1991 HMMWV overheating

[bmdupont]

Looking for some advice, took it out for an extended drive 20-30 miles and near the end had radiator fluid spilling out.

Noticed that my temp gauge is not working.
Questions.
1. bad gauge or bad sensor?
2. if the temp gauge isn't working would that prevent that cool down fan from turning on? It usually kicks in at some point when i'm driving but didn't think much because it was a cooler day.

Figured its a bad gauge unless the fan is controlled by the sensor then ill start there.

Thank you!

 

[Retiredwarhorses]

Man, not,sure where to start....but first off, a temp gauge has nothing to do with overheating.
if the gauge was not working, driving the truck was not a good idea, at least not for distances.
did the fan come on? You would know...as it sounds like a jet taking off, if it did not, that’s why you overheated most likely.
problem now is, did you do any damage to the engine.
where was the coolant coming from? The only place it should is from the overflow tube on the coolant tank.
the cap is rated at 15psi, they are a problem if the cap won’t hold its rated pressur.

 

[bmdupont]

Man, not,sure where to start....but first off, a temp gauge has nothing to do with overheating.
if the gauge was not working, driving the truck was not a good idea, at least not for distances.
did the fan come on? You would know...as it sounds like a jet taking off, if it did not, that’s why you overheated most likely.
problem now is, did you do any damage to the engine.
where was the coolant coming from? The only place it should is from the overflow tube on the coolant tank.
the cap is rated at 15psi, they are a problem if the cap won’t hold its rated pressur.

Yeah the fan didn’t come on, what would initiate the fan? Temp sensor?

coolant was coming out of overflow only. It was in the last 30 seconds of our 30min trip.

 

[Retiredwarhorses]

Take your pick...thermostatic switch, Bad TDM, bad Cadillac valve...I just replaced a bad Cadillac valve yesterday, wasnt the Cadillac valve solenoid, something was clogged in the valve.
75% of the time its the thermostatic switch, 20% TDM, the rest Cadillac valve.
unplug the TDM and see that fan runs constant....the default is supposed to be locked on, but I’ve seen bad thermostatic switch that won’t open and deny 24v to the TDM...often.
once the TDM is unplugged and the fan is running constant, you can drive as long as you want....but fix the gauge issue.

 

[98G]

Or maybe it was overfilled to begin with and just puking out the excess....

 

[Retiredwarhorses]

Anything is possibl, but PMCS would have uncovered that. So I guess we will never know.
But a bad temp gauge leaves it all to speculation. No fan and no idea of air temp and how the truck was driven
leaves another black hole....what I can say, is that no fan will overheat the truck.

 

[TOBASH]

You need to go through the forum threads and prior forum posts regarding activation of the Cadillac valve And Cadillac valve electronic control. You can put switches onto the Cadillac valve electronic control to automatically run your fan full-time or to turn it off entirely when you go through water and ford deep water.

Understanding this system will help you debug your rig. In the meantime, the first thing you should do is check the internal spring thermostat to ensure it is working properly.

Then you need to make sure that the Cadillac valve and the electronic module are also functioning.

On my vehicle, the thermostat dashboard gauge was not working because the wires were on backwards. You need to look for that.

Retired War Horses is absolutely correct. You should never drive any vehicle where the gauges do not function and the thermostat gauge is absolutely 100% essential to ensure that you do not turn your engine into a paperweight.

Best of luck,

Tobash

 

[Milcommoguy]

Sounds like this was an extended test drive. If this rig is new to you, limited history, With a PMCS & TM's in hand...it should have been first on the list. With the hood up it's like looking into a crystal ball, everyone sees something different and becomes difficult to come to a clear conclusion.

Your on the right trail. Make some tests, know what does what to when and how. That's in the TM.

Gauges not working. that's a separate circuit / sensor to inform the driver. No fun flying blind.
Temp sensor for fan control in conjunction with delay module, Cadillac valve, clutch and power steering pump. Another stand alone / shared system.
Overfilled reservoir tank ?? fill only to line. Do not top off like you could get away with in that old Chevy.

