fan always on... help - PeachParts Mercedes-Benz Forum

sjsfiji · #1 05-16-2004, 10:32 PM

Hi All,

I just finished cleaning the engine compartment with degreaser and water. I covered all the necessary things: ingition, batter, fusebox, and electicals on left fender.

My fan seems to be running at high RPMs all the time, as if the engine is hot. It didn't do it for the first couple of hours and then it's just on all the time whether hot or cold.

Non of the fuses w/i the fuse box is blown.

Any help would be appreciated...

thanks,

h_shek · #2 05-16-2004, 11:08 PM

did you by accident unpluged the engine temp sensor plug? On the 124, if the plug is unpluged, the fans will spin on high....

sjsfiji · #3 05-16-2004, 11:10 PM

Location please....

h_shek · #4 05-17-2004, 01:51 AM

not too familer with your engine but it should be located next to the thermo-stat housing. its a 2 pins plug. i'll try find a photo for you.

h_shek · #5 05-17-2004, 01:58 AM

sorry can't find a picture on my PC. but its the CTS (Coolant Temperature Sensor) I am referring to.

h_shek · #6 05-17-2004, 02:00 AM

here is a picture for CTS. I think its the one used by your car

sjsfiji · #7 05-17-2004, 02:29 AM

H-SHEK... thanks for all your efforts.... I'll try to locate the sensor and make sure it's plugged... thanks....

One thing though, the CTS (pictured) is referred to as "Engine temp sensor" in Fastlane, which I guess measures the coolant temp. Do you know what this "Water temp sensor" is for?

It gets confusing but I'm almost there....

thanks,

h_shek · #8 05-17-2004, 12:13 PM

sorry mate... I have a W140 which is on a M104 engine. I think yours is a M103 so I cannot help you too much.

The fans come on when:

1) CTS gives the right signal (in ohms); and
2) when the A/C system pressure is above a certain bars.

To have the fan turn on, all we have to do is lower the ohm on the CTS sensor or alternatively, play with the pressure sensor on the A/C system.

To give you more details, have a look at Jimf's web site whom tells you all about the aux.fan and a mod. that he sales.

http://pages.prodigy.net/jforgione/MB_CTS.html

So in your case, either you have unpluged your CTS sensor or you have stuffed the refrig pressure sensor.

sjsfiji · #9 05-17-2004, 12:22 PM

h-shek et al,

I found the sensor on the left side of the engine near where the air cleaner housing is. I took the 2-wire connector off and the fan slowed down.

Can I conclude this to be a faulty sensor? If so, how to replace...i.e. drain coolant, remove cover for access, etc...

thanks,

h_shek · #10 05-17-2004, 09:22 PM

sorry i need to know exactly which sensor you are refering to. Can you show us a picture? Do you mean the CTS sensor that goes near, or almost at, the thermostat???

If it is, the sensor can be replaced by a mere screw off and screw on. You will loss some coolant, so it will be messy, but thats all it takes.

PLEASE DO THIS WHEN THE ENGINE IS COLD!!!!

Before replacing, you should use a multimeter to check the ohm readings on the sensor. Its strange the sensor can go bad from a wash! try cleaning it a bit and re-plug the wires. could be as simple as lose contact!

sjsfiji · #11 05-18-2004, 02:20 AM

h_shek,

The picture you posted is the actual sensor. Number "1" shows it's location. Number "2" shows the plug I disconnected and the fan speed went into LOW. I doubt if it's the connection since the speed changes when connected and disconnected.

At first I thought maybe I shorted the connectors together by getting it wet. Then I thought the fan should go to HIGH when the sensor is CLOSED, and back to LOW when the sensor is OPEN. When the connector is connected the fan is on HIGH, therefore, the sensor is CLOSED. When the connector is disconnected, the FAN is on LOW, therefore the circuit is OPEN (connector not shorted). It appears the connector is fine and the sensor may have gone bad.

If I get a hold of a digital multimeter, how should I check if the sensor is bad.

thanks,

sjsfiji · #12 05-18-2004, 02:21 AM

Here's a close up...

h_shek · #13 05-18-2004, 02:41 AM

dude....

the 2 pole connector is the reading that feeds to the fan. Its based upon resistance and the lower the resistance, the faster the fan runs.

The single pole connector should be the one that feed the temperature information to the cluster.

How to find out?

get a multimeter and measure the resistance across the 2 pole on the sensor. It should change from a very high ohm reading to a very low ohm reading as the car heat up. If you say when you plug your connector in the sensor, the fan spin high, my guess is your reading should be almost zero thus the car computer thinks the car is way hot!

If however, the ohm reading is not low, (sorry I don't have the temp Vs resistance table for your car) and is verying from a very high number to a low number as the car warm up, than you may have a different problem. When I say very high, I am refering to something in the thousands and very low means something in the low hundreds. Perhalps you can use the W140 temp. V resistance table for reference.

I think someone here post such readings for the 124 before, do a search.

One thing, your car has the 2 pole and the single pole connector integrated into one sensor. Unlike W140 where we have seperate sensors. If your cluster is reading with the right temperature, that should mean your sensor is not broken. As I say I am no expert in 124, nor 140, just trying to help you. It would be difficult to think the sensor is half broken!

if you think the new sensor is not expensive, just chnge it and see!

BTW, did you check the refrig. sensor switch?

h_shek · #14 05-18-2004, 02:50 AM

another EASY test...

go buy some resistors in verying resistance., unplug your harness from the sensor and connect the resistors across the 2 holes on the harness.

This will give a high/low ohm value to the car computer, (high/low depends on your resistor) and your fan should now adjust to STOP, LOW, MEDIUM or HIGH.

What resistors to buy, again, do a search for the table.

sjsfiji · #15 05-18-2004, 02:52 AM

h_shek,

thanks for the quick response. Yes, the temp in the cluster is reading just fine... 'round 80 - 85 deg C (like always).

I'll try, or maybe buy, a multimeter... It'll be handy in the future...

My AC is broken a not being used... I'm not sure if that sensor you mentioned would apply since it has been used for years. Is that the right assumption, or that sensor is still in the equation?

thanks,