How does the shutter stat work?

A few weeks ago, my shutter stat started leaking air out of the little pin hole. I replaced the two smaller orings and the larger black one this week. I took it out for a drive and it seemed to work or at least it didn't leak air. The truck never gets over 160 with normal driving to I thought everything was fine until my trip to Haspin for the rally today.

We took to the trails and the truck was doing fine until one of the larger hills, the temp went up according to the gauge to about 200. After the second hill I was about 220 so I headed back to camp to let the truck cool because the fan never came on. When I raised the hood, I noticed that the plunger on the shutter stat was sticking out the back about 1/4". After sitting for a while, the plunger went back in but then it started leaking air, this time not out the little pin hole but around the plunger.

We took the shutter stat apart again, re-lubed the rings and tried the trails again. Same thing happened.

So how does this thing work? What is causing the problem? What are the parts supposed to be doing?

Any help explaining this would be appreciated to help me figure out how to fix it.

Find out 1st, does air hold the shutters open or closed? Not familiar with your particular truck..... shutters & the fan can work either way: Nominally "open", or nominally "closed". If all air is bled out of the truck, you will know something. Open or closed. Fan free or held tight? On OTR trucks, fan clutches can be over-ridden to get home - usually by screwing a bolt in the back. Same with those shutters. On mine (avatar), air held the shutters closed & the fan engaged. Then there was a shutter-stat. It screwed into the water jacket & controlled air to both. I put small valves in those lines, so I could control things. PROBABLY your air cylinder is bad or your shutter-stat. Trick: a brake shoe rivet, put into the air line at a junction, will stop the air flow when the line is re-hooked up. Sometimes there is an electric switch, thermostacticaly controlled, that controls an electric air valve.

Get your fan turning, 100%, and your shutters open, 100%, and "Keep on Truckin"!

Rosco, when you say

Find out 1st, does air hold the shutters open or closed?

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What does that mean? How do I find out? I know my fan is free because it turns in the wind while sitting with the hood open.

Thanks for the link Stone. I have read that thread along with several others previously and while it was informative, it didn't really answer what I want to know. Unless, I just missed it. It is a long thread and it was getting late.

I guess what I am asking, is how all these pieces function together. When the truck gets up to temp when the fan should engage, what are these parts doing?

Could I have gotten the wrong size orings? The parts all fit together but some are tight.

Does the notch in part A need to line up with the hole in Part B in my pic? I don't see any way to keep them lined up when re-assembling the parts.

How freely should part D slide into Part A? Mine is a tight fit.

My part C is plastic and doesn't have the oring on the end of it. There is a little oring on the inside of the large dia cylinder of part D but I don't know how you would change it.

Are we talking about the shutterstat on the vehicle in your signature?

Swamp Donkey said:

Are we talking about the shutterstat on the vehicle in your signature?

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Yes he is M923a2.

As Dave said, yes.

What Rosco stated is correct, although we don't have shutters on our trucks. Shutters are rarely seen today as they were used on older vehicles as a means to control air flow to the radiator. This would be the same as closing the blinds on an open window.

This is where the name shutterstat came from, as this was used to control the shutters. In our trucks the most correct term would be "fan actuator" but since shutterstat has been used for so long, it has become the norm.

Going with shutterstat, they come in either "normally open" or "normally closed", abbreviated to NO and NC respectively. NC would mean that no air would travel through the shutterstat until activation temperature was reached. Since our fan clutches require air pressure to engage the clutch, our shutterstats are NC. When activation temperature is reached, the shutterstat opens releasing pressure to the clutch so it can engage.

So, why does the little SOB got to be so complicated if all it's got to do is open and close? I know the theory behind it, and can successfully troubleshoot with that, but I'm still working on the part names and the air flow during opening and closing. I sent a request in to Evans Tempcon last week for a air flow diagram so I can see what each particular part does.

Basically temperature change is not instantaneous. Since temperature controls the the shutterstat, the shutterstat would open slowly as the temperature rises. There is a clutch that is engaged by the shutterstat though, and a clutch that is slowly engaged is a clutch that is quickly worn. So it was designed with an internal poppet valve if you will. The poppet valve holds full pressure. When the thermal capsule creates enough pressure to open the poppet valve, the fan clutch is instantly engaged while keeping clutch wear to a minimum.

All this has to be done with a single supply line and it has to be automated. It still has to reverse the process to disengage the clutch. There are a lot of parts to preform a simple task but they all must work in conjunction to preform it properly.

