Okay buoys and gulls, I want to wire my two auxiliary cooling fans to run constantly when the engine is running (this is Texas and I NEED all the cooling I can get for my 4.2 liter 400E). Where would I pick up the positive current for this arrangement and where is the wiring that actually feeds the fans? I would need to by-pass the pressure switch on the A/C filter/dryer/accumulator. I intend to install a toggle switch in the panel whereby I can turn these two fans off or on as the need arises, but I will run a separate wire lead from the new wiring that I will install for that purpose. Any help will be very appreciated.

 

I think it would be easier to adjust your thermostat to engage the fans at a lower temperature. I imaging a lower cut-in temperature would be better than keeping them on. Even in Texas, cruising on the interstate would mean that the car would be running cool anyway.

If anything, set your thermostat to engage at 85 degrees celcius. Again, only my opinion but I think it is a FAR better solution than custom work and wire hunting...

 

........how do you adjust for the fans to come on sooner?
BTW, I was out in my garage just messing around and checking "things" (I'm an ollllld retired mechanical engineer with three M-B's and I like the peace and quite of my garage.) and guess what I found? I found one of the wires going to that humongous resistor that sits behind the left headlight on the fender well, broken at the lug. I thought I was going to have to pull the entire fender to get to that little sucker, but, I got it out and got a new lug soldered in place, but one of the nuts that holds the lug in place is shot, so it's gonna be tomorrow before I can locate one of those.............and the beat goes on.....

 

Get the connections to the resistor sorted out first before you start modifying things. Bad connections to that resistor will result in the fans not running at high engine temperatures, even though they may run as commanded by the A/C pressure switch.

That resistor is the series dropping resistor which allows the fans to run at a high and low speed. The electric fans for both A/C and engine high temp are controlled by the climate control controller in the dash.

The A/C system makes used of a phenomenon known as "super heating" to increase the efficiency of the system. The A/C clutch is engaged, but the fans are not, until the refrigerant pressure builds. Then a pressure switch turns on the fans which causes a sudden drop on temperature in the condenser which increases the system cooling efficiency (A/C systems are basically Carnot heat cycle engines, so the greater the delta T that results from delayed fan operation results in greater efficiency). Bypassing the fans so they run all the time will defeat this mechanism and lower the efficiency of the A/C (something one does not want to do in Texas).

There is a separate temperature sensor that the climate controller uses to check the engine temperature. When it gets too hot, the controller closes a different relay which turns on the fans via the humongous resistor. So if the resistor or its connections are bad, there will be no auxiliary cooling from the fans.

For these reasons, I wouldn't modify the system until such time as you know it is working to factory specs and it is still inadequate.

That said, V8s are known to have heat issues and to that end there are several modifications out there which "fool" the climate controller into thinking that the engine is hotter that it is. These result in the fans turning on at a lower "high engine temperature" thus mediating the heat issues. The modification consists of adding a resistor in parallel with the temp sensor which lowers the effective resistance, thus making the controller think that engine is hotter than it is which, in turn, makes the fans come on a lower real temperature. I'm looking for a link I have (somewhere) to a guy in LA that provides pre-modified temp sensors, but I can't find it at the moment. Until then, you might try doing a search on it.

Hope this helps.

edit: . . found the link: http://www.k6jrf.com/MB_S500.html#MYCH

 

Thanks guys, for your help. I'll get that humongous resistor back in the engine bay in the A.M. I had to come up with two 8mm nuts because the ones that were originally there were really corroded due to over-heating and loose connections (or, I guess I should say that the loose connections caused them to be over-heated). Anyway, I located two of those little buggers. I will take a look at what the temps are, once I've re-installed the resistor and then decide if I want/need to go any further. But, you are correct, it was a bad idea of mine to have the auxiliary fans going constantly (I guess that's why M-B employs engineers, to figure that stuff out, huh?).

 

Can we see these Aux fans you've picked up?