Fan runs all the time, think it's the fan sensor.

Based on the temperature gauge my fan used to cycle on about 220 and then off around 210. Then it started cycling on and off once then stayed on all the time.

Based on my troubleshooting it appeared that the fan sensor on the water bridge was not working properly. When cold fan was off, could turn on by unplugging the TDM. When it warmed up enough to turn on the fan I could unplug the fan senso, connect the wires together and that would turn off the fan. Seemed pretty clear that the fan sensor wasn't doing it's job.

Ordered a new sensor from Oshkosh. While waiting for it I borrowed one from another HMMWV since we were driving 60 miles to a parade on the 4th of July. Just put it in the night before so the trip was the test. Well, the fan never came on at all. Had to unplug the TDM and let the fan run all the time.

Received the new sensor from Oshkosh. Installed it and went for a test. Fan came on early, about 180 on my gauge. Of course it ran all the time then because it never cools down below 215 on the gauge with the fan running.

Called Oshkosh to see if they had any thoughts. They had no idea and told me to call the Hummer guy across the street, we do business with him all the time, he should be able to answer your questions. Thought that was strange, but made the call anyway. Hummer guy was courteous, but did not want to advise on another vendors parts. Completely understand.

Next I set up a test to monitor voltage on both sides of the fan temp sensor. At startup 27.2vdc on both sides. When it reached 180 on my guage the voltage dropped to 15.3 on the TDM side and the fan engaged. My infared temp sensor said the temp was more like 160. So of course the fan will never go off because the temperature never goes back down below that threshold with the fan running.

If you floor the accelerator the TDM is working, shuts the fan off for 15-20 seconds.

Seems to me the new fan sensor is bad (changes state in the wrong temperature range)? I sent a couple of emails to Oshkosh asking for an RMA to return the one I purchased. Have not recieved any response. Not sure I want to buy any more parts from them.

Is my logic / troubleshooting correct here? Fan sensor should allow volatage through keeping the fan off until about 220 degress, then cut voltage to turn the fan on until the temperature goes back down to around 210?

I'm dealing with something similar.
As I understand it, it's possible for some functions of the TDM to work and others not.
I'm going to order a new TDM as soon as I have some cash to spare.

What I can't get past is that if bypass the fan switch and close/open the connection manually the fan works as it should.
Here's some pictures of my test and the new fan sensor I purchased.

So the question is did you replace the Thermostatic switch or did you replace the TDM

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I replaced the fan sensor and it does not appear to operate at the right temperature range

Picture 1 or picture 2

WantOne said:

I replaced the fan sensor and it does not appear to operate at the right temperature range

Click to expand...

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Have you checked to see if the radiator is hot? If the thermostat doesnt open, the fan will keep trying to cool the engine.

Action said:

Have you checked to see if the radiator is hot? If the thermostat doesnt open, the fan will keep trying to cool the engine.

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And the temp gauge would show very hot on an engine with a bad thermostat. Sure sounds like either the TDM isn’t sending power through it when the thermostatic switch tells it to (when it is cold enough) or the switch itself is never closing once it opens.

Upper radiator hose is hot and has pressure in it.

Put my original fan sensor back in with the same volatage monitoring setup. Fan off initially, power on both sides of sensor. When my temp gauge gets up just above 220 fan comes on and I see the voltage go away on one side of sensor. Fan stays on even though temperature decreases to about 215 on gauge, I never read more than 185 with infrared thermometer around the sensor and thermostat area. After stopping for fuel fan was off initially then came on and stayed on. Does seem that either the fan sensor is not working as it should or the engine temperature is not getting below the threshold. The sensor is marked 215 degrees. The new one I tried was even worse. Came on earlier. With fan running my temp gauge is 215-220, again readind with infrared at sensor and thermostat housing never more than 185. Will try a new thermostat I guess.

Make sure it the right one that Humvee specific high flow. I found in mine it was the wrong one. Check your power steering fluid level as this is what runs to the fan clutch. If fluid is low the fan will stay on. This starting sound like the Cadilac valve is sticking. Went through the same issues.