Kinda vague isn't it? As I said, I know the basic operation and how to troubleshoot problems but I'm still working on the intricate details of the inner workings. This is where the diagram I requested would come in handy.

Answers to some of your questions:

1. The O-rings should be tight but that's kind of a feel thing. Tight is better than loose in this case. The O-rings will flatten over time so you need to compensate for this.
2. The orientation of the notch doesn't matter.
3. Part D should be a tight fit into part A. The large O-ring on part D is critical and the cause of most folk's issues.
4. Have you tried pushing the plastic plunger in while it is sticking that 1/4" out? I'm curious if it is pressure holding it open or if it gets stuck while purging.

Enough typing for tonight. Maybe some of this will help.

Ok, one more answer. The little O-ring that looks like it can't be changed, actually can be changed. It takes a little finesse but can be done. I had a shutterstat I picked up a while back that had a cracked housing. Some of the insides were scarred as a result. I decided to figure out how to get to the little O-ring by doing a little surgery.

There are 4 upsets around the perimeter of the hole where the O-ring goes. Those upsets hold in a brass washer. The washer holds in the O-ring. You'll need to get a cutter tip for a Dremel with a perfectly flat end as seen in the picture below. With the end perfectly flat against the washer you can grind the upsets back flush with the perimeter allowing removal of the washer. Use low speed and take your time. That corner around the perimeter needs to stay squared off so you can make proper upsets when you reassemble.

In my case, the piece was no good so I just wanted the washer out to get to the O-ring. It only takes a second to make the perimeter look like mine in the picture. It would be hard to make upsets with it like that.

Take a pick and remove the washer and the O-ring is directly under it. To reassemble, make sure all your shaving have been removed. Push the washer back in until it is touching the O-ring. If you only ground off the old upsets and the rest of the perimeter is in good shape, the corner of a flat tip screwdriver can be used to make new upsets. Make sure the washer stays in contact with the O-ring. Another set of hands to keep pressure on the washer will help here while making the upsets. I wouldn't consider this part of a typical rebuild but if you can visually see something wrong with the O-ring, it might be necessary.

Thanks Swamp.

4. Have you tried pushing the plastic plunger in while it is sticking that 1/4" out? I'm curious if it is pressure holding it open or if it gets stuck while purging.

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I tried pushing in it with a flat head screwdriver but I did not apply much force. I wasn't sure what was going on so I did not want to damage it.

Just to add a little more info, there was air leaking around the plunger, even when it was protruding out the back. Both times, it went back in where it was flush with the end but still leaked air. The first time I know for sure that once the plunger went back in, air started coming out of the pin hole instead of around the plunger.

The M931a2 with a Cummins 6CTA 8.3L engine shutterstat has been reengineered/redesigned for more reliable performance. The older Evans unit has repeated failures and in my opinion, if you have a failure or are concerned about it, I would replace the unit with the new redesigned part obtainable at via Oshkosh for approximately $225 w/shipping. The NOS Evans units usually cost about $250 or more. Here's the OshKosh part link; http://www.oshkoshequipment.com/products/detail/42/604/Fan-Clutch-Thermostat-Shutterstat . Verify your correct part and temperature on their M939 series truck model/part list.

Updated Clutch D=Fan Shutterstat Actuator
Air IN at Aluminum end cap. Air OUT at the blue colored cap(marked OUT on valve body). Third plug w/hole is the vent. Brass nipple and sleeve needed to cover extended thermal bulb shaft. Use Teflon paste to seal these fittings.


Old Evans actuator internal photos. Small air IN debris and O-rings causes repeated failures even with NOS units.

I just found a great source for the Shutter Stat for the M923, and other models that use the NHC250. Only $82. They said that they try to keep a few of these on the shelf. They were out when I called but said that they were going to make some new ones. Took a few weeks and finally received it. Looks great. I had rebuilt my old one with new o-rings but am keeping this new one as a spare.

www.rossairworks.com out of Elizabeth, NJ.

Part number is 1047-35000-29. It is a 185 degree shutter stat.

