question about the coolant temperature sensor

[vincenzo masiello]

i’m an electrical engineer and been diagnosing, dealing with, and designing circuits and wiring and control systems for decades and have a good knowledge of car electronics.
my question is:
in an ignored car cooling system, a coolant sensor is usually covered with a layer of mud-like dirt inside the engine and the contacts in its connector (which are right under the thermostat housing and the sensor itself) and the wires get severely corroded through the years adding, probably, a few ohms to the resistance of the sensor that works with a negative temperature coefficient. the small increase would not be noticeable when the engine is cold and the sensor resistance is in the thousands of ohms, but when the engine reaches its standard operating temperature, the change should be significant (try adding 0.5 to 3000 and then 0.5 to 12, for instance). that should lead to the computer and the dash gauge, and even the fan to think that the car is cooler that it is. is my analysis correct?

i just inspected the sensor in my car. there is a wire that is about 5″ with a connector on one side to the sensor and another connector on the other side to another wire coming out of the main harness that goes everywhere (including the computer). i removed the little wire and cleaned all four sides with an electronic spray cleaner. then measured the resistance of the two conductors and it was two small for my fluke meter to recognize. then measured the resistance of the sensor (car was sitting for about 3 hrs) and it was about 850 ohms. then measured the voltage of the other side and it was only 1.2 volts when the key was at ON. that worries me because i expected something like 5 volts. should i dig into the harness?

i have a 2008 grand prix that probably never made it to the middle of the gauge (210 F) and the fan never ran without AC!
is it possible that the two factors i mentioned above is causing my gauge to show 185 average?
last weak i did a thorough flush and i’m about to put the manufacturer recommended coolant (dex-cool for GM) and replaced the thermostat and ordered an AC Delco radiator cap and did several trips with an obd2 temperature scanner connected and the temperature was always between 182 and 195 even though the new thermostat is of the 195 kind which is what the manufacturer recommends to keep the engine at 210 for optimum performance. do i need to replace/clean the sensor too?

Topic

[vincenzo masiello]

update:
i drove the car for about 30 mins and measured the resistance and it was 230 ohms. both hot and cold resistances of this sensor are far from manufacturer’s specs.
then, while sensor disconnected, i connected resistors in its place and with a resistor of 100 ohms, my gauge was at 3/4 where the red zone starts and with 39 ohms, it was T max.
so, my conclusion is that the wiring and gauge and the whole system is good, but the coolant sensor is bad. it looks like they have a lifetime of a few years, and then their range becomes narrow (or they become less sensitive to temperature change).

there is only one other probability, which is that, even though i purged the system and used the spill-free funnel and air valve, but the sensor lives in a bubble of air or steam and is not working correctly.

any opinions are appreciated

[Timothy Salomon]

Since the coolant temperature sensor is out of range, I think you’re ready to replace it. It shouldn’t read lower temperatures than the thermostat is made to open at. Most likely it is just the sensor. The low readings seem to prove this.

[vincenzo masiello]

thank you sir for replying.
I just ordered a new OEM sensor and should get it in a couple of days. It was only $20.

The problem is that Haynes made books about all years of grand prix till GM stopped making them, but they only skipped the last year (2008) and it’s a lot of browsing for me to find a schematic. According to other similar GM engines of same year, the two conductors that go to that sensor are:
1. ground inside the ECM.
&
2. at the connection between a pullup resistor and an analog-to-digital converter(ADC, usually very high input impedance) inside the ECM.

So, when i measured the voltage coming to the sensor when it was disconnected, I expected the beautiful 5V, while when measuring voltage with the sensor connected (kinda hard in reality) a lower voltage may make sence due to the voltage divider between the pullup R and the resistance of the sensor.

