...don't quite understand the question/issue?

There should be two on the t-stat housing--one for the gauge and another for the aux fan. I believe they are of different thread diameters and even if they weren't mixing them up is no big deal.

 

If I was you I would change all three sensors

ECU 008-542-32-17-64
Aux 008-542-45-17
Gauge sensor A 005 542 26 17

These are for the M116. make sure they fit your engine...

 

I am sorry, I did not do give a good explanation in starting this thread. I am trying to figure out if the two sensors on the thermostat housing need some sort of thread seal or washer.

Thanks

 

I am sorry, I did not do give a good explanation in starting this thread. I am trying to figure out if the two sensors on the thermostat housing need some sort of thread seal or washer.

Thanks

Hi there,

Both sensors have sealing rings that are included should one buy new sensors.
By the look in these screenshots, and my Gen 2 experiences, I expect they are aluminum.

In addition, according to my early morning research, though they show as separate parts on each diagram those parts indicate the very same part number.

M

 

From memory I think they have steel washers? Someone may know better.

Sometimes MB uses these instead, where I'd have expected copper ones and once I read why on here I was very impressed.

 

Aha, MBL has it. As usual 👍

 

Update - I need some advice and direction. The engine temperature gauge moves to 120F within one minute of starting the engine, but the thermostat housing is not warm to the touch.

After diagnosing and replacing the OVP relay, the vehicle was able to start, but I noticed that the temp gauge was moving up quickly. Not knowing all the service history, I decided to change the coolant, thermostat, and temperature sensors on the thermostat housing; as well as the belts. Temperature sensor A005 542 10 17 was found to be in bad shape and was replaced with a Mercedes part of the same number. After starting the car the temperature went up to 120F but the engine felt cool. I pulled the wire from the sensor and the temp gauge seemed to turn off.

When changing the belts I found that the water pump seemed to turn freely without grinding and no play in the shaft. Also no visible coolant leaks.

Any assistance would be greatly appreciated.

Thank you.

 

Update - I need some advice and direction. The engine temperature gauge moves to 120F within one minute of starting the engine, but the thermostat housing is not warm to the touch.

After diagnosing and replacing the OVP relay, the vehicle was able to start, but I noticed that the temp gauge was moving up quickly. Not knowing all the service history, I decided to change the coolant, thermostat, and temperature sensors on the thermostat housing; as well as the belts. Temperature sensor A005 542 10 17 was found to be in bad shape and was replaced with a Mercedes part of the same number. After starting the car the temperature went up to 120F but the engine felt cool. I pulled the wire from the sensor and the temp gauge seemed to turn off.

When changing the belts I found that the water pump seemed to turn freely without grinding and no play in the shaft. Also no visible coolant leaks.

Any assistance would be greatly appreciated.

Thank you.

Dang!

Yes, from your description, it does sound as an electrical anomaly.

I could be wrong here, but from afar, it sounds as though there could be something else/another circuit trying to find ground through the temperature gauge.
Instrument cluster grounds.... ?
Scratching my head here, I'm wondering if perchance there is a bad engine to chassis ground here?

As an aside here, we saw clutch cables fail prematurely do to that very condition/cause & I do remember a spectacularly unnerving speedometer failure on my 1980 Volvo one morning a really long time ago. LOL. Yes, there was odd indicator movement, then a lot of evil smelling smoke, in my face & all at once. I digress, sorry....

Here is the temp gauge electrical schematic for you. Hope it sheds light.
Looking at the circuit, its pretty simple really. You disconnect at the sender.. voila... so I don't believe the cause to be an issue downstream of the temperature gauge.....

There's obviously a lot of equipment powered up once the ignition is ON.. like everything..

To isolate the cause, how about removing all the fuses, save the ones needed to run the engine.
Start the engine & observe the temp gauge and add fuses [one at a time] see if you can duplicate the condition.
Should you be able to recreate it, you'll have have a pretty good handle on 'what it is NOT' & conversely, where to look.

Here are some more diagrams of Power Distribution this time.

Again, good luck.

M

 

Here is an update on what I found today. I decided to check the resistance and the voltage from temperature sensor A005 542 10 17 from a cold engine to the 120C mark:

Ambient Temperature 24.5C

Cold key off resistance: 0.293k ohms
Cold Key in start position: 8.66 v

80C key off resistance: 0.169k ohms
80C Key in start position: 8.7 v

120C key off resistance: 0.570k ohms
120C Key in start position: 8.8 v

The upper and lower hoses, and the radiator, were cool to the touch at 120C.

