Fan engaging at high RPMs with buzzing sound

[joeberg]

I am having an issue I haven't seen before. I have searched and checked the TM's to no avail. I have a 94 M998A1 with a fan that engages every time I rev my engine past a certain point. When it engages at high RPMs, there is a buzzing/vibrating/resonating sound with it that goes away a second or two after engaging or after disengaging when I left off the accelerator. It doesn't make this sound when engaging at lower RPMs. It only makes the noise when engaging at high speed.

If I unplug the TDM, the fan is engaged from the start and is already engaged at high rev and doesn't make the sound. It almost sounds like a resonating or grating sound. I am wondering if it could be the fan clutch slipping because it is engaging at high rev, but my first concern is why it is engaging at high RPMs to begin with. It is engaging regardless of the engine temperature and seems to do it when I hit the same RPM range every time whether I am in neutral or in 3rd gear at about 55mph. I have access to 4 other HMMWVs and none appear to do this. I did have a belt break and have to change the belts a week ago but I honestly don't know if it was doing this prior. Does anyone have any thoughts to what may be happening or what to look for? Thanks in advance!

 

[Retiredwarhorses]

You could have a cracked or broken fan blade from you broken belt....out of balance fan blade will on do this when engaged. As far as engagement goes...assuming everything in the fan clutch is good, you could be loosing fluid flow from your PS pump at high RPM...meaning the pump might be failing. But I'm more inclined to TS the other components first.
also make sure the fan blade is not contacting the fan shroud....

 

[kc5mzd]

Probably was doing same thing before the belts broke but didn't make as much noise due to slipping belts. I would start by making sure the fan circuit is connected to the thermostat switch and not a sensor like oil pressure. Also a bad ground can do strange things like turning stuff on or off when the engine is loaded or unloaded or at high rpms.

 

[joeberg]

Thanks for the suggestions. I hope to be able to dig more on Friday. I figured this wasn't normal behavior. If anything, I thought the fan was to disengage at higher speeds to prevent problems with the fan due to airflow. I did check the sensors and verified it is tied in to the temp and not oil. I will add a grounding harness next which was already on my list. If not that, I will have to investigate what might cause loss of pressure at higher RPMs. I will report back my findings and thanks again!

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[joeberg]

UPDATE: I installed a ground harness and it didn't make any difference. I went ahead and installed a tach at the same time. I also switched out the TDM with a working one at the same time. The fan is kicking in at around 1700-2000 RPMs based on the tach. I have also been researching the TDM and the testing methodology in the TMs. So far I have found that it is getting a signal from the temperature switch to turn on the fan at high revs. At this point, I believe something it wrong with the thermal switch and although it seems to be working find at turning the fan on when the temperature gets to 220, it is also opening the circuit at high RPMs which causes the fan to of course come on. I suspect it is a mechanical issue with the switch and could be caused by higher coolant pressure. I am going to switch it with one of my other Humvees to confirm.

 

[Retiredwarhorses]

The thermostatic switch just opens and closes based on temp....I've seen more then one fail.
the fan comes on when it's denied 24v at any point from the thermo switch to the TDM to the Cadillac valve solenoid.

 

[joeberg]

I know how the switch is "supposed" to work, but this is strange and has nothing to do with temperature. I just bypassed the switch and tied the leads together and confirmed the fan does not come on at high revs when bypassed. I also ohm-ed out the switch while reving and the resistance greatly increases to a virtual open circuit when I get over 1700rpms. I have never seen a switch do this and cannot explain it but at least I know what the issue is. I know I can pick these up on ebay. Is there an equivalent part that could be picked up from an auto parts store locally?

thanks everyone for the assist.

 

[Retiredwarhorses]

I know how the switch is "supposed" to work, but this is strange and has nothing to do with temperature. I just bypassed the switch and tied the leads together and confirmed the fan does not come on at high revs when bypassed. I also ohm-ed out the switch while reving and the resistance greatly increases to a virtual open circuit when I get over 1700rpms. I have never seen a switch do this and cannot explain it but at least I know what the issue is. I know I can pick these up on ebay. Is there an equivalent part that could be picked up from an auto parts store locally?

thanks everyone for the assist.

the switch is broken....simple, replace it.

