All,

Thanks for your help so far, this has been a great community and I've learned a ton. Because of quarantine, I have a lot of time to work on my 'new' 1982 300D. Forgive me if I've been posting too many topics! Most of my issues have been fixed by searching, but 2 of them I haven't been able to search my way to a solution yet:

My coolant gauge isn't working, I don't think. When the car is off, the needle is pegged down. When I turn it on, it rises to just above 40 and stays there no matter how much I drive. Bad sensor? Or perhaps it's running too cool?

My foglights don't work. To diagnose this, I assume I will need to test some components of the system with my multimeter, but I don't have much electrical experience and don't know how to do tests.

Thanks again!

Coolant: Do the easy stuff first - pull the thermostat. Does it open at the right temp in boiling water? Is there even one installed?

Fogs: Using a multimeter is pretty easy, and there are tutorials on youtube.
That said, same applies as above - easy stuff first. Check/change the fuse - they're cheap and you should have them on hand anyway. Then move on to checking/changing the bulbs unless you have a reason to suspect the switch or wiring.

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1983 300D Midnight Blue
4 speed, W115 intake mani, non-EGR exhaust mani, KKK K26, 3 pc Euro bumpers, Lo-spec Euro headlights, AL129X, hubcaps, custom A/C

1987 300TD Smoke Silver
Euro headlights, thermostat relocation, coolant bypass mod, rebuilt Becker + Jehnert + Helix audio, OEM Oris roof boxes and surfboard racks, Euro towbar

1983 300TD Reed (Moss) Green / Dark Olive MB Tex Euro spec

1983 300TD Silver Blue / Blue cloth Euro spec, OM648 and 722.6, 15" hubcaps, W126 S2 brakes

Possibly the thermostat is not present or is stuck open. The thermostat is designed to bring the engine coolant up to a pre-determined temperature.

Measure the resistance values of the sensor (unplugged, DVOM posts on body of sensor and pin)before cold start and warm-up and see if it changes .
The sensor body
https://www.pelicanparts.com/More_Info/0055421017.htm?pn=005-542-10-17-M82&KeepHeader=Y&ByPassCat=Y&null=

...incorporates a certain 92-027-004 (VDO #) thermistor used in many (incl. aftermarket) applications.

Then go to resistance values chart on page 38:
https://www.vdo-instruments.com/media/instructions/TU00-0776-5104620%20Data%20Sheets%20Temperature%20Switches_short_version.pdf

Another link for quick fahrenheit temp/resistance reference:
https://www.vdo.com/media/746715/de_betriebsanleitung-singleviu_22-201801.pdf

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Get an infrared thermometer and check the temp at the thermostat housing and it will likely be 40C. If it is replace the thermostat.

For the foglights it's often the bulb. Jumper battery voltage to them and see if they light.

Please advise year and model.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Thank you all. Very helpful replies.
I may just do a coolant flush, and replace the thermostat anyway - if that fixes the issue then great. If not, I'll be back here with you all.

For the fog lights, can I jump power to the bulb itself? If it doesn't light, then I have 2 bad bulbs.

I know the basics of the multimeter, but when I pull a harness (for example, from a light bulb) all I see are a bunch of pins and I never know where to put my leads.

Folks,

Foglights were simply the bulbs.. so that's good.

I also drove the car pretty hard yesterday on the freeway and saw a slight increase in temperature, so it looks like you all were correct: bad or missing t-stat.

Question, assuming the worst, and the previous owner drove the car cool like this for a while, what kind of remediation should I consider doing? (besides a new t-stat of course) A diesel purge? It drives nicely, but I don't know what thousands of miles of a cold engine might do..

Remediation re: bad thermostat...drive it and enjoy it!!! Nothing to remediate.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Thanks sugar bear you’re awesome! Parts get here on Tuesday.

Right now, do a valve adjust with the engine stone cold then warm it up and do a hot oil & filter change .

You'll need a set of special bent up 14MM open end wrenches, several are commercially made, I vastly prefer the set made by an enthusiast as one is *slightly* longer and so nestles underneath the shorter one making the job so much easier and faster it's amazing .



Yes, Diesel purge and replace both filters after, make sure you get the clear plastic intake screen so you'll know the *instant* any crud or fungus begins to bedevil you .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

VWnate - the last owner said he's had the valves done 'recently' but who knows when that would be.

The valve job intimidates me a little bit. Is it a of reasonable difficulty for the average driveway DIYer?

Understand the intimidation but you'll want to know how to do the adjustment over the life of the vehicle. Get a manual, read it, study it, ask us questions and ask around if any older mechanics in the area are willing to teach or show you on a couple of valves. Some techs will, some won't and some don't know how.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Not terribly difficult, maybe scary the first time but look it up in these forums so you get an idea of the tools needed .

At least once every year .

If you ever had an old air cooled VW it's similar .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

It is pretty staightforward job (Job number 05-210), but it can be a little indimidating the first time.

I prefer to do the adjustment on a stone cold engine, with the front wheels on ramps.
Usually, I also remove the glow plugs.
That way I'm not fighting engine compression when rotating the engine clockwise
via the deep 27mm socket on the crankshaft bolt (while under the car).


To save time, as each lobe comes into position I check/adjust the corresponding valve's clearance.
After all ten valves have been checked/adjusted, I then recheck each valve clearance at least one more time.


For future reference, here is a a link to the Disc 2 of the W123 FSM hosted by MBUSA at startekinfo.com:
https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/matrix6.htm

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Thanks everyone, I'll put a valve adjustment on my ever-growing 'to fix' list.
How long for a newbie to complete this job?