Block & T-stat sending units same? - PeachParts Mercedes-Benz Forum

renman · #1 07-23-2012, 09:46 AM

1984 W123 300TD
Are the temperature sending units in the block by the glow plug and near the thermostat the same part? The EPC diagram only indicates one part # and its for the sensor by the thermostat housing. How doe's one test this device to verify it works correctly? My instrument temp gauge is pegged at 6 o-clock reading nothing when the car is running.

Cliff

qwerty · #2 07-23-2012, 09:58 AM

The "sensor" on the thermostat is not a sending unit; it's a switch. The sending unit for the gauge is on the cylinder head.

renman · #3 07-23-2012, 12:19 PM

Thanks, I have tracked down the parts now form various sources. Is there a way to determine if the sensor or switch is okay? The price for the pieces isn't unreasonable, and I have read on the forum that A lot of owners are changing them out a a matter of routine service. My car has 275,000 miles so it may be a matter of routine to replace.

eatont9999 · #4 07-23-2012, 12:32 PM

Quote:

Originally Posted by qwerty

The "sensor" on the thermostat is not a sending unit; it's a switch. The sending unit for the gauge is on the cylinder head.

Do you know what it happens to switch on or off? I know the vacuum switch one is for EGR but what about the electrical one?

qwerty · #5 07-23-2012, 01:06 PM

Quote:

Originally Posted by eatont9999

Do you know what it happens to switch on or off? I know the vacuum switch one is for EGR but what about the electrical one?

The thermostat housing switch is open when the engine is cold. It prevents the cabin blower from operating when heat is called for but not avaialble.

Quote:

Originally Posted by renman

Is there a way to determine if the sensor or switch is okay?

The sender is basically a rheostat. You can check it with an Ohm meter.

eatont9999 · #6 07-23-2012, 01:23 PM

Quote:

Originally Posted by qwerty

The thermostat housing switch is open when the engine is cold. It prevents the cabin blower from operating when heat is called for but not avaialble.

The sender is basically a rheostat. You can check it with an Ohm meter.

Well, that explains a few WTF moments from last winter. I have never owned a car that shut the blower off until the engine warmed up. Good idea, though. I'm just used to the manual implementation.

charmalu · #7 07-23-2012, 02:14 PM

Renman, does your 84TD have 2 sensors on the left side of the engine?
My 85 300D has 2, one between 1st and 2nd GP and one between the 2nd and 3rd GP. both look the same.

My Indy mentioned to me one time when I was having a temp problem, to swap the wires between the two sensors to see if one was the problem.

One is for the temp gauge and don`t remember what the other one is for.

Charlie

renman · #8 07-23-2012, 09:45 PM

This is Thermostat housing.

This is the drivers side Block sensor

This is FAE Coolant fan sensor which look similar to either above. has same black base ring. I am confused.

Cliff
P.S. Got tired of tyring to post images. Just figured out but all the text is gone doing copy past.

renman · #9 07-25-2012, 10:11 AM

Bump.

Can anyone help identify the pictures in the above pictures with reference to the location on the engine and the new part picture I posted. Would a bad sensor or switch cause the temp gauge to peg down at the 6-oclock position? Or do I need to follow a different track to determine the problem and fix it.

Cliff

joshuajeeper · #10 07-26-2012, 09:19 PM

The rear sensor on the cylinder head is for the gauge, the front sensor on the cylinder head is for the EGR computer (84 Cali - 85), the sensor mounted vertically on the T-stat is for the Climate Control blower shutoff, and the sensor mounted horizontally on the T-stat is for the transmission cold shift delay solenoid ('85~).

Part numbers:
Gauge - A005 542 10 17
EGR - A006 542 34 17 (84-85)
ACC - A005 545 63 24 (85)
Trans - A005 545 70 24 (85)

renman · #11 07-27-2012, 11:10 AM

Thanks for the post info Joshuajeeper.

Part of the confusion emanated from not finding all of the parts identity on the EPC site. I also have used as a reference for research a variety web parts catalogs which all list the two parts I am looking for with either no description as to where they go or the description is vague in similarity but not the same.

PROBLEM # INSTRUMENT TEMP GAUGE DOESN'T REGISTER OR MOVE AT ALL.
This is what I ended up with.
Vehicle:1984, MERCEDES BENZ, 300TD, L5-2998cc 3.0L DIESEL1 Fae Coolant Temperature Sensor
{left; At Rear Of Cyl Head; For Preglow Of Glow Plug; 1 Per Veh}; Years:1984-1985 FAE 80233085615

So plugging in your A005 542 10 17 for the temp sensor for the gauge, I googled the item and it looks exactly the same and is described as a temp sensor, but the part # is linked to a W140 vehicle. Go figure. The Coolant Temperature Sensor listed above looks the same, but notice the description of its purpose.

PROBLEM # 2 AUXILARY FAN DOESN"T COME ON AT ALL
1 Behr Cooling Fan Switch
{at Thermostat Housing; For Auxiliary Fan; 1 Per Veh}; Years:1984-1985 BEHR

Googling the ACC - A005 545 63 24 Returns the A/C Lock Out Sensor description, which looks to be the same as what is described as an auxilary fan switch, even though the two parts aren't exactly the same in the picture.
Im sure had I had the original MB parts #s I would have been alot farther down the road, but the two sensors above have shipped and if neither quick repairs get the job done, I have an appointment to have the car looked at next week. I am okay diagnosing mechanical and wrenching, but complex electrical problems like shorts and bad grounds baffle me.

Thanks again.

joshuajeeper · #12 07-27-2012, 06:07 PM

Some of these sensors have multiple uses across various models & years but all the listed numbers do go where I listed for a W123 5cyl turbo diesel. A quick way to test the gauge and wiring is to jump the coolant temp sensor for the gauge straight to ground and see if the gauge goes to max with the key on. If it does then its a sensor issue or there is no coolant around the sensor.

If you are talking about the fan in front of the condensor then the coolant temp sensors have nothing to do with it. It is controlled off of a temperature switch on the receiver dryer only which closes at 144deg F. If you're talking about the blower motor, then it would be the coolant switch on the thermostat.

Also the auxiliary fan in front of the condensor is designed for the A/C system only and isn't designed to compliment the fan clutch for engine coolant temperature.

Hope that helps and I'm glad to help with the part numbers. They can be a pain to get ahold of sometimes!

Alastair · #13 07-28-2012, 02:22 PM

The two temp-sensors in the cyl-head have different characteristics...

They are Not interchangeable. One is for the temp-gauge the other for the glow-plug timer/relay.

As to which is which--I cant remember!

qwerty · #14 07-28-2012, 03:00 PM

Quote:

Originally Posted by Alastair

The two temp-sensors in the cyl-head have different characteristics...

They are Not interchangeable. One is for the temp-gauge the other for the glow-plug timer/relay.

Not on an '84 300TD.