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Question re failure of low speed fan

 
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Old 24 Nov 2012, 07:29 pm
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Default Question re failure of low speed fan

Hi all, my overheating issues with my sons 2001 PT LTD continue. So far I've changed the radiator(because it was badly cracked), the rad cap, thermostat and replaced the coolant with HOAT.

At first when warmed up the coolant was boiling in the reservoir but then after changing the rad cap that problem stopped. However the car was getting very warm when idling at 2,000 revs, it never got up to the red but made it to the three quarter mark.

Today i pulled the fan assy to test it properly, hooking the middle connection to ground, i connected in turn the left and right connectors to +12V. Only one of them made the fan spin, I believe it is the high speed that is spinning. Measuring resistance shows that the high speed side is very low ohms and the low speed connection to ground is open circuit. So I'm satisfied that the fan is shot and needs to be replaced.

My question is this, if the low speed side is out of commission, can this alone cause the car to overheat? I ask this because as the engine heats up to the appropriate temp, the ECT should detect it and signal the PCM to switch the ground(via the relay) to turn on the high speed fan to pull heat out of the rad to keep the temp stable. I think the low speed is for AC but doesn't do much to keep the engine temp cool. Am I reading this wrong?

thanks
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Old 24 Nov 2012, 07:40 pm
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Default Re: Question re failure of low speed fan

if the low side is bad, yes, it will cause the car to overheat. there is absolutely no doubt.

pick up a dorman fan on either rockauto or amazon. less than $100. and a lifetime warranty.
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Old 24 Nov 2012, 08:42 pm
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Default Re: Question re failure of low speed fan

Agree with soyomb.

Although the low speed is quiet and half (or lower) than the high speed, it's still enough to cool the compressor and help cool the coolant and as above, it will cause overheating.
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Old 24 Nov 2012, 10:43 pm
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Default Re: Question re failure of low speed fan

I agree with Crewzin and soyomb but have some detail to add that I learned on another Chrysler with a two-speed fan setup.

The temp sensor isn't so much a switch but a transducer. That allows the PCM to read the actual temp and compare it to specific values. If it can't turn the low speed fan on, then the temp gauge will begin to climb above it's normal operating mark. When the coolant temp reaches the upper limit or anytime the a/c is activated, the high speed fan comes on and stays on as long as the a/c switch is active or until the coolant temp drops below the lower set point.

To answer your specific question about the overheating, as long as the high speed fan is working correctly, you should not have to worry about over-heating. But, the cooling system MUST be able to hold adequate pressure (16 psi, correct me if I'm wrong please) in order to keep from overheating. Coolant's boiling point is directly affected by how much pressure it's being kept under. If it's atmospheric pressure from a leaking cap or other open component of the cooling system, it boils easily and below the lower setpoint for the fan. At the design pressure (16?) of the cooling system, the boiling point is well above the high temp setpoint.

Hope this helps.
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Old 24 Nov 2012, 11:05 pm
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Default Re: Question re failure of low speed fan

yep whAT THEY ll sAID
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Old 26 Nov 2012, 10:21 pm
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Default Re: Question re failure of low speed fan

Thanks for the replies guys. Ordered a Dorman fan kit from Amazon today, should arrive Wednesday. Hopefully this is the final fix for the overheat problem!!
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Old 26 Nov 2012, 10:54 pm
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Default Re: Question re failure of low speed fan

Quote:
Originally Posted by bandaide View Post
I agree with Crewzin and soyomb but have some detail to add that I learned on another Chrysler with a two-speed fan setup.

The temp sensor isn't so much a switch but a transducer. That allows the PCM to read the actual temp and compare it to specific values. If it can't turn the low speed fan on, then the temp gauge will begin to climb above it's normal operating mark. When the coolant temp reaches the upper limit or anytime the a/c is activated, the high speed fan comes on and stays on as long as the a/c switch is active or until the coolant temp drops below the lower set point.

To answer your specific question about the overheating, as long as the high speed fan is working correctly, you should not have to worry about over-heating. But, the cooling system MUST be able to hold adequate pressure (16 psi, correct me if I'm wrong please) in order to keep from overheating. Coolant's boiling point is directly affected by how much pressure it's being kept under. If it's atmospheric pressure from a leaking cap or other open component of the cooling system, it boils easily and below the lower setpoint for the fan. At the design pressure (16?) of the cooling system, the boiling point is well above the high temp setpoint.

Hope this helps.
Hey bandaide. Nice explanation. I'd like to correct one statement and add a little on another.

The coolant sensor is a thermistor, a resistor that changes value with temperature. The PCM sends out a 5 volt reference signal to the sensor. The voltage goes through the resistor and back to the PCM. The return voltage is what the PCM uses to determine the coolant temperature. What the PCM does with this info to control the fan is explained very well.

The cooling system holding pressure is crucial to proper system operation as you stated. A 50/50 water/anti-freeze mix has it's boiling point raised 8 degrees for every pound of pressure. At atmospheric pressure it will boil at 212 - 218 degrees.

Ron
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Old 03 Dec 2012, 07:27 pm
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Default Re: Question re failure of low speed fan

Well got the new fan, fitted it on Saturday, checked it out before and it worked on lo and hi speeds. Filled up the rad with HOAT via the top connection until HOAT spilled out, filled up the top hose with same, quickly connected the top hose to top rad connection and proceeded to bleed the system with the engine idling.

Did this for 25 mins and the temp needle never moved from just under the half way mark. Took it for a test run and the needle moved above and below the half way point by only a smidge. Once in stop and go traffic, the needle moved up towards the three quarters mark but never hit the red nor did the warning come on.

But once we got to the gas station to fill her up, noticed steam and gurgling coming from the overflow. Note that I did have the overflow filled to the "HOT" mark when the engine was cold and prior to starting any of this so technically it was pretty full. When I checked it at the gas station the bottle was pretty full, above the "HOT".

My son drove to his job, 3 miles away, parked it then drove it back later and left it overnight. When he drove home the needle went up to the 3/4 mark but not beyond. Checking in the morning i pulled the rad cap off and could see that the fill tube was dry down to the thermostat but the overflow was still at the "HOT" mark on the overflow.

I put the cap back on, started it and drove it in to the garage, all of 100 yards. I pulled the rad cap off and there was a release of pressure when I opened it which surprised me.

So i'm now thinking that the Headgasket is a goner and I'm getting combustion gases in the coolant. I will run a block test first before I decide whether or not to pull the head or junk the car.

One question, should there be a gasket between the coolant tower (where the thermo is) and the block? I don't have one there and am wondering if that could be letting air in but not let water out.

cheers to all
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