Bad Fan Clutch Troubleshooting Case Study (GM 4.8L, 5.3L, 6.0L)

By: Abraham Torres-Arredondo
Created: 18 August 2012
Updated: 31 March 2024
Articl Id: 240

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Quickly Checking The Thermostat's Operation

Now that I had eliminated a blown head gasket, the next step was to check the operation of the thermostat (even though Oscar had already replaced it with a new one). Specifically that it was opening up and letting the hot engine coolant circulate into the radiator at around 190° F.

This is another very easy test and it's done with a scan tool with Live Data capability. The only other requirement is that you have to start out with a cold engine.

I'm gonna' briefly describe what I did here, but if you want to see the step-by-step thermostat test, you can find it here: Thermostat Test (although this test is for the 4.3L, 5.0L and 5.7L engines, the info still applies).

The next step was to check the thermostat. This is what I did:

I connected my generic scan tool and went to its Live Data mode.
- I scrolled down to the PID that showed me the coolant's temperature.
- Once there, I made sure the coolant temperature showed that the engine was cold.
Checked the temperature of the upper radiator hose and made sure that it was cold to the touch.
I started the engine and let it warm up.
As the engine started to heat up, I kept my eyes on the coolant temperature the scan tool was reporting.
- Every couple of minutes, I checked the temperature of the upper hose.
When the scan tool reported 190° F, I checked the temperature of the upper radiator hose
1. The hose was starting to feel hot.
  - This was the result I was expecting to let me know that the thermostat was indeed opening up.
Once the scan tool reported 195-200° F, the upper radiator hose was completely hot.
- This confirmed that the thermostat was indeed doing its job of letting the hot coolant circulate into the radiator.

Now, in case you're wondering, If the upper radiator hose had not gotten hot (at around 190° F), I then would have known that the thermostat was not opening up and therefore I could conclude that Oscar had installed the thermostat incorrectly or had installed a defective one.

Having eliminated the thermostat, I was off to the next test.

Checking The Fan Clutch

By this point, Oscar's van had been idling for about 30 to 45 minutes and the engine was now at normal operating temperature and this meant that the fan clutch was also hot and hopefully operating in its engaged mode. So the next step was to manually check to see if the fan blades were locking up or freely spinning.

You and I can check this by turning off the now warmed up engine and spinning the fan blades by hand. If the fan clutch is in its Engaged Mode, then the fan blades should not be able to complete one or two revolutions.

This is what I did:

Before I turned off the engine, I checked the coolant's temperature one more time, with the scan tool, and it read around 200° F (which also indicated that the engine was not overheating yet).
Since the engine was still running, I turned it off.
I then grabbed the edge of one of the fan blades and manually spinned it.
The fan blade DID NOT spin freely.

Since the fan blades did not spin freely, I now knew that the fan clutch was in its Engaged mode. I was starting to think that the fan clutch was OK but I have seen this before where the fan clutch blades do not spin freely yet the fan clutch is bad.

Also, the van was not overheating. So, I took it on a road test. I hadn't even gone 5 blocks when the van started to overheat. Not only that, but when I came to a stop the engine idle would get very high (the PCM was commanding a high Idle since it saw that the engine was overheating and was trying to spin the fan clutch faster with a higher idle).

I had one more test to do and this was to manually lock up the fan clutch and road test it again.

Manually Locking Up The Thermal Fan Clutch

Manually locking up a fan clutch to see if it's truly fried or not is an old mechanic's technique, and this was what I did next.

Before I jump into how I did it on the van, let me say that this technique (of manually locking up the fan clutch) involves placing the end of the bimetallic coil in another position (on the fan clutch itself) so that the shaft it's attached to is turned counter-clockwise.

When the shaft is manually turned in this way, the fan clutch immediately goes into and stays in its Engaged Mode all of the time. The end result is that it starts to pull in more air across the radiator. On the majority of older vehicles, the fan clutch is designed in such a way to you're able to this very easily.

