Fan Clutch Running at full speed non stop. Need places to look

[Jeff Bayley]

My 1997 Royal has the clutch on the radiator fan running at full speed non stop. We know the ECM is not turning it on. We know the relay's and breakers are fine. If we turn the ignition off it disengages and turns freely so we don't think it's the actual clutch component. We replaced the engine temp sensor that was faulty for the last 1,000 miles and were hopeful that would remedy the fan but it appears not.

What else to check ?

The high pressure on the A/C system was considered as a suspect but the tech reasoned that it was not showing up on the ECM. But as we are composing this post together I let him know that the dash air is a converter item and probably would not show on the ECM. The service tech would like to know there the pressure switch would be located to check it.

Look forward to help here in western MN where the corn is plentiful and the techs are honest and trying to help me.

[ajducote]

Jeff,

This may not be much help, but on my coach when I turn on the dash air, the clutch locks up and stays on until I turn off the dash air. It may be possible that the circuitry that controls the fan thinks the dash air is on?

When the fan clutch is on, is the A/C compressor also on?
On my coach when I had my dash air fixed I needed a control box replaced that was in the dash area because my fan clutch would not engage when I turned on the dash air. Maybe your control box failed in the other way. It never turns off the fan clutch?

[truk4u]

Jeff,

The dash air is a Prevost item, not converter installed. Like Andre suggested, if the dash A/C is on, the fan will run continuously.

[Jeff Bayley]

Thanks for our replies. Hope to get a few more by mid day Thursday before shoving off from this shop.

I think you guys might be right about it having something to do with the dash air which is why the tech would like to check the high pressure switch if he can find it. I know the fan will run if the dash air is on. It is not on, but the fan continues to run and I am getting lousy mileage. I've been checking it between fill ups and have been getting between 5 and 5.5. I can't say how long it's been running full time but I've gone at least 5,000 miles on this trip so far so the cost will continue to rack up if I don't rectify it.

How do I locate and test the control box Andre speaks of ? Am I going to need to get to Prevost for this because I'm in western MN and not close by but Chicago has a place called Central States Coach that I like and is pretty good. Right now I'm at a Freightliner dealer that is Detroit certified and the guys are pretty good (and affordable) but they're at an stand still on this unless I can get them pointed in the right direction again tomorrow. Besides the dash air, my coach has an auxiliary fan and condenser (I think it's a separate condenser) that is all the way up high near the ceiling with a separate 4 speed switch for the fan for that 2nd unit. I know that works off the compressor for the dash air also. I'm fairly certain the A/C switch on the dash must be on for the 2nd blower to produce cold air just as the dash air would. What I don't know is if there could be something faulty inside that 2nd upper unit besides the possible control box Andre speaks of but with A/C switch on the dash off I don't think that 2nd blower unit is the culprit. Seems like if it's A/C control box related that I maybe should be getting A/C 100% of the time if that's what is kicking the fan to run 100% of the time. Would that jive ? Guess the answer is "maybe so maybe not". It was suggested that it might be a bad ground wire and they looked breifley but I stopped the blood letting on the clock and said I would see what if I could find something out form the POG brain trust.

[ajducote]

Jeff,

The control box is a Prevost part. Mine was under the dash near the entrance door. I had my A/C fixed at Excalibur Coach in Sanford, FL. Doug the owner had to use some Prevost wiring diagrams and a couple of jumper wires to determine the part was bad. If I understood him correctly, it is basically a box of relays that is used for the dash air and heat system. The dash air and the one up by the ceiling are all 1 in the same system. The compressor on the engine is turned on by the switch on the dash and it pumps freon to both the dash and the unit by the ceiling. The 4 postion fan switch just runs the fan up top. Most of the time I do not run that fan (the top one) because it is very loud.

My low and high pressure switches are behind the small door just to the rear of the drivers side tag wheel. Now that is on a 45 foot coach. If you have a 40 foot it would be somewhere else.

You could check the dash switch to make sure it is OK by pulling one of the wires from it an see if the fan clutch disengages. It might be a bad switch.

[Jeff Bayley]

Andre- you wrote " Mine was under the dash near the entrance door. I had my A/C fixed at Excalibur Coach in Sanford, FL. Doug the owner had to use some Prevost wiring diagrams and a couple of jumper wires to determine the part was bad"

Do you mean inside the coach behind the dash or access it by dropping the bumper? I think a call to the Prevost hot line is my next move but the more ready I am with the parts located and ready to test the better.

[ajducote]

Jeff,

I was behind the dash panel inside the bus, near the entrance door. I will try to find ny receipt and it might have the part number.

