Validating M998 temperature gauge

[Radium]

I purchased a 1989 M998, 6.5L NA, 3-speed. So far I've only driven about 100 miles.

Today the temp gauge went to 240 degrees right as I reached home. I checked the antifreeze and nothing was boiling or venting out the overflow. After some research I discovered that it's common for the temp gauge to be faulty. I'd like to validate that it's the temp gauge, so I used a laser temperature sensor and got a reading of 166.6 degrees from the thermostat housing, while the temperature gauge showed 200 degrees; a 33 degree difference.

As a sanity check, can someone confirm that where I took the laser reading the correct location? I took a picture and you can see the red dot from the laser in the image. I'll order a new gauge, but just want to make sure I'm not overlooking something.

Note that last week the temp gauge seemed to work... once the temp reached around 220 degrees a loud fan kicked in and brought the temp down to around 200 degrees almost instantly.

The only difference today was that it was raining and somewhat cold. I had the heat turned on to defog the front windshield.

 

[Ajax MD]

With regards to your photo, when I shoot the IR gun, I also hit the area with the thermostatic switch just next to that. It usually reads hotter.

Make sure the battery is fresh in your IR thermometer and don't shoot it from too much distance or you'll be checking the temperature of a wider area. Try it from no more than 11 inches away.

My truck behaves the same way, indicated temperature is hotter than the IR gun. Clean the main grounding point behind the instrument cluster and ensure that all your wire connections to the gauge are clean and tight. I need to do this myself.

 

[911joeblow]

IR is notoriously inaccurate for reading internal coolant temps. Don't shoot at shiny metal, rubber hose is best. That all being said those temps are sounding correct. A lot of guys chase gas engine temps thinking cooler is better (180-190F) but that is absolutely false on a diesel. You need temperature to get proper burn. Here is a blog we wrote about all this: https://paradoxbydesign.com/blogs/news/the-humvee-diesel-engine-cooling-paradox

 

[TOBASH]

Trust the gauge and not the laser thermostat.

Dummy lights are there to prevent us from being dummies. Please don’t be a dummy.

Best,

T

 

[Radium]

Thanks for the feedback!

I took the truck for a long drive yesterday and the temp gauge seems to work again. The temp stayed between 210-220 degrees - whenever the temp went to 220 degrees the fan kicked on and cooled the engine down.

The only thing I did different yesterday was I didn't run the heat. I'll take the truck for another long drive this time running the heat to see if I can re-create the issue where the temp stayed at 240 degrees.

 

[DREDnot]

 

[papakb]

Works one day and not the next. This is so typical of a bad instrument cluster with a bad ground. Pull the dash aside and make sure the connection on the firewall in front of the heater controls is clean and tight.

 

[Radium]

Great, now I want a thermal camera!! I'll just add it to the bucket list.

 

[cwc]

When using a spot laser thermometer the field of view should also be taken into consideration. The field of view can be much larger than suggested by the "dot". They are great for things like racecar tires, where the target is simple and consistent and you just want a quick number. For something more complex, with multiple surfaces potentially in view, the camera helps to ensure the surface in question is being evaluated. IR of bare metal is tricky. There is a concise FLIR article on practical considerations for managing emissivity issues. Search "use low-cost materials to increase target emissivity". I do a fair bit of IR thermography in an industrial setting - a can of flat black paint is always at hand. Also, the external surface temperature will be somewhat below the internal fluid temperature, no matter what surface is scanned.

Sent from my E6910 using Tapatalk

 

[DREDnot]

Great, now I want a thermal camera!! I'll just add it to the bucket list.

I took that pic with this plugged into my phone

FLIR ONE Gen 3 Professional Thermal Camera for iOS and Android Smart Phones | Teledyne FLIR

The FLIR ONE Gen 3 is a smartphone thermal imaging camera that helps professionals quickly identify problems. Enhance your work with advanced MSX® technology.

www.flir.com

 

[fpchief]

I took that pic with this plugged into my phone

FLIR ONE Gen 3 Professional Thermal Camera for iOS and Android Smart Phones | Teledyne FLIR

The FLIR ONE Gen 3 is a smartphone thermal imaging camera that helps professionals quickly identify problems. Enhance your work with advanced MSX® technology.

www.flir.com

I'll tell you what....FLIR has come a long way in not very many years. For true FLIR for less than $200 is nuts.