1960s 1969 DeVille Water Temp Light Dimly Lit

[Jim Miller]

I'm curious. Where is the best place to aim an IR gun to take a temp reading? On my 41 there is a lot of exposed block to check. But in the newer ones with the valve covers and exhaust manifolds not so much. 

[TJ Hopland]

When I'm checking against a gauge or light I will probe the area with the sender.  Its not going to be the exact same as the coolant in the area but typically the area where the sender is isn't all that thick or close to some major heat source its gonna give you a decent idea.

[The Tassie Devil(le)]

I too use the Sender unit for testing, but also the top Radiator Hose, once the Thermostat opens.

Bruce.

[James Landi]

'I'm curious. Where is the best place to aim an IR gun to take a temp reading? On my 41 there is a lot of exposed block to check. But in the newer ones with the valve covers and exhaust manifolds not so much.'

With flat heads with 'tender' valve seats and cylinder heads that are prone to crack, I'd focus on the top of the engine.  In my limited experience, I've found that water jacket passages tend to get blocked up at the head gasket, so that coolant can't circulate in the cylinder head; therefore, you're searching for areas of extreme heat at the cylinder head in particular.  So old flat heads I've worked on in old 50's boats had electric heat gauges at the very top/front of the engine (inboards mounted on an angle), apparently that's where a lack of circulation in the head would first report a coolant issue. Please keep us in the loop and tell us what you find.

[R Simone]

I finally took some temperatures today in five places with a quality Temp Gun:

1. Radiator Hose @ T-Stat
2. Upper Radiator Inlet Hose
3. Lower Radiator Outlet Hose
4. At Temp Switch in front of intake manifold/under AC Comp
5. Middle of Radiator

The Ambient temp was 82°F - I checked it before start up, ~15 min in, and at ~30 min, I let it idle and then did some driving as well. Despite the WATER TEMP light being illuminated by the time I did the 15 min check the temps never even got over 180°F at any of the readings.  My understanding is that IR temp guns might read a little low, but regardless it doesn't seem like I'm anywhere near overheating (actually maybe a bit the opposite?). I am using the correct style 180°F T-Stat as pictured which is a "Fail Safe".

[TJ Hopland]

Ya the reading could be a little off but that light sender is usually fairly high like at least 230 so if you are reading 180 you would think worst case the coolant in that area could be what 200? which is a long way from 230+. 

I'm not sure what they mean by fail safe stats, sounds like something marketing came up with but it is a good sign that its got the disc at the bottom which blocks the bypass port which helps a little bit when things really get cookin.

I can't remember on these can you reasonably disconnect the wire off that sender?  Or isn't there enough room under the compressor?   If you disconnect it and the light goes out then its a faulty or incorrect sender that is just starting to trigger too soon.  If it still comes on then its to the diagrams to see if there is anything other than damaged wire between the sender and light that could cause it to come on.

[James Landi]

You've proof  that the sender unit is faulty or, as TJ  suggests, perhaps a wire to the sender that's causing the problem..  Maybe the "triggering" element in that little original unit is way off the  factory  calibration and specifications.  Leaves me wondering if there is some kind of temporary "work around"  such as an electric probe (like a kitchen meat thermometer)  that will provide you with an accurate heat reading that does not require you to perform invasive surgery into the cooling system-- and some of these are wireless.  The challenge, of course, would require you to know just where to place such a probe and to come up with some accurate way to interpret the reading of normal "metal temperature" at the location of the probe's placement. Perhaps a correlation with your IF gun's reading might serve this purpose, provide peace of mind, and get you back on the road this summer with the least possible effort. 

[R Simone]

The sender connection point itself is a PITA to get to (Under the AC Comp), however there is an easily accessible connection between the sender and the ignition coil.  I will play with that and go from there with inspecting the wiring back into the car.  At any rate this is a much more palatable gremlin vs a new overheating engine! I now have some peace of mind that I'm not cooking my engine while I sort things out. Thanks all and I'll report back when I find the fix.