Overheating 🥵

[Stevev]

Good morning and Happy New Year
On my 1989 M998 my fan was always engaged
I replaced my Cadillac valve and now getting 120 psi to fan and it is disengaged all is good runs great but now won’t engage
Temp climbs to 240 and I still have 120 psi
My sensor is also new my truck was running at 160
Before I replaced the valve so I think the
Temp gauge is correct
Any ideas?
Could the tdm be bad? It giving 26v all the time to the valve when running
Thanks
Steve

 

[Dock Rocker]

Is it just spinning all the time or is the clutch actually engaged?

Unplug the TDM and take it for a spin. With the TDM unplugged the fan will fail to engaged. When you do this does it run hot?

If it still runs hot I would do a radiator flush and see what comes out. Have you done one lately?


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[Stevev]

Is it just spinning all the time or is the clutch actually engaged?

Unplug the TDM and take it for a spin. With the TDM unplugged the fan will fail to engaged. When you do this does it run hot?

If it still runs hot I would do a radiator flush and see what comes out. Have you done one lately?


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Thanks for the replay
It is definitely engaging and disengaging
Now
When I unplug the tdm the fan moves and engages and cools back off
I’m wondering if the tdm can go bad and send 24v all the time? I thought when I changed the Cadillac valve and now getting the fan to disengage my problem was solved...lol
She runs great now tho
Thanks for the help

 

[DREDnot]

From what I've read here on SS, the TDM is not known for reliability. It has a reputation as the most likely culprit in overheating issues involving the fan clutch.

 

[Stevev]

From what I've read here on SS, the TDM is not known for reliability. It has a reputation as the most likely culprit in overheating issues involving the fan clutch.

That’s what I was thinking
I’m going to drive it for a while and see if the fan kicks in
From what I read it may not kick in in certain driving conditions so maybe it will kick in if I drive it normal without pushing the engine

 

[DREDnot]

My first impression driving mine on the road is that they seem to run at what looks like too hot on the gauge. Gave me anxiety the first few times I drove it any distance.

Mine has always kicked on right at the 220 hash mark. It cycles consistently when I'm driving in traffic 40-50 MPH. I found that gives the most consistent conditions to cause the fan to engage and when it does, you know it!

The time/distance it takes to first get to engagement temperature will vary with the outside temps, as will the time between fan engagements.

This is how mine has functioned since I got it a year(850mi) ago. It is still running the fresh and functional coolant it came with.

I've been gathering the parts to go through the cooling system completely and hopefully replacing all the controls and sensors with genuine NOS will give me the maximum cooling capacity that the stock system has to offer.

 

[Stevev]

My first impression driving mine on the road is that they seem to run at what looks like too hot on the gauge. Gave me anxiety the first few times I drove it any distance.

Mine has always kicked on right at the 220 hash mark. It cycles consistently when I'm driving in traffic 40-50 MPH. I found that gives the most consistent conditions to cause the fan to engage and when it does, you know it!

The time/distance it takes to first get to engagement temperature will vary with the outside temps, as will the time between fan engagements.

This is how mine has functioned since I got it a year(850mi) ago. It is still running the fresh and functional coolant it came with.

I've been gathering the parts to go through the cooling system completely and hopefully replacing all the controls and sensors with genuine NOS will give me the maximum cooling capacity that the stock system has to offer.

Thanks for the info
I’ve had mine since oct
Just going through it fixing what isn’t working
Even tho it drives me nuts figuring out what’s wrong I love this thing
Lol

 

[Dock Rocker]

Just unplug the TDM and drive it. That will have the fan engaged 100% of the time. If it still runs hot you have a radiator problem. If not you need to start trouble shooting.

Also do you know for sure if your gauge is correct? They are not known to be reliable.


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[Stevev]

The fan was always on when I bought it
I tested the psi at the fan clutch and it was 50
New Cadillac valve fixed it psi is now 120
But the fan clutch is getting 24volts still from the tdm when the temp is 220-240

 

[DREDnot]

But the fan clutch is getting 24volts still from the tdm when the temp is 220-240

Another possibility is the fan switch in the crossover being bad. Check for continuity when cold, and again when over 220. It signals when to engage the fan

 

[REF]

Don't forget about the micro switch on the throttle lever, it's sole purpose is to kick the fan off when fording. if the switch sticks it will prevent the TDM from allowing the fan to come on.

