Verifying temp gauge with IR Thermometer - Defender Source
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Old September 7th, 2011, 08:54 AM
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mark kellgren
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Verifying temp gauge with IR Thermometer

So I have a VDO (had it for years). Over last several months, I've started having overtemp issues as indicated by guage. Easily get to 220-230 on highway at speed. Last couple 2-3 hour trips, I've pulled over after reaching 225 on highway, opened bonnet, and hit radiator hoses (upper and lower) with my handy, dandy IR gauge. When the VDO is reading 220ish, the IR gauge is reading 180-190ish on the hoses. Are the hoses going to accurately reflect the temp that would be observed in the t-stat housing where the sender is at?

I replaced the sender last weekend, and I'm getting much more stable readings, but haven't been on highway yet to see if it is spiking. also replaced the factory 18awg ground wire for the gauge with the recommended dedicated 14awg wire direct to fuseblock ground per VDO installation instructions. I want to confirm my gauge is accurate, rather than sweating bullets on highway hoping I'm right.

If there is a more accurate place to hit with the IR gauge, I'd like to hear it.
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Old September 7th, 2011, 09:19 AM
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John Karlsson
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Have you tried pointing the IR thermometer at the sender?
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Old September 7th, 2011, 11:11 AM
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mark kellgren
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Nope. Wasn't sure if the t-stat outer housing would reflect water temp in the housing accurately or not. Looking for feedback in any form to reduce dangerous assumptions on my part.
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Old September 7th, 2011, 01:50 PM
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I wouldn't hit the hose but rather the metal adjacent. I've been doing this on another vehicle (I've got an old sports car with Smith and Lucas so Lord knows what is right) as well as my D90.

If you hit the tstat housing with the eng comp hot I can't believe it's more than a few degress diff.

I've also found hitting the exhaust right off the head instructional- its amazing how much the temp varies from cylinder to cylinder. I've seen 40 to 60 degree diff with the centers being hottest and closetst in temp. A "cool" cylinder is supposed to mean intake or vacuum leak on carburated. On my D90 I'm planning on rebuilt injectors.

If you want to be dead cert on the tstat take it out and put it in boiling water. Otherwise I'd settle the reading on the IR and recablibrate my interpretation of the gage.

js
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Old September 7th, 2011, 01:59 PM
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steve
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I always aim the IR gun at the sender on the t-stat housing.... dunno if that is right but seems like a good spot.

When you are driving and put a load on the alternator (i.e. lights, heater, stereo) does the temp change? The grounding gremlins in defenders always seem to manifest in my temp gauge.
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Old September 7th, 2011, 07:24 PM
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Have you tried your gun on a known temperature. I have found the ggaugges to be spot on with the ir
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Old September 7th, 2011, 10:07 PM
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might want to check the voltage @ the temp gauge.
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Old September 15th, 2011, 02:17 PM
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mark kellgren
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Ok, so here's what I've learned. In another stread I stated that I was going to replace the sending wire with a 14awg...WRONG! I tried all different combinations, and it ends up that the VDO instructions were the right way to go. The factory ign+ and sendor wire (both appear 18awg) and a 14AWG gauge dedicated ground is the most accurate setup. I still get what appear around 10degF variance between gauge (higher) and IR scan of sender/housing, so either gauge is still a bit off or there is a temp diff between coolant in housing at sender and external surface.

I tried all the combinations prior to this revelation with the VDO gauge wiring. I went 14 awg ground, power and sender, and that ended up about 30-40 degF higher than the IR scans. I was hitting 235F on the highway that way, and when I pulled over and lifted the bonnet, the IR was reading 189F at the housing, no pressure on the expansion cap, no coolant loss, and engine was running fine. I was also getting a reading jump from just a 14awg sender with factory gauge + and grnd. I'm not sure exactly how the gauge works, but those resistance variations matter quite a bit.

The error in all these cases seem to become more magnified as temp increases past 200F.

With the correct setup now, I am running normal except when it's over 85F outside and i'm on highway doing 65+ mph, then it will start climbing up to about 210. I still have some work to do on my hoses and cooling system capacity however, as I have some known flow restrictions between my heat exchanger and webasto.
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