1998 Olds Intrigue coolant temp sensor question
 

Hi all,

On the 3.8L there is a "temperature sending unit" screwed into the aluminum lower intake, which inconveniently faces toward the ground on the underside/overhang of the intake, under the upper rad hose (below the thermostat housing). When I bought this car, that sensor had had the wiring chopped off flush with the pigtail connector, so in other words, the sensor was just acting as a hole plug, not connected to the wiring system whatsoever. Also missing was the harness that went from the sensor to the main harness. Behind the front valve cover near the middle fuel injector there is a female socket on the primary harness which was laying there empty, so apparently, a sub-harness went from that socket over to the temp sending unit, but it was no longer in the vehicle.

I had another 1998 Intrigue, so I cannibalized that sub-harness from it.

NOTE: the MIL light in my cluster has always been permanently on since I bought the car, and it turns out (see below) that the reason is because this sensor is not connected/operating.

Despite the foregoing, the temperature indicator needle gauge in my instrument cluster accurately reports engine temperature from startup until operating temperature is reached (just below the middle of the range, the needle stops climbing), and the PCM is controlling the engine fans correctly when the sensor is not connected.

Question: How? What other sensor/system in the car could be reporting coolant temperature information to the instrument panel (and triggering the correct operation of the cooling fans)?

I ask because I suspect the answer may be critical to understanding/resolving the following issues.

NOTE: I read in a response post from Mike on an old 2010 string about cooling fans on an older Olds Ciera the following: "Try this, unplug your temperature sending unit sensor and the fans should immediately come on and stay on" (the thread was about inoperative cooling fans). So, according to this logic, my fans should be running at all times. But they are not. They turn on and shut off in the normal fashion even though this sensor has never been connected since I bought the car.

Used an ODB II scanner the other day (buddy has a high end one). Error codes relating to the engine coolant temperature sensor came up (several related codes), including "DTC P1115 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage" and "DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage:". The scanner also reported engine coolant temperature of -38 F. Absurd. A reflection of the sender being unplugged.

Installed the harness/connector into the car thus making the sensor be "seen" by the PCM (for the first time since I owned the car and after many thousands of miles of driving it without any overheating or cooling issues). MIL light went out. Ran further scans and the aforementioned error codes gone. Scanner reported engine temperature that climbed to mid-160 F range, so voila, all is well, right?

Wrong.

Sitting there celebrating, and suddenly the car boils over, coolant puking into the overflow tank and steam coming out tank profusely. Shut off car, cooled it down, bled and refilled the radiator. Car running, parked, executing further ODB scans, and bang, boils over again. Again bleed and refill cooling system. Repeat the process, and a few minutes in, bang, again boils over. Bleed and refill the rad. Decide to drive-test the car. Scanner on whole time. Consistently reports engine temp of around 150F on scanner. I notice that for the first time ever since owning the car, the temp needle in the instrument panel only climbs about roughly halfway to its normal operating position. Won't go higher as we continue driving. Scanner reports from 150F to 160F. Start driving aggressively under load to try to get the number to climb. Eventually reaches 171F on the scanner after lots of super aggressive revving load driving (not very hot at all according to the PCM), and car boils over again.

Limp it home, bleed and refill cooling system. Disconnect the sensor that had just been for the first time ever connected, and voila, electrical fans come on (and then shut off, and come on again) as normal, with car idling. Temperature needle in instrument cluster goes back to normal behavior, working its way up to just below the mid mark, and staying there. Fans come on and off over time, car idles nicely, no boiling over and the temp needle in the instrument cluster is settling at the correct position (while there no temp numbers on the gauge, I estimate the spot it stops at to be around 200F based on the fact it was considerably lower during the above-described testing and the scanner was reporting 150-170F).

And, of course, the MIL light is back on, and the scanner reports the coolant temp error codes again.

So, basically, if I hook up this sensor, the PCM is happy and stops reporting error codes and my MIL light goes out, but the cooling fans basically stop operating, and it is only a matter of minutes sitting idling until the car boils over. I don't know what temperature is the trigger for the PCM to instruct the fans to come on, and it is possible that because the PCM is not "seeing" a temp over 170F with the sensor connected (despite the car boiling over), it is simply not going to signal the fans to come on.

So, I have unplugged the sensor (in fact removed the sub-harness altogether) and the car is back to behaving well and I have been driving it extensively the last couple of days with perfect cooling behavior. But, the MIL light is back on and the PCM is throwing error codes.

My buddy with the scanner says the coolant temp sender should only be a two-wire sensor with a two-wire harness (his scanner under information screens says the PCM only "reads" two wires in the main wiring harness for engine temp). He says this is why the PCM is not seeing accurate temperature, because I have the wrong sensor and wrong sub-harness installed. I don't buy his theory. All the sensors listed on parts pages show 3 prongs/terminals.

Possibly, the sensor is the correct part, but faulty/malfunctioning? It is sending a max signal to the PCM of 170F even when the engine is red-lining with heat, and that is whey the cooling fans are (unsurprisingly) not engaging? I think I will just try to install a new sensor and see if the needle in the dash behaves "normally", and indeed, whether the cooling fans come on as the car heats up, sitting idling.