[Jeff Bayley]

I just removed that access panel last week to clean the dash air intake filter. I presume that's the same one. You take the lighted grab bar off first and then it slides out toward the door. Is that the way to get to it ?

By the way, I need new lights for inside the grab bar. I ordered them up before for the window lights but several years ago. Think the place that makes them up to your length is in Elkhart. Anyone know the name of the place ?

[ajducote]

I just removed that access panel last week to clean the dash air intake filter. I presume that's the same one. You take the lighted grab bar off first and then it slides out toward the door. Is that the way to get to it ?



Yes, that is it. After you take the panel off, look for a box about 6 inches long, 3 inches wide, with a bundle of wires attached.

[Jeff Bayley]

After a lengthy process of Prevost Hot Line tracking down a tech he put me on hold after we were just getting into the meat of things and disconnect followed. I found out that the fan stops turning when the test light goes to the post of relay number 84 in the rear compartment breaker box. Is this the final conclusion that there is a shorted wire that has to be hunted down some where between the ECM and that relay ? Fun. If other possible (simpler) possibilities anyone might suggest please tell.

The good news is I need to the dash air at all times anyway in this MN heat. So I'm getting my money's worth out of the fan running at least since I need the air.

[Mark3101]

Where in MN are you at? Just curious as we live in Duluth.

[Jon Wehrenberg]

Jeff, Certain comments slip right by my brain, and then all of a sudden they come into my awareness with a loud noise. Kind of scary.

But I wanted to comment on a comment you made. I understand when the AC is on the fan engages. I am not clear if it comes on when the compressor cycles, or if it is on when the AC switch is on.

Drawing upon my own experiences the worst mileage I have ever gotten is on this last trip we took to Maine. The roads were hilly, I was towing the H2, and the bus OTR was never turned off due to the outside temps. We got 7 MPG.

Under more average conditions I usually see 7.5 MPG and if it is warm the OTR is on I do not believe I want to experience even the slightest discomfot so I am not shy about running the OTR, and when we hit the rest areas for lunch I turn on the generator and run at least two AC units. It makes the toilet seat cold, but thats the price we have to pay for comfort.

My point however is that if our fan is on when the AC is engaged it will not drop our mileage to 5 or 5.5 MPG. How fast are you driving? Are you idling a lot? Are you towing a heavy vehicle? For a Series 60 of your vintage you should see no less than 7.0 I may be wrong but I'll bet others in the 95 to 98 or 99 vintage all see at least 7 unless they drive at 70 or more.

[jack14r]

Jon,OTR does not turn on the fan but dash air commands the engine fan to run.The OTR has the 24 volt condenser fans.

[Jon Wehrenberg]

Jack,

I understand the OTR may not always engage the fan, and that we have the 24V evaporator and condenser fans. But no matter how we slice it, we are likely loading the engine, maybe more so than just driver's air.

That 24V to the fans is not free. The alternator is loaded to supply the power and those fans do draw a lot of power. At the same time we are loading the engine with the big compressor. As a result the engine is being asked to produce a lot more power than normal to drive the alternator and the compressor and as such is generating more heat which in turn is engaging the fan.

What I do not know is if there is any comparison between mileages on driver's air coaches and OTR coaches. It seems counter to my rationalization that Prevost would design a system that blows air in the front only, but uses more fuel than one designed to cool the entire coach. I suspect the driver's air coaches engage the fan only when the compressor is running. So its duty cycle shouldn't be much different than mine. But if it is, Jeff's fan is puilling about 30 HP (a guess), while I am likely doing much worse driving a loaded alternator, the compressor and added fan engagement. Just a guess.

[Jeff Bayley]

I can't comment on whether my fan turns only when the compressor engages or when the switch is on because it's turning 100% of the time now but Jack seemed to indicated that the fan will run with the dash A/C switch turned on. I didn't start checking my mileage until half way into this trip so I don't know how long my fan has been in error but the mileage is 5.5 at it's best. I keep track of the generator hours and account for .5 gallons per hour on it. I used to have a led foot but my speed is around 60 and on cruise when ever possible. What else could be dragging my mileage down besides this fan ? My estimate of 5,000 miles this trip is about right. The difference between the 1.5 miles per gallon translates into an extra 195 gallons or between $500-$550. I'm waiting here for the tech to probe just a bit more for the bad ground wire. He grounded the test light to relay's 78 and 79 (high and low speed fan relays) in the rear bay and the fan stopped. But I'm not going to invest too much time in letting him look for it since I'm running the dash air anyway for now and I'll try to track the bad ground myself later when it's not 90% with the same humidly.

[Jon Wehrenberg]

Somewhere on this forum someone posted things that impact mileage such as speed, weight, etc. I suspect that for your mileage to be reduced by 21% (from 7 mpg) more than just a fan being engaged is the cause.

