Author Topic: Cooling Fan Test- Is there one?  (Read 341 times)

mrkozzy

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Cooling Fan Test- Is there one?
« on: 04 April 2021, 11:58 PM »
Hi all.

Any simple method of testing my front electric cooling fan?

The car temperature gauge went a little above normal the other day and I'm worried.  ( It never ever moves off the usual mark). Water, coolant etc etc are all fine and at the correct levels.

temperature climbed about 3-5 degrees, minimal I know, but its doesn't do that in my car! Ever.

Couldn't find the fan working, however as it is no longer summer, perhaps its not meant to.

Suggestions welcome

MrKozzy

Nabstud

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Re: Cooling Fan Test- Is there one?
« Reply #1 on: 05 April 2021, 03:43 AM »
There is a coolant temp sensor next to the temp gauge sensor, it makes the fan come on when the coolant reaches 100C. If you can find it, pull it off and earth the wire, that will make the fan come on (with ignition turned in).

Basically the front electric fan almost never comes on, and if it does the temp gauge will be past the 3/4 mark anyway.

Maybe the radiator is slowly blocking up?
Have you checked the viscous fan hub?
1975 280S - Australian delivered

UTn_boy

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Re: Cooling Fan Test- Is there one?
« Reply #2 on: 05 April 2021, 09:43 PM »
This fan will only come on for two reasons:
1) If when the air conditioning is on the temperature switch on the top of the A/C drier senses the high side temperature is at or above the temperature the switch closes at
2) If the 100 degree (C) switch senses the engine coolant temperature is at or above 100 degrees (C). 

Since it's getting cooler there, I'm assuming that even if the A/C were on the fan wouldn't come on in cooler weather.  Your temperature gauge didn't reach 100 degrees, so the engine wasn't hot enough to make the fan come on. 

To actually test the fan, simply pull the wire connector off of the 100 degree switch on the engine, switch on ignition (run position), and ground this wire to any ground.  Fan should come on.  The second test requires unplugging the two wires from the top switch on the A/C drier.  Once unplugged, short the two wires together on the vehicle wire harness side.  Fan should come on. 

If the fan doesn't operate by one/both of those methods, then the fan, wiring, or realy(s) are at fault.  If only one of the tests works, then you'd need to address that particular circuitry for the non working side of the test. 

In general, this fan should only be on in extreme cases of high engine temperatures or when the A/C is on sitting in traffic on a hot day.  It will not, and should not, come on at any other time. 

I suggest you renew your viscous fan coupling, and if it's fairly new then buy another one.  Newer ones are junk and are short lived.  If you really want to you could buy a good one from Mercedes that'll outlast you, but be prepared to cough up $800-$1,200. 
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slfan

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Re: Cooling Fan Test- Is there one?
« Reply #3 on: 06 April 2021, 05:49 PM »
Great post! Thank you for the write-up.

Quick question:  what do you mean by shorting the wire together on the VEHICLE WIRE HARNESS SIDE.  I did not follow this last part.

Thanks & best regards,


This fan will only come on for two reasons:
1) If when the air conditioning is on the temperature switch on the top of the A/C drier senses the high side temperature is at or above the temperature the switch closes at
2) If the 100 degree (C) switch senses the engine coolant temperature is at or above 100 degrees (C). 

Since it's getting cooler there, I'm assuming that even if the A/C were on the fan wouldn't come on in cooler weather.  Your temperature gauge didn't reach 100 degrees, so the engine wasn't hot enough to make the fan come on. 

To actually test the fan, simply pull the wire connector off of the 100 degree switch on the engine, switch on ignition (run position), and ground this wire to any ground.  Fan should come on.  The second test requires unplugging the two wires from the top switch on the A/C drier.  Once unplugged, short the two wires together on the vehicle wire harness side.  Fan should come on. 

If the fan doesn't operate by one/both of those methods, then the fan, wiring, or realy(s) are at fault.  If only one of the tests works, then you'd need to address that particular circuitry for the non working side of the test. 

In general, this fan should only be on in extreme cases of high engine temperatures or when the A/C is on sitting in traffic on a hot day.  It will not, and should not, come on at any other time. 

I suggest you renew your viscous fan coupling, and if it's fairly new then buy another one.  Newer ones are junk and are short lived.  If you really want to you could buy a good one from Mercedes that'll outlast you, but be prepared to cough up $800-$1,200.
1978 - 450SEL 6.9 - 3170
1978 - 450SEL 6.9 "Parts Car" - 2973

revilla

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Re: Cooling Fan Test- Is there one?
« Reply #4 on: 06 April 2021, 10:07 PM »
To short the 2 wires together on the side of the harness that goes towards the engine compartment. It was a way to highlight and avoid the “temptation” to shorten the two bare connectors on the pressure switch itself, which would do exactly nothing as it won’t be energized once the plug is disconnected.  Essentially, by shortening on the vehicle wire harness side you’ll be simulating as if the pressure switch was in closed-circuit position.  Which should make the fan run momentarily while in test mode.

Hope it helps

Cheers
Robert
W116 1977 280SEL & 1979 280SE

slfan

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Re: Cooling Fan Test- Is there one?
« Reply #5 on: 10 April 2021, 06:13 PM »
Revilla,

Thank you and understood.  I tried both tests and my fan did nor turn on.  When I grounded the wire leading to the engine temperature I would hear a “click” sound from somewhere below or behind the fuse box.  Any idea?

Thank you and best regards,
Angel


To short the 2 wires together on the side of the harness that goes towards the engine compartment. It was a way to highlight and avoid the “temptation” to shorten the two bare connectors on the pressure switch itself, which would do exactly nothing as it won’t be energized once the plug is disconnected.  Essentially, by shortening on the vehicle wire harness side you’ll be simulating as if the pressure switch was in closed-circuit position.  Which should make the fan run momentarily while in test mode.

Hope it helps

Cheers
1978 - 450SEL 6.9 - 3170
1978 - 450SEL 6.9 "Parts Car" - 2973

Nabstud

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Re: Cooling Fan Test- Is there one?
« Reply #6 on: 10 April 2021, 09:38 PM »
Check the fuse box for blown fuses
1975 280S - Australian delivered

slfan

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Re: Cooling Fan Test- Is there one?
« Reply #7 on: 13 April 2021, 04:43 PM »
Okay. Thank you very much.
Best regards,
1978 - 450SEL 6.9 - 3170
1978 - 450SEL 6.9 "Parts Car" - 2973

Nabstud

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Re: Cooling Fan Test- Is there one?
« Reply #8 on: 13 April 2021, 05:55 PM »
If the fuses are ok, next step would be to check if there is any power at the fan terminals. With ignition on and pressure switch shorted/100C switch earthed, put a 12V test light at the fan terminals. If it lights up the fan needs to be checked out, if it doesn’t light up you need to work out why the power isn’t getting there.

Let us know how you go!!
1975 280S - Australian delivered

slfan

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Re: Cooling Fan Test- Is there one?
« Reply #9 on: 17 April 2021, 06:54 PM »
Nabstud,

Thank you for the guidance.  Will check this out this week and let all know the outcome.

Best regards,
Angel
1978 - 450SEL 6.9 - 3170
1978 - 450SEL 6.9 "Parts Car" - 2973