300E 4Matic - Aux Fan not operating

[justino]

My 300E (1991) is running hot, nothing new there but after investigation the front electric auxillary fans appear not to be operating. The viscous fan passes the carrot test, but I have to say I don't recall hearing the fan noise evryone mentions.

I've done the usual, checked fuses, shorted out the temp sender with ignition on and heard the relay clicking, so presume that is ok. In the relay section of the fuse box there are another 2 fuses although I'm not sure what these are for. Only 1 fuse slot was occupied. Filling the other made no difference.

I'm starting to run out of options, I cant even find the wires for the electric fans so I could check for voltage, or hard wire them to check their operation.

My final thought is that the air con is not operative. I know it has no gas, but not sure if that is the only reason. Just wondered if the two were linked?

 

[Andy Cottrell]

Hi,

Don't know if this will help but....
In my 94 E300D there are 2 relays in the "relay box" that control the speed of the aux.fan. One has a 15amp fuse ( the one closest the bonnet), the other a 30amp fuse. They are located on the left hand side looking towards the back.
Because my viscous fan fails the carrot test and I can't buy the tools to do the job in the UK ( another story) I've added 2 1.2kohm resistors in parallel at the thermoswitch in the cylinder head so that the aux fan comes on at 80-85 ish on the temp gauge, whether the air con is switched on or not.
Shorting the thermoswitch on the cylinder head and switching the ignition on causes my aux fan to rotate.
The supply to my aux fan goes through a "connector block" that is located on the "chassis rail" behind the passenger headlight and above the aircon refrigerent "resevoir".

Hope this may be of some help. Good luck.

 

[justino]

Andy

Thanks, I'm trying to do the same by getting the aux fan running, but it is dead at the moment. Funny I had 1 of the fuses missing, but replacing it made no difference. Shorting my temp sensor doesn't make the fan come on. I need to fing=d the connector and test for voltage. Do you have a better description of where it is?

Thanks

Justin

 

[Andy Cottrell]

Hi,

My connector is behind the passenger side headlamp, about 6 inches towards the rear and connected to the top of the "chassis leg" on the inner edge. Don,t know the layout under your bonnet but on mine if you look down from the top of the PAS pump/resevoir toward the inner wing, my connector is on the edge of the "chassis rail". It is held in place with a metal clip. There appear to be four connectors within but I can't see to measure the voltages on the pins at the moment ( not outside lights and a torch with duff batteries!).

Good luck.

 

[stwat]

If hardwireing, ie "shorting" the temp censor does'nt make the fan run then id suspect the fan is shot mate.

Stu

 

[justino]

I have shorted out the temp sensor, not hardwired the fan yet. My first port of call will be to check to see if the fans are getting any power, if they are then I would tend to agree with you that the fan is shot. If I short the temp sensor and no power is being received then the problem may lay elsewhere in the wiring.

 

[justino]

I've established voltage is getting to the fan and managed to get 1 of the fans running, slowly to start with, speeded up afterwards. I disconnected the temp sender from the head, and after a short time the fan kicked into life.

I think the fans are a little tired, but will replace the temp sender first, this may coax the fans into life. Its a shame they are concealed, probably a squirt of WD40 into the motor would do the trick

 

[stwat]

I seem to rember reading that the fans are twin speed. ie, they first come in at a certain temp at a lower speed and if the temp continues to rise, they speed up.

Though this could well be a false memory on my part. LOL

Stu

 

[justino]

I think this is correct. I'll run the car up to temp tonight and check. I'll also look at the resistance across the temp sender to see if this is working. These are detailed at http://pages.prodigy.net/jforgione/MB_WW.html

 

[justino]

correction, this page http://pages.prodigy.net/jforgione/MB_CTS.html

 

[Andy Cottrell]

Hi,

When I investigated my aux fan I found that with the engine cold ( before starting) the resistance across the thermo sensor was 6 kohms. The fan operated when the resistance reached 415 ohms and cut off at 530 ohms.
Hope this may be of some help.

 

[Ramius]

Had the same issue mate on my 300te24........turned out to be nothing more than the thermostat.......best advice i can offer on that one. £7 at ECP and all is good again.

 

[justino]

I've got one of the fans (their are 2 aux fans) working like a dream. Basically I drilled a small hole in the fan housing and filled the motor with WD40. I shorted the temp sender out and then tapped the centre spindle of the fan with a small hammer. It stuttered into life, but now runs fine. Once all is fine I will silicone the small (2mm) hole up.

Now I know one fan works, I was able to test the temp sender, so I let it run upto temp, and hey presto at about 105 ish the fan kicked in.

I can't get the other fan working though despite using the same technique. I'm wondering if this runs off a different relay which may be kapput?

Justin

 

[justino]

Ramius, thanks for the advice, decided to change the thermostat, £6.70 from ECP, and its now running perfectly at about 95 degrees.

Only if I run it upto temp and then leave it ticking over does it go over 100, and then at 105 ish the electric fan kicks in. Only 1 of the fans work, but this is still good enough to drop it down to 95, then it turns off again.

My aircon is out of gas, so I don't know if the fans will work activated by the pressure switch, but I may not even bother re filling seen as I bought the Merc as a weekend hack about so is no big deal

 

[johnrr]

auxilliary fans

had a similar problem on my '95 E320TE auxilliary fans not working. They do have two speeds: low speed all the time air con is working to cool the condenser, and high speed only if the engine reaches 100/105 degrees, to assist the viscous coupled engine driven fan. The 'connector' on the chassis rail behind the N/S headlamp and near the self-levelling pump is in fact a resistor. The feed for the low speed goes through this, and the feed for high speed goes to the other end and so by-passes the resistor. On my car one of the terminal pins on this resistor had gone high resistance inside and was not passing any current. I removed the resistor, and by drilling out the insulation at the bottom of the faulty terminal, was ablt to undo it, clean it up and re-assemble. However one of the fans was still faulty (they tend to suck in road debris) and works intermittently. I'll try the trick of drilling a small hole in the shroud and squirt some stuff in next.

 

[justino]

I've established that my viscous fan is also working today. A side effect of the thermostat not working is that the radiator doesn't get hot enough and in turn there is not enough residual heat arounfd the fan for the viscous fluid to stiffen up. Whilst revving in neutral whilst in a queue of traffic I heard for the first time ther tell tale sound of the fan - great!

Now everything seems ok I'm not too bothered that one of the aux fans doesn't work, I don't seem to need it. One trick is to tap the spindle of the fan with a small hammer - thats what I did. After that it stuttered into life. Once its running you can lubricate it, keep it running by disconnecting the temp sender on the head, and it should free it self up.