high speed aux fan operation - PeachParts Mercedes-Benz Forum

jsmith · #1 07-31-2002, 12:15 PM

my cooling system has not really been acting up all summer and has stood up to the heat waves we've been getting here in the east. i do know that the high speed fan kicks in when temps rise and i had been experimenting with different resistors for the cooling mod. today when i pulled off the sensor harness i was surprised that the fan did not kick in. after about 5 seconds in started up and stayed on for about 10 seconds then went off again. i turned on the AC and the fan went on at low speed. all this time temps were at around 90C. i know that the cooling mod works because my temps never go over 100C even with stop and go driving and AC on. when i stop the car after highway driving the temps go up to around 100C and if they are still up when i restart the engine, the high speed fan kicks in - so i know it's working fine.

i always thought that when that thermosensor circuit was open, the computer knew to turn on the fan at high speed. should i be concerned? i waited for a little bit with the thermosensor unplugged and the fan didn't kick in at all after the initial 10 seconds that it went on (the temps were at a steady 90C). the system seems to be doing its job, it's just that this unexpected behavior stumped me...

Arthur Dalton · #2 07-31-2002, 12:46 PM

If you have the mod resistor in parallel with the ECT, then when you unplug the ect, the harness still has a bridge resistor in it
[ the mod resistor].
This resistance value by itself is way to high for fan operation cut -in spec.

When the ecu senses an open [ which you do not have w/mod still in the uplugged harness] ,
it switches fan by default.

Remember, when you are putting the mod resistor in parallel with resistance of the ect, you are lowering the Total resistance
[at a given temp] of the ect and fooling the ecu..
In a parallel resistance circuit, the total resistance is always LOWER than the Smallest resistor in that circuit. So, if the cut-in spec of the ecu is , say 300 ohms, adding the mod of , say 1500 ohms, The total ect R. is now 250.
So , as you can see , the ecu will now detect an earlier cut -in temp by being fooled by the new temp/ohms set-up.

jsmith · #3 07-31-2002, 01:22 PM

Arthur,
As usual, you've debugged my little "problem". I had made up two inline "cooling mod harnesses" which allows me to switch them or to revert to stock completely. I was switching harnesses and it had gone on when I took off the harness that was in place. That accounts for the 5 second lapse because I disconnected the thermosensor end first, then the ECT side. It was on for 10 seconds until I plugged the other harness in on the ECT side! Duh...
Thanks

Arthur Dalton · #4 07-31-2002, 02:05 PM

Cool .. literally..

Here is some food for thought

Low fan is a/c pres. activated and accomplished via resistor V. reduction.. [ which is a big draw, but besides the point]

Now , if we had a thermo sw. to sens under hood/cond. air
[ an electric water heater top element thermo would fit the bill here , as it is adjustable and the top one has both n/on-n/off terminals ] , and we wired that in parallel with the low speed resistor, set the cut-in for extreme heat/ load conditions,
you would in effect now have both a low and high speed a/c condensor fan.... which may be just enough to keep the a/c cond cool enough on extreme load as to not raise the eng temp to the ect temp cut-in.
It seems that the high fan ect is usually called for at extreme a/c
heat load as the radiator intake air is right up there from the hot condensor... so maybe........

This would still be an indepenent circuit cuz we are still using the a'/c pressure sw , but just changing the fan speed to 2 speeds , now both pressure[low] and temp[ high] sensed..
I have not tried any of this , but it has been rolling aroud in my head...

jsmith · #5 07-31-2002, 05:38 PM

Arthur,

For the cooling mod, I am using a lower cut in point from your recommendations for summer time use. This mod would negate my need for having two different cooling mod harnesses. With my current setup there would probably be an overlap in the cut in points of the ECT and your proposed pressure switch activated high speed fan.

Cheers

Arthur Dalton · #6 07-31-2002, 05:51 PM

Prob would be...
My thoughts are more geared to the concept that the ect sensor could use a higher cut-in as with the low speed being temp sensed for high speed a/c cooling , the eng temps would not run as high to start with....

the other advantage of the 2 speed a/c set-up , aside from better a/c cooling, is that as the cars speed allowed for ram air/cooling, the temp sens would cut out and the low fan would [or should] continue to run as long as high side pressure called for assistance...

I have noticed that then later models now incorporate a low fan regardless of high side pressure [ runs with comp. engagement] and a higher speed for excessive high side pressure...