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  #1  
Old 04-18-2011, 07:46 AM
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Radiator fan stuck ON!

Thanks in advance for any tips on figuring out why my radiator fan is wanting to run non-stop!! I unplugged it for the Minnesota winter

-- but now we're actually above freezing a bit so...

Thanks,
todd

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  #2  
Old 04-18-2011, 08:10 AM
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Any tips on figuring out what vehicle you have?
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Old 04-18-2011, 09:19 AM
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And what fan? If its the electric one it could be a bad temp control switch.
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1984 300D 500k miles
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  #4  
Old 04-22-2011, 12:34 AM
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w123 Radiator fan always ON?

my 79 300td radiator fan (the one in front of the radiator) doesn't want to shut off -- any tips??

greatly appreciated...
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  #5  
Old 04-22-2011, 01:36 AM
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One of the vehicles is listed as a 79 300TD
From what I can remember from the Forum the fan goes on if the Coolant temp it goes above a certain temp or any time the A/C is on.

Assuming you Coolant is in a normal temp range disconnecting the Wire on the Temp Sensor while the Fan is running should tell you if it is the Sending unit.
If it continues to run after disconnecting the Wire at the Temp Sensor it must be some problem in the A/C circuit.

Another thought is that if the Coolant Temp Fan always has + voltage to it all of the time and is turned on by Grounding it through the Temp Sensor or the A/C circuit it could be the insulation has rubbed through on the Ground Wire and is grounding on the Chassis.
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  #6  
Old 04-22-2011, 11:29 AM
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Quote:
Originally Posted by Diesel911 View Post
One of the vehicles is listed as a 79 300TD
From what I can remember from the Forum the fan goes on if the Coolant temp it goes above a certain temp or any time the A/C is on.

Assuming you Coolant is in a normal temp range disconnecting the Wire on the Temp Sensor while the Fan is running should tell you if it is the Sending unit.
If it continues to run after disconnecting the Wire at the Temp Sensor it must be some problem in the A/C circuit.

Another thought is that if the Coolant Temp Fan always has + voltage to it all of the time and is turned on by Grounding it through the Temp Sensor or the A/C circuit it could be the insulation has rubbed through on the Ground Wire and is grounding on the Chassis.
Looks like I am partly wrong. There is no Engine Coolant Temp sensor.

Also the only good pic I could find is from a 1982 model.

The only temp sensor is the A/C temp sensor and there is a Ralay. It looks like you can pull the Relay to shut off the Fan and worry about it another time.
Or consult your Fues Chart And Pull the Fuse.
Attached Thumbnails
Radiator fan stuck ON!-aux-fan-1.jpg   Radiator fan stuck ON!-aux-fan-2.jpg   Radiator fan stuck ON!-aux-fan-3.jpg  
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  #7  
Old 04-22-2011, 01:26 PM
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Originally Posted by Diesel911 View Post
LIt looks like you can pull the Relay to shut off the Fan and worry about it another time.
The relay is the most likely culprit. Stuck on. Easy to verify by disconnecting the switch.
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  #8  
Old 04-22-2011, 08:44 PM
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radiator fan on...

Great info -- thanks everyone...

Now, as I am not a mechanic and still learning about what is what...the relay that controls this fan -- you're saying IT could be in need of replacement? Isn't this relay located next on in front of the fuse box on the wheel well?? or not?

huge thanks.
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  #9  
Old 04-22-2011, 09:17 PM
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Originally Posted by peachykeen View Post
Great info -- thanks everyone...

Now, as I am not a mechanic and still learning about what is what...the relay that controls this fan -- you're saying IT could be in need of replacement? Isn't this relay located next on in front of the fuse box on the wheel well?? or not?

huge thanks.
I am not so sure it is the Relay itself.
When the A/C Temp Switch is hot enough it grounds itself and completes the circuit. When that happens by way of an Electro Magnet inside of the Relay it closes the Relay and the Fan goes on.

If you remove the Fuse that goes to the A/C Temp Switch (see your Fuse Chart) and the Relay turns off the Fan there is a good chance the Temp Switch is no good.