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I checked the power steering fluid level, it’s good. If the Cadillac valve was sticking why can I manually turn fan off any time it’s running by flooring the accelerator and engaging the TDM? Turns off for 15-20 seconds then comes back on. I can also turn the fan off when running by bypassing the fan sensor and connecting the two wires together.

Not sure why it matters where I bought the fan sensor. There are 20 different places you can get them. Even thought I purchased from Oshkosh it was advertised as the correct replacement for the HMMWV as you can see in the pictures I posted. Not sure I will purchase from them again as they have been completely unresponsive to my emails after the purchase.

I had the same issue last year, replaced the sensor, time delay module,cadillac valve, and fan clutch with no results. Ended up being the NOS replacement cadillac valve was faulty and a second replacement fixed the issue. Here's hoping your fix comes easier

General Hood said:

I had the same issue last year, replaced the sensor, time delay module,cadillac valve, and fan clutch with no results. Ended up being the NOS replacement cadillac valve was faulty and a second replacement fixed the issue. Here's hoping your fix comes easier

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Thermostat, Thermostatic switch, TDM, Have New Cadillac valve, not installed. Got better on cooling. Replaced the powerstering cooler and powerstering pump. Drastic improvement can't tell you why.

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It’s not a fan sensor switch....it’s a thermostatic switch. It helps to use the correct terminology for the part when searching TM’s and ordering parts.

Lots of great responses. Haven't read this in a while and got to say... Read the book. https://hummerhuren.files.wordpress.com/2013/01/hmmwv_test-diagnostics.pdf

This is the easiest, straight forward, free, how to fix it, with pictures, (that I need) HMMWV manual. Right out of the gate, Section 1, COOLING SYSTEM. FAN THERMOSTATIC SWITCH TESTING.

YES, these switches go bad, in all the ways that a switch can go bad, Open, closed, intermittent, works, use to work and the list goes on. It is KEY to the fan system working with relation to engine temperature and as long as all the other parts are OK. It is a N.C. (normally closed) switch. COLD closed, HOT, open.


If switch is suspect AND ALL of the other parts are good ( BIG IF HERE) Remove both of the 458 leads to switch. Start truck, keep hands in pockets. Fan should engage. Remove hands from pockets. Next place short (shorting) jumper wire between the leads removed from switch leading to engine harness. Fan should dis-engage or free wheel. Be careful and thoughtful of rotating parts.

If fan does not cycle properly during a good hot road test replace switch. (cheap insurance for old truck) Be careful not to over-temp engine. should switch not open. (Switch in hot water in pot is good too) Any wires to this system unplugged will force fan on (another if) If hydraulics are good.

Another crazy reason to owning a HMMWV and then timing the cycling of the fan while on a road trip. Its the heartbeat of the the fun.

Going crazy yet?

CAM

I replaced the thermostat with a 180 degree high flow miloden that was referenced in some other thread. Put my old fan sensor "thermostatic switch" back in. After a couple of road tests it seems to be operating normally. Fan comes on when my temp gauge reads 220 degress. Didn't time the actual run time, but it runs for a short period, then shuts off. Temp decreases slightly. Then when temperature goes back up to 220 on the gauge fan comes on, runs, then shuts off. I did not test the old thermostat, maybe I should to validate but it seems that even though the temperature gauge showed a lower temperature when the fan was running the temperature at the water bridge was not low enough to activate the sensor/switch and shut the fan off? Yes I know the temperature gauge reads from a different location, just reporting my observations. Returned the new sensor/switch I purchased from Oshkosh.

That thermostatic switch is a Taiwan replacement - Non Original. - YMMV
Any original parts should have a NSN number and most importantly, a CAGE code
of the manufacturer. Any parts without a CAGE code could be suspect.

NEVER use the dash temp gauge for diagnostic work. The Thermostatic switch and
the Gauge sender are on two different parts of the engine, the actual readings will
never be the same.
The gauges and sender are notoriously inaccurate.
If you drive the truck and the fan cycles on and off, it is probably working ok.
Always check the actual temp like you were doing with an Infrared tester at the
metal crossover pipe right behind the thermostat. That will give the most consistent
readings.

Fake:
MAYBE OK: