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1995 Buick Park Avenue - Tricky temp issue

I have been getting a false Trans Fluid Temp reading on my scanner. It seems to be stuck at 304 even when cold Today it briefly dropped to a more normal 130-140 before returning to that 304 number instantly.Sometimes it comes up as a P0173 code This car is a bitch to replace the sensor in the tranny because the wiring is inside the side case and not accessivle from the bottom pan.Is it possible this is more of an external wiring issue? Every false reading sets off the check engine light but otherwise the car seems to shift and drive nomally and all the other numbers are good. I am thinking of investing 10 cents to cover the check engine light with a piece of electrical tape or keeping the scanner on all the time and just erasing the code as it appears.I dread dumping hundred and hundreds of dollars in labor to replace a $20 part inside the tranny side case only to find out that was not the problem The car only has 95K miles on it

I don’t think your trans temperature reading has anything to do with the P0173 code. This code is triggered when the air-fuel ratio is too lean or to rich and exceeds the values the ECM (computer) is programmed to and the ECM is unable to compensate for the lack of air or gas.

I would focus on that problem first.

As long as the wiring between the computer and the sensor checks out ok as far