What I find odd is that I have a Temp Switch on my Thermostat Housing, yet the Diagram does not show such a Switch being hooked to the Aux Fan Circuit?
Also when I looked up sensors for a 79 300D it had;Cooling Fan Sensor; Auxiliary Fan Switch; 212 Degree F
So if you see a sensor on your Thermostat Housing, disconnect the Wire and if the Fan turns off you found the problem.

Do the same with the A/C Temp Sensor or pull the Fuse and if that fan goes off that is the prolem.

If the Fan is still going find out where the Relay is and pull that out.

If that Fan is still going after that you have a short circuit going to ground in one of the Wires.
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Last edited by Diesel911; 04-22-2011 at 09:29 PM.
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  #10  
Old 04-22-2011, 09:27 PM
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Originally Posted by Diesel911 View Post
What I find odd is that I have a Temp Switch on my Thermostat Housing, yet the Diagram does not show such a Switch being hooked to the Aux Fan Circuit?
That temp switch prevents heater blower operation with a cold engine. It has nothing to do with the aux fan.
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  #11  
Old 04-22-2011, 09:33 PM
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Originally Posted by tangofox007 View Post
That temp switch prevents heater blower operation with a cold engine. It has nothing to do with the aux fan.
Well that explains it!
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Old 04-22-2011, 09:47 PM
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Originally Posted by peachykeen View Post
Great info -- thanks everyone...

Now, as I am not a mechanic and still learning about what is what...the relay that controls this fan -- you're saying IT could be in need of replacement? Isn't this relay located next on in front of the fuse box on the wheel well?? or not?

huge thanks.

You say you unplugged it. Do you mean you unplugged the fan motor itself, under the front bumper (more/less)?

Or do you mean the temp switch at the receiver drier? The receiver drier is the black cylinder behind the right headlight. Find two wires with spade connectors. They should be connected to a brass fitting screwed into the black thing.

1) If not, and they're connected together, there's your problem. Someone jumpered your aux fan.

1a) Undo whatever you did to "unplug" the fan; i.e. plug it back in. If it's running, disconnect the wires from each other or "un-jumper" them. Does it stop running? You're done.

2) But if they are connected to the switch (the brass thing is temperature switch) ...

2a) Plug your fan back in. I assume it starts running. Then disconnect one wire from the R/D. Does it stop? If so, you've got a bad switch. (This presumes you're not trying to run your A/C and the receiver drier isn't hot).

2b) If it runs with the wires disconnected, then you've got a bad (stuck) relay. It should be on the left fender, maybe the most outboard/rearwards, possibly under the cover. Carefully disconnect that one (it MAY be warmer than the others as a tip to help locate the right one). Does it stop now? It's a bad relay.

Post back with your results. You can buy and install either a bad switch or a bad relay easily. The R/D temp switch, unlike the pressure switch, doesn't open up the A/C system.
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  #13  
Old 04-25-2011, 09:58 PM
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Thanks for all the help...wow...

OK, I unplugged the aux. fan about a 6" or so from the fan itself..so about immediately to the side of the radiator...

Someone mentioned the heater blower motor -- it just stopped working a couple weeks ago..after I discovered the fan running constantly....-- would there be a connection between the two acting this strange? -- I replaced the blown fuses for the blower and it keeps doing it...

I am not and haven't been running the A/C...to answer one of the questions..

I'll be able to try some of your suggestions here later this week...THANKS SO MUCH for the insight...

todd
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  #14  
Old 04-25-2011, 10:06 PM
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that socket disconnects the power feed to the fan, with it connected, it runs if something is calling for it.
next step would be to unplug one of the wires going to the temp switch on the reciever dryer and see if it stops running. if so, that switch is bad.
next would be to unplug the relay that turns on the blower. you could follow the wire you unplugged by the condenser, and see which relay it goes to. or find a chart to the wiring of the thing.
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"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

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1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

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Old 04-25-2011, 10:06 PM
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the blower is likely siezed bushings or bad brushes in the blower motor itself. MOST 123 blowers are worn out by now.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
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