Fan clutch questions

I'm about to replace the fan clutch on my 165K mile '94 XJ. I'm going to use a thermostatic model ($50 at C.A.P.), but my question is: how do I know what temperature the clutch will engage at? And, is the engagement temperature adjustable with the little bi-mettalic coil on the front?

Thanks!

Andy

AndyS said:

I'm about to replace the fan clutch on my 165K mile '94 XJ. I'm going to use a thermostatic model ($50 at C.A.P.), but my question is: how do I know what temperature the clutch will engage at? And, is the engagement temperature adjustable with the little bi-mettalic coil on the front?

Thanks!

Andy

Click to expand...

As you surmise, the bimetallic coil sets the engagement temp, and is NOT adjustable.

Thanks Jeff; that's what I figured, but I was wondering if you could wind the coil tighter or looser to change the temperature that it engaged at.

Also, are fan clutches marked or stanped to indicate at what temperature they lock up at?

I just don't want to get one that "happens to fit" my XJ, but locks up at 170 degrees, in which case it'd be running all the time, or set too high which'd have no different effect than my worn out clutch...

Andy

AndyS said:

Thanks Jeff; that's what I figured, but I was wondering if you could wind the coil tighter or looser to change the temperature that it engaged at.

Also, are fan clutches marked or stanped to indicate at what temperature they lock up at?

I just don't want to get one that "happens to fit" my XJ, but locks up at 170 degrees, in which case it'd be running all the time, or set too high which'd have no different effect than my worn out clutch...

Andy

Click to expand...

A decent parts house selling top grade parts will supply a fan clutch that is set to OEM specs.

Fan clutches aren't like the clutch for your transmission - they don't "lock up" all at once, and at a specific temperature.

A fan clutch uses a thermally-actuated viscous coupling, which may be aided by the use of the bimetallic coil (on the front) in some hard-use applications. The viscous coupling gradually engages over a temperature range, which results in a variable fan speed until full "lockup" (at which point it is merely not slipping so much. A fan clutch never really "locks up" unless you drill a hole in it and put a bolt in it.)

I've never seen a unit (on any vehicle!) marked with an engagement temperature or range, and I've never felt a need to test one.

If you are concerned (although I don't understand why - would you please enlighten me?) about fan drag or anything like that, it is possible to remove the string-driven thing entirely and replace it with an electrical fan - but many who have done this have later gone back and reinstalled the belt-driven fan.

Me, I'm going the other way. I've got a set of drawings I'm working on for a "fan clutch eliminator," where the fan will be coupled to the pully with a solid chunk of metal. I don't mind having the fan run all the time, I'm tired of changing fan clutches, and I've demonstrated before that lowering the operating temperature of the engine does little to impact fuel efficiency or tailpipe emissions (haven't had a chance to run the engine at various temperatures on a dyno yet, so I can't empirically evaluate the effect on power output. I shan't even guess, at the moment.)

5-90

5-90 said:

If you are concerned (although I don't understand why - would you please enlighten me?) about fan drag or anything like that, it is possible to remove the string-driven thing entirely and replace it with an electrical fan - but many who have done this have later gone back and reinstalled the belt-driven fan.

Me, I'm going the other way. I've got a set of drawings I'm working on for a "fan clutch eliminator," where the fan will be coupled to the pully with a solid chunk of metal. I don't mind having the fan run all the time, I'm tired of changing fan clutches, and I've demonstrated before that lowering the operating temperature of the engine does little to impact fuel efficiency or tailpipe emissions (haven't had a chance to run the engine at various temperatures on a dyno yet, so I can't empirically evaluate the effect on power output. I shan't even guess, at the moment.)

5-90

Click to expand...

My only concern with running the fan more than it needs to is the negative impact on power and mileage. My mileage is utter crap already, and I don't need to knock it down another 2-3 mpg. Plus, the ol' girl is getting a bit tired on the highway, so having the A/C running AND a fan pulling down the motor will make my 3.55's feel like 2.73's.... (it already feels like 3.07's with my 31's...)

If it's an older rig (like more than 10 years or so...) you may be having a similar problem to mine. The sensors get old, and as that happens, they lose sensitivity and the fuel mileage starts to suffer.

Check and/or change the HEGO, MAP, TPS, and IAT for starts - they're the "big four" for fuel metering. The CTS is also a good idea to keep up, but its main input goes into determining "Open Loop" or "Closed Loop" operation.

Changing those sensors usually results in a fuel mileage boost right away.

Also, while a certain amount of torque is needed to spin the fan, we've got it to spare, so if your engine is in tune, there's nothing to worry about...

