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FYI:Temperature Dial is not a Precise Indication of Coolant Temp

 
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Old 16 May 2017, 11:36 am
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Default FYI:Temperature Dial is not a Precise Indication of Coolant Temp

Overheating seems to be one of the most common newcomer problems on this board. What people don't realize is that the PT Cruiser temperature gauge is used to indicate "normal" versus "abnormal" temperatures and not a whole lot more. The temperature readings are very non-linear and a large temperature swing barely moves the needle when you are within the "normal" operating range of the PT Cruiser engine. That "normal" operating range is anywhere between about 185 degrees F and 220 degrees F. That means you wonít be able to tell exactly when the low speed fan should turn on or off and when the high speed fan should turn on or off just by looking at the temperature gauge and reading the table in the service manual:


Iíve attached several digital readouts of coolant temperatures from the OBD2 diagnostic port and what the temperature looks like on the temperature gauge of the PT Cruiser.

152.6F The car is just starting to warm up



185F Still warming up



186.6 When coming down in temp, this is where my thermostat typically closes and the temp starts going back up



194F My thermostat is rated at 195F. On long freeway drives, the temperature usually sits around here and is steady



201F Still warming up



204.8F



206.6F



208.4F This is when the thermostat opened up for the first time this morning. The temperature immediately started dropping after this and bottomed out at 186F before going back up




On another occasion when I didnít bleed all the air out, the temperature reached 250F. This is what it looked like on the gauge.
BAD!!


If you want to know what your engine module is really reading from the coolant temperature sensor, use an OBD2 reader, whether it is a stand alone diagnostic scanner(~$50 from Harbor Freight), a wireless Bluetooth or Wifi module linked to your phone (~$20) or a Scangauge or Ultragauge (~$80 - $100), the digital readout will give you a more exact reading of temperature than the instrument dial. Of course it is only as good as the temperature sensor itself and if that's bad, your readings will be bad. A temperature gun (another $20 item from harbor Freight) can also come in handy and is a good "third party" indicator.

Last edited by rsrocket1; 22 Aug 2017 at 12:03 am.
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Old 16 May 2017, 12:02 pm
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Default Re: FYI:Temperature Dial is not a Precise Indication of Coolant Temp

This is very good info to know. Especially now since it is getting hotter and the overheating issues are starting to show up. Without a scanner that shows live data it is very difficult to know what is happening.
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Old 16 May 2017, 01:23 pm
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Default Re: FYI:Temperature Dial is not a Precise Indication of Coolant Temp

My 01 has a very similar curve. at 185 it is just above the lower mid marker, doesn't reach the bottom of the middle marker til it hits 195, it stays there without any noticeable movement up to at least 217. It hasn't gone above that temp since I installed the ultraguage three months ago, so I have no idea when it will start rising past it's "normal" position touching bottom of middle marker line.

I have a superstant 195 thermostat in the car. At highway speeds, temp typically runs in the 184 - 188 range. the idiot gauge reads that about a third of the the way up between lower mid mark and mid mark.
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Old 16 May 2017, 03:38 pm
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Default Re: FYI:Temperature Dial is not a Precise Indication of Coolant Temp

Plenty of cars have these idiot gauges. They don't tell you what the actual temperature is, they only alert you if there is a problem. We have been talking about them here for years. The purpose was that people would complain when their temps were normal, so the "normal range" was put where you expected the gauge to be. I think many of the new oil pressure gauges are similar. Mine is dead center from around 195-220.
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Old 16 May 2017, 05:00 pm
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Arrow Re: FYI:Temperature Dial is not a Precise Indication of Coolant Temp

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Originally Posted by Fk5 View Post
Plenty of cars have these idiot gauges. They don't tell you what the actual temperature is, they only alert you if there is a problem. We have been talking about them here for years. The purpose was that people would complain when their temps were normal, so the "normal range" was put where you expected the gauge to be. I think many of the new oil pressure gauges are similar. Mine is dead center from around 195-220.
I know the idiot gauge has sweet spot for idiots so they don't complain.. I didn't realize just how large that range is until I put the ultragauge in. It still is good to know my car's gauge isn't reading abnormally, seems all cruisers have that same reading.
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Old 16 May 2017, 07:37 pm
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Default Re: FYI:Temperature Dial is not a Precise Indication of Coolant Temp

Just to add I did this same report for GEN 1 PT Cruisers. Remember that the reporting using the Diagnostic connector is only as good as the information sent from the coolant sensor. To be accurate the use of a hand held should confirm all results or testing perimeters.


