Where's the Coolant Temp Sensor and how do I test it? '86 300E
 

I've read the description of various ailments caused by a faulty coolant temperature sensor, and I'm beginning to think that it may be responsible for my cold & warm start trouble. Can anyone describe where this sensor is located on a W124 300E, and how I can test it? I've seached, but I only found vague descriptions...
Thanks guys!
Mika

I believe its on the thermostat housing.

Sounds like you're referring to the "Water Temp Sensor" a.k.a. "Temp Sensor for CIS". on my m103 (190e 2.6), it's on the top of the intake manifold, the closest sensor to the firewall.

If you're referring to the Temp. sensor for the temp. gauge in the cluster (probably not), it's also on top of the intake, the next-closest sensor to the firewall.

OK, can anyone elaborate on how to actually test it, i.e. how many ohms over which terminals, etc.? My 300E has been having some issues lately and I would like to rule this sensor out.

Thanks.

Ohm's range from the temp chart in the /Engine Manual-Combustion Manual-07.3 Mechanical/electronic gasoline injection system (KE injection)-121 Testing electrical components of KE injection system. page 60

Temperature °C........Resistance kΩ
-20........................15.7
-10.........................9.2
0............................5.9
10...........................3.7
20...........................2.5
30...........................1.7
40...........................1.18
50...........................0.84
60...........................0.60
70...........................0.435
80...........................0.325
90...........................0.247
Same resistance values apply to KE intake air temp sensor.
I think this chart applies to these 2 sensors for all KE systems but don't know for sure.

Is yours 2 prong? http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1FX0MB1RS1IG0SWQP6&year=1986&make=MB&model=300-E-002&category=C&part=Water+Temp.+Sensor&appEngines=_any
Just un plug it and measure the resistance across the 2 prongs, compare to engine temperature...

2.8 kohms at roomtemp.

Glad to see you testing it before replacement. I think it's $50.


Michael

Requesting further clarification. I am trying to order this sensor from FastLane, but want to make sure I DON'T order the one for the temperature gauge. It's confusing because this part is supposedly called "water temp sensor" but they also sell a sender, which tells me it's for a gauge. Am I wrong?

Which one do I actually need?

WATER TEMP SENSOR

ENGINE TEMP SENSOR

You need Bosch part # 0 280 130 044. Good luck.

Well, mine measured 2.7K Ohms at about 35 or 40 degrees C and then it measured .37K Ohms at about 85 C. My problem is occurring when the engine is at operating temperature and according to the chart, it is only about 50 Ohms too high at that point. I'm hesitant to buy a new one at this point.

Thanks for everyones feedback.

Larry,

Mine just was out of range and the next year I decided to replace it. Emission testing showed a bunch better. It alway had this slightly rich at idle carb smell to the exhaust. I also replaced the o2 sensor and between the 2 I picked up some mileage.
I don't remember them being expensive. The switch (non gauge sensor) there were 2 different ones. It is engine serial number dependent and that might make it clear which one to get via a catalog. Memory says the gauge/injector sensor was $35 dealerwholesale and maybe $25 for the early style (3 pin green switch). Oh, the gauge sensor is a brass body no plastic except the insulator around the single pin.

Talk with a real parts guy and this should be easy.

Michael

Larry the CIS-E needs to hit 80C to be at operating temp and be stoic. SO if you are low then it thinks your engine is below 80C and thus enriching mixture to much. Replace that bad boy as it is only about $40-$50 part. See your system never thinks it gets to 80C to adjust fuel/air to the proper mix. It is thus making it rich to compensate for the colder engine. Thus wreaking havoc on a warm start and warm idle. You will be getting too much fuel when starting at that temp. The next point is 100C where it will lean a hair to attempt to cool the chamber and thus the engine.

Also Larry if you have the 4 prong one and KE-III jetronic which has the vacuum line going to the Ignition Controller on the wheel well then the sensor also control advance as well.

You need the Water Tempo Sensor, yeah I know they named them backwards. Top one!

Hello

I have a 1987 300E

The car sat for over 1 day. I just went outside and pulled the two connectors off the sensor and I measured each point on the sensor (one at a time) to ground.

One of the points measured 1248 ohms and the other 1258 ohms. About a 8% difference so am not too worried that they both don't match 100%.

The outside temp was 68F or 20C

These values don't match the values listed in one of the above posts that lists Temp vs Resistance.

Actually I just reread the post and they measured it not each one to ground like I did but between the two points so 1248 + 1258 = 2506 ohms which matches the chart (20C = 2.5k)

Are we talking about the same senor? I am talking about the green sensor screwed into the top of the head closest to the firewall. The sensor that has the two individual wires/connectors.

Bosch part # 0280130044

wanted to confirm the above sensor is what this thread is about.

Also want to know if anyone knows how these fail, should I take more readings as the engine heats up?

The part is relatively inexpensive and easy to replace for peace of mind. If you want to bench test it. Run 2 long leads to the connector and connect the other ends to the sensor. Use a thermometer and you can either dip the probe end into water and take readings of the temp and resistance and note when the fans turn on.....or just use a heat gun and very careful heat the probe end for faster results. TBH, unless you're planning on modifying it, who cares about the values, readings/resistance, etc. As long as your fans turn on when the temp reaches the specified on value, I believe at 105°C, then you're good to go. Or you can simply get rid of that sensor and swap it for an on/off temp switch which will function the same way, minus having resistors/values to worry about.

thanks for the reply. I am talking about a different coolant temperature sensor. I am talking about the two prong temp sensor which is two sensors in one. One wire goes to the fuel injection control and the other sensor goes to the ignition control.

I care about if it is accurate as if not the ignition and fuel injection control systems will not have accurate inputs and therefore not manage the fuel and ignition systems properly.

You mention the radiator aux fan switch.

yes I will take a couple of more measurements as it warms up to see if the sensor is accurate at higher temperatures.