 

[Fate3d]

I know how the switch is "supposed" to work, but this is strange and has nothing to do with temperature. I just bypassed the switch and tied the leads together and confirmed the fan does not come on at high revs when bypassed. I also ohm-ed out the switch while reving and the resistance greatly increases to a virtual open circuit when I get over 1700rpms. I have never seen a switch do this and cannot explain it but at least I know what the issue is. I know I can pick these up on ebay. Is there an equivalent part that could be picked up from an auto parts store locally?

thanks everyone for the assist.

I also try to find an alt option switch but that was a no go. Just buy one off ebay for $45 like I did. To me sounds like the switch has a short. Hook up the meter to it and move the wires to check for a break in the wire. Can be a easy fix.

 

[joeberg]

I also try to find an alt option switch but that was a no go. Just buy one off ebay for $45 like I did. To me sounds like the switch has a short. Hook up the meter to it and move the wires to check for a break in the wire. Can be a easy fix.

That's what I did. I checked for shorts and breaks and no go. The way the resistance builds as the RPMs increase almost make me think someone installed a pressure transducer instead of a thermostat (yes I am checking the right one). The resistance seems to max out at about 200k ohms at the peak (over 2000rpms). I just have never seem one fail this way. In the past either they worked or they didn't which is why this threw me off. Thanks.

 

[kc5mzd]

I know how the switch is "supposed" to work, but this is strange and has nothing to do with temperature. I just bypassed the switch and tied the leads together and confirmed the fan does not come on at high revs when bypassed. I also ohm-ed out the switch while reving and the resistance greatly increases to a virtual open circuit when I get over 1700rpms. I have never seen a switch do this and cannot explain it but at least I know what the issue is. I know I can pick these up on ebay. Is there an equivalent part that could be picked up from an auto parts store locally?

thanks everyone for the assist.

Sounds like you are on the right track. I like the $45 ebay suggestion best. If you wanted to check the auto parts stores your best bet would to take the old one in along with the temp range or the temp it turns on at and whether it is normally (when room temp) open or closed. ***You need to find a auto parts store that hires counter people because of their parts experience and not because they were taught a foreign language at birth (very important).*** I have had the best luck at either Napa or if there is a Fleet Pride near you they are usually the best at matching parts.

 

[Action]

Could it be just a faulty wire? maybe a grounding issue?

I remember back in high school, some guys that didn't know how to install amps and subs, would get a whine in the speakers that got higher as they gave more throttle.
Sounds similar.

 

[Fate3d]

Best way to test it is to pull it out and test it with a meter. Set it to continuity and should show no continuity and use a heat gun and watch it connect then it works. They might have put in the wrong senor.

 

[joeberg]

UPDATE: I replaced the thermostat with a new one and it resolved the problems as expected. It definitely seems to have more pep now that the fan isn't cutting on every time I pass 1700RPMs. Old thermostat functioned fine as a thermostat other than causing the fan to cut on at higher RPMs. There doesn't appear to be a bad connection. It didn't become a completely open circuit at higher RPMs but the resistance simply increased as the RPMs increased until it became a virtually open circuit (around 200,000KOhms). At least I can put that behind me. Thanks everyone for the help.

 

[Action]

The thermostat controls water flow. It isn't electric. How would a bad thermostat cause the fan to come on at high RPM? other than the engine getting hot?

 

[joeberg]

I don't follow. This thermostat is electric. It passes 24v to control the solenoid to turn on and off the hydraulic fan.

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[Action]

Are you talking about the time delay module bu the washer fluid tank? It is a solid state relay.
The thermostat is round and at the end of the big rubber hose coming from the top of the radiator. It is inside the silver housing that the hose hooks to. It opens when hot to let water flow through the radiator.
The thermostat in your home is electric, but not the one in your HMMWV.

 

[joeberg]

You are talking about a different thermostat. The thermal switch I am speaking of feeds into the TDM. It is an electrical component. It has two wires coming from it. Essentially it is an electrical switch and has nothing to do with water flow. When the temperature exceeds ~220F it opens the circuit which signals the TDM to open the solenoid to turn on the hydraulic cooling fan. This thermostat is on top of the manifold at the top of the engine. Below is the same thermal switch from Kascar.

https://www.real4wd.com/zedSuite/catalog/partdetail.aspx?PartNo=12338599

 

[Action]

That is a thermostatic fan switch or temperature switch, not a thermostat. That is why I didn't understand what you were saying. The Humvee has only one thermostat, and it is in the coolant flow.