ECU Temp vs Gauge Temp

[Euology101]

Hello all,

First time poster, but a lurker for a while.

I just have a quick question:

I have a 1999 XJ, and as far as I'm aware it only has one coolant temp sensor, on the thermostat housing. (My previous pre-96 4.0, have always had 2 sensors, so this is a first for me)

According to the gauge on the dash, I range between just under or just over 210, pretty much consistently. On a cool morning highway drive to work, its half way between the lines just below 210. On a hot afternoon sitting in NYC traffic, AC on, its half way between 210 and the line above.

I have a OBD2 Wifi Dongle, which I use OBD Fusion, to watch things like coolant temp, and so forth.

According to this, my real range of Temp is anywhere from 193 (Highway on a cool morning) to 223, NYC Traffic.

I'm sure 90% of the people talking about overheating, are basing most of their numbers off of the mechanical gauge in the dash, but are these numbers I'm pulling from the ECU, good, bad, normal?

Thanks,
Justin

 

[sidewaysstarion]

The numbers on the gauge are fed from the ECU, 210 is normal operating temp for a 99 and 225 is when the secondarily fan comes on, if you are hitting 235+ then I would start to worry.

 

[Euology101]

The numbers on the gauge are fed from the ECU, 210 is normal operating temp for a 99 and 225 is when the secondarily fan comes on, if you are hitting 235+ then I would start to worry.

I think my question is more why is there such a variation in what the Dash gauge shows and what I see digitally, via the OBD. I know the dash gauge is not the most accurate in terms of scale, but it definitely does not feel like it throws a 30 degree variation, that I'm seeing via obd.

So I guess based on this, I'm probably in a good range? 223 on a daily occurrence doesn't seem bad? My cooling system in general should be doing pretty well considering I can easily coast down the highway at 193, going 65-70mph.

Only reason I care is because I'm leaving the Humidity behind in a few months, and moving to SoCal, so I want to make sure the system is good before I hit even warmer weather.

 

[Rod Knee]

I have the quicklink dongle that fits into the obd2 port which I read on my phone. The quicklink temps often read 5 to 10 degrees cooler than my instrument panel gauge. I also have a laser therm which I read the temp at the thermostat housihg which also is usually a bit cooler than the gauge FWIW.

 

[sidewaysstarion]

Only reason I care is because I'm leaving the Humidity behind in a few months, and moving to SoCal, so I want to make sure the system is good before I hit even warmer weather.

SoCal isn't that warm at all (I sometimes go wheeling in 110+* heat in vegas). Also 223* it nothing to worry about and it totally within the normal temp range.

 

[IllianaXJ]

You are fine.

I run into the same thing on my Yukon. I use a ScanGaugeII to monitor engine/trans temp and a couple other things when towing. You will have to learn to ignore the data changing a degree or two all the time and just trust the gauge cluster or you will drive yourself crazy. Before I knew exactly what the ECU was seeing in real time I didn't worry about temps as long as the cluster showed everything was in range. Once I installed the ScanGauge I started to obsess over the coolant temp rising 1-2*, when I knew it really was fine.

At least Chrysler gave us gauges that actually move versus gauges that go to the middle of the range and stay there like Ford.

 

[Euology101]

SoCal isn't that warm at all (I sometimes go wheeling in 110+* heat in vegas). Also 223* it nothing to worry about and it totally within the normal temp range.

Your probably right, during the summer NYC tends to be hotter, than SoCal. I spent 2 months out there from May-June, and I swear it never got above 75, where as it was 90 in NYC.

Once I get out there, you'll have to let me know some good places to go. I'm not really into rock climbing, but I do like to explore, and get off road/camp, that kind of stuff. I'll be in your general area, currently looking in Orange County with a slight possibility on SD, depending on work. - XJ is set up more overlanding style.

Glad to know these temps are normal. Appreciate your help!

 

[Euology101]

You are fine.

I run into the same thing on my Yukon. I use a ScanGaugeII to monitor engine/trans temp and a couple other things when towing. You will have to learn to ignore the data changing a degree or two all the time and just trust the gauge cluster or you will drive yourself crazy. Before I knew exactly what the ECU was seeing in real time I didn't worry about temps as long as the cluster showed everything was in range. Once I installed the ScanGauge I started to obsess over the coolant temp rising 1-2*, when I knew it really was fine.

At least Chrysler gave us gauges that actually move versus gauges that go to the middle of the range and stay there like Ford.