 

[Stevev]

Don't forget about the micro switch on the throttle lever, it's sole purpose is to kick the fan off when fording. if the switch sticks it will prevent the TDM from allowing the fan to come on.

Good call thanks you
Can I just leave it unplugged
I won’t be going through any rivers lol

 

[REF]

If memory serves the switch is normally open so unplugging should work, I haven't tried it BUT look at it first. It's just a small micro switch on the side of your injector pump with a lever that pushes the switch when the throttle is pushed wide open. push the lever with your finger make sure it moves freely and if so listen for a click when you push the lever. If the lever is free and the switch clicks then unplug the leads and put an ohm meter between them and check to see if the switch is opening and closing. You mad just need a little oil on the lever.

 

[Stevev]

If memory serves the switch is normally open so unplugging should work, I haven't tried it BUT look at it first. It's just a small micro switch on the side of your injector pump with a lever that pushes the switch when the throttle is pushed wide open. push the lever with your finger make sure it moves freely and if so listen for a click when you push the lever. If the lever is free and the switch clicks then unplug the leads and put an ohm meter between them and check to see if the switch is opening and closing. You mad just need a little oil on the lever.

Thanks bud I’ll do it tonight

 

[REF]

The nice thing about the switch is that anytime you need full power, like passing a car ( LOL) . it will kick the fan out and make the freed up HP available to the wheels.

 

[Milcommoguy]

Not the "sole" purpose, but important when going for a swim. More importantly IMO... It is also tied to the kick down solenoid on the TH400 transmissions. Probably fines more use there when passing. Still disengages fan for that added extra 20 seconds of boost.

Just saying: As with anything they do go bad. If not closing at WOT, it will not trigger TDM to start fan disengage timer and transmission KDS. If jacked up and closed all the time, fan won't cycle and transmission will not shift out of first. Switch is N.O. and closes when WOT. It is a special switch assembly and could be modified with a micro switch type. Not worth the headache. Go OEM.

Hummwv fans make poor boating propellers. CAMO

 

[Stevev]

Not the "sole" purpose, but important when going for a swim. More importantly IMO... It is also tied to the kick down solenoid on the TH400 transmissions. Probably fines more use there when passing. Still disengages fan for that added extra 20 seconds of boost.

Just saying: As with anything they do go bad. If not closing at WOT, it will not trigger TDM to start fan disengage timer and transmission KDS. If jacked up and closed all the time, fan won't cycle and transmission will not shift out of first. Switch is N.O. and closes when WOT. It is a special switch assembly and could be modified with a micro switch type. Not worth the headache. Go OEM.

Hummwv fans make poor boating propellers. CAMO

I just tested the micro switch on the throttle
The switch is normally open and makes continuity when you peg the throttle
So I assume the fan should be engaged at 220degees or more when the engine isn’t wide open
I just ordered a new TDM mine seems to give 24v all the time to the Cadillac valve keeping the fan clutched and the psi stayed at 125psi
I tried to follow the TM on testing the TDM and it seemed to ohm out much lower then what it should
Hope the new one does the trick
Thanks for all the help

 

[Andyrv6av8r]

Here's a good video about fan troubleshooting.

 

[Stevev]

Here's a good video about fan troubleshooting.

Great video
Thanks
Im at the point that the video shows and clutch isn’t engaging at 220 so hopefully it’s a bad tdm

 

[avman]

Good morning and Happy New Year
On my 1989 M998 my fan was always engaged
I replaced my Cadillac valve and now getting 120 psi to fan and it is disengaged all is good runs great but now won’t engage
Temp climbs to 240 and I still have 120 psi
My sensor is also new my truck was running at 160
Before I replaced the valve so I think the
Temp gauge is correct
Any ideas?
Could the tdm be bad? It giving 26v all the time to the valve when running
Thanks
Steve

There are pads in the fan. Everything worked fine on my hmmwv and was overheating. Replaced these pads and no more overheating issues. You will see small screws on the back of the fan. They are easy to replace. Got my new pads off of eBay a few years ago