>Ideally, what is the best temp in your opinion for the fan to turn on? Is it possible to get the fan to come on at low speed settings based on a 90c temperature, even though they only turn on low speed with pressure? If so, how? >>

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124.032 HFM/SFI-104 system chassis run best at 95C Range.

The resistor is simply put across the sensor . [ the sensor used up to your chassis is the two wire/blu sensor the parts book shows, but you have the later sensor , so the only difference is the casing]. This is a parallel bridging of the sensor and b/c the sensors value drops with heat increases, the new R bridge changes the total R factor of the sensor to a lower value.. [ par resistor circuits Total R is always less than the smallest R] ..which the N22 module sees as 105 cut-in , but you now have tricked the module into seeing a cut-in at a lower temp. What you have done is actually put the sensor OUT of factory calibration, resistance-wise.
And that is all there is to it...BUT..
The N22 module that looks at this sensors values also uses that same info for other a/c operations...so, if you go to far out of spec . you can also effect other operations [ like comp cut-off at high ambients]
SO , I find a happy medium at a 100C cut-in..which will get you approx 95 cut-out..thereby keeping the coolant range in a good temp operating range for HFM system, and not killing your fan motors and alternator more than needed.
As far as 90C on low fan, that system is seperate, so many guys just run a toggle in the cabin across the High pressure sw and they can manually get a low fan anytime they want it..this is a good feature for towing or traffic jambs, etc..b/c it gives the operator a fan system manual over-ride w/o upsetting any of the stock circuit. Once the over-ride sw is not in use , you are right back to Normal System operation. The other advantage is with a toggle , you are still using the low fan circuits relay/fuse/resistor/etc..so you do not have to be concerned with any other feeds or parts ..just a simple $2 switch a some lamp-cord.
It comes in handy .... [ if one wanted to do this automatic, a 90C sensor could be added to the system in place of toggle, but I like the availibility of a manual switch cuz it give the operator Over-ride control over all systems]
So... a resistor for sensor cut-in for a $1, and a toggle for $2 for added feature of manual low fan..what else could a guy want for Xmas...!!!
On your system, before doing any mod on your sensor, I would first check it out as to why it does not trigger @105. That trigger spec may just be a bad sensor. And if the sesnors value if off from the get-go, the mod R bridging will be off also..the mod requires a correct sensor Resistance to start with b/c the value change is a combo of the bridge and the sensors R for TOTAL R factor.... [Parallel Resistance circuit]


Go here too..this has the newer sensor bridging picture. Just scroll down on first page... And the charts are there too

http://www.k6jrf.com/MB_S500.html

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A Dalton