You hit it on the nail. I would watch my gauge because it moved a lot, and so I picked up the OBD dongle, and was like :wierd: when I saw the deviation digitally. Now that I know it's good, I'll stop watching it. Just installed a transmission temp gauge and upgraded the stock cooler, but that is sort of hidden, so I don't watch it as much.

 

[Muddzz]

The instrument panel gauge works from the CCD bus connected to the comp. circuit in the instrument panel and gauge.

The OBD2 scanner connected to the PCM I assume uses CTS input directly from the PCM.

CTS ========> PCM === CCD Bus ====> instr. panel => temp gauge

CTS ========> PCM =======> OBD2 scanner.

The instr. panel gauge is more of a replacement for a coolant temperature warning light.

If you're running to around 223 deg. F usually means your aux. fan is working (On - ~118 deg. F. / OFF - ~ 209 deg. F.)

What I did was install a manual aux. fan override switch (wired so aux. fan works from a manual switch or PCM)

When under higher heat conditions I have an option to turn the aux. fan before ~118 deg. F, the engine generally runs cooler under the same terrain and air temp. conditions, (taking less time to cool down to normal temps)

 

[Euology101]

The instrument panel gauge works from the CCD bus connected to the comp. circuit in the instrument panel and gauge.

The OBD2 scanner connected to the PCM I assume uses CTS input directly from the PCM.

CTS ========> PCM === CCD Bus ====> instr. panel => temp gauge

CTS ========> PCM =======> OBD2 scanner.

The instr. panel gauge is more of a replacement for a coolant temperature warning light.

If you're running to around 223 deg. F usually means your aux. fan is working (On - ~118 deg. F. / OFF - ~ 209 deg. F.)

What I did was install a manual aux. fan override switch (wired so aux. fan works from a manual switch or PCM)

When under higher heat conditions I have an option to turn the aux. fan before ~118 deg. F, the engine generally runs cooler under the same terrain and air temp. conditions, (taking less time to cool down to normal temps)

I have the Aux Fan relay/switch write-up bookmarked, and it's fairly high on my list of things to do. I want to be able to just kick it on as soon as I see traffic building up. I have noticed even with the extra load of the AC, I can drop temps to like 217-219 just because the AC turns on the Electric fan.

Although I've noticed the AC really kills the power and bogs down my 4.0. I had AC in my YJ, and I don't remember it being bogged down when I switched it on, though that was a 5-speed, so maybe I just manually over rode it by downshifting.

 

[md21722]

I have a 1997 and 2001. My 2001 almost never goes a hair past 210 and yet the fan will cycle if I'm idling for a while and its hot out. My 1997 by comparison just aims for the next hash mark past 210 often without the fan. Originally I thought my 1997 was maybe having a cooling system problem. I hooked up a ODBII and started driving around with it.

Then I found that my 2001 can be at 226, fan cycling, and the needle is just over 210. At 218F my 1997 will be 2/3-3/4 to the hash mark past two and only then the fan will come on.

The different model years are programmed differently for fan operation so this is to be expected. 223-226 is normal starting point for 2001 and 218 is normal for 1997.

It appears that in later years the gauges were dumbed down into more of an OK - not-OK gauge.

Jeep 4.0's come with 195F thermostats which generally mean they are fully opened at around 215F.

I would start being more concerned if the dash gauge moves past the hash mark after 210, or if it reads less than one half of a hash mark before 210.

 

[Euology101]

I have a 1997 and 2001. My 2001 almost never goes a hair past 210 and yet the fan will cycle if I'm idling for a while and its hot out. My 1997 by comparison just aims for the next hash mark past 210 often without the fan. Originally I thought my 1997 was maybe having a cooling system problem. I hooked up a ODBII and started driving around with it.

Then I found that my 2001 can be at 226, fan cycling, and the needle is just over 210. At 218F my 1997 will be 2/3-3/4 to the hash mark past two and only then the fan will come on.

The different model years are programmed differently for fan operation so this is to be expected. 223-226 is normal starting point for 2001 and 218 is normal for 1997.

It appears that in later years the gauges were dumbed down into more of an OK - not-OK gauge.

Jeep 4.0's come with 195F thermostats which generally mean they are fully opened at around 215F.

I would start being more concerned if the dash gauge moves past the hash mark after 210, or if it reads less than one half of a hash mark before 210.

Good info, and thanks for the year to year comparison, makes me feel more confident, and I'll stop worrying about this. Now onto the 800 other noises, smells, vibrations, etc that make me search for answers.

