Fan Trouble 2

layoutshooter · Apr 12, 2008

I had the A/C on my truck checked out. All the switches/sensors are working. The coolent
is at the right levels and the pressures are correct. The fan still comes on and runs the entire time the A/C is on. On Freightliner told me that on the MB engines it could be a problem with the EGR or EGR sensor, OR , the thermostats need replaced. The engine has been running cool for a while, will replace the thermostats soon. If that does not fix the problem we will have to look at the EGR. Layoutshooter

piper1 · Apr 12, 2008

Lay, it sounds like the high pressure switch is triggering the fan. The switch may be "working" but is it working properly and for that matter is the A/C ok. If the fan problem only happens with the A/C on then don't look any farther than the A/C.

In most trucks, the fan is turned on when the pressure builds too high on the liquid or high side on the A/C. Once turned on it is then usually time controlled by the ECM to turn off after the pressure drops. A standard setting is 180 seconds.

I would get them to put a set of guages on the a/c and ensure the pressures are normal. The high side should fluctuate between 120 and 200 or so if the system is ok and the low side between 30 and 70. If the high side pressures are too high, you have a plugged condensor or the system has too much freon in it. A good rule of thumb to check for too much freon is with the system OFF you should have a psi reading near or slightly above the ambient temperature. When running if the high side is too high and the low side is too low, the expansion valve (sometimes called the block valve) needs replaced.

If the pressures are ok then, with the system running, unhook the high pressure switch wires. if its a NO switch the fan should quit (remember the timer) if it is a NC switch, it you short the wires together on the truck end of the plug the fan should quit (again after the timer expires).

Unfortunately, A/C is another one of those things that if you don't end up with the right mechanic it will get expensive. A/c and it's controls are fairly simple if the guy understands the concepts involved.

Bottom line, you only have a problem with the A/C on, so don't look any farther than it and how it controls the fan. If the engine is running cool, thermostats will not solve your fan problem.

Frustrating isn't it?

layoutshooter · Apr 13, 2008

According to the A/C tech all the pressures were within specs. They spent several hours running tests on it both with a cold engine and when it was up to temp. They checked the figures with 2 other shops, all agreeded that all the pressures were correct. The one shop told me about the EGR, thermostat and ECM that might be causing it. Who knows. All I know is that is driving me nutser than I already am. Layoutshooter

piper1 · Apr 13, 2008

Guess we might have to chalk this one up as one of the joys of owning a truck.

Here's a dumb question I should have asked,

Does your fan have an on-off clutch or a viscous clutch?

layoutshooter · Apr 13, 2008

WOW, you lost me on that one. Where would I find that out? My manual sucks, it does not even have the basics in it. Like, no fluid capacities etc. I don't even know what you are talking about on that one. Could you please explain that in terms that a mechanical dummy like me can understand? That way I can insure that I will learn something today!!!! LOL
Thanks for the help, I need all I can get. Layoutshooter

highway star · Apr 13, 2008

piper1 said:
Does your fan have an on-off clutch or a viscous clutch?

Would a viscous clutch just verbally abuse you, or would it try to physically assault you?

layoutshooter · Apr 13, 2008

I am not sure but I bet if you put your hand in there you would get bit pretty bad!!!
Layoutshooter

piper1 · Apr 13, 2008

I've never been verbally abused by a viscous clutch but I have had more than enough physical abuse from them, the do bite really hard!

An on off clutch is simply that, usually air operated the fan is either fully on or off. Easy way to tell is with the truck shut off if you grab the fan it will either freewheel or be locked. If you can turn it but feel some resistance it is a viscous clutch.

Viscous clutches usually sense the air temp behind the rad and will ramp up and down in speed to control it. They are much more common on smaller engines. Viscous clutches are almost always NOT electrically controlled and work solely on the air temp behind the rad.

Dunno what you have as I can't remember what you have for a truck.

layoutshooter · Apr 13, 2008

Thank you for the info!! I learned my one thing for the day!!! I have a M2 business model
M112. It has a MBE 900 450HP. It is a on/off fan. I KNOW WHAT YOY MEAN NOW!!!!!
Thanks again, Layoutshooter