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Old 11-06-2000, 05:50 PM
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Join Date: Oct 2000
Location: Jacksonvill, FL, USA
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This is for a 1983 240D. The teperature gauge is not registering. The other gauges seem to be working. How do I determine if it is the gauge or the sending unit?

If it is the gauge, can I replace just the gause or does the whole section of the instrument cluster need to be replaced?
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Old 11-06-2000, 06:08 PM
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Location: Sherman Oaks, CA
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I'm not a tech, but from my understanding, it would most likely be the sending unit and not the guage.

I believe it wouldn't be a major ordeal to change the gauge itself, but I would think getting the sending unit test and replaced if needed should solve the problem.

Wait for a tech to answerthis one. i think it's probably the sending unit.


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Old 11-06-2000, 07:00 PM
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I have a 1990 190E. Same thing. The sending unit is cheap and easy to replace. You can go one of two ways. If your guage works after the replacement, then that was the problem. Unfortunately, if you wind up like me, after replacing the sending unit it still didn't work, then it's the guage. Dealer wants $$$$ for the whole dash assembly. A dismantler can give you just that guage that has the temperature (and I believe the gas, etc) on it for less. I hope Benzmac can give you (and I) some pointers on replacing it ... i.e. how do you remove the dash, etc.

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Old 11-07-2000, 11:16 AM
Join Date: Aug 2000
Location: Holland, MI
Posts: 1,316
Hi, Tref! Try this...

Do you have access to a Volt-Ohm Meter (VOM) and a basic knowledge of its use? It's not too difficult, and I'll assume you do, or have a friend who can help.

With the engine cold, and ignition switch off, find and disconnect the temperature sending unit lead. Set the VOM to Ohms and measure resistance from the connector on the sensor to ground (chassis, engine block, etc.) I don't know proper values, but something under 100 or 1,000 or so would be a good guess. More than that, like 1,000,000 or more, and the sending unit is "open" or "bad" Let's assume you have a "good" sensor.

You can warm up the engine and take another measurement with the engine hot. The "hot" reading should be quite different than the "cold" reading.

You can also remove the sensor and take resistance measurements from the connector to the threads when it's at room temperature and after dunking it in hot water; remember it's hot when you grab it!

Sensor looks good? OK, let's try the gauge.

Set the VOM to Volts. Select at least 15 volts full scale, or the meter might go POOF!) (Ask me how I know this sometime... )

Turn the ignition on, and look for voltage from the sensor wire lead to ground. I don't know how many volts would be "good" but zero is "bad."

MOST gauges are connected on one end to battery voltage (12v, 5v, etc.) in the instrument cluster, and are "grounded" through a sensor. The sensor changes its resistance depending on what it's sensing (temperature for coolant, pressure for oil, level for fuel, etc.)

Lower resistance allows more current to flow through the gauge, while higher resistance allows less. The gauge basically indicates current, and the needle moves up or down accordingly. You guessed it! Some move up with more current, some move down.

Service manuals and electrical schematics are ESSENTIAL for troubleshooting.

Hope this works, and it's only the sensor that's bad. Good luck!

BCingU, Jim

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Old 11-08-2000, 07:08 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
One simple quick test is to ground the sensor wire. If there is no reaction on the gauge the problem is in the gauge or gauge circuitry.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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