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Unread 05-09-2013, 08:28 PM   #1
mschi772
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1997 XJ Cherokee 
 
Join Date: Apr 2012
Location: Racine, WI
Posts: 2,361
Ideal on/off temp for electric fan conversion

I've been poking around, and all I can find is a whole bunch of guesswork.

I'm installing some electric fans in place of my stock fans and am trying to figure-out how best to control them. I know Dirtbound, for example, slaps a [very overpriced] switch into the heater hose that turns one on at 185 and another at 205, but that location looks sloppy, using 185 at the stat housing as an ON temp seems dumb because it's a 195 stat, and I have no idea what their chosen switch has for OFF temps.

I've got two new radiators (don't ask), and one is for an older XJ that used to have a thermal switch in the driver's side tank to activate the aux fan. That seems like a nice place to mount a switch/sensor, but I don't have much of a clue of what temps I should use for on/off at that location to best serve the engine. I may also be really fancy and get a DCC controller (despite their TERRIBLE reputation for customer communication) to use with the other radiator instead, but I'd still like to know what temp to set it to.

I'm looking for an educated response that can be backed-up with an explanation. I know the stat is 195 and that the 4.0 should run anywhere from 195-220, but I don't have the slightest clue of how to efficiently control the fans to maintain a stable and safe engine temp.

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Unread 05-09-2013, 09:20 PM   #2
djb383
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1998 XJ Cherokee 
 
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'98 FSM says "on" at 218F, "off" at 209F. Play with the "on/off" settings from there. Doubt if there's any such thing as "ideal".
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Unread 05-09-2013, 10:05 PM   #3
Carves
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1998 XJ Cherokee 
 
Join Date: Oct 2008
Location: Western N.S.W., Australia
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Quote:
Originally Posted by mschi772 View Post
I've been poking around, and all I can find is a whole bunch of guesswork.

I'm installing some electric fans in place of my stock fans and am trying to figure-out how best to control them. I know Dirtbound, for example, slaps a [very overpriced] switch into the heater hose that turns one on at 185 and another at 205, but that location looks sloppy, using 185 at the stat housing as an ON temp seems dumb because it's a 195 stat, and I have no idea what their chosen switch has for OFF temps.

I've got two new radiators (don't ask), and one is for an older XJ that used to have a thermal switch in the driver's side tank to activate the aux fan. That seems like a nice place to mount a switch/sensor, but I don't have much of a clue of what temps I should use for on/off at that location to best serve the engine. I may also be really fancy and get a DCC controller (despite their TERRIBLE reputation for customer communication) to use with the other radiator instead, but I'd still like to know what temp to set it to.

I'm looking for an educated response that can be backed-up with an explanation. I know the stat is 195 and that the 4.0 should run anywhere from 195-220, but I don't have the slightest clue of how to efficiently control the fans to maintain a stable and safe engine temp.

Likely reason for all the guess work is probably coz there has been too much reliance on the belief that the XJ ... "must have" ... a 195* t'stat ... and that it likes to run hot,
... and then theres some folks - that don't want the fans coming on at early and useful temps ... coz they are too noisy ..


Essentially want you need to ascertain is ...

What temp should the coolant coming out of the radiator should be ... for the engine to add its heat to ... to provide a satisfactory temp at the thermostat housing. ??

... and the engine heat should be - whats generated under an average load at least,
... not just a, no load, at idle, parked in a snow drift, sorta engine heat ..

Then find a fan switch that controls whatever fan combination is fitted - to maintain that, cooled, coolant temp.


Using the stock XJ setup as an example:

Fan Clutch operating parameters:(Note that the temps are the air temps directly behind the radiator.)

Fan drive engagement should have started to occur at between 74 to 82 C (165 to 180 F). Engagement is distinguishable by a definite increase in fan flow noise (roaring).

Fan drive disengagement should have started to occur at between 57 to 79 C (135 to 175 F). A definite decrease of fan flow noise (roaring) should be noticed.
.

Soooo ... using a little guesswork ... If one fits an "equivalent cfm capacity" electric fan to replace the stock clutch fan ... then a switch in the outlet side of the rad would probably want to operate at temps of about, +10*f, of the air temps.

edit: ... I based that +10* temp on the RHD setup where the clutchfan is fitted at the outlet end of the rad ... On a LHD it might be more applicable to actually use the factory stated air temps ... as the coolant temps for a, outlet end mounted switch ... since the coolant still has to pass through the last half of the rad from the LHD clutchfan.

Note clutchfan, off/disengagement temp - of between 135*f - 175*f ..


Factory elec aux fan:

Comes on at approx 218*f
... NOT to maintain a 218*f running temp ... but to provide additional cooling ... to lower the coolant temp - to a more acceptable level.

Best to think of the 218*f temp - as the engines ... OMFG - I'm starting to overheat, temp ... rather than a running temp .. IMHO ..


What setup you fit will also make a difference ... fan sizes, CFM ratings, one on or all on, two or three fans etc.,

If you get the parts shop to check a, temp sensor/switch suppliers catalogue,
... You'll find theres a range of switches with different temps - something like this ....

http://www.tridon.com.au/Products/Pr...5&G=483&P=2014

An adjustable one is actually, probably better, for the trial and error process of getting a new fan system to run right ... but the reliability of the adjustable ones ... and the accuracy/security, of the, probe in the fin, setup - leaves a bit to be desired imho.

You may even find you will be better off running a 190* t'stat than a 195*.

Have fun with it.
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Unread 05-09-2013, 10:28 PM   #4
djb383
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From way down under, u sure are blowing a lot holes in the "XJ was designed to and must run at 210F" temp theory/thinking.
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Unread 05-09-2013, 11:25 PM   #5
Carves
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Quote:
Originally Posted by djb383 View Post
From way down under, u sure are blowing a lot holes in the "XJ was designed to and must run at 210F" temp theory/thinking.




ohhhh I'm sure it was intended that the thing run at about 200*/205* .... It can't do much else with a 195* t'stat fitted.

Keeping toes warm in Toledo and hippies happy in California is a primary consideration in US vehicle design Im sure ..

Just a shame that so little "design" went into keeping the vehicle from exceeding that running temp ... in anything other than, perfect driving scenarios in perfect weather conditions.
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