Note that I have not tried this in my M923 yet. I got this part number from someone on another forum.

goober said:

I just found a great source for the Shutter Stat for the M923, and other models that use the NHC250. Only $82. They said that they try to keep a few of these on the shelf. They were out when I called but said that they were going to make some new ones. Took a few weeks and finally received it. Looks great. I had rebuilt my old one with new o-rings but am keeping this new one as a spare.

www.rossairworks.com out of Elizabeth, NJ.

http://store.rossairworks.com/default.aspx?page=Item Search Results

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You have installed this and it's working properly? What is the part number? Your search link goes to a blank search page.

tracks4me said:

Well gang, my M931a2 with a Cummins 6CTA 8.3L engine also developed a Fan Clutch Thermostat Shutterstat problem with only 220 miles on it and built in late 2012. The clutch fan operated erratically and not at the correct 200 degrees Fahrenheit as specified. So, I drained about 4 gallons of coolant out from the lower right side of radiator, removed the shutterstat air input and outlet hoses and removed the shutterstat. I did a visual inspection and disassembled it by unscrewing(lefty loosey) the Aluminum end cap and the 3/8" thermocoupler nut and shaft to look for the failure causes. I noted no physical damage but did see small particles of debris that had contaminated the interior possibly causing the needle valve and slide to become stuck at times. This would explain why when I tapped on the shutterstat while the truck was running, that the fan clutch would engage. HOWEVER, in my research I noted that the shutterstat has been reengineered/redesigned for more reliable performance. In my opinion, if you have a failure or are concerned about it, especially on hot days this summer, I would replace the unit with the new redesigned part obtainable at a better than NOS price via Oshkosh for approximately $225 w/shipping. Here's the part link. http://www.oshkoshequipment.com/products/detail/42/604/Fan-Clutch-Thermostat-Shutterstat

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I hope you noted exactly how far the thermal capsule stem was threaded in the shutterstat before you disassembled it...

Hi, Swamp Donkey. I updated the post with the part number.

I did not try it out yet. However, I received the part number and source from another poster on the M939 / 5 Ton Facebook page a month or two ago.

Swamp Donkey said:

I hope you noted exactly how far the thermal capsule stem was threaded in the shutterstat before you disassembled it...

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Absolutely. The rebuilt shutter stat has been working just fine for months after the new O-rings. Be sure to clean up all the surfaces in there with some 2000 grit sandpaper or fine steel wool if the surfaces have any buildup or corrosion. Also be sure to use quality O-rings and not just the ones you find at Harbor Freight.

goober said:

Hi, Swamp Donkey. I updated the post with the part number.

I did not try it out yet. However, I received the part number and source from another poster on the M939 / 5 Ton Facebook page a month or two ago.

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Just curious, but why did you decide to go with a 185° instead of the 190°? The thermostat begins to open at 180°, so having a 185° shutterstat doesn't leave the thermostat much time to do its job.

I thought that part number looked familiar. I have the 190° sitting on my bench, yet to be installed. I had intended to test it to see if it was a viable alternative but, as with so many other things, I haven't gotten around to it yet. That's why I was curious if you'd tried it yet.

What is this Facebook page you speak of and how do I get to it?

Swamp Donkey said:

Just curious, but why did you decide to go with a 185° instead of the 190°? The thermostat begins to open at 180°, so having a 185° shutterstat doesn't leave the thermostat much time to do its job.

I thought that part number looked familiar. I have the 190° sitting on my bench, yet to be installed. I had intended to test it to see if it was a viable alternative but, as with so many other things, I haven't gotten around to it yet. That's why I was curious if you'd tried it yet.

What is this Facebook page you speak of and how do I get to it?

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I seem to recall that the TM for my 1984 M923 NHC250 engine calls for a factory 185 degree thermostat. It runs fine out here in hot Southern California with that and the fan only kicks on when climbing hills for a distance. I would just put in the same that came out or whatever the TMs call for.

On Facebook, do a search for M923 5 Ton and you'll see a member page for all of the 5 ton series vehicles.

goober said:

I seem to recall that the TM for my 1984 M923 NHC250 engine calls for a factory 185 degree thermostat. It runs fine out here in hot Southern California with that and the fan only kicks on when climbing hills for a distance. I would just put in the same that came out or whatever the TMs call for.

On Facebook, do a search for M923 5 Ton and you'll see a member page for all of the 5 ton series vehicles.

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In reference to the thermostat:



In reference to the shutterstat:



Yeah, we have the same truck. I was mainly curious how it was working with the slightly lower activation temperature. Every shutterstat I've pulled on the M939 and M939A1 has been a 190° unit. It's good to know that it works right though. Alternatives to expensive and sometimes hard to find replacement parts are always a good thing.

These newer ones do pop up on ebay from time to time. I got the 190° new for $34 with free shipping in February. He only had one from an overstock sale though. At the time I figured wasting $34 if it didn't work was a good compromise in the hunt for an alternative. Good to know that I'm $34 ahead instead, since I haven't had time to test it.