Don't over complicate it. Simple steps, simple tests, shorter test drives. When you get it right, it will want to go a lot farther.

If this rig is new to you... one NEEDs to go top to bottom, front to back to assure safety, reliability, performance from a 30 year ? old rig.

My weegie board says you'll have it figured out soon. Good Luck, CAMO

 

[bmdupont]

Appreciate the replies, had the truck for 5 years. My first guess is the temp sensor.

Just thought it was an interesting coincidence the gauge just stopped working this spring and this happened.

I lack understanding of how the temp sensor, gauge and fan are connected or aren’t connected. I read some older posts about other users with over heating issues and still did not understand If there was a connection between gauge and fan. Does not sound like they are based on replies.

My first step will be to clean the gauge connections, have had this problem before with other things not working due to bad connections and dust.

Have a new temp sensor, gasket and radiator cap on order. Then it will be replacing the sending unit. If it goes beyond that I will have to take it to someone as it’s out of my league.


Sent from my iPhone using Tapatalk

 

[TOBASH]

Gauges on these vehicles can be a crap shoot. You need to open up, use a voltmeter on each wire. Additionally each wire has a metal crimp with a number. You need to go to the electrical forum and print out the wiring number translations. The TM's/Technical Manuals will have diagrams. Finding them sucks, but well worth while.

You need to find the grounding bolt and reground it as they fill up with crud and stop properly grounding.

You need to ensure gauges are connected properly to their wires. Sometimes people get the wires backwards.

Not hard to work on the gauges, but potentially time intensive. Helps to have elbows that bend backwards.

My gauges on my rig were all wired wrong. I figures it out and now my gauges are A1.

 

[Milcommoguy]

Can't say for sure... I don't think it's out of your league. You're just on first base and heading to second. Big hitter on the bench here. Just be clear on sending the correct signs. (symptoms) Can't get home without runing the bases in order while playing smart and hard.

Watch out for less than top quality parts. Good way to get put out. OUCH

Check the play book (TM) to clear the possibility of grounding out and not seeing the gauges or anything working right. Really want to have them property grounded. Easy to execute before spending money.

When was the last time the waterboy check the cooler. That stuff could be nasty. Auh !

Lots going on here. We understand the need for a designated runner too.

Up in the peanut gallery rooting for team SS, and bmdupont, CAMO

 

[papakb]

There's an excellent flow chart in the TMs for troubleshooting overheating problems you might want to look at and a couple of good videos on You Tube.

Something I didn't see mentioned here is if the thermostat is stuck open or has been removed the coolant can actually flow too fast and won't give up it's heat in the cooling stack.

Have you done the basic "unplug the TDM" to see if the fan clutch is moving?

How'd your power steering feel? Remember it's power steering fluid that operates the fan clutch.

The temp gauge sensor is between the alternator and the block. The fan clutch sensor is the one on the crossover manifold.

 

[bmdupont]

Thanks For the help, I will check the TMs to troubleshoot.

I have not tried unplugging the tdm i will try. Power steering feels normal so far.


Sent from my iPhone using Tapatalk

 

[Andyrv6av8r]

This is always good info to start with,

 

[bmdupont]

Got around to working on the overheating. Things I have done and still have the problem.
1. Replaced the thermostat that controls the fan turning on super fan.
2. Changed the thermostat valve and gasket in the hose. Hope I didn’t mess this up? Made sure it was installed the correct way.
3. Disconnected thermostat and the fan clutch engaged but didn’t go whooom like it used to when engine got to cooling temp. - I don’t understand why it wouldn’t be loud like it used to?
4. I have a new temp thermostat for the gauge but still gotta figure out how to get a wrench in there without taking something else out.
5. Turned it on and let it run for a little bit tonight to see if the fan would go whomp and cool it down. It did not engage and it was plenty hot.
6. Replaced coolant tank cap

First question is about the fan clutch and if it would be as loud as normal doing the manual wire pull to start it?
Second question if it should be as loud then what’s next?


