91jimmy, changed water pump..

hi all, i just changed the water pump in my 91jimmy...
all seems to be well, however i am a little concerned about the temperature now...
it used to run just a tick above middle on the temp gauge, but now runs
lower than that, goes even lower when moving, then comes up again at idle
could it be the fan clutch? or thermostat?
or is this normal?
thanks for your help
mikey

How much does the temp fluctuate between idle and cruise? A bad t-stat will hardly ever reach operating temp. You can check the fan clutch be rotating it by hand and see if it is "locked"/ engaged.

at idle, once warmed up, it's at like 190 and while driving it goes down to around 160 or so...it used to sit steady at 200 all the time...
i will check the fan clutch, i do know it turns by hand with the engine off....
and it should turn with some resistance with the engine warm.....

The fan clutch is fluid filled, when the fluid is cold, the clutch is engaged, (locked). As the engine warms, warm air is drawn through the radiator and thins the fluid which disengages the clutch. If the radiator runs low on coolant and the engine begins to overheat, the radiator cools and can no longer draw warm air over the clutch, the fluid thickens and the clutch engages. When starting the engine cold, below approximately 35F, you should hear the fan "roar" until the clutch disengages. The "roar" sound will come and go until the clutch fluid warms and thins out, disengaging the clutch. The clutch should never be locked totally, you should always be able to turn it by hand. The temperature of the fluid will determine how much resistance there is.

If the coolant level is full, you have a thermostat that is not closing completely causing the temperature fluctuation.

Old Master: Going by what you said the fan clutch is engaged when the oil is cold. If this were true then the fan would be engaged when the truck is cold and disengaged when hot, which is opposite of what you want. The clutch on these fans is fluid filled but I believe they are centripetal. The fan is engaged when at lower RPMs which allows for cooling when sitting and at slow speeds. Driving at highway speeds keep the engine RPMs higher so the fluid is forced to an outer chamber within the clutch, this disengages the fan. Some centripetal fans also react thermally, I believe this adjusts how easy the fluid can flow between chambers but am not sure. You can hear when the centripetal clutch disengages if from a stop you start going in 1st gear (if an auto trany just lock out the rest), let the truck rev up to around 1800 to 2000 RPMs, and hold it there. Make sure your radio is turned off along with the vent fan, you will hear the cooling fan roar at first but then fade away (that is if your fan clutch works).

Old Master: Going by what you said the fan clutch is engaged when the oil is cold. Correct. If this were true then the fan would be engaged when the truck is cold and disengaged when hot, Also correct. which is opposite of what you want. No, the main reason for using a fan clutch is to disengage the fan to conserve fuel and horsepower, and reduce the "roar" sound. The clutch on these fans is fluid filled but I believe they are centripetal. The fan is engaged when at lower RPMs which allows for cooling when sitting and at slow speeds. Correct, very little air is drawn across the radiator while at stops, the fluid thickens and the clutch engages. Driving at highway speeds keep the engine RPMs higher Along with increased air flow across the radiator, the fluid thins and disengages the clutch. so the fluid is forced to an outer chamber within the clutch, this disengages the fan. Some centripetal fans also react thermally, I believe this adjusts how easy the fluid can flow between chambers but am not sure. You can hear when the centripetal clutch disengages if from a stop you start going in 1st gear (if an auto trany just lock out the rest), let the truck rev up to around 1800 to 2000 RPMs, and hold it there. Make sure your radio is turned off along with the vent fan, you will hear the cooling fan roar at first but then fade away (that is if your fan clutch works).

The fan should never lock completely to the clutch. With the clutch able to rotate independently of the fan, the friction from the slippage in the clutch, along with the air warmed by the coolant in the radiator being drawn across the clutch, the fluid warms up and thins out. When the silicone fluid thins, the clutch disengages, the fan spins more freely, and the "roar" stops. You mentioned hearing the "roar" when cold; if the fan doesn't roar when cold, there's a problem. Another built in feature of a fan clutch occurs when the coolant level in the radiator drops too low. The warm air from the coolant is not available to assist with warming the clutch so the fluid starts to thicken, which engages the clutch providing maximum cooling.

This web page may help you understand a little better. It covers both types of fan clutches
http://www.chevyhiperformance.com/techarticles/148_0504_cooling_system_info/

After some online research I think we both may be wrong, I know a first for me :lol: yeah right!!!
http://autorepair.about.com/library/faqs/bl407c.htm
This site seems to do a good job and coincides with most other sites. There were a couple of different beliefs but they didn't seem to plausible saying things like the fluid becomes thicker when hot, that only happens with very few fluids on earth. I love learning something new.

I'm not sure where he got his information. He contradicts himself in his explanation and it doesn't sound like he really knows. The person that posted in that article had 3 clutches do the same thing, I wonder what the chances are of getting 3 bad ones. Each replacement did the same thing, (worked properly). The GM shop manual for S/T body vehicles confirms clutch operation exactly how I explained it.

Actually his explaination of the operation is correct, however the means he lists for diagnosing is not. It appears that he copied the operation explaination from somewhere and then guessed at how to test it.

Some of the information posted here is correct and some of it is not.