Let me explain this in more detail:

If you have ever noticed, the end of the Bimetallic spring is placed inside a groove or set against a tab that protrudes from the fan clutch itself.
The thermal fan clutch will then have another groove or tab that's placed opposite of the one that the end of the bimetallic coil is placed in (not all fan clutches have this setup, but most do).
So then, to manually lock up the fan clutch, all you do is remove the end (of the bimetallic coil) from its groove or tab and place it on the groove or tab directly opposite of the one it's in (if the fan clutch is so equipped).
You then start and road test the vehicle and see if this solves the overheating problem. If it does, then you now know that the fan clutch is bad and needs to be replaced.

On Oscar's Express van, it wasn't possible to place the bimetallic coil in another position because the fan clutch simply did not have this option but I was able to manually lock up the fan clutch anyway, using a method that's a bit unorthodox (but that works) and I'll explain it in detail in the next couple of paragraphs.

Manually Locking Up The Thermal Fan Clutch (Part II)

As mentioned before, Oscar's fan clutch didn't give me the option of locking up the fan clutch by repositioning the bimetallic coil but I was able to do it anyway by removing the bimetallic coil and using a piece of metal (in the shape and size of the end of the coil) in its place to lock the shaft in its counter-clockwise position with epoxy.

Now, this, I'll confess, is a pretty extreme way of testing the fan clutch and it may not be everyone's cup of tea. But, keep in mind that I had already verified several important things and these were:

No coolant leaks from anywhere on the engine, radiator, or water pump.
Thermostat was functioning correctly.
No indications of a blown head gasket.

And so, I felt confident that this was the only way to check the true condition of the fan clutch. Not only that, but Oscar had already spent quite a bit of money on parts that had not solved the problem and did not want to throw anymore money at the van unless he was sure that the part was gonna' solve the problem.

This is what I did:

I removed part of the fan shroud (to have good access to the bimetallic coil).
Removed the bimetallic coil.
I then found a small piece of metal that was about the same thickness as the end of the bimetallic coil.
I placed one end of this piece of metal into the groove on the shaft.
Using this piece of metal as a lever, I then turned the shaft to its maximum counter-clockwise position and then expoxied it (to lock the shaft in place).
After waiting the time the epoxy needed to set, I road tested the van.

And the overheating problem was solved! That's right, no matter how far I drove the van (on the highway or not), Oscar's van did not overheat anymore.

All Oscar needed to do was to run down to the auto parts store and buy a new fan clutch.

Fan Clutch Troubleshooting Summary

It used to be that checking a thermal fan clutch was an easy matter since they all could be very easily locked into their Engaged Mode by simply moving the end of the bimetallic coil to another spot on the fan clutch assembly.

Nowadays, most modern thermal fan clutches don't have that feature anymore but it doesn't mean that they can't be tested.

If you're able to think outside the box and get a little creative, you can still manually lock up the fan clutch to test it. Here are three other very important things you should also check on the fan clutch assembly:

FAN NOISE at all times, whether the engine is at normal operating temperature or not:
- It's normal to hear a roaring sound (as the air gets pulled across the radiator) when you first start up a cold engine.
- This roaring sound, with a correctly functioning fan clutch, dissipates after a few minutes (as the clutch re-distributes the silicone fluid back to its normal disengaged mode).
- It's when the roaring sound continues under all high engine speed conditions ( 2500 rpm and up ) that IS NOT normal and indicates that the fan clutch assembly is locked up due to an internal failure.
- This condition can be further tested with the engine off and as you turn the fan blades by hand, you'll notice that the fan cannot be rotated and/or you'll feel a rough grating as you turn the fan by hand.
LOOSENESS (excessive lateral play):
1. With the engine running and under various temperature conditions, there's a visible lateral movement that can be observed at the tip of the fan blade.
2. With the engine off, you can grab the tip of the fan blade and physically check this movement.
3. The amount of play (movement) is normal up to a certain point (since this play is due to the type of bearing used). Approximately 1/4" maximum lateral movement measured at the fan tip is allowable depending on make and model (you'll need to check the manufacturer's specifications to see if the lateral movement of the fan clutch on your vehicle is normal).
SILICONE FLUID LEAK from the bearing area:
- Silicone fluid leaking is a sign that the fan clutch needs to be replaced.

Remember, check these things also:

Water pump for coolant leaks.
Radiator for coolant leaks.
Operation of thermostat.
Indications that the head gasket has blown.

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