Coolant Temp sensor verification - Mercedes-Benz Forum
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post #1 of 19 (permalink) Unread 07-15-2019, 09:45 AM Thread Starter
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Coolant Temp sensor verification

Hi guys,

I've now read a variety of posts suggesting different information - the car is a 1991 300E 2.6L engine and I'm attempting to locate and replace the coolant temp sensors as part of a 200K refresh. In various posts I've read that two sensors are located on the thermostat housing (single pin sends data to cluster and multiple pin to the fuel system). What sensor is located at the rear of the head (green top) and is it utilized on all engines?
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post #2 of 19 (permalink) Unread 07-15-2019, 10:12 AM
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The rear most sensor on the head is the engine temperature sensor. It to, sends info to the ECU.

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post #3 of 19 (permalink) Unread 07-15-2019, 10:28 AM
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if the sensors aren't broken, don't 'fix' them. those sensors generally last the lifetime of the engine, unless they get physically broken.
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post #4 of 19 (permalink) Unread 07-15-2019, 10:45 AM Thread Starter
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Originally Posted by LeftCoastGeek View Post
if the sensors aren't broken, don't 'fix' them. those sensors generally last the lifetime of the engine, unless they get physically broken.
I've had recent issues with the car running quite hot (see attached) and intermittent issues with the Coolant Warning light coming on - the coolant levels are perfect, so my concern is that the sensors are playing tricks on the car. While I know these cars are hot blooded, i'm concerned that running this high temp daily may stress the engine. The fans kick on, and California head isn't helping, but still - or is this normal?
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post #5 of 19 (permalink) Unread 07-15-2019, 10:45 AM Thread Starter
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Vic

The rear most sensor on the head is the engine temperature sensor. It to, sends info to the ECU.

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Ah! Thank you! This clarifies it for me!
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post #6 of 19 (permalink) Unread 07-15-2019, 11:06 AM
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I wouldn't find the temperature in your pic to be alarming. Maybe more importantly, does the car cool down a bit when it's moving?

Miles on the radiator/car? Have you removed the shroud around the aux fans and cleaned the debris collected between the shroud and the condenser?

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post #7 of 19 (permalink) Unread 07-15-2019, 11:06 AM
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The lamp is for low level in the tank.
The sensor in the rear of the engine is for the ecu. Sensor for the coolingfan is on the front. 2 or 3 pin.
The sensor for the temp.gauge are also in the front, 1pin or the 1 pin on a 3pin sensor.

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post #8 of 19 (permalink) Unread 07-15-2019, 11:11 AM Thread Starter
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I wouldn't find the temperature in your pic to be alarming. Maybe more importantly, does the car cool down a bit when it's moving?

Miles on the radiator/car? Have you removed the shroud around the aux fans and cleaned the debris collected between the shroud and the condenser?

Jayare

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I'd have to check service records for when the radiator was replaced - the car has 195K miles - I'll also need to search how to remove the aux fan shroud and see if there is debris behind the shroud! All great feedback - really appreciate it!
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that low level sensor gets gummed up, especially if funky coolants have been used... its stuck in the bottom of the forward facing side of the coolant overflow tank, and can be removed and cleaned.

hows the centrifugal clutch on the mechanical fan?

as W201 says, the cooling fans are run by one of the front sensors... the rear sensor for ECU will cause lousy gas mileage if its wrong, but not much else... ok, it could make hard starting if it told the ECU that the engine was hot when it wasn't.

reading a hair above the 100 tick on the temp gauge is pretty normal at an extended stoplight in hot weather. it should promptly go down when you're moving.
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post #10 of 19 (permalink) Unread 07-15-2019, 11:43 AM Thread Starter
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that low level sensor gets gummed up, especially if funky coolants have been used... its stuck in the bottom of the forward facing side of the coolant overflow tank, and can be removed and cleaned.

hows the centrifugal clutch on the mechanical fan?

as W201 says, the cooling fans are run by one of the front sensors... the rear sensor for ECU will cause lousy gas mileage if its wrong, but not much else... ok, it could make hard starting if it told the ECU that the engine was hot when it wasn't.

reading a hair above the 100 tick on the temp gauge is pretty normal at an extended stoplight in hot weather. it should promptly go down when you're moving.
I appreciate all of the feedback! I will drain the overflow tank and remove the level sensor/replace it as they're fairly cheap, as well as check for any debris on the fan shroud!
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