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Old 05-29-2002, 06:31 PM
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Join Date: Mar 2002
Posts: 76
Unhappy Electirc Fan Not comming on

1989 300CE.

My electric fan is not switching on when I turn the A/C on, ,. Can someone point me in the right direction to troubleshoot this?.

Fuse is fine and It did come on when temp rose past the 100c mark when I had a fan clutch failure so it looks like the fan is ok.

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Old 05-29-2002, 08:46 PM
engatwork's Avatar
Join Date: May 2000
Location: Soperton, Ga. USA
Posts: 11,942
Does it come on after a few minutes of running the a/c? It should not come on the minute you turn the a/c on.
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Old 05-29-2002, 08:48 PM
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Not at All . . . .

The fan does not come on at all.
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Old 05-29-2002, 09:53 PM
Holson Adi's Avatar
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Join Date: Jul 2001
Location: Boston, MA
Posts: 2,561
On my W123 the fan only comes on when it's a hot day out and the A/C has been worked for a while.
2008 BMW 335i Coupe
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Old 05-29-2002, 09:57 PM
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Join Date: Jun 2001
Location: Omaha, NE
Posts: 962
Start with a search on "aux fans." There are many posts on this topic. There are a couple of simple tests of this system that are well detailed in the archives.
'93 400E
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Old 05-29-2002, 11:30 PM
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Posts: 76
Thanks for the Input

Thanks for the input.

I did a search on Electric fans last night but did'nt seem to find anything relevant. Aux fans may do the trick.

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Old 05-30-2002, 07:41 AM
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Join Date: Feb 2002
Location: Philippines
Posts: 111
Hi CE4me.

There are two events that trigger the auxillary fans. The first is the refrigerant pressure switch, which turn on the first stage (low speed) when refrigerant pressure passes a certain pressure threshold. The second trigger is the ECT sensor, which turns on the fans at stage 2 (high speed) at a pre-set engine coolant temperature.

To test if your pressure switch is working, pull out the plug at the top of the freon receiver. This should turn on the fans in low.

To test if the ECT sensor is working, pull out the connector on top of the thermostat housing. This will trigger High speed.

This works for the W202. On some models you might need to jumper together the terminals on the connector.

If no go, check out the fuse supplying power to the two aux fan relays. The fuses are identified on the label beneath the fusebox cover.

If all else fails, see if the fans themselves are functional by applying 12 VDC directly to their terminal. Locate the dropping resistor which is connected to the fans and apply 12VDC. Watch your fingers!

Hope this helps
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Old 05-30-2002, 09:17 AM
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As was told to me for my 560sel - if your freon is a bit low - the pressure will not build up enough to trigger the pressure switch to turn the fan on. You may still be getting AC but you should check if the freon level has dropped.
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Old 05-30-2002, 10:37 AM
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Join Date: Mar 2002
Posts: 76
Talking Thanks again

Thanks Again.

I searched for "Aux fan" and that did bring up some info about the sensor for the high pressure and the other switch for the temp. Since the fan did come on when I had a bad fan clutch, the temp sensor must be working.

I was'nt aware of the pressure switch on the receiver/drier. (I was wondering why there were 2 switches there though.).

My freon level is a bit on the low side. My low side looks ok 30-40psi but the high side pressure looks to be a bit lower than normal. The range I beleive was always in the 225-275psi range.
I'll jump the switch today, test the fan and possibly top off the r-12. The air could actually be a bit colder as well.
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