Fan engagement not dependable

[rossbart]

Have a super nice M1123 as my farm feed truck. And, I’m trying to figure out why the fan engagement is unreliable. The oem setup worked fine most of the time. But, occasionally the fan would miss its engagement point and if you weren’t watching the gauge pretty closely you’d look up and be running 240. The only consistent thing I noticed is that it pretty much never failed in colder weather. The failures were always in hot days after the truck had been running awhile. Almost like high underwood temperature was causing something to change?
Well, I’ve never been crazy about how hot these rigs run from factory. And, I actually had a diesel shop pull and rebuild my IP (due to it leaking from bottom of case) and he suggested swapping to a cooler thermostat and fan engagement sensor. So, since I was theorizing that the faulty component was the sensor (so was going to replace it anyway) I found an aftermarket sensor that was set to 185 on. I paired that with a new 160 degree thermostat. And, during the 98% of the time it’s not glitching it keeps the engine (on gauge) right between 180 & 185 perfectly.
However, it still acts exactly like the initial setup. Sometimes on a hot day, and after under hood temps get hot, it will sometimes miss its engagement point and will never catch it. But, if you park it and let things cool down and restart it, it might work just fine?
I installed a manual override switch inline with the Cadillac valve wires so I can force the fan on without having to open the hood. But, I’m afraid a farmhand (or even I) might not catch the glitch and could overheat it badly. So, I’m hoping that someone might have some insight into this.
I have gone through all the diagnostics from black dog customs YouTube video and everything seems to be in spec. I have ordered a new time delay module and will see if that fixes anything.
Otherwise, does anyone have any ideas or experienced something similar to this?

 

[Mogman]

The only component I can think of that would possibly fail due to under hood heat would be the delay module, it is the only component with semi conductors inside in that system.
Also you have eliminated the cadillac valve due to your bypass switch.
Did you put the bypass switch in the thermal switch circuit or between the delay module and the cadillac valve?

 

[rossbart]

The only component I can think of that would possibly fail due to under hood heat would be the delay module, it is the only component with semi conductors inside in that system.
Also you have eliminated the cadillac valve due to your bypass switch.
Did you put the bypass switch in the thermal switch circuit or between the delay module and the cadillac valve?

This is a poor quality picture but just had foot surgery so can’t go take a better one. I ran the fan toggle switch through one leg of the two wire plug between TDM and Cadillac valve.
This picture was the result of my ohm test on the Cadillac valve. My understanding is that acceptable resistance is anything between 58 and 78 ohms. I’m clearly at the high end of that but only by .3 ohm?
I do have a new TDM coming my way on FedEx. I’ll swap that out when it comes in. Hopefully that will be the culprit. Thanks for your insight & help….

 

[Mogman]

That would be a good enough reading, but heat failure would not show up when cold and simple resistance reading are only a marginal way to test a device.
0.3 ohms is "close enough"
Good luck with your project!!!

 

[Hummer Guy]

Have a super nice M1123 as my farm feed truck. And, I’m trying to figure out why the fan engagement is unreliable. The oem setup worked fine most of the time. But, occasionally the fan would miss its engagement point and if you weren’t watching the gauge pretty closely you’d look up and be running 240. The only consistent thing I noticed is that it pretty much never failed in colder weather. The failures were always in hot days after the truck had been running awhile. Almost like high underwood temperature was causing something to change?
Well, I’ve never been crazy about how hot these rigs run from factory. And, I actually had a diesel shop pull and rebuild my IP (due to it leaking from bottom of case) and he suggested swapping to a cooler thermostat and fan engagement sensor. So, since I was theorizing that the faulty component was the sensor (so was going to replace it anyway) I found an aftermarket sensor that was set to 185 on. I paired that with a new 160 degree thermostat. And, during the 98% of the time it’s not glitching it keeps the engine (on gauge) right between 180 & 185 perfectly.
However, it still acts exactly like the initial setup. Sometimes on a hot day, and after under hood temps get hot, it will sometimes miss its engagement point and will never catch it. But, if you park it and let things cool down and restart it, it might work just fine?
I installed a manual override switch inline with the Cadillac valve wires so I can force the fan on without having to open the hood. But, I’m afraid a farmhand (or even I) might not catch the glitch and could overheat it badly. So, I’m hoping that someone might have some insight into this.
I have gone through all the diagnostics from black dog customs YouTube video and everything seems to be in spec. I have ordered a new time delay module and will see if that fixes anything.
Otherwise, does anyone have any ideas or experienced something similar to this?

Had this happen as well, you should look into the viscous fan clutch, much better setup

 

[HoveringHMMWV]

I had a similar but slightly different intermittent problem with the fan engagement on my M1151A1. I found that the wiring harness near the windshield washer reservoir had been nicked during the demill process. With vehicle vibrations, this created an intermittent open circuit on very random occasions which caused my fan to behave erratically. I would suggest a detailed inspection of the wiring bundle for damage.