An otherwise swell running 81 SD has lately shown the following symptoms.....
Coolant temperature gauge is reading unusually low - in 40s, and will not register any higher.
Seems this started late last year when the car seemed to be running nice and cool and consistantly registering in the low 80s even during hot weather and climate control running full time. Now I'm wondering if it was registering a lower then actual temp.
When I check under the hood the engine does not seem to be running at any abnormal temperature and coolant level is fine.
If there is a temperature sending unit somewhere that is a replacement item after a cerntain amount of miles or age ?
The 81 SD has 190,000 miles.
Thanks !

Replace the thermostat (20 min job) and see if it helps. Most of the time, this is the problem.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope :)
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Thanks, Benzmac -

Just curious: Wouldn't a faulty thermostat cause overheating ?
Or is there some sort of a sending unit that is part of the thermostat that gets out of calibration ?
Thanks again.

If a thermostat breaks closed it will cause overheating, if it breaks open the engine will run consistently cool as if never gets an opportunity to warm up.

I do understand somewhat the consequences of different thermostat malfunctions.
However I was suspecting something else because even after prolonged highway cruising the temperature never rises to what I think should be normal range between 80-90 on the gauge.
Seems to me given a certain amount of time the engine would manage to warm up dispite the thermostat being stuck "open".
I would love it if it's just the thermostat.
Just wanted to make sure it's not something else others have encountered before proceeding.
Thanks for the feedbacks, by the way. :)

During cool times the thermostat is necessary for maintaining engine temp. This means if its stuck open the engine won't heat up. Let me tell you that a stuck open thermostat won't make a hoot of difference to engine temperature in Florida most of the year (with the posible exception of diesels and M110 motors). If you run 70 for 5 miles on a 80+ deg day and are reading in the 40's then you have a gauge problem.

The sensor is on the left side of the cylinder head and has a single wire. I never remember the ohmic values but disconnecting it should cause the gauge to go hot or cold with the key on. Grounding the wire should make it go full range the other way. The old 126 dash printed circuits used to get bad connections where the fuel and temp gauges screw to the board. The sensor is cheap enough it might be worth it to just replace it from a diagnostic sense.

If you would like an ohm value let me know and I'll measure one; if I don't have it written down somewhere in the shop.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Hi, Steve -

Thanks so much for the info.
Just what I was hoping to find for a better understanding of things beyond just the thermostat function.
I will look up the ohm value in the 617.951 engine manual - which is on loan to someone else....

Alan
81 300SD