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Checking the FMTV fan system

tennmogger

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Here are some notes on the engine cooling, fan system components.

The temperature switch is located on the coolant piping in front of the engine. This is a switch, not a sensor, and has two wires. It is a Kysor 07950, 200 deg, NC-normally closed when cold (reads zero resistance), two wire. In operation, the switch will be closed unless the coolant temperature goes above 200 deg F, then the switch opens. Check the resistance with an Ohmmeter for near zero Ohms.

So does it open above 200 deg? The switch can easily be tested. Borrow a pan of water, set on the stove, insert the switch, and boil the water. The switch should open about the time the water boils.

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Yeah, I still prefer an analog meter. The needle to the right is switch closed, to the left, open, with water boiling.

This switch was working.

With the switch closed, 24v DC is applied to the fan solenoid valve (behind the passenger kick plate), and air pressure is applied to the fan clutch. The fan is driven only by the viscous coupling unless the coolant is over 200 deg.

This is the fan solenoid valve: Take off the kick plate and the solenoid is hanging in the upper right side, easily reached from a stool/stump/concrete block on the ground.


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The easiest-to-reach tube is the supply of air. The hard to reach tube is the tube to the fan clutch. Remember, there is power to the solenoid under normal operating conditions (less than 200 deg). There is air to the fan, unlocking it's clutch. With engine off, the fan can be turned by hand, feeling the viscous coupling. Remember, there has to be air on the system for this to work.

Find the wire from the solenoid, mark the socket it came from (number 34 on mine), and unplug it. This removes 24v from the solenoid and the fan solenoid should dump, and fan clutch should connect and run under power with engine on.

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The air dump can be tested with engine off. The fan will not turn by hand after air is dumped. This explains a discrepancy in some of the threads: will the fan turn or not? It will turn if there is air on the system. The fan will not turn if there is no air because the clutch is locked.

It's fairly obvious if the temperature switch is not closed while cool. Replace it.

So what if the solenoid does not work, and this is apparently a common problem. The solenoid can be removed in two ways, either complete assembly (which I preferred), or taken apart where it was mounted. I did it both ways as seen in the pictures.

Four bolts can be removed and the mounting bracket comes out. One bolt is hard to get to (!!) and I did not replace it. There is no clearance underneath the tubing fitting to the fan air hose (left rear as you look at it).

Then unscrew the two hoses and unseat them from the fitting. There will be air loss. Best to bleed the air system first.

054.jpg055.jpg


The solenoid can be tested with a 24 volt source. I used a 24v radio battery. Make and break the connection to the connector of the solenoid. If your solenoid is like mine, it will not cycle. Have to take it apart. If it does cycle the result is really subtle. Some people report unsticking their solenoid with application of oil.

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There are four screws through the rear of the solenoid coil assembly, the black rectangle. Two of those have no glyptol/point in the slot. Unscrew those. The solenoid will be free of the base where the tubing fittings are located. There are seals in between so work in a good spot to catch loose items.


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Remove the remaining 2 screws, the ones with glyptol. The solenoid comes apart and there are small parts and o-rings and the valve seat and seal.

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There is access to the valve solenoid slug with the seal in the end. The slug was sticking. I lubricated it then reassembled.

With the solenoid reinstalled but not plugged in, applying 24v to the solenoid would cycle the fan on and off.

When plugging in the solenoid, notice there are two available sockets! Hope you marked the right one to use.



058.jpg

But, if you didn't, you can start the engine and measure the voltage between the contacts in both those sockets. One will have 24v on it. That's the one to plug in. There is 24v on this one coming through the temp switch.

Once it's all back together you can test your work by unplugging that connector again. The fan should turn on.
 
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Floridianson

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Working on mine now. Fan is locked up when aired up. First problem is someone nice disconnected the air line off of the fan. With the line off it should have been leaking air out of it when truck is aired up but not mine. OK Mine is a three air line system 1997 M1088. I took it apart and both plungers are free and O rings good. Still not working. I will try the Temp. switch and see if it is staying on. Thanks for the right up has helped a lot. Will report back with fix or just buy another truck.
 

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Floridianson

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Yea I am not getting any air out of the line that connects to the clutch. I hope it is just the water temp switch, If there is a big problem I will just convert it to a manual air switch. Something like that is on a Deuce front axle ingagement. They also make one for the drop axle or air tail gate's on dump trucks.
 