5-90

I've been thinking about changing the O2 sensor, as it's been in there for a good 70-80K miles, but I haven't thought about the others. Do you have any suggestions for how to test the other sensors? I thought that ones like the TPS were in the category of working/not working, rather than slowly deteriorating...

Thanks!

What year? That would help...

The HEGO sensors are different, but the rest of the sensors are quite similar. You'll want a hand vacuum pump, a reasonably accurate thermometer, and an accurate multimeter (analogue is better, if you have one to do the tests...

Hook the hand vacuum pump up to the MAP sensor, and hook the multimeter up to pins B and C (for RENIX, later models should be similar. I just don't have an FSM handy, and I don't feel like going out and getting one, to be perfectly honest...)
Note the resistance reading. Slowly apply vacuum with the hand pump, and watch the meter reading. It should chance steadily as the vacuum increases - I believe it will increase. If it should "jump" or "hang," the sensor has developed a flat spot and should be replaced.

Unplug the TPS and find out which terminals (for yours) will vary in resistance as the throttle blade is moved (for 1987-1990, it's terminals B and C.) Again, note the reading, and make sure it varies smoothly as you move the throttle blade. Flat spots indicate replacement as before. We'd have to know what year your XJ is before we can go into adjustment procedure...

The IAT sensor will want to be removed for checking - it's another resistance gage, but it's resistance varies with temperature. I usually put it about a foot or so in front of a hair dryer on LOW, with the thermometer right in front of it and an analogue ohmmeter hooked up to it. The resistance should decrease as the sensor element gets hotter - there's a table below.

Like I said, these four sensors have the most to do with fuel metering. The CPS can cause trouble - but it's usually "No Start" or "Stalling" caused by a failing signal, and they tend to go all at once when they finally do go (not much of a decay time, usually.) Those are best checked by looking for a pulse of some sort - RENIX and HO are different, and I mostly work on RENIX so far.

Here's the chart for both the CTS and IAT sensors - they follow a standard GM thermistor curve...

212*F 100*C 185 ohms
160*F 70*C 450 ohms
100*F 38*C 1600 ohms
70*F 20*C 3400 ohms
40*F 4*C 7500 ohms
20*F -7*C 13500 ohms
0*F -18*C 25000 ohms
-40*F -40*C 100700 ohms
I've noted variations of up to about 15% in these values without causing any major performance issues. Depending upon your hair dryer, you may have to switch to HIGH for the higher values. The lower ones usually aren't a problem - chances are, it's not generally that cold. If you are worried, put the thing in the freezer (for IAT) or in a salted ice-water bath or put some alcohol in the deep freeze for a while to get below freezing temperatures for the CTS.

5-90

I have considered permanently engaging my fan. Not sure of the downside, might do it for awhile and see if there are any bad effects. I think in winter I would like to have it run like stock with the fan clutch though. Ive been thinking over the mechanism in my head for awhile now.

1994, 4.0, AW4. 165K miles. Powertrain is all stock, save for a K&N drop in filter and a Borla header and 2.5" exhaust system.

5-90 said:

Fan clutches aren't like the clutch for your transmission - they don't "lock up" all at once, and at a specific temperature.

A fan clutch uses a thermally-actuated viscous coupling, which may be aided by the use of the bimetallic coil (on the front) in some hard-use applications. The viscous coupling gradually engages over a temperature range, which results in a variable fan speed until full "lockup" (at which point it is merely not slipping so much. A fan clutch never really "locks up" unless you drill a hole in it and put a bolt in it.)

I've never seen a unit (on any vehicle!) marked with an engagement temperature or range, and I've never felt a need to test one.

If you are concerned (although I don't understand why - would you please enlighten me?) about fan drag or anything like that, it is possible to remove the string-driven thing entirely and replace it with an electrical fan - but many who have done this have later gone back and reinstalled the belt-driven fan.

Me, I'm going the other way. I've got a set of drawings I'm working on for a "fan clutch eliminator," where the fan will be coupled to the pully with a solid chunk of metal. I don't mind having the fan run all the time, I'm tired of changing fan clutches, and I've demonstrated before that lowering the operating temperature of the engine does little to impact fuel efficiency or tailpipe emissions (haven't had a chance to run the engine at various temperatures on a dyno yet, so I can't empirically evaluate the effect on power output. I shan't even guess, at the moment.)

5-90

Click to expand...

When you build that block make it a bit shorter so the fan shroud can be slipped by the fan w/o breaking it

When I did my waterpump a couple weeks ago I broke the shroud [98], however it broke inboard of the bottom tab so I cut the other side off same place so my fan shroud is now upside down U shaped but has both tabs still intact, slips right by the fan. Have noticed no adverse effects in the 90F + temps and 100% humidity we have had here for the past week or so.