 

[Muddzz]

I made a typo mistake in the previous post. It should read ~218 deg. F. instead of ~118 deg. F. ~218 deg. F. = On / ~209 deg. F. = Off

2000 - 2001 the A/C aux fan control is different from 97-99. An A/C line pressure switch is used to turn on or off the aux. fan regardless of the compressor clutch being engaged or disengaged.

A/C aux fan control for earlier models when the A/C compressor clutch is on the aux. fan should also be on.

2000-2001 - I have read the temperature set point for On is ~223 deg. F. instead of ~219 and OFF is higher than ~209/210. I can't remember I think it's turned OFF at around ~217 deg. F.

My cooling system normally is between 195 and ~200 deg. F. while on-road. Can significantly rise in hotter weather, while off-road in heavy sand, steep inclines, etc.

Here's a diagram I drew up. My actual aux. fan wiring installation is a little different, but fairly similar.

 

[Euology101]

I made a typo mistake in the previous post. It should read ~218 deg. F. instead of ~118 deg. F. ~218 deg. F. = On / ~209 deg. F. = Off

2000 - 2001 the A/C aux fan control is different from 97-99. An A/C line pressure switch is used to turn on or off the aux. fan regardless of the compressor clutch being engaged or disengaged.

A/C aux fan control for earlier models when the A/C compressor clutch is on the aux. fan should also be on.

2000-2001 - I have read the temperature set point for On is ~223 deg. F. instead of ~219 and OFF is higher than ~209/210. I can't remember I think it's turned OFF at around ~217 deg. F.

My cooling system normally is between 195 and ~200 deg. F. while on-road. Can significantly rise in hotter weather, while off-road in heavy sand, steep inclines, etc.

Here's a diagram I drew up. My actual aux. fan wiring installation is a little different, but fairly similar.

Would this diagram be the same for a 1999?

 

[Muddzz]

Would this diagram be the same for a 1999?

I don't think would make any difference between 97 - 01 and earlier years that are wired the same.

The OE has it's own relay that's controlled by the PCM that uses input from the CTS when to turn on or off the aux fan.

All your doing is cutting the OE PCM aux. fan control circuit

Aux. Fan Relay (PDC) ========= cut == Aux Fan

and adding another relay that will power the aux. fan. directly from the battery rather power from the OE PDC aux. fan relay.

You can add an (optional diode) to further isolate power from the manual switch that's connected to the OE PDC aux. fan relay output pos+.

 

[Muddzz]

Would this diagram be the same for a 1999?

I don't think would make any difference between 97 - 01 and earlier years that are wired the same.

The OE has it's own relay that's controlled by the PCM that uses input from the CTS when to turn on or off the aux fan.

All your doing is cutting the OE PCM aux. fan control circuit

CTS ------> PCM ---trigger-----> Aux. Fan Relay (PDC) ===power====== cut ==> Aux Fan

and adding another relay that will power the aux. fan. directly from the battery rather power from the OE PDC aux. fan relay.

CTS ------> PCM ---trigger-----> Aux. Fan Relay (PDC) --------> Second aux. fan relay ==============> Aux. Fan
Battery ==========================================|

You can add an (optional diode) to further isolate power from the manual switch that's connected to the OE PDC aux. fan relay output pos+.

 

[Muddzz]

Sorry for the multiple posts. I'm having trouble when trying to post or edit a message. The Naxja server keeps going down or is too busy.

Here's another diagram ... You need to cut the OE aux fan wiring just before the plug-in and wire the OE hot wire and the switch wire to the second relay #86 when using a standard 30/40 relay.

AUX FAN <====(oe plug)=== (87)Second Relay

The optional diode further isolates the OE and manual switch pos+ trigger wires.

In this design the second relay is doing all the work supplying power to the aux. fan. The PCM and manual switch only act as trigger switches for the second relay.

I'm using an isolator relay vs a 30/40 relay and haven't had any problem. You just need to make sure the connections are properly spliced, protected such as with heat shrink tubing and aren't able to come loose. When making connections and splices I sometimes use solder other times use spade connectors, some connectors come already water proofed while other can be water proofed using heat shrink, etc.

Anyway this is the method I use. Other people may use other methods. I don't believe in splicing into the OEM wiring unless absolutely necessary. I keep all my aux. equipment on a separate power circuit from the OEM wiring.

