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temperature gauge accuracy

md21722

NAXJA Forum User
Location
TN
Hey all,

I was wondering why my 97 seemed to be running hotter than my 01. While I still haven't figured it all out, I found that the gauge display on each are "calibrated" differently. The signal goes from the sending to the PCM which in turn tells the dash gauge what to display.

I believe they are designed to show a warm up cycle and then show an hot/overheat condition rather than true temperature.

As an example, my 01 XJ seems to show warm up accurately figuring 27.5 degrees per hash mark between 100-210F. However, at 221F it is still hanging on the 210F hash mark.

My 97 XJ will show 208-212F as the right side of the hash mark and 218-223 as 3/4 of the way to the hash mark after 210F.

These temp readings are from an ODBII reader connected while driving.

It does seem that the 97 runs a few degrees warmer, say 207-208 when the 01 runs 199-203. However, even that difference could be from a sending unit. The 97 may have "warmed up" a few degrees with a new sender while the 01 still has its factory sending unit.
 
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The first mistake was assuming the factory guages were accurate. Granted the newer CANBUS Guage units/senders are far more accurate when compared to the previous generation of guages used in the Jeep lineup, you should take what they say with a grain or twelve of salt. Even the stock speedo can be off by 10% from the factory (mine was).
If you want an accurate reading, install a set of quality guages. The stock stuff is there just so we have something to look at at stop lights, and something to worry about now and then.
 
I'm not sure I'd call it a "mistake" at all. The ODBII tool and the dash gauges clearly shows me that the 97 runs a little bit warmer (according to the sending unit) than the 01. As an example, driving home from work, I see that the gauge of the 97 is higher than the 01 and the ODBII confirms it. What is different is how they display after 210F.

Jeep may have detuned the gauge by 2001 so people don't freak out as they are set up to run warmer. The auxiliary fan is not even programmed to come on until 223F and then shuts off at 217F. On my 97, the PCM signals the aux fan to come on at 218F.
 
The dashboard gauges are pretty much OK/Not OK indicators, not calibrated measuring devices. As long as my 98 and my 00 don't overheat, and display approximately 210*, their actual operating temps really don't matter that much to me.
 
The dashboard gauges are pretty much OK/Not OK indicators, not calibrated measuring devices. As long as my 98 and my 00 don't overheat, and display approximately 210*, their actual operating temps really don't matter that much to me.

The approximately 210 is what threw me. I did not expect the two Jeeps (97 and 01) to display any differently. My guess is that the gauges themselves are fine, but that the PCM programming changed over the years so that the 2001 appears to run cooler at temps over 210. I think the OK/Not OK reasoning is a good one.
 
The other day I was driving up a mountain trail in 100 degree heat and the AC on. The scan gauge read 245 degrees, but the gauge in the cluster was at the first line past 210. I thought that was interesting. I imagine, because of this, that some people might assume the first line is 220 degrees, but for my 99 sport, it is 245. 235 degrees is about halfway between 210 and the next line.
 
That's pretty high. I believe the PCM sets the hash before red at 254 and buries it at 262.

On my 01, at 226 there is a needle width between 210 and the needle.

On my 97, at 218 its halfway between 210 and the next hash mark.

Maybe the programming changed around 1999.
 
in my experience if i notice my jeep getting hot i would change the thermostat to a 180 deg one. I dont know how your jeep would be affected in the winter as you live in colorado but here in cali mine runs nice and cool even on 100 deg days. thats just my two cents. i know on most stock jeeps they have had a 190 or 195 deg one from factory.
 
The degree rating on a thermostat, is when it starts to open, not the operating temperature of the engine it is installed in. A well maintained XJ open cooling system with a 190* thermostat should easily hold approximately 210*-225* all day long under any conditions except the most extreme desert temperatures.
 
While on the subject,,my 96 will read 195* at the sensor in the T-stat housing with the Snap-on "brick" scanner,while the rear temp sender for the Autometer gauge will read 210*.
This difference will stay the same thru all normal operations that I have had it in.
 
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