Sent from my iPhone using Tapatalk

 

[Bulldogger]

Got around to working on the overheating. Things I have done and still have the problem.
1. Replaced the thermostat that controls the fan turning on super fan.
2. Changed the thermostat valve and gasket in the hose. Hope I didn’t mess this up? Made sure it was installed the correct way.
3. Disconnected thermostat and the fan clutch engaged but didn’t go whooom like it used to when engine got to cooling temp. - I don’t understand why it wouldn’t be loud like it used to?
4. I have a new temp thermostat for the gauge but still gotta figure out how to get a wrench in there without taking something else out.
5. Turned it on and let it run for a little bit tonight to see if the fan would go whomp and cool it down. It did not engage and it was plenty hot.
6. Replaced coolant tank cap

First question is about the fan clutch and if it would be as loud as normal doing the manual wire pull to start it?
Second question if it should be as loud then what’s next?


Sent from my iPhone using Tapatalk

The fan is not loud when it engages at idle. When clutched in it turns at engine speed, so the "jet taking off" sound we often describe it tied to high idle, i.e. driving along the road.

If you've verified it indeed clutches in, then at least that part is working. There are threads here about how to use a board or something softer to tickle the fan blades to see if they fight back, meaning they are clutched in and not freewheeling.

1. Do you mean the TDM that screws into the coolant crossover pipe? All I can offer is that the sensor must be matched to your engine and more importantly the Smart Box, but you probably figured that out when placing the order.
2. Should be fine, as long as it was the correct part #.
3. see above
4. Not sure what you mean by "temp thermostat for the gauge"
5. If you were at idle, it is very hard to sense when and if the fan engages, as noted above.
6. Usually a good idea

If you have the time and TM, it might help to post the exact part numbers and nomenclature you're using. Using layman's terms may cause unintended confusion as folks try to sort through this in the heads.

We'll get you there though!

Bulldogger

Don't understand what you mean in number 4.

 

[bmdupont]

Here is the thermostatic switch installed. Link
#4 is temperature transmitter that will hopefully solve my gauge not working. Link

After looking at the thermostatic switch i installed it doesn't look OEM, didn't have the tag like the one i pulled out but otherwise looked the same. It also threaded deeper than the one that was installed.

 

[Milcommoguy]

Fan goes round and round and following along with this thread, my head is spinning too.

It's good that your'er working the problem. MY process is to have or get a clear understanding of the system. Get your colored pencils and mark up the drawings with notes. Work it out on paper, the brain will follow. Plus side is no dirty hands. Now that one as a BETTER understanding of the goings on... one should be able to have a logical place to start. I have found this to be simple and most helpful >

https://hummerhuren.files.wordpress.com/2013/01/hmmwv_test-diagnostics.pdf < YEP. I look to it ever time. Good read to fall asleep with too.

Well, it is a military machine... so lets divide and conquer. We all do it at some level when troubleshooting. This HumV fan system is about as simple as it gets (for what they were thinking at the time) SOoo don't over think it.

Start somewhere then check for presents of or lack of... voltage, pressure, travel, action or reaction. Track it back to the drawings. schematic. Move thru the working stuff with knowldge gained above. Prove it to self or system and check it off. Next.......

Stuck or going in circles ? STOP ! take a break and back to the drawing board. I know we know this stuff, But sometime trying to hold it all in the brain is a drain, especially something new or foreign to us.. Add in the "it's just a fan" it can be like being lost in the woods. Use the MAP and be a the MAN and you will walk right out. Compass (tools) helps too.

Two cents of help...and some salt.

There's no TDM screwed into a pipe. TDM lives on the left foot well. Fan doesn't have anything to do with Smart box either. Just get voltage from it.

Sender for temperature gauge. Lower front of left cyl head. Might be easier to work / seen from below. Has nothing to do with fan, only gauge.

Is a bit hard to determine if fan is or isn't engaged at idle... UNTIL all is working and you learn its slight movement. Keep you hands out of there.

