Temperature gauge permanently low

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tyrrellnick
B Class
Posts: 22
Joined: Tue 17 Jan, 2017 2:22 pm
Model you own: w116

Temperature gauge permanently low

Post by tyrrellnick » Fri 05 May, 2017 1:42 pm

Hello again. It's creeping up to the weekend and now that I've got the timing on the engine sorted, and a way forward on my heater issues, I'm tackling the next niggling thing.

The temperature needle never rises above about 45-50 degrees in the dash indicator, even after a good half an hour of driving. I initially thought it could be a thermostat stuck open, so I replaced it with a new one, but it made no difference. So it seems there must be something else. I presume the answer is either the read-out is wrong (ie. the water is getting to 80+degrees but the indicator isn't representing that), or something is happening to keep the water temperature well below where it should be.

There don't appear to be any cooling system leaks, so I'm interested to know where you experts would start on trouble shooting this. Please remember I'm a bit of a newb, so any and all feedback is appreciated. I've been learning a great deal over recent weeks, and it's really satisfying!

Cheers

Nick

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Bartman4800
300 SLR
Posts: 1727
Joined: Fri 10 Oct, 2008 12:10 am
Model you own: w111
Location: Perth WA

Re: Temperature gauge permanently low

Post by Bartman4800 » Fri 05 May, 2017 2:12 pm

First of all try to determine what temperature the engine actually is.

The thermostat housing should be that hot that you cannot hold your hand onto it.
If you can get your hands on an infrared thermometer, that would be helpful. They are less than 50 aud at Supercheap

If your engine is still running cold, there must be a bypass circumventing your thermostat, or the thermostat is actually opening too early.

It is not uncommon that new thermostats do not work correctly. You could test it in the saucepan at home. The (fully) opening temp is stamped into it and should be around 80 deg.

If the thermos is correct, is it mounted correctly? Spring should be at the bottom.

Check that first, if that all pans out, you must have an illegitimate bypass somewhere


Bart
1963 220 Sb Sedan "Kermit" (Australian Assembly)
1960 220 Sb Sedan "Zum Schlachten" (Early German Assembly, with a torsion bar spring for the bonnet) - Stored in Country WA
1981 Subaru Brumby 1.8 with Weber and 5-speed box "little utie" - Sold to another enthusiast!
2006 Ford Focus "missus car"
2002 VW Passat V6 30V Station Wagon (my new commuter bus)

tyrrellnick
B Class
Posts: 22
Joined: Tue 17 Jan, 2017 2:22 pm
Model you own: w116

Re: Temperature gauge permanently low

Post by tyrrellnick » Fri 05 May, 2017 2:36 pm

Bartman4800 wrote:First of all try to determine what temperature the engine actually is.

The thermostat housing should be that hot that you cannot hold your hand onto it.
If you can get your hands on an infrared thermometer, that would be helpful. They are less than 50 aud at Supercheap

If your engine is still running cold, there must be a bypass circumventing your thermostat, or the thermostat is actually opening too early.

It is not uncommon that new thermostats do not work correctly. You could test it in the saucepan at home. The (fully) opening temp is stamped into it and should be around 80 deg.

If the thermos is correct, is it mounted correctly? Spring should be at the bottom.

Check that first, if that all pans out, you must have an illegitimate bypass somewhere


Bart

Thanks Bart. I'll check my hand on the thermostat housing when I drive home. It'll be after about 30mins of driving, so should definitely be hot. The thermostat came from MB Spares and is an 87deg unit. Mounted correctly yes, given the SLS compressor stuck right above it, I couldn't have gotten it in upside down!

So you think it's more likely to be an illegitimate bypass rather than an electronics issue with the gauge (I had assumed as much given the gauge does move from cold to 45degrees)? Is there a temperature sender that could be faulty? That's something I've never messed with, wouldn't even know where to look for it.

Thanks again.

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Bartman4800
300 SLR
Posts: 1727
Joined: Fri 10 Oct, 2008 12:10 am
Model you own: w111
Location: Perth WA

Re: Temperature gauge permanently low

Post by Bartman4800 » Fri 05 May, 2017 4:28 pm

Hi Nick,

If you established the engine is around 80 deg C by another means than the dashboard display, then yes your dial is not displaying the correct temp.

An interweb exerpt on how a temperature gauge works:

Basically, an electric temperature gauge is a voltmeter. The scale on the gauge face is reading temperature but the instrument itself is reading voltage. The gauge itself is comprised of a bimetallic (two different metals fastened together) "hairpin" assembly. This assembly is attached to the needle.

The gauge requires an electric circuit and a sending unit in order to read temperature. The sending unit is a temperature-sensitive material that is part of a variable resistance, water-sealed unit that sits in the coolant stream in the engine. As the engine warms up the resistance in the sending unit is lowered gradually until the system reaches maximum heat. The sending unit is the "ground" portion of the circuit.

In the completed circuit the battery voltage passes from one side of the gauge, through the bimetallic spring and onward to the sending unit, which is grounded to the engine. When the engine is cold the resistance is high, so little current passes through the gauge. This small current doesn't heat up the bimetallic spring, so the gauge reads a low temperature. As the engine warms and the sending unit's resistance lowers more current passes through the gauge and the needle reads higher and higher because the bimetallic spring expands further.

Electric gauges can fail to read accurately because the sending units fatigue or rust over, or simply lose their connection to ground. The bimetallic spring can also fatigue over time, rendering the gauge inaccurate or inoperable.


Bart
1963 220 Sb Sedan "Kermit" (Australian Assembly)
1960 220 Sb Sedan "Zum Schlachten" (Early German Assembly, with a torsion bar spring for the bonnet) - Stored in Country WA
1981 Subaru Brumby 1.8 with Weber and 5-speed box "little utie" - Sold to another enthusiast!
2006 Ford Focus "missus car"
2002 VW Passat V6 30V Station Wagon (my new commuter bus)

tyrrellnick
B Class
Posts: 22
Joined: Tue 17 Jan, 2017 2:22 pm
Model you own: w116

Re: Temperature gauge permanently low

Post by tyrrellnick » Mon 05 Jun, 2017 2:52 pm

Thanks all, I got a replacement temp sensor and replaced it. Works perfectly now.

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