fan clutch questions

[BDaugherty]

I'm going to look at a 2003 3500 48re truck tomorrow. The complaint is that it starts to overheat during slow speed operation and possibly at idle. Fan clutch is not engaging, but will engage by turning a/c on. Thermostat and coolant changed 1 month ago with heater core. Radiator is original 200K mile. Water pump and fan clutch are about 3 years old Is there something that could cause the fan to not engage unless the a/c is turned on, but the temp gauge still works?



When I look at it, I will test the fan and temp sensor. Is there also a temp switch or more than 1 temp sensor? Am I looking at a possible ecm problem or maybe a reflash?

 

[Bob4x4]

The engine should be at normal operating temperature.

Set the parking brake and verify the transmission is in park or neutral.
Set air conditioner (if equipped) and blower fan to OFF.
Start and allow engine to reach normal operating temperatures.
Stop engine, connect the DRB III and select appropriate model year and engine option.
Check for and correct existing DTC's
Using Tool 6801, connect pin 1 of the electronically controlled viscous fan drive connector, located at the lower fan shroud to battery ground (Electronically Controlled Viscous Fan Drive Connector).
Using the DRB III, verify that DTC 0480 set.
Start the engine.
Go to the SENSOR screen and observe the fan speed.
Run the engine at 2500 rpm.
NOTE: It maybe take 15 minutes before fan speed increases.

The fan speed should increase according to the table below.
If fan speed does not increase, replace the electronically control viscous fan drive. ELECTRONICALLY CONTROLLED VISCOUS FAN DRIVE SPEEDS
ENGINE RPM FAN RPM (Min)
500 490
1000 950
1500 1420
2000 1850
2500 2230
3000 2440


If the fan speed does increase and there is still a concern, refer to the appropriate Powertrain Diagnosis Manual to diagnosis the electronically controlled viscous fan drive control circuit.
CAUTION: If the viscous fan drive is replaced because of mechanical damage, the cooling fan blades should also be inspected. Inspect for fatigue cracks or chips that could result in excessive vibration. Replace fan blade assembly if any of these conditions are found.

 

[AH64ID]

I would think that overheating at slow speed and idle conditions is a thermostat issue, the cooling system is so large that I cannot imagine the load required to cause an overheat issue at low load.

I can barely keep my truck at 190° with slow speed and idle, it usually drops to 180°.

 

[Bob4x4]

A tstat becomes more important under load if it is not opening enough... ... ... ... .....

 

[BDaugherty]

He had the overheat problem this past summer, but to a lesser degree... possibly due to having the a/c on. It mostly happens when he is maneuvering trailers. I didn't know anything about the problem when I changed the thermostat, it was done as preventive maintenance since the cooling system was drained and it was factory original. I will follow the test that Bob posted and see what I can come up with. I feel like it has to be a fan clutch problem, but it seems odd that it would engage with the a/c, but not temperature... or maybe I just need to see the whole deal for myself.

 

[AH64ID]

Good luck, you may also start by scanning for codes as a fan underspeed should set one.

I am still not certain you could put enough load on the engine in the manner you described to create an overheat condition, even if you removed the fan.

A tstat becomes more important under load if it is not opening enough... ... ... ... .....

Which is the common failure, but what if it's not opening at all under light loads? These motor just don't make enough heat to overheat at low load/speed unless there is a water pump or thermostat issue.

 

[Bob4x4]

It's really not clear if it actually overheats (pukes coolant) or is just warmer than the owner is comfortable with. Never assuming anything is always the best way to approach a customer concern,and get as much info as possible to be able to verify the concern.

 

[AH64ID]

True, I just have a hard time seeing either with a normal functioning pump and thermostat.

Do you know the temp where the fan starts to be commanded on? Or when it goes to 100?

I want to say 100% isn't until 215°, but I do not know for sure.

 

[Bob4x4]

It can be read with a scanner so not something I try to add to the memory bank. My Data storage capacity drops with my age.

 

[Prairie Dog]

Deleted - no time to verify and I don't want to spread possibly wrong info.

 

[AH64ID]

Interesting, I was under the impression the fan was 100% controlled by the ECM.

 

[Prairie Dog]

You may be right AH, I know it's ultimate speed is controlled by the ECM but I'd have to verify the rest - could be my memory mixing things up!

 

[AH64ID]

I'm sure Bob will let us know.

I have heard mine come one once when the ECT didn't warrant it, but the underhood temps were high and I presumed it was from the difference in IAT (pre-turbo) and ambient temp.

EDIT:

I just pulled this from the service manual for my 2005.

The Engine Control Module (ECM) controls the level of engagement of the electronically controlled viscous fan
clutch by monitoring coolant temperature, intake manifold temperature, air conditioning pressure and transmission oil temperature. Based on cooling requirements, the ECM sends a signal to the viscous fan clutch to increase or
decrease the fan speed.

Fan speed is monitored by the ECM. Fan speeds above or below a calibrated threshold will set a DTC. Circuit
concerns will also set fan clutch DTC's.

NOISE
NOTE: It is normal for fan noise to be louder (roaring) when:
- The underhood temperature is above the engagement point for the viscous drive coupling. This may occur
when ambient (outside air temperature) is very high.
- Engine loads and temperatures are high such as when towing a trailer.
- Cool silicone fluid within the fan drive unit is being redistributed back to its normal disengaged (warm) position.
This can occur during the first 15 seconds to one minute after engine start-up on a cold engine.


So what is unclear to me is note #1. Does that happen because the ECM tells the fan to engage or because the coupling gets hot and engages?

It also states manifold temp, which surprises me, I would have thought it would have used pre-turbo temp??

 

[Prairie Dog]

You suppose they mean intake manifold temp via the IAT?
Also the note about temps under the hood causing the viscous portion to engage the fan when temps are high under there suggest maybe air temp alone can engage it?

Wait a minute, you have three note #2's AH! Are you trying to confuse me? :-laf

 

[Bob4x4]

You pulled that up quick AH64ID

 

[BDaugherty]

It's really not clear if it actually overheats (pukes coolant) or is just warmer than the owner is comfortable with. Never assuming anything is always the best way to approach a customer concern,and get as much info as possible to be able to verify the concern.

It didn't get hot enough to puke. The coolant level was in the normal range and the temp was around 230, but the fan didn't "roar" until the a/c was turned on. The owner is pretty good about giving accurate info and he has owned the truck since day 1. I will hook the scanner to it and check for codes and then start running the fan test. I feel pretty good about the thermostat because he had similar symptoms with 2 different ones, but if I have the cooling system open for any reason, I will swap it out again. I'm leaning more towards a fan clutch or temp sensor, but I guess it could be water pump related. Good thing I have a nice heavy trailer sitting in the yard to hook to for testing purposes.

 

[AH64ID]

You suppose they mean intake manifold temp via the IAT?
Also the note about temps under the hood causing the viscous portion to engage the fan when temps are high under there suggest maybe air temp alone can engage it?

Wait a minute, you have three note #2's AH! Are you trying to confuse me? :-laf

From what I have been able to tell in the FSM the Intake Manifold Temp is the temp derived from the MAP sensor on the intake manifold, and IAT is the temp pre-turbo.

That's just how it copied over, they aren't numbered in the FSM. . I removed the little 2's.