Thermostatic switch (manifold) really doesn't turn ON. Really. It turn OFF and removes voltage to system, then the Cadillac valve does its thing.

Radiator cap is a main part of overall cooling system. Any leak will reduce cooling performance. Only fill to line on tank. It's not really an over flow IMO.

Don't use cheap knock off parts. Pay the price now or double down later. And high price MIGHT not be the high quality one you want. BEWARE

Can you easily rotate fan with engine OFF? YES That's A PROBLEM. NO. Time for "air hose" test to eliminate this major monkey motion part. Having and KNOWING this is working allows piece of mind to get on the the next components. Cadillac valve, correct hose connections, pump pressure (advance test)

Could run the electrical testing of system. Got right voltages and resistance values at the right places AT the right times. Back to the drawings for test points.

Is the cooling system flowing clean-clear at this point. Don't waste antifreeze till all fixed up. Mudded up and rusty, another something to fix / flush, heater too.

Worked fine when it left the factory. Don't need to reinvent the wheel. Stay to the drawings. OEM - Factory - MIL SPEC. (put that in there for RWH)

Don't make up new ways on how it works. The facts are black and white... and it matters. Keep the drawings close by. Nothing magic going on here.

Your going through the motions GOOD. On target. Just a little to the right of center. Make the adjustments and stay focused. Both eyes open.

Not saying any thing bad about any SS crew. if I call out or say YOU, WE, ME, US, Dem Guys. It' just me, like you (opps said me & you) wanting to get this fixed.

On line... and on target diagnostics is not always that easy. (really challenging) Got a few elements / variables going on here

Like driving somewhere with extra back seat drivers. "YOUR" in the front seat. We'll get there ??? YES we will. GOOD LUCK.

Dizzy and waiting for the Greyhound , CAMO

Boy that a lot of stuff.

 

[911joeblow]

So many of these get refilled with river water or worse, make sure you are dealing with clean coolant. Jack up the front of the truck and run it for a bit. There are pockets at the back of the block that can trap air you can help bleed out by lifting the front of the truck. Everything else in this thread is spot on, I just wanted to add some additional not mentioned.

 

[MattNC]

Quick thoughts to add after reading this.

First is if you want to check the fan at high idle there is the throttle lock under the dash. After letting it run for a minute bump up the RPMs and disconnect the thermostat sensor and see if you get the jet engine sound we all know well. If it engages but can't move the air as it should that would help you focus on the hydraulic engagement vs if it fully engages and moves air you could focus on the controls side of the system.

This may have become clear but nobody seemed to state it outright. The reading on the temp gauge in no way is directly connected to the cooling system turning on or off or thermostats opening. If all is well is it a good secondary measure showing the system performing, but you could yank the whole gauge and wire and it would have no impact on the cooling system doing its thing.

Buy a cheap IR gun from autozone or elsewhere. Probably $30. They are great for spot checking temps on the engine and seeing the thermostat opening. Check temps when you start the engine and you should see the coolant recirculating in the engine and beginning to rise. The thermostat doesn't let the coolant into the radiator until it hits the temp its rated for. So if you had a 190 degree thermostat for example you should see the coolant rising after startup in the engine loop and crossover pipe until it reaches 190. All during that time the temp on hose to the radiator should be relatively cool, well under the temp on the crossover pipe. At 190 the magic should happen and it should open and now allow coolant into the radiator. Now the readings between the crossover pipe and the hose leading from the thermostat to the radiator should be really close to each other, hopefully around 190. If you hit 190 and the crossover pipe continues to rise and the hose to the radiator doesn't stop immediately as the thermostat has failed. Inverse to your problem is the stuck open thermostat and you would see the temps match between the two points from startup and it would struggle to get to 190 without driving it.

Last point I'll add since my day job is working on this. The IR gun is also handy to shoot friends and family with directly on the forehead (not in the eye). Above 99 or so there could be a problem and get it checked, not on the steel soldiers website though.