Excellent idea. I've often thought the temp gauge should have an audio component, say the sound of a shrieking tea pot when the temp is going high. I've often tried to check my gauges once a minute or so. But then I get lazy and complacent and don't look at them anywhere close to that often. I'm convinced I dodged a bullet on the SDL. Somehow the coolant level light caught my eye. I was somewhat worried though, Diseasel made the correct call that the symptoms likely indicated a bad radiator. There had been no obvious leak in a hose so I was hoping it was the radiator - it was. Better than a cracked head.

I gather the newer cars have computers that will make noise in the case of emergency. The buzzer you speak of might be the best we can hope for on these older cars. One would think a retrofit coolant temp sensor could be made that had an audio alarm. I've never looked for one.

*EDIT*

Sure enough:

https://www.gowesty.com/product/-/24196/gowesty-audible-coolant-temp-alarm-kit?v=

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1986 300SDL, 362K
1984 300D, 138K

This is the best of intentions for those of us with these old M-Bs that we have such affection for. I just hope that I look at the right moment. That being the sort of statement that makes engineers queasy.

I'm glad that you caught it! Many years ago while driving my 1983 300D, I happened to notice the temperature gauge needle rising as it should not under typical driving conditions, immediately pulled over, and shutoff the engine. Failed water pump.

True story:
Around 2006, I decided one day to order a replacement radiator for the 1983 because it was 23 years old and a wise professional tech. had told me something along the lines of: You've gotten your money's worth, don't roll the dice anymore with that beautiful and well-cared for car. At home after a freeway drive the day the replacement radiator had been delivered, I noticed that the original radiator had failed and there was coolant spray all over the front of the car. Such experiences age a person. A few years later, I replaced the head gasket and the head was *not* cracked. Whew.
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As for the radiator tank...
On my 1985 300D, I have that tank - with the "cap" (126-500-00-84).
I may be simple minded and showing my ignorance, but here's my thinking:
If I had the correct sensor that fit the tank (126-500-23-49), there might be, say, 2 - 4 (I'm guessing) wires that come off of it. With some research, I could probably figure out the power, ground, etc. and could also probably figure out how to situate the wires so that they would activate the buzzer under the dashboard. Worst case scenario: Add another buzzer in the cabin and...coolant level drops below the sensor and voila...I'm hearing a sound that I hoped never to hear but nonetheless, glad to hear it!P

To my way of thinking, such a project as this is akin to (read: improving upon a good design by making it safer and more reliable) installing relays for the headlights to bypass sending more than a trickle of current through the dashboard rotary headlight switch - a project I have successfully done. This, to avoid a fire under the dashboard that truly, would ruin my day.

So, if anyone reading this knows anything about the sensor (e.g., which one fits? what is the sensor's wiring configuration?) that is specified for this capped tank and would be willing to share the details, that would be sweet.

Thanks-

Way more likely to have an old coolant hose go bad or a rotted metal coolant line near the firewall.

I have used a a fair amount of these from junkyards with no issue. They're not likely to catastrophically fail. but a hose or a pinhole in steel will.

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I think it's a great idea. And the coolant switch, a medium priced model, costs about the same as the plug and also functions as a plug. I'm pretty sure I could suss out what wiring would work. A multimeter and some trial and error. Also thinking about using a light along with it. Not sure how much amperage that switch can handle. I'll see if I can find that.

Here's a thread about using 4 BMW E30 window switches in place of a ruined power seat switch on my long gone '81 300SD. Took some trial and error.

w126 Seat switch replacement: Success! 4 used window switches work after all

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1986 300SDL, 362K
1984 300D, 138K

Yes, this is all doable. Might be a bit of legwork, some steady patience for ascertaining the details, but likely doable.

As for your thread about the 4 BMW E30 window switches in place of a ruined power seat switch on your long gone '81 300SD, that's a great read. Some of my favorites:
"It was quite a puzzle to work out but oh, baby, it was satisfying when it worked and it works like a charm." - How I felt after my headlight relay project.

"...the seat switches strike me as a bit of stretch, a little too much cute and not enough solid function" - A rarity for this era of M-Bs but one does come across such things from time to time. Even the most disciplined engineers can fall prey to the marketing department.

"I like to put my seat back all the way back and take a brief nap during lunch or break..." - one of my favorite activities inspired by my dogs, LOL!

Bottom line: Good work on this seat switch project!

Thanks.

I saved that seat switch bundle. I have a 300SD driver's seat the right color, I have an idea to put it in my 300D and use that switch, mounted on the front of the lower part.

Did you ever do a write up about the headlight relay switch install?

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1986 300SDL, 362K
1984 300D, 138K

Short answer:
No.

Longer (but condensed) answer:
I searched the archives here for relevant information (there's a fair amount) and, finding enough, essentially combined it enable me to find my way. Not complicated once one gets the logic of the electrical system - took me a bit, somewhat of a learning curve...I'm not a professional auto. technician*. Being able to access (and study!) the circuit diagrams in the applicable FSM Electrical Troubleshooting Manual was required, I think. I wanted to be absolutely certain that I was correctly dealing with all of the involved wires - and there is a fair quantity of them.

*I needed a tutorial on relays used in an automobile, LOL. This, for a guy who successfully did a head gasket replacement on a 617.952! Live and learn.

Relays can be tricky. I drove two different BMW E30 325i(s, plural) as daily drivers for about 16 years, many long trips. I learned soon enough to always have some fuel pump relay units in the glove box. Could be had real cheap and Pick n Pull for years, easy to test.

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1986 300SDL, 362K
1984 300D, 138K

They certainly can be. HOWEVER, once you figure out which pin does what, they can be FUN to play around with! Like my dog working the squeaker out of a squeaky toy.

Smart.

LOL. Spares of the smallest (and inexpensive) of items in the car can relieve a remarkable amount of pressures in a technical person's life! The fuel injector plug for the injector adjacent to the #5 cylinder on my 617 comes to mind. With that little rascal failed or missing, one quickly discovers just how efficient the fuel pump is at moving fuel through the system - Hah!

OMG, THAT would be a drag. Similar to what I've heard if one of the oil cooler lines goes out on a 617. The oil pump will pump your oil onto the roadway before you know what's happening. THAT would be a place to have an audio alarm. I swapped both of mine. One or both (I forget) was/were leaking. Not a real easy job but oh well.

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1986 300SDL, 362K
1984 300D, 138K

Definitely. Fortunately, I have never experienced it but I've read of others who have. Not pleasant.

No, not an especially fun job but I've found that it gets easier the more often I've done it. I have a philosophy about those oil cooler lines that I debate with nobody (LOL) but follow rigorously:
1. During my general inspection under the car every 6 weeks, I inspect the lines for signs of leakage or damage.
2. Every 10 years or 100,000 miles (I drive *WAY* less than 100,000 miles over 10 years), I replace the lines.
So far, so good.