1994 E320 Aux fan on and off problem
 

These fans are driving me nuts. They come on and off whenever they want. On a cold day I will turn the key to the on position and the fans will run. The next day they won't. Sometimes they turn on and off every few seconds then they may stay running. Can' figure it out. Any suggestions I'll appreciate it. Thanks

Check for a good connection at the blue temp sensor on the thermostat housing.

As Dalton pointed out a bad blue 2 pin sensor on the water pump can cause that problem.

Also the relay that controls either speed could be bad.

Outstanding I checked the wire and it was almost broken off. Thanks

Sometimes we actually get one right...:)

no, you guys nail it just about all the time!

. . . and that probably means somebody . . . won't say who . . . has a new engine wiring harness in their future . . . :confused:

Thanks for posting the solution to your problem. It's frustrating searching through threads and not finding out what the actual problems are.
Lots of good help on this forum.

Aux Fans - Constantly ON
 

Hey guys,

My aux. fans are running constantly, even when A/C is off. I thought the car might have had a cool-harness installed and after some guidance from Jim, figured out that wasn't the case.

Of the threehe sensors on the thermostat housing, I've replaced the one on the far right - when facing the front of the car - thinking that is the switch for the fan. I've also replaced the relay. No change. would the other two sensors on the thermostat housing play a role here? What about the resistor? Any other ideas?

Thanks!

RJ.
'92 500e

This is the sensor for aux fan coolant temp. sensing..
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU28V0O97SM&year=1992&make=MB&model=500-E-001&category=R&part=A%2FC+ACC+Temp+Sensor

..but, be aware that the a/c high pressure sw also controls the fan w/ac ON ..that is the relay that I would also check .

When you say they are running constantly, is that in low or high?

They're constantly running in High, regardless of whether a/c is on or not.

Of the three sensors, the two-prong sensor is the one on the right (drivers side), there's a single-prong sensor on the left (which I replaced last night, we'll see if it makes a difference) and a four prong, circular sensor in the center. Haven't replaced the center sensor yet.

[QUOTE=rjfjr1;1698475]Of the three sensors, the two-prong sensor is the one on the right (drivers side), there's a single-prong sensor on the left (which I replaced last night, we'll see if it makes a difference) and a four prong, circular sensor in the center. Haven't replaced the center sensor yet.[/QUOTE

Single wire sensor is the temp gauge sender unit.
The 4 wire is for CIS system
The one I posted is the aux fan high speed sensor. [ blu 2 wire picture]

If there is an open circuit on the sensor or the wires going to it, the ACC panel defaults into High Fan.
An easy test for this is to unplug the sensor and bridge the connectors wires with a 1K ohm resistor. If that shuts down fan, the sensor is faulty. If not, then pull the relay and see if fan stops.
Report findings.

OK, unplugged the two pin sensor, fans shut off...bridged the two holes in the plug with a 1k resistor, stayed off, plugged back in, they turned back on.

With new temp sensor (single pin), they're behaving better, they stay off for a while until temp rises to around 85 or so on the dash guage. They go from off to both on (seems like high), with no middle ground.

Any ideas? Based on this?

Thanks!