Having done the 100 ohm and 39 ohm tests and observing the gauge made me now think that in my specific case, the circuit inside the ECM is probably wired differently….puzzling

And that’s why, I’m looking for other opinions, hopefully more experienced than MHO, in addition to the fact that i’m not familiar with CTS resistance range shrinking after a while (thought it only becomes an open circuit when it’s bad)

BTW, this is a great video that’s very close to what i’m talking about https://www.youtube.com/watch?v=GdUSvwZ4bPY

[zero]

[quote=”Timothy S” post=166451]Since the coolant temperature sensor is out of range, I think you’re ready to replace it. It shouldn’t read lower temperatures than the thermostat is made to open at. Most likely it is just the sensor. The low readings seem to prove this.[/quote]

That makes no sense. The ECT sensor has to be able to read temperatures far below the thermostat opening temperature. The coolant temperature is one of the parameters used to determine the temperature of the engine so the engine can operate appropriately. If it’s 0 degrees outside and the car has been sitting overnight, it will read 0 degrees.

Your best bet would be to check for voltage drop inside the harness, with it disconnected if you suspect a wiring issue.

The dashboard gauges are only indicators and are essentially there for the drivers enjoyment. If you want to read what the sensor is outputting accurately, most scantools that are more than code readers will be able to show the actual value.

[vincenzo masiello]

thank you, agree.
now im thinking that i bought the new sensor just to compare it’s response to heat with old one (over the stove test like this https://www.youtube.com/watch?v=YxV6M7Ror5c) just because it’s cheaper, easier, and less risky than tackling the wires and computer side or trying to contact a dealer for info or finding another car of the same kind to compare 😛

[vincenzo masiello]

[quote=”Timothy S” post=166451]Since the coolant temperature sensor is out of range, I think you’re ready to replace it. It shouldn’t read lower temperatures than the thermostat is made to open at. Most likely it is just the sensor. The low readings seem to prove this.[/quote]

my statement that the resistance range has changed was only an educated guess, because i dont have another reference for the temperature of the coolant inside the cooling system when the engine is hot and it’s dangerous to open the radiator cap to measure.
i based my guess on the fact that driving any car for 20 mins on a highway and city traffic and with a new thermostat should make it reach the middle or more.
i will do the stove test as soon as i receive the new one and confirm

thank you again mr. timothy

[Steve Johnson]

You can measure the actual temp of the coolant with an infrared pyrometer. Check to see if the thermostat is operating properly and see if the temps match your scanner,or/and temperature -resistance chart.

[vincenzo masiello]

[ebay][ebay][/ebay][/ebay]I think that the inrared tool is a great tool but is too expensive for this specific task when it won’t measure the real temperature of the liquid inside the cooling system. but i’m getting a digital contact thermometer that i can make it’s tip touch the engine block when it’s hot just to get an idea and to prove to myself that the sensor resistance is indeed not reflecting the temperature of the engine. (like tis little cheap thing[ebay]http://www.ebay.com/itm/LED-Display-Digital-Temperature-Meter-50-to-110-Gauge-Thermometer-Sensor-New-/262363407835?hash=item3d1613e5db:g:iqgAAOSwa-dWsGo7[/ebay])
I’ll be very happy if it’s just a sensor thing, because otherwise, i’d have to continuity test the two wires to the connectors on the ECM, and if one or both don’t beep with all the dozens of ends on the ECM 3 connectors, that’s a nightmare. I pray that no one get’s a wiring problem like that.

[zero]

You can pick up infrared thermometers pretty cheap, you don’t need a super expensive one. I paid like $20 for mine on sale.

Once the engine is up to temperature and coolant is flowing through the rad hoses, the rad hose connected to the thermostat should be within a couple of degrees. If the rad cap isn’t on during the warm up, you can check the temperature of the coolant physically there too.

[vincenzo masiello]

Thank you lots DaFirns for the push towards those things. I initially looked up ebay and they were $75-100 and then I remembered seeing them at harbor freight (where electrical/electronic tools shouldn’t be bought) for around $35. I’ll keep that in mind and get a $20 one day. for now, the new sensor is coming in the mail today, and I will put it with the old one in hot water and see if there’s a large difference in resistance measurement with two or the same multemeters.