Floridianson

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Ok something strange. I unplugged the Temp. switch wire engine running and my rear fans came on and the engine fan was already engaged. I plugged it back together and the rear fans stopped and the engine fan disengaged for the first time. When I had the Temp. switch unplugged and the rear fans running if I used the in cab fan rocker switch it shut off the rear fans. I don't think it fixed its self? Yes I know the rocker switch does stop the engine and rear fans when everything is correct.
 
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NDT

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You said "fan is locked up when aired up". This occurs when the clutch sticks and often corrects itself, but sometimes not. If not, see my thread on the subject.
 

TNriverjet

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This is great information. My fan is always locked. I'm hoping a few percussion impacts on the clutch will release it as NDT describes in his thread. If not, I will start troubleshooting the electrical switch and air solenoid as done by tnmogger.
 

TNriverjet

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Well, the temp switch checks out. I pulled off the solenoid and did a disassemble with clean and lube. When I reinstalled there was a constant air leak at the bottom port. I don't think my solenoid is working properly. So, today, I took it back out and accidentally broke off the small air fitting threads inside the solenoid. IMG_3280.jpg I was able to remove the threaded piece, but no one in town has anything like it. I am now looking for a replacement solenoid. I would also like to put air to the fan supply line and make sure my clutch is working, but haven't figured out a good way to do that yet

For a temporary fix I plugged the ports of the solenoid with small bolts and teflon tape. As expected, the fan is still locked up as it always was, so no change there. I was then still able to drive the truck today.
 

Smike740

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How far apart did you take the solenoid? The black plastic body which houses the coil can be seperated from the brass piece and the valve core and spring can be removed and cleaned. I had the same symptom as you and was able to pull it apart and clean up the valve core, now everything is working as it should.

New replacement valves are available from Burkert Online, i am not sure if they have the fitting you need, perhaps try mcmaster carr.
 

TNriverjet

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Thanks! I did completely disassemble the solenoid and clean/lube. The biggest problem is that I broke the smaller air line fitting (supplying the fan) upon reinstallation. Even buying a new solenoid, I would have to switch my fittings to the new one. At this point I've emailed Will (suprman) and he is checking for one on a parts truck he has. I would also buy someone's old unit just for the fittings if anyone has one available.

On another front, I was able to get 120 psi on the line to the fan with my portable air tank. Unfortunately, the clutch would not disengage, so I think I found my real problem. My son and I did try hitting the clutch both with air applied and without. It did not break loose as some others have reported. I did not have time to take the shroud and fan apart, so it will have to wait until next weekend.
 

mkcoen

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If you do wind up taking it apart make sure to inspect the fan hub carefully. Had I been warned before hand I probably would have switched to the new fan clutch and fan when I had everything disassembled for the AC pulley install. The new style is much more robust than the original setup.
 

Rene M

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A few notes from personal experience.

A kysor switch will not read steam only liquid. if the coolant is low the switch will stay in to open " unlocked mode".

If the vent " which looks like a plug with grain jammed in it" is blocked the valve will not work.

In the old days we used to rebuild the clutch in the field. the kits are still available and it is a very simple procedure.
 

Rene M

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Oh, there is a new style air solenoid far more common and a ton less prone to failure.. Let me get a number and a picture for you guys.
 

Kswiney

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Thanks! I did completely disassemble the solenoid and clean/lube. The biggest problem is that I broke the smaller air line fitting (supplying the fan) upon reinstallation. Even buying a new solenoid, I would have to switch my fittings to the new one. At this point I've emailed Will (suprman) and he is checking for one on a parts truck he has. I would also buy someone's old unit just for the fittings if anyone has one available.

On another front, I was able to get 120 psi on the line to the fan with my portable air tank. Unfortunately, the clutch would not disengage, so I think I found my real problem. My son and I did try hitting the clutch both with air applied and without. It did not break loose as some others have reported. I did not have time to take the shroud and fan apart, so it will have to wait until next weekend.


Did you find that brass fitting. I broke mine as well the small one.
 

TNriverjet

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Did you find that brass fitting. I broke mine as well the small one.
No, I searched parts houses all over town. I ended up buying a used solenoid from suprman. He actually sold me a double solenoid unit, so I now have additional spare parts. PM me if you can't find what you need.
 
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