (I do know which ends point towards block and radiator. Pointing up towards hood is my question)?

Thanks again,
mark

the spring part stays in the head.I'd go with lower temp stat,as these engine get old.Scale and rust coat the engine,and rad.also grounds,I run extra ground from gauge cluster,and extra from negative battery to block,and chassis,on all my cars.They barely ground the cars when built.My temp sender where it plugs was bad,and I had to resolder,and splice in new wire.

Do you run extra ground from block or battery?

Sorry I'm at work and read your post fast. Just reread it and saw my ? Was answered in your 1st post.
Thanks,
Mark

My motor is a 603...but I've found that running a lower temp thermostat in an older motor (1986) with 436,000 miles seems to keep things cooler especially in the summer desert south west.

So in summer I run what I think the folks over at the Mercedes Classic Center USA in Irvine CA refer to as a "Tropical T-Stat" that opens around 75°. I think you could call them asking for "Tom" if I recall correctly. We had to find a "compatible" size as there was no listing for my SDL. I think the Tropical Stat was listed available for an older SL vehicle and we crossed referenced the fit.

I've done all the cooling system updating including citrus flushing and it still needed at cooler t-stat for the summer.

So maybe call the MB Classis Center at (949) 598-4850 and see if you can order one up. Also check out the Center's site if you've never seen it. Very cool: https://www.mbusa.com/mercedes/enthusiast/classic_center

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1986 300SDL 440,xxx

Thanks for the link and info.

Grounds are the most common cause for electrical mysteries. When confronted with one I simple run an extra ground first just because - and it usually fixes it. On Mutt the oil temp gage sender is in a rubber line that shunts the oil cooler connections (I don't run an oil cooler for my short runs on the track) so of course it wasn't grounded. I simple stripped a wire about 2" and wrapped it around the Stewart-Warner sender then secured it with a hose clamp then ran the other end to an engine ground. This works like a charm. Also, I have several wires from the engine to the chassis as I simply take no chances with grounding.

Dan