[vincenzo masiello]

[b]ok, so I finished reflushing (emptying-starting for 10-20 min-filling radiator) 10 times (5 with distilled water to get all the old antifreeze out and five with 60/40 dexcool to bring it back to specs) purged the system the best I could, put new OEM thermostat-195 and a new radiator cap.
I just received a new OEM coolant sensor and compared it’s reading to the old one and they both read almost the same resistance for the same temp (which was for both 2.3 Kohms cold, and a little over 200 ohms at engine operating temperature).
I measured the voltage coming to the sensor (with the sensor disconnected), and one wire reads 5V to battery ground and 1.2V to the other wire, and then checked continuity between these two wires and the ECM connector and found their location at the connector and they were good.

My engine is still, pretty much, in the middle between 1/4 and 1/2 the gauge which is about 192F, which is below the temperature that the thermostat opens, and the gauge never gets to the middle (210F) no matter what, but it does look unstable to me because it get’s closer to the middle (I saw a little over 200F, once) when driving in traffic, while it approaches 1/4 (177F) when driving on a highway at night when there are no hills.

The normal behavour that I’m sure everyone knows is that it shouldn’t fluctuate that much, and it should get about 20 degrees hotter.

Are there any people who have experienced something similar?
Please help[/b]

[vincenzo masiello]

I read somewhere that the cooling fans have two (may be more) speeds. Is that thue?

If there are speeds, then is it working by the computer using pulse-width modulation (PWM) or is it only two speeds due to some wiring trick in the fan’s DC motor armature winding?

If it’s the latter case, then there should be two relays (one for each speed) and there is a chance that only one is malfunctioning causing the closed loop temperature control system to get messed up and bounce.

I drove, yesterday, for several hours after bringing to coolant back to around 60% and purging the system well. Using the OBDII scanner and a wireless microphone I attached to the fan to see when the work. I noticed the following:
1. It takes a relatively long time of serious city driving to bring the temp to around 210 (about 30 mins).
2. The thermostat opens at around 197 and closes at around 191.
3. Most importantly, the fans work high speed when AC or heat is on, but not without. They only work high speed at around 219 which is kind of too much unless there is a low speed operation that was supposed to start at a much lower temp in the range of 200-210.

Anyone with a 2004-2008 Grand Prix who knows about fan speeds?

[zero]

Generally there are 2 separate fan speed circuits. You would have to look at a wiring diagram for your car to know how they work exactly.

Usually the fans will come on once the thermostat has opened.

If the fan has 2 separate input points you can physically apply voltage to the circuits separately. If the motor on the fans is drawing excess current, it’s possible that it isn’t working at all on low speed and not going fast enough on high speed. Again, you would have to refer to an actual wiring diagram of your make and model.

[vincenzo masiello]

The only year/model that Haynes doesn’t make a manual for is this one. I could get a close schematic from googling for a few hours last weekend but I don’t know how to post it here. In my under-the-hood fuse box, there are 3 fuses and 2 relays which doesn’t look like the best schematic I found that has 2 fuses and three relays. So, I may have found the wrong schematic.

I’ll test the 2 fuses and three relays that I found in the morning.

[vincenzo masiello]

I tested the fuses ans relays and they were all good. I even measured the resistance of the contacts of the relays when they close and then operated a rated load on each and everything was good. Then did some driving and observing last night and noticed that this time there were fans on at around 210. Forgot to mention that I put the 192 degrees OEM thermostat and a new rubber ring around it (the old ring was cracked and swollen).

May be taking the relays out and back in made them all work somehow. Anyway, the temperature now almost never gets higher than 212.

The most important thing is that after a long research, I found a very interesting website with a few articles about this thread that I started, I hope it helps many of us. The sensor does not last forever. Here are two samples:

Diagnostic Solutions: Engine Coolant Temperature Sensors

Saturn Tech Tip: Diagnosing a Cracked Engine Coolant Temperature Sensor

Thanks to everyone who read and/or replied here.

It looks like I will have to ignore the warning that I received about changing or messing with the sensor, since I already bought a new AC Delco (made in Israel) one.

And, BTW, I mentioned something wrong above, when I said that the voltage coming to the sensor was 1.2V. I measured the voltage between the two wires coming to the sensor when it was unplugged and that was wrong. I just measured each of the wires with respect to battery ground and they were 0 and 5 volts.

[Author]

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