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Old 07-03-2010, 03:10 PM
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Simple Coolant Temperature Alarm DIY

For about US$5 just about anyone can build an alarm that will let you know that your engine's coolant temperature is getting high. It may alert you to a problem before your engine is ruined. This is a "pay attention" alarm, not a "you're dead" alarm. This design is applicable specifically to the OM603 and other engines that use a switch in the water jacket to turn on the auxiliary fan. Here is a picture of two such switches used by Mercedes.



Not all engines have such a switch but there is a work-around for those that don't. The work-around is more expensive (of course).

The alarm takes only two electronic components, an LED and a resistor, so it should be buildable even by folks with no electronics experience. (The LED has a tiny IC in it to make it blink about once a second. You can't adjust the blink rate, sorry.) I have deliberately kept this DIY as simple as possible; there are many ways to make it more complicated if you really want to. Although designed for my '87 300D Turbo (W124, OM603) it can be adapted to almost any car.

Parts list
1. Blinking red LED, Radio Shack part number 276-312 or equivalent, about US$2.
2. [optional] LED holder, Radio Shack part number 276-079 or equivalent, about US$1.50.
3. Resistor, 330 Ohms, 1/2 Watt, Radio Shack part number 271-1113 or equivalent, pack of five about US$1.
4. Wire, 18-24 gauge stranded.
5. Tape or shrink tubing for insulation.

The thermostat housing in the early 603 engines has two ports (tapped holes). One is a sensor for the Electronic Diesel System, the other is a dual switch. One part of the switch turns on the auxiliary electric fan in high speed at about 105C; the other part turns off the a/c at about 120C. The exact numbers vary from model to model. I figure that if the engine gets hot enough to turn on the auxiliary fan, I ought to know about it.



We are interested in the part of the switch that turns on the aux fan via a relay that lives in the fuse/relay box. Mercedes calls the switch "S25/5" and it is installed in the housing to which the upper radiator hose is attached. The wires that go to the three switches and sensors are shown in the following drawing.



S25/5 has three pins; one is by itself and the other two are together, numbered "1" and "2." All you need to do is to attach another wire to the connector for pin "2" and run it into the passenger compartment as the following drawing shows.



Now connect the blinking LED and the resistor together and then to something that will have 12 volts when the engine is running. Put the LED where it will point straight at your eyes when you are driving. It is not bright enough to be dangerous but bright enough that you will not miss it when it blinks at you. (The LED is made of clear plastic but the light is red.) Do not connect the LED directly to 12 volts or you will burn it out. That's what the resistor is for -- it limits the current that can flow through the LED.



I installed my LED in the instrument cluster next to the temperature gauge. This is not recommended for beginners as there isn't a lot of working room and you could damage something expensive. Here's what it looks like (I didn't use the Radio Shack LEDs).



The other LED is for the oil pressure alarm (DIY in another thread). I use the "Economy" gauge to monitor the vacuum level in the line to my transmission; the four-gauge cluster is from an '89 300E and is otherwise identical to the 300D gauges.

Modification for buzzer.
If you prefer a buzzer, just remove the LED and resistor and substitute a buzzer. Radio Shack has many kinds; just make sure you get one for 12 Volts. You can also add a switch for silencing the buzzer, just don't forget to turn the switch back on! You can use the LED to remind you that the buzzer is "OFF;" I'll let that be your job to design.

Modification for later engines.
Later model engines don't have a switch "S25/5." They use the climate control unit (CCU) to turn on the aux fan. If you have one of these cars, run your wire from pin 11 of connector X2, on the left side of the push button unit (PBU). An easy way to do this is to make a small loop from solid wire, put it over the pin, and then put the connector back on. The solid wire isn't very flexible so solder it to stranded wire. The rest of the alarm wiring is the same.



Modification for engines that don't have an aux fan switch.
A temperature-sensitive radiator switch can be installed in almost any car. There's an example in this web site:

Flex-a-Lite

This switch can also be used to control the aux fan directly and the thermostat allows you to set the temperature at which it comes on.

Conclusion
This could be a life-saver for those of us with alloy-headed engines. The only thing I might add is a "test" switch to turn the aux fan on manually, thus making sure that it and the LED actually work.

Good luck!
Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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  #2  
Old 07-03-2010, 05:55 PM
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Thanks

Jeremy,

Simple,concise.
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  #3  
Old 07-19-2011, 09:54 PM
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Jeremy, I love this man. I just had my coolant fan fuse blow and caught the temp slowly rising and wondered what was going on. The part numbers etc are great!

So my question is:
How can I make it chirp? I am fluent with relays, diagrams, etc Just wondering how to transfer this to an audible alarm. I'm thinking a door chime relay attached instead of the LED and/or use both. I'd prefer a more annoying alarm like, a lady quietly saying "hey your car is overheating, turn it off dumb ass or die." However, I would settle for a chime, chirp, or wailing sound.