 

[Muddzz]

I haven't tried this method. You may be able to connect the manual switch relay directly to the aux fan or splice into the OE wiring. Control of aux. fan power and control would be from two relays instead of one.

You would need to figure out where to splice the OEM aux fan wiring or connect the manual aux. fan relay directly to the aux. fan.

 

[JeepNoob]

So I got to thinking about this last night/today after see those diagrams... I found something similar posted on -

To me, this seems like a much easier to understand way of doing a manual fan override and I thought it made a lot of sense, but then after talking to a guy at work (Trust me, he's a Ford service engineer who's been working on the technical hotline since when they still talked to techs over the phone. Now it's mostly all web-based. And he really knows his electrical/electronic shit.) why even bother running a secondary relay and fuses and stuff in the first place? Would this diagram with the way it's laid out work more on an earlier system versus something with OBD II like mine? I KNOW it would work on something with OBD II like mine, but after doing a bit of digging earlier tonight, it seems like there's always battery voltage on the control/low-amp side of the fan relay with the ignition in run. I'm sure with OBD II that the control side of the relay is switched by the PCM on the ground side whenever the A/C is on or the PCM sees the engine temp getting too hot. I found a wiring diagram earlier that basically said the temp sensor closes whenever it reaches 190, but the temp sensor was also completing the voltage electrical path to the control side of the relay, which didn't see to jive with anything I was seeing. Might have to check tomorrow when the engine's cold, but it seemed like I was seeing battery voltage at the control side of the relay whenever the key was in run. Which kinda brings up the the what this fellow service engineer mentioned to me, why even bother running 2 more fuses and a relay and a bunch of wiring when you could just splice into the wiring in the fuse/relay box, run maybe one wire back through the firewall and just provide the relay with a new, manually-controlled path to ground through a dash-mounted switch?

 

[Muddzz]

So I got to thinking about this last night/today after see those diagrams... I found something similar posted on -

To me, this seems like a much easier to understand way of doing a manual fan override and I thought it made a lot of sense, but then after talking to a guy at work (Trust me, he's a Ford service engineer who's been working on the technical hotline since when they still talked to techs over the phone. Now it's mostly all web-based. And he really knows his electrical/electronic shit.) why even bother running a secondary relay and fuses and stuff in the first place? Would this diagram with the way it's laid out work more on an earlier system versus something with OBD II like mine? I KNOW it would work on something with OBD II like mine, but after doing a bit of digging earlier tonight, it seems like there's always battery voltage on the control/low-amp side of the fan relay with the ignition in run. I'm sure with OBD II that the control side of the relay is switched by the PCM on the ground side whenever the A/C is on or the PCM sees the engine temp getting too hot. I found a wiring diagram earlier that basically said the temp sensor closes whenever it reaches 190, but the temp sensor was also completing the voltage electrical path to the control side of the relay, which didn't see to jive with anything I was seeing. Might have to check tomorrow when the engine's cold, but it seemed like I was seeing battery voltage at the control side of the relay whenever the key was in run. Which kinda brings up the the what this fellow service engineer mentioned to me, why even bother running 2 more fuses and a relay and a bunch of wiring when you could just splice into the wiring in the fuse/relay box, run maybe one wire back through the firewall and just provide the relay with a new, manually-controlled path to ground through a dash-mounted switch?

Like i said there are other methods to wire in a manual aux. fan switch. I prefer this method as I don't need to mess as much with the OEM harness.

OBD, especially OBD-2 can become sensitive, a single wire or sensor can disable your vehicle, vehicle's lighting can stop working, other types of problems can occur down the road, years later.

I wired in a separate aux. fuse panel that's powered from the main battery and also have a second battery with a heavy duty high amp switch so the 2nd battery can be charged from the main battery & alternator. ISO relay(s) can also be used, aren't normally preferred over using a high amp manual type switch when installing two batteries. There are different types of iso relays, how they open and close, type of contacts, etc.

Except for my tow bar lights and aux fan all the aux. equipment is on a separate wiring harness.

The Jeep's signal, brake and running/tail lights are also able to connect to a tow vehicle.

For each tow vehicle bulb connection a diode is installed. This is important when using your OE lighting for towing. The diodes also have heat sinks.

tow light circuit __diode___
oe light circuit __________| ___(bulb)

You can wire in a single diode or purchase ones with heat sinks designed for tow vehicles, vehicle lighting, etc..

Splicing into the oe harness takes more work, sometimes diodes are necessary to prevent potential problems.