91 300E

My auxilary fans are only running with the a/c on.

They are not coming on when the car gets hot sitting in traffic. There are 2 temp switches from what I gather.

Which switch does what? alldata doesn't give a clear explanation.

The 2 wire blue sensor . It will not come on until 105C.
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AO123CS42AR0R5UPD&year=1991&make=MB&model=300-E-002&category=R&part=A%2FC+ACC+Temp+Sensor&appEngines=_any
If you are exceeding 105C, then your problem is most likely the fuse on top of the relay behind fuse box.

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A Dalton

I pulled off the fuse box cover. the 2 relays listed as aux fan relays at location B and C do not have a fuse on top like the OVP relay.

OK
You have the earlier chassis ..it is in the fuse box or a seperate one outside the box.

The test for High fan is to turn ON the key and pull the connector off the sensor..that defaults to high fan..if that works , then the fuse and circuit most likely have not reached the 105C cut-in.
If still NO, check the fuse ..

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A Dalton

if you unplug the blue 2 wire sensor with ignition on the fans should run.one of mercedes brighter ideas so that if the sensor went bad the fans would still run and not overheat the car.

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David S Poole
European Performance
Dallas, TX
4696880422

"Fortune favors the prepared mind"
1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
http://www.w108.org/gallery/albums/A...1159.thumb.jpg

That's what Default means........

If these do not find the problem, the next place to look is the wiring at the Resistor behind the drivers headlamp..the wires burn off there after age...it is possible to still have low fan and no high fan if the wire from the relay is burnt at this resistor...but check the other suggestions first...

A rule of thumb on these aux fan circuits is the problem is on the sensor circuit if the relay does not click when the connector is pulled ..but if one hears the relay clicking , but no fan..then the problem is on the Load circuit [ meaning fuse/relay/wiring, or fan motor.]
You know fan motor is fine b.c you have low fan..

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A Dalton

ok nothing when I disconnect the sensor...not even a click..

i assume the resistor is the one to the right of the reciever dryer...I'll start checking there..any other ideas....

fuse 7 is the aux fan...checks ok.....no wires broken...

I assume the relay is fine because the fan does work with a/c on.

hmmm? where to start checks with the ohm meter???

fuse D good also....

< >>

You do not understand..
There are TWO relays ..one for a/c fan [ low speed] and one for HIGH speed ..[ temp sensor].

Do you hear a relay click when you remove the blu 2 wires sensor ???????????????????????????/

Swap the relays and see what happens.

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A Dalton

get back to you in a bit ...i'll try that..

bingo....bad relay....thanks...I don't know why i was being a block head and didn't try that first.....

thanks for all your help.

another question...because you guys really know these cars....

I replaced the vacuum pod for the defrost flap behind the glove box...i still have air leakage from the defrost vents when I want only air to blow out the main vents.

I tried a spare hvac panel...didn't change anything....

do I have a problem with the vacuum tree....or do I need look at the recirc vacuum pods????? those will be a pain to replace...looks like dash pull..

also sometime I hear a clicking like a relay...when ac system turned off, if I push the Off button in and hold it the clicking goes away....

There is more than one vacuum pod for the AC vents. The dashboard has to come out to change them all. Lots of money or a pain int the @$$ to DIY. Very common problem with these vehicles. Do a search...

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J.H.
'86 300E