1988 300e Aux Fan Runs on High When Temp Switch Disconnected
 

Hello, I have an '88 300e and the aux fan will not run when the car really heats up. But when I disconnect the temp switch with the key on, it comes on high. Bridging the connections does nothing, it keeps running on high. When I plug it back up, it won't come on no matter what the temp.

Can anyone help?

lmarty1359
'88 300e
'74 450sl

Trip On for that sensor is 105/107 C .
Are you reaching that temp???

No, it's really cool but when I disconnect the temp switch, the fan blows high.

That was not the question.
The question is :
Do you realize the high fan does not trigger until 105C ??????????????????????

I understand. I'm working on the car now and I let it run until the temp was above 105C ... the fan did not come on.

Ok
Take an ohm reading across the sensor pins when temp gauge says 105.

[ sensor unplugged]

Starting in 1998 MB started using that variable resistance circuit switch.
Most likely that switch is the problem, & yes the aux fan should go to HIGH.

Thanks, but can there be ANY resistance if the fan starts to run when I unplug the sensor? I thought the fan could not run until you jumped the two female connectors on the temp plug?

You do not understand the sensor and/or the circuit.

The sensor is a Thermistor .. not a switch...meaning the resistance of the sensor changes with the temp. [ inversely...meaning it is a Negative Coeffecient Thermistor]...NCT

So, when the Control Panel sees the sensors resistance go from high to low resistance from the sensor , it triggers the high fan....[ using a switching transistor/circuit in the CP]
BUT, if the sensor resistance is incorrect, the CP does not trigger , and if it is open, it goes into Default mode and switches ON the high fan for engine protection in case of sensor or wiring failure. When you unplugged the sensor , you are simply putting the CP into DEFAULT ON mode you then get HIGH FAN. That is how we test for high fan..we simply unplug the sensor justlike you did .That tells us that the fan circuit is working, but if it does not work at high coolant temp, we suspect a bad sensor...so, as I posted, we have to take a RESISTANCE reading at the sensor to see what is up.

This sensor is not a switched sensor as you suspect ..it is a THERMISTOR and triggers a Transistor balanced circuit at a specified Temp/Resistor value..in ithe words , it is Electronic switching, not mechanical sensor switching.

There is a temp/resistance chart for this sensor, but if you just do my test, I will tell you if the sensor is the problem.

You never jumper a thermistor sensor, you simply unplug it and ohm value it.

Thanks Arthur, silly me. I bought the shop manual for this car from MBZ ... must have been a waste of money. It says to test the aux fan on an '88 124.030, engine 103.983, jump the pins on the temp switch. I'll put my ohm meter on it and see where it's at.

You jumper the pins for LOW AUX FAN at the low fan sensor... We are not talking about the low fan sensor..we are addressing your HIGH FAN circuit...high fan is Thermistor sensed , low fan is not a thermistor and requires sensor jumper for fan activation ..different circuit...
The earlier 300e had the mechanical switch for fan, but that ended @ 9/ '87 production.
Do you have the Blue , 2 wire sensor?? If yes..then we have a thermistor circuit which is the standard component on your 124.030 chassis in 1988.

Here is the old system sensor ..I do not believe you have this system as it was ended as of production date of 8/'87 on chassis model 124.030 [ note serial # change on part listing] This is sensor S25/5 on the schematic.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2EI1B4MYE2F1112JFD&year=1987&make=MB&model=300-E-002&category=G&part=Engine+Temp.+Sensor

Here is the sensor [thermistor] that I believe you have . This is sensor B10/8 and it is the blue/2 wire termistor NTC sensor. If you see this sensor, then my previous tests apply.
Here is the Factory Specs Chart per Benz Factory Manual ..
Temp C / Ohms: - Thermistor/ACC temp input Sensor B10/8
100 C / 300-400
110 C / 230- 290
120 C / 180-220
This is what that sensors ohmic values should read at those coolant temps.
If they are not to spec, that is why your high fan does not activate and the diagnosis/remedy is to change the sensor.
I am fairly sure I am correct b/c you get a high fan with disconnect of sensor..that test could not work on the mechanical sw of the '87 models and I know these systems fairly well.
Is this below sensor the one you have ? It should be this thermistor for your 1988 chassis. This type sensor replaced the earlier 105/115 switch and the test procedures are very different b/c it is a thermistor with an OHM value rather than an ON/OFF switch [ as the '87 model used]

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2EI1B4MYE2F1118W7W&year=1988&make=MB&model=300-E-002&category=R&part=A%2FC+ACC+Temp+Sensor&appEngines=_any
I have the wiring schematic for the fan circuits on 124.030 and it shows both the modification change and dates for the S25/5 to the B10/8 production circuit changeover, if you need it.