But the reason I have posted is to find out how the car is able to operate normally the cooling fans and accurately report temperature on the instrument cluster needle with the temp sensor unplugged?

Even if replacing the sensor fixes the issue, I a desperate to get to the bottom of that issue.

Thanks, I will let you all know if switching the sensor at least results in accurate temp reporting in the instrument panel and results in the cooling fans operating.

Cheers

That was a lot to read.

I believe the PCM is designed to turn the low speed fans on when it sees a disconnection from the coolant temp sensor. My 95 and 97 Bonneville did this when I purposely disconnected the temp sensor.

Now, under normal operation, low speed fans will turn on at 212, and high speed will turn on at 226.

I'm beginning to wonder if your coolant pump is working/damaged, or your t-stat is stuck closed/too high of a temp. Once your engine warms up, feel the lower rad hose and see if its warm like the upper hose.

Thanks Mike. Seems my fans do run all the time when the sensor is unplugged. Also my lower rad hose is not really heating up (certainly nowhere near as hot as the upper which is mega-toasty). I do have a brand new excellent functioning radiator, and because my fans are on (it appears high) at all times, and because it is not yet very warm in my area, I think it may be that the coolant is being cooled so well that it is not hot when it exits the bottom into the lower hose, so the lack of heat in the lower hose may be inconclusive. I would rather do some other test/check to make sure my pump is pumping. What I plan to do follows; your comments appreciated.

I bought a brand new AC Delco temperature sending sensor today and swapped it for the prior one. No change/improvement. Same: when I connect the circuit, my fans shut off, and they won't come on again before the car boils over.

I am skeptical that I have either a t-stat or pump issue, as I rebuilt this motor last summer (July 2019) with brand new AC Delco water pump and new t-stat installed during the build. Very low mileage on both, less than 6000 miles, car has never yet overheated or shown any irregularity in cooling function (other than the MIL light on). Also, the temperature sending sensor circuit was unplugged when I bought the car, before I ever did the rebuild, which I did not know/suspect because the temperature needle in the instrument cluster was and is still working with the temp sender sensor unplugged. But, the prior owner must have unplugged it (and threw away the wiring) for a reason which predated my having installed a brand new water pump and t-stat.

Question: when you had your temp sending unit unplugged and your fans were running constantly, were you still getting (accurate) temperature indications by the temp needle in your instrument cluster? I am. This confuses me, as the temperature sending unit is unplugged and I cannot imagine where else the car is getting the data from. There is a sensor in the radiator tank, but it is a level sensor only as far as I know. Aside from that, I am unaware of any other sensor it could possibly be pulling coolant temp data from (under the hood anyway). I would like to isolate/defeat this secondary information source, just in case it is coming into conflict with the "real" temperature sending unit when I plug it in.

Ideas on that?

Can you respond to the following numbers, which have to do with my intention to pull the t-stat out before I take the next testing steps. If I pull the thermostat out entirely:

1. I should be easily able to visually see if my water pump is working by seeing the churning in the filler neck of the radiator (moving/circulating coolant) as soon as I start the motor with the cap off. Also, if I stick my finger in the cold coolant after start up, and before long I start to feel warmer, warmer, hot coolant moving over my finger this means I have a properly working water pump. Correct?
2. Also, if the pump is then visibly circulating the coolant, and I reinstall the rad cap, my lower rad hose should start to get hot (or at least warm) as the car continues to idle with no thermostat installed if I temporarily disconnect the electrical fans from running. Correct?
3. I was also thinking that after confirming the pump is circulating coolant, there will certainly be coolant flow past that sensor with the t-stat gone, so once I plug its circuit back in, I should see correct temp reporting in the instrument cluster gauge and I should see my fans turn on once the temp comes up to operating temp. The only way there would continue to be no fan turn on and the car boiling over while the PCM is reporting 150F from the sensor is if there is a problem with the electrical circuit (bad relay, etc, etc). By removing the t-stat and visibly seeing coolant circulation by the pump, this eliminates all possibilities except for electrical, if there is still an issue remaining. Correct?

1. No, unless your T-stat is open, I don't think you will see any coolant movement.

2. The lower hose, should be significantly cooler to the touch vs the upper hose as long as you have airflow through and the pump is circulating and the t-stat opens. Speaking of the t-stat, is it installed correctly? I can't remember if these ones are 'idiot proof'.

3. (A) You should see whatever temp the engine is producing due to the temp sensor is before the t-stat. (B) You probably won't get up to operating temp with the t-stat out. Ironic, you can still overheat an engine without the t-stat. (C) You could try and rmv the t-stat and watch coolant flow in the fill neck, but I don't know how much movement you will see.

Another option, I have one and its a very useful tool. Find yourself a thermal imager. So you can 'see' what the cooling system is doing.

What was your fix, if your issue has been resolved? I have a 99 Intrigue doing the exact same thing. Also, in response to the question why you are reading temp on the dash with the sensor unplugged, is that you are probably reading a default from the PCM. The three wire sensor is for VIN code K (3.8L engine). According to the wiring diagram, the light green wire goes directly to the instrument cluster while the black and yellow wires go to the PCM, which controls the fans in all operating conditions. Thanks in advance.

I'm glad you got this. To much reading for me.
Merry Christmas.

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