What about a turn signal relay attached prior to the LED and then to a noise maker of some sort? When it clicks it would make something chirp instead of a bulb blink.



Also make sure the test switch is routed through a Bosch automotive 4 pin relay so the switch doesn't burn out. This is also a great way to turn on the fan if the system fails.

Last edited by stewfish; 07-19-2011 at 10:14 PM.
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Old 07-19-2011, 10:02 PM
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so we are now going VW on mercedes benz diesels.

their diesels use this type of blinking LED to highlight a low coolant level or engine overheating near 105 degrees. They also have a nice low oil pressure alarm that sounds when the level drops while driving
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Old 07-19-2011, 10:10 PM
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I know my wife would have never seen that temp gauge! I tell her to look at hers every time she looks at the MPH, but I don't think that one is setting in. She is the greatest though.
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Old 07-19-2011, 10:16 PM
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while being on the same note - my cars high speed cooling fan fuse was blown - new 30A fuse and we back in business (not that it ever needed it) But I have a problem now.

When the engine has been sitting for about 6 or 8 hrs, the high speed fan kicks on when I start the car - and switches off after about 5 minutes (its not an immediate cut out - its more like a on - off) situation for half a second and finally off

Disconnecting the temp sensor does nothing (the wiring harness condition is good btw) Am I looking at a bad CCU (because the heat/cool functions work nicely)
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Old 07-19-2011, 10:19 PM
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Always check the grounds. Clean and add dielectric grease
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Old 07-20-2011, 08:55 AM
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A back up beeper. But how to change the beep to sound a bit different. Resistor at the speaker mechanism?

http://www.amazon.com/Wolo-Model-BA-52-Back-Up-Alarm/dp/B00029XI2Q
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  #9  
Old 07-20-2011, 01:44 PM
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Back-up beeper

Quote:
Originally Posted by stewfish View Post
A back up beeper. But how to change the beep to sound a bit different. Resistor at the speaker mechanism?

http://www.amazon.com/Wolo-Model-BA-52-Back-Up-Alarm/dp/B00029XI2Q
A back-up beeper is designed to be heard outside so it may be a little loud in the passenger compartment. You could always muffle the sound with foam rubber. It also depends on what you mean by "different." Tone? Pitch? Loudness? Beep rate?

Changing the sound might be possible if you put a resistor in series with the power lead. The size (ohms and watts) will depend on how much current the beeper draws, which I do not know. You will have to experiment.

If the beeper is a simple mechanical buzzer, it might have a tuning screw. If it is entirely electronic, one of the internal components might affect the pitch of the sound but that would take some experimentation. Probably easier to get another beeper than to rebuild this one.

Jeremy
__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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Old 07-20-2011, 04:50 PM
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Ok, so I went to radio shack. I saw a couple Piezo buzzers some with slow pulse and normal pulse and different currents. I went with normal. It is 3.0 to 28 volts DC. I touched the ends to the battery and the sound is perfect. Different than any other auto sounds e.g. back up beeper, burglar alarms, door chimes etc, but is not too loud as well like a burglar alarm. It is louder than a door chime though, but it didn't seem too loud. I figure some electrical tape etc can dampen it if I need to.

The pic doesn't show the red and black wires but it has them. And it is not 90 Decibels.
Radio shack Part number 273-066:
http://www.radioshack.com/product/index.jsp?productId=2062399

Couple questions since I don't know much about milliamps. The buzzer has 12mA MAX at 12v DC do I need a resistor like the LED so it won't blow? Thanks again1 I think this will all get the wife or a friend's attention enough to call me and ask what's going on.

= .012 Amps

I'll do some research, but I'll add an update if I figure it out.

Update:
To determine the appropriate sized dropping resistor for your application, take Supply Voltage minus Buzzer Voltage divided by Buzzer current. So in my case (14.4V - 12V) / .012A = 200 ohms Right?

But how do I know what the buzzer voltage is since it's 3.0 to 28V?

Is it (14.4v supply -14.4v draw) / .012A = 0 ohms = No resistor

Original guide:
To determine the appropriate sized dropping resistor for your application, take Supply Voltage minus LED Voltage divided by LED current. So in my case (14.4V - 3.0V) / .035A = 325ohms, rounded up to next highest value of 330ohms (I'm using a 1/2W resistor). Note: there are 1000 milliamps in an amp, so 35mA is entered as .035A.

Last edited by stewfish; 07-20-2011 at 05:37 PM.
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  #11  
Old 07-20-2011, 06:26 PM
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If the buzzer is rated 3 to 28 VDC then you don't need a dropping resistor. Just the switch and the buzzer. I just double-checked the schematic in ETM; you'll be fine.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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