The fan is a equiped with a clutch that is partially filled with silicon oil. When the engine hasn't been driven in a while the fan will be engaged, not because the fluid is cold and thicker, but because the fluid has migrated to the clutch. As the engine runs the fluid migrates back to the storage area and the clutch begins to disengage. During normal operation, as warm air comes through the radiator it warms the bi-metal coil inside the clutch. As the coil heats it expands and begins to open allowing more fluid from the storage area to enter the fluid coupling. The additional fluid engages the clutch, and the fan begins to cool the engine. As the engine cools, cooler air will be moved across the fan from the radiator. As the cooler air passes over the fan, it begins to cool the bi-metal coil, which then contracts, and the fluid then migrates from the fluid coupling back to the storage area. The fan then begins to disengage.

Good information Blazee! Some clutches use a bi-metal coil and others use strictly silicone fluid. Bottom line is that the temperature of the clutch determines how freely the fan rotates. When the clutch is cold the fan rotates with more resistance. When the clutch is warm the fan rotates with less resistance.

old master, maybe I understand you wrong but think you have things backwards. You want the fan to run when engine coolant is above a specified temp. As the coolant increases temp the radiator will dissipate more heat, causing the bi-metallic coil to react (it is close enough to the radiator for this). This reaction allows more oil to be seen at the clutch plates, which will allow additional force transmission (higher RPMs). Again, you want the clutch to be engaged with higher coolant temps and disengaged with lower coolant temps.

The main objective of the clutch is to keep air moving across the radiator at slower vehicle speeds when airflow is not present like it is while the vehicle is moving. When vehicle speed increases, so does airflow so the clutch disengages to reduce "roar" and increase fuel mileage. Think about a transverse mounted engine using an electric fan, the fan only runs when the vehicle is traveling slowly or at idle for extended periods of time. Electric fans have an advantage over fan clutches as they can be switched on and off actively. Whereas the fan clutch relys strictly on passive air flow temperature.

You are absolutely correct that you want a fan to run fast at when stopped and slow down as speed increases. Also, you are correct that at higher speeds more air will be forced through the radiator and past the fan clutch. What I disagree with you on is the heat transfer aspect. Tracking a defined volume of air passing by the radiator, at high speeds very little heat energy would transfer (the air would see little temperature change) and at low speeds there would be much more heat energy transfered (the air would see a greater temperature change). Because of this the air seen by the bi-metallic coil will be cooler at higher speeds and warmer at slower speeds. Now the reason higher air speeds (flow rates) will cool the radiator better is because heat is transfered at a higher rate when the temperature between the hot at cold substances (radiator and air in our case) is greater. Supplying more "new" air to the radiator will allow a higher rate of heat transfer because it is cooler then the air currently in and around the radiator. This is also why it feels cooler if you stand in front of a fan during the summer.
P.S. sorry for the long posts.

When the engine is shut down after reaching normal operating temperature, the silicone fluid in the clutch “settles” in one area of the clutch. It usually takes approximately 2-4 hours for this to happen.

When the engine is re-started, the fan spins approximately 80% to 90% of water pump RPM and the fan will “roar”. As the silicone fluid is redistributed evenly within the clutch, the clutch gradually disengages; the fan begins to free wheel, and the "roar" decreases.

When the engine is running, airflow is directed through the radiator and fan shroud, and across the thermostatic coil on the clutch. The fan will continue to free wheel until the air temperature, (at the coil) reaches approximately 150F. Internal components of the clutch gradually redistribute the silicone fluid and begin to engage the clutch. When airflow temperature across the coil reaches approximately 195F, the clutch is fully engaged and the “roar” becomes noticeable again. As the clutch engages, airflow is increased across radiator allowing it to dissipate more heat. As the air temperature at the coil decreases, the clutch starts to disengage, and the cycle starts over. Think of the fan clutch as a “temperature regulator” or “thermostat” for the radiator. Hope this helps.

The Old Master nailed it in his last post - I'm glad we got that mystery solved. :)

I fully agree with you in how it works and think are and have been on the same page with that. The only part I was talking about was if the fan engages or disengages as temperature increases. You said, "When the clutch is cold the fan rotates with more resistance. When the clutch is warm the fan rotates with less resistance." Did you mean the clutch pulls more air with warm and less when cold? I take more resistance as engaged and less resistance and disengaged because increased clutch resistance means more power transfer. In the end I think we are talking the same thing just in different ways...violent disagreement is how my father used to put it.

I think you're right about us being on the same page. “Warm” and “cold” are relative terms. Maybe I can clarify my explanation though. When the clutch is cold, as it is after setting overnight, the fan turns with more resistance until the fluid is distributed evenly within the clutch. Once the fluid is evenly distributed and the clutch becomes warm, up to approximately 150F, the fan spins with less resistance. When cruising with the engine at operating temperature, airflow across the thermostatic coil is below 150F so the clutch remains disengaged. When the vehicle comes to a stop, so does airflow across the radiator. The thermostatic coil now solely relies on the fan, turning at approximately 20% of water pump RPM, to draw air through the radiator. The decrease in airflow causes an increase in temperature at the thermostatic coil. When the thermostatic coil reaches 150F, the clutch begins to engage in an effort to decrease radiator and thermostatic coil temperature. If or when the temperature drops below 150F the clutch will disengage. If or when the thermostatic coil temperature reaches 195F, the clutch becomes fully engaged and maximum cooling of the radiator is obtained. The inability of the clutch to engage when hot, could explain why an engine overheats only while idling. On the other hand, the inability of the clutch to disengage will cause excessive fan noise, (roar) and decreased fuel mileage.

Looks like old_master has this one wrapped up, his last few posts have described it perfectly.
:thumbsup: