The single wire sensor has nothng to do with the FANS.
I only gave you the single sensors value chart so you could confirm the accuracy of your temp gauge on the DASH ..that is all that sensor does..it reports the coolant temp back to the gauge. Nothing else.
Once we know that your temp gauge is accurate, then you tell me what TEMP that gauge reads when the HIGH fan comes ON.
I believe either your gauge OR 2 wire sensor circuit are incorrect , but I need to know the gauge calibration firsts b/c we are using that reading to check the high fan cut-in temp .
The single sensor also has nothing to do with the a/c fan [ low fan] ........... it is just there to tell you the ENGINE COOLANT TEMP...and that is what we need to know and is what we will go by...
You want to see the high fan activate when you see 105C at the dash gauge...if not , the gauge is WRONG or the Sensor circuit is WRONG. Soooooooo, we check the dash gauges accuracy FIRST before we condemn anything..................if the gauge is incorrect, we don't want to be checking the fan circuits with an incorrect COOLANT TEMP READING.................we want to know the EXACT/Correct coolant temp before we use that info for diagnosis...that is what I gave you the temp/ohms chart for ...to check the gauge against the single wire sensor. [ Post # 25]

OK, from the three data points you provided (chart?) I'm interpolating that at 59ohms, fans are turning on at 85 degrees which is roughly what the temp. gauge reads.

Replaced Dual pin sensor two weeks ago.

What concerns me is that when I disconnect the dual pin sensor, the fans shut off. In a prior post, you mentioned:

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.

Thanks again,

RJ

__________________
RJ
'92 500E
'75 Triumph TR 6

PS, car is cooling off now, temp guage just crossed 80 deg. and read 65 ohm.

__________________
RJ
'92 500E
'75 Triumph TR 6

Ok...that is close enough for the gauge to be used ..
Now, compare the ohm resistance across the blu 2 wire sensor
[ unplugged] with this chart, using the dash gauge readings as your temp input.
Remember , each index mark on the gauge is 20C. [ I see some guys read 105C as 85c , not knowing that]

Temp Ohms:

60C- 900-1800 ohms
85C- 460-650
100C- 300-400

* note
You want the highest temp reading here for testing the sensors accuracy...the lower temps ones mean very little.....

OK,

at the 60C mark, reading is 1250 ohm
at the 80C mark, reading is 670 ohm
at 85C (est), reading was 530 ohm

Hottest I could get was just shy of 90C est. where reading was 443ohm just wouldn't get any hotter.

__________________
RJ
'92 500E
'75 Triumph TR 6

....and did the high fan ever come ON with the sensor plugged back in????

Yes, whenever its above 85(est) fan goes on (high)

__________________
RJ
'92 500E
'75 Triumph TR 6

Well, that would indicate the driver circuit in the N22 module is out of calibration...should not trigger until ohm value of termistor/sensor drops to approx. 250 ohm. It's triggering threshold is too early..


Another test:

Unplug sensor and take an ohm reading across the plug terminals with key OFF.
This should be around 1200 Ohms. That is the resistance of the termistor/op-amp triggering circuit in the N22 module.
If this is not close, then there may be a bridging resistor at the x-26 plug
[ at fuse box] from a previous owner to bring the sensor to a lower cut-in.
...or wire harnesss may be shorting...just possibilities that would also be in line with your losing the default/unplug feature of the N22. This test will verify either condition.........
Get that reading and also get me the sensor reading as soon as fans do come on High. [ just unplug the connector at that exact time and take a quick ohm value/reading of the sensor.........]

OK, took a number of readings just to be sure I interpreted your tests correctly.

With key out and cold read 299 ohms across the female plug, 980 when hot, but both were inconsistent readings.

With key on and engaged so everything was clicking and humming, but engine off, female plug read 1075 cold, 1174 when engine was warm

Fan comes on consistently at around 1180 across female plug, 510-515 at sensor (male) 85C on temp guage.

you reference a resistor at x-26 plug in fuse box. would this be under the fuses, if so, where? as far as I can tell, I have 16 numbered fuses and then A-H along sides. Plus up to 12 relays (but the aux. fan relay takes up two sockets).

__________________
RJ
'92 500E
'75 Triumph TR 6

Ok

The ohm reading across the plug [ unplugged from sensor] with key off should be around 1200-1300 ohms, regardless of temp...this value does not change to any degree with temp changes ..it is the sensor that changes w/temp. So, if you are getting different readings at that female plug [ you mention 300 to 1000 ], you have a problem in either the wires from the sensor plug to the N22 module or a bad N22 .
You mentioned that you had changed that plug ..WHY was that..did you have bad insulation at that harness?????
The x26 plug is around that fuse box somewhere and that is just a plug in the harness that is between the sensor and the module for firewall disconnect.
Your sensors values are Ok , but your N22 module is triggering early.
The sensor feed at N22 is pin #8...you should have perfect cont [ trace ohms]between that pin and one side of the sensor....

Here are some comparablereadings I just did on a working chassis ...[w/o a bridging resistor. [ie/stock wiring]

At 20C amibent :
Sensor 6.3K
Plug -1250

At 80C:
Sensor -625
Plug- 1250
This is using the temp gauge as temp reading source.

The default will not trigger until approx 80C. The fan will trigger below default by jumper at plug.

[QUOTE=Arthur Dalton;1710320]Ok

you have a problem in either the wires from the sensor plug to the N22 module or a bad N22 ."

OK, Where is the N22 Module and is it difficult to replace?

You mentioned that you had changed that plug ..WHY was that..did you have bad insulation at that harness?????

Plug had "melted" according to mechanic. harness looks to be in pretty good shape.


The x26 plug is around that fuse box somewhere and that is just a plug in the harness that is between the sensor and the module for firewall disconnect. can you tell/show me what this plug might look like?


Your sensors values are Ok , but your N22 module is triggering early.
The sensor feed at N22 is pin #8...you should have perfect cont [ trace ohms]between that pin and one side of the sensor....

I'll try this, once I find the N22 module


Thanks again for your help!

__________________
RJ
'92 500E
'75 Triumph TR 6

N22 is part of the CC Push Button Unit in the dash.........

Check MENU#24 for the web address (or phone number) to "Beckmann"; they can check it out first for a nominal charge and then repair it if needed.

Of course, you can a new one but it's a bit expensive.

__________________
Regards . . . . JimF
-------------------
'94 S500 Cpe

Visit my Mercedes Web Page

Mystery SOLVED! (I think)

Poking around again today, examining sensor and wiring harness. While harness exterior looks fine, nice and flexible, the wires inside have absolutely no insulation, it's turned to powder and comes sprinkling out the end of the harness when shaken...wow! Obviously, the harness is shorting out. Schedule to replace the harness next Friday, I'll let you know.

Thanks Again for all the guidance.

__________________
RJ
'92 500E
'75 Triumph TR 6

Very Good..
It tracked down to either the N22 or the wire connections between the N22 and the sensor.. any shorting of those two sensor feed wires from sensor to N22 would change the resistance at the N22 input...which would trigger the cut-in signal to aux fan high speed relay at incorrect temp ...N22 is seeing an incorrect resistance/temp value due to bad wiring between the two parts [ sensor/module]