Any opinion on what I found?
Could there be an air pocket giving a false reading?
Perhaps the water pump is not working?
A faulty sensor?
The wrong sensor?
Go to wiring diagrams and look for bad ground connections?

On the plus side, she runs!

 

How are you making the resistance measurements? As temperature rises your measurements first trend down, then back up. That is not possible as you would have two different temperatures with the same resistance, which would be indistinguishable from one another. Most resistance type temperature probes trend from high resistance at low temperatures to low resistance at high temperatures. Here is an example of a typical resistance vs. temperature plot.

How are you making the temperature measurements, or are you just quoting the readings from the gauge? What are the actual temperatures?

 

John350, what you found in my numbers also reveals my apparent inability to accurately transcribe my notes! I made the correction in bold. The resistance and voltage measurements are coming from my multimeter which is directly connected to the sensor and the ground for resistance, and in-line from the single pole to the wiring connector for the voltage. The temperatures are coming from the dash gauge. Do you have a recommendation for measuring the actual temperature and the thermostat housing? Thank you.

UPDATED READINGS

Ambient Temperature 24.5C

Cold key off resistance: 0.293k ohms
Cold Key in start position: 8.66 v

80C key off resistance: 0.169k ohms
80C Key in start position: 8.7 v

120C key off resistance: 0.057k ohms
120C Key in start position: 8.8 v

 

OK, the resistance readings are reasonable. (I assume you removed the connector when making them). The voltage readings don't tell us much. The way to do that would be to put the (+) probe on the terminal and the (-) probe on a good ground, but it still wouldn't tell us much. The gauge responds to current, not voltage. What does it read when you unplug the sensor? What when you ground the lead that belongs on the sensor?

Use an IR temp probe to measure the temperature. Some IR body temp thermometers have a setting for baby bottles that has a pretty wide range.

 

I would wager a bad ground on the gauge cluster. Could also be a defective sensor. i've had that happen, with a Bosch part too, before. It happens.

 

I would start with replacing all three temp sensors....
They are not expensive and then go from there....
For the air pocket...
When car is cold..
Open radiator cap.
Start car..
Every minute squeeze on top radiator hose few times gently...
Keep radiator cap off.
Keep doing that until car gets hot. Operating temp.
Close cap.
Also as John said get a thermo gun and measure housing where sensor is.
If that does not work
It could be bad temp guage inside dash.

 

The aux sensor was replaced with the gauge sensor. I will order a new dash gauge sensor and ECU sensor. (Where is the ECU sensor?) Also, I will order a thermo gun to make sure of the actual heat, and I will work on bleeding the air. My fear is that temps are reading correctly but somehow the water pump is not working.

I will likely not be working on the car until the following weekend and I do not want anyone to think that I ghosted the forum. The same flash flood that destroyed our two C230s also damaged a meniscus in my knee during the cleanup. Fixing my knee tomorrow (Tuesday).

 

My fear is that temps are reading correctly but somehow the water pump is not working.

I think that's pretty unlikely, unless the belt is off.

 

Hello, post knee surgery and some unexpected travel, I am again working on the dashboard engine temperature gauge issue on my 84 W126. The engine is not overheating, and here are the two ways that I tested the system:

1) I purchased a Kelin Tools IR temperature sensor (IR10) and took aim at the thermostat housing, with an emissivity setting of 0.20. With the engine running, the temperature from the IR never rose above 75.2°C and I could feel the upper hose could no longer be squeezed, which means the thermostat and the water pump are working. The dash gauge was pegged above 120°C. I also noticed that the fuel gauge was pegged above full.

2) Referencing the attached image, I drilled a hole through a sour cream container and threaded an extra temperature sensor through the lid, with a ground wire wrapped in the space just above the threads of the sensor. There was also a small hole in the lid for a thermometer. The container was filled with boiling water and brought to the car. The ground wire was attached to the ground that is attached to the thermostat housing. The sensor wire, yellow in the photo, was attached to the top of the sensor and to the wire connector of the sensor. At this point, the temperature of the water was 87.7°C as read by the thermometer. The key was turned to the 2nd position and the gauges became active. Despite the water temperature reading from the thermometer of 87.7°C, the dash gauge was pegged above 120°C.

Now I a beginning the search for a grounding issue with the dash gauges for a grounding issue for the dash gauges. Best to everyone.

 

Now I a beginning the search for a grounding issue with the dash gauges

I agree. If you go back and look at the schematic in post #9 you'll see a "T" shaped connection between the two gauges. This is the gauge ground, and if removed, I would expect both gauges to peg regardless of what the sensor is doing.