Good write up MR rsrocket1

Temperature range on dash gauge
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Last edited by NitroPT; 17 May 2017 at 08:58 am. Reason: http://www.ptcruiserlinks.com/forum/general-pt-cruiser-discussions/53439-temperature-range-dash-gauge.html
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Old 17 May 2017, 07:42 am
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Default Re: FYI:Temperature Dial is not a Precise Indication of Coolant Temp

Yep. As my daddy always said, them dash gauges are just for show and to let you know when something's really wrong.

And the problem with the UltraGauge and other devices that work off the OBDII connector is that they are usually reading off the same sensors as the dash gauges. They are just putting an uncalibrated number to the same signal.

When compared to coolant readings with a calibrated thermometer, the best I've found are quality brand mechanical gauges. I'm sure there's some aftermarket electric gauges with their own sensors that are as good too. I got one of these for my Dodge Charger.

Also, these IR temperature guns are great. But they only measure the surface temp of liquids. To get the most accurate readings, a thermometer or probe is required that can be challenged against a primary source for each use. But for all of this, what's "good enough" depends on what you are doing. The dash gauge is likely fine for most daily drivers. And the UtraGauge and aftermarket gauges seem to work well for lots of folks that want more.
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Old 17 May 2017, 07:56 am
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Default Re: FYI:Temperature Dial is not a Precise Indication of Coolant Temp

Absolutely right Handy.

The thermostat only opens and closes in the "ballpark area" too, but the computer controls the cooling fan relays based on the sensor reading so the fan logic should follow the OBD2 readout. You are right in that if the sensor is bad, the OBD digital readings and the dial will not be accurate (garbage in/garbage out).
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Old 17 May 2017, 08:56 am
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Default Re: FYI:Temperature Dial is not a Precise Indication of Coolant Temp

Hmm funny my CONFIRMED via technical specifications STANT Thermostat operates at 179.6 open. The Ultra Gauge shows 179.6 and confirmed by using a IF handheld at the T-stat housing ? I have also asked many that use the UG and the STANT and they also have found this to be true. Over the decades many have thought tat after market mechanical were more accurate then an electrical IMHO was because many did not really know how to read the gauges. Oh and there has been countless magazine articles with manufactures like Stewart Warner, Sun, AutoMeter etc., trying to help correct that old MYTH.

Nuff said.

I have experimented to readjust the calibration in the PCM having to do with the fan controller a better on/off setting. I prefer a lower full fan activation and turned off all the low speed activators. Seems to work for me.

Temperature range on dash gauge
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Last edited by NitroPT; 17 May 2017 at 08:58 am.
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Old 17 May 2017, 09:10 am
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Default Re: FYI:Temperature Dial is not a Precise Indication of Coolant Temp

Quote:
Originally Posted by NitroPT View Post
Hmm funny my CONFIRMED via technical specifications STANT Thermostat operates at 179.6 open. The Ultra Gauge shows 179.6 and confirmed by using a IF handheld at the T-stat housing ? I have also asked many that use the UG and the STANT and they also have found this to be true. Over the decades many have thought tat after market mechanical were more accurate then an electrical IMHO was because many did not really know how to read the gauges. Oh and there has been countless magazine articles with manufactures like Stewart Warner, Sun, AutoMeter etc., trying to help correct that old MYTH.

Nuff said.

I have experimented to readjust the calibration in the PCM having to do with the fan controller a better on/off setting. I prefer a lower full fan activation and turned off all the low speed activators. Seems to work for me.
Yep. I believe there was a time that the mechanical gauges may have been more accurate than the electric counterparts. Especially the cheaper brands including older models made by Stewart Warner and Sun. I consider that "old hand" experience and not a myth because it was true. As you point out, it's likely not the case today with the good quality electric gauges. That's why I bought an electric one myself. There's no overarching reason one shouldn't be as good as the other. And most should be more accurate than the OEM sensor which has pretty low resolution and a logarithmic slope rather than linear.
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