I don't know if this is relevant, but when the 8V92 engines were being used some coaches had radiator shutters and some had none. The fans on those coaches were always being driven and the shutters would increase fuel efficiency by blocking air flow,

For those that want to see the impact of shutters on how much drag a fan blade has, stick your hand over the end of the vacuum cleaner hose and you will hear an immediate increase in motor speed. By blocking air flow a vacuum is created and the motor spins easier in the vacuum.

My point however is that there was nowhere near a 21% gain in mileage on coaches with the shutters compared to those without. If the fan did cause a 21% increase in the load as soon as we started uphill in our coaches and the fan kicked on we would immediately feel the engine lugging down and I have no clue from my driver's seat to tell me the fan is cutting in or out.

[Jeff Bayley]

I found the cause of my fan running 100% of the time. Detroit Diesel put a new ECM in about 4 years ago. They either put the wrong ECM, or failed to use the correct adapter. One of the 5 or 6 connectors was never plugged in. It was tie wrapped up. We traced wire number 555 from the relay all the way down to the ECM looking for a break in the wire with a tone generator. It lead us all the way to connector that was tie wrapped up instead of being plugged in. The technician not only deliberatly sent me down the road with a "will fit and start the engine" ECM, but what else on those 5 pins was not connected that I don't know about ? The plug that was not connected to the ECM has 2 wires. So one other wire never made it back to home plate either and I haven't figured out what it operates yet. There are 5 pins total on the plug that is baron with nothing on it (on the ECM side). There's another plug tied up as well which may have connected to the old ECM.

Of course I now have to go back to Detroit and explain to them that I can't hear whether my fan is running from 42" away and it took my 4 years (only 20,000 miles though) to finally have a shop point out to me that they observed the fan running non stop. I wonder if they bothered to download the right software which I found needs to be matched to the serial number of the engine and corresponding rear end and other drive train related which may or may not have variables. I would really like to have someone else verify their work before taking it back to Detroit to know if they A) gave me a suitable ECM and B) they downloaded the right software version to the ECM prior to install.

You go to the "certified" and "authorized" and "factory trained" shop and you still take it up the short hairs. You pay full retail but get shade tree work. The more I tinker with the coach, the more I see why some members try as hard as possible to do all the work they can themselves.

[johnklopp]

Jeff,
Interesting problem. I'm a little surprised that the engine fan is commanded on to provide condenser air flow. Obviously condenser air flow is required, but to supply it by delivering a huge amount of air flow that mostly goes around the condenser is crude. The reduction in millage could be large and it will be interesting to see how much of an increase you get when the fan cycles with the compressor.

I'm going to take a look at my coach and see it there is and easy way to add eclectic condenser fans typically used on most cars to eliminate the radiator fan operation. I believe 2 small 12 VDC fans 10 or 12 inch diameter would provide the required air flow in moderate temperatures. In high ambient locations the radiator fan will provide additional condenser air flow based on engine temperature. Control of the 2 small condenser fans will be provide by pressure switches set to turn on the first fan at a refrigerant pressure of 150 PSI and the second fan at 175 PSI The second fan would cycle off at 150 PSI and the first off at 125 PSI. The entire modification should only cost around $150.

[Jon Wehrenberg]

I would be interested in seeing the results of that. One thing I have trouble with is that most folks treat such items as the engine cooling fan, OTR compressor, and the generator as fuel consumers (ya think?), but ignore that electrical loads on the alternator cost fuel also. The only difference between any of them is which is most efficient? I don't think anyone had documented the cost in fuel running the same load, but from inverters, generator, alternator or engine.

[johnklopp]

Jon,

Good point, however the savings will result due to the reduction of total air flow. If the mechanical drive and electric drives were considered to be equal efficiency, the electric condenser fans would use less power. The reason is the radiator fan is moving perhaps 1000 cubic feet of air per minute (CFM) and the condenser fans 200 CFM. The electric fans will use 20% of the power to run the radiator fan based on the rough estimate of airflow. Obviously the radiator fan will run for an undetermined amount of time and no savings would take place during its normal operation. Just to add confusion to the equation, the CFM requirement and the horse power to create the airflow will changed due to elevation. The reason is that the reduced density will require a proportional increase in mass flow to achieve the same heat transfer.

[Jon Wehrenberg]

I have OTR so I don't know where or how the driver's air condenser sits and how air flow is directed across it, but I suspect all our coaches have a radiator and a charge air intercooler at the least, and possibly an oil cooler for the power steering.

That means with no AC on, the engine driven fan is going to be operating some percentage of the time, negating the need for supplemental cooling. The DDEC temps will control that if the AC is taken away from it. I have no clue what percentage of the time my engine driven fan runs, but I suspect it is a high percentage. This summer with 90 plus degree days it was likely engaged at least on the low setting most of the time, and all of the time when the bus was going up a grade.

So the equation necessary to determine if there is a potential savings with an electric cooling fan to supplement the bus engine driven fan is more difficult to develop because when it is hot out the bus fan is running a lot and when it is cool out you will not be running AC.

If the purpose is to save a few CCs of fuel why not go all the way and tap into the large amount of fresh water on board and just add a mister system to spray water across all heat exchangers and just fill it up at camgrounds every night?

[travelite]

On the S60, typically the fan has two speeds, "fast" and "really fast". On my coach fast rpm is 80% of engine rpm and really fast is a fixed 1800 rpm. What speeds does the Prevost S60 fan spin at? I'm wondering if "fast" is sufficient for the chassis A/C.

The DDEC III and IV turns fan speed to "really fast" ( that is, full speed) at 205 deg F and back to "fast" (that is, partial speed) at 195 deg F or after 180 seconds.

[Jon Wehrenberg]

At the rally Bill Jensen of Prevost would be able to answer the question of speeds. I can't.

Whatever they are, they are clearly adequate because in this summer's heat, towing the H2 with the OTR air on so the engine is pulling hard plus driving the compressor going up long steep grades I could watch the temps climb, the fan kick in and the temps drop down and stabilize. The OTR system uses a large pair of DC motors to drive the condenser and evaporator fans so the alternator is also loaded. I reset the ProDriver with every oil change, so using that as a guide to mileage I average 7.5 mpg, and that value was higher before the diesel fuel specs changed so I am sure the cooling fan has an impact on mileage, but I still suspect, in order of importance, the factors that influence my mileage are speed, strong winds, especially head or tail winds, hills, towing a heavy toad, stop and go driving, running the OTR and excessive idling.

If I could track the fan operating time it might show up on the list, but I do know the engine gets better MPG in hot weather than in cold weather, all other things being equal so increased fan operation in the summer may be offset to an extent by the impact of colder temps on mileage.

[travelite]

Whatever they are, they are clearly adequate....

Jon,

I could have been clearer. What I meant was can the chassis air operate sufficiently on the slow fan speed setting; i.e., is the fast speed really necessary? To answer this we need to know the actual fan speeds and then we can estimate the CFM of air moved. It would be useful to characterize the Prevost fan speeds. I suspect that minimum air flows are toughest to achieve when the engine is idling in stop and go traffic; it would be interesting to see the difference in slow and fast fan speeds at engine idle . Anyone know what the fan speeds are from idle to 1800 rpm on both the low speed and the high speed settings. A photo-tachometer can give you this information.

[Jon Wehrenberg]

This is solely a guess and I would love to find what is really happening, but I suspect we never see high fan speeds at idle or stop and go driving.

If I am remembering accurately I don't see the highest fan speed until I am pulling under load, such as high road speeds or going up hills. I don't think our engines generate a lot of heat at idle, but as horsepower is produced, the engine inefficiency in converting fuel to horspower shows up in the form of unusable heat that we get rid of via our cooling systems. If we are not producing much horsepower such as in stop and go driving or idling I think we have significantly less heat to get rid of thus less fan operation.

If I recall someone added a pair of lights to the dash to show when the first and second stages of fan operation were happening. That is what we need to get a better idea of the length of time the fans operate and in which speed range. But as to calculating the air moved based on fan speed I think in Prevost land we would have a problem. Last year Roger, Jim Keller and I were comparing our cooling systems in an attempt to see if we could find the differences in cooling that might relate to a question Jim had. We were all surprised to find in my 97, Jim's 99 and Roger's 03 that there were differences in fan blade length and more significantly in the clearance between the radiator shroud and the fan blade tips. We got into this because Jim was hearing a noise and felt a vibration he thought was coming from his fan kicking in. Our quest for knowledge led us into trying to figure out why one coach would have a big gap between blade tips and the shroud, and another would have minimal clearance suggesting one had a more efficient air flow than another. We also tried to figure why there would be a difference in fan diameter.

So the bottom line is an answer about fan speeds that might apply to my coach, may not apply to Jim's or Roger's.

I can say with some degree of accuracy that if I run my coach hard enough to get the second speed to kick in on the fan, by the time I get it slowed down and stopped so I can measure the fan speed using a hand held tach I will be below the high speed temperature and without putting it back in motion and up a steep hill or at high speed it will not get hot enough to kick on the second stage. I don't have driver's air so I cannot engage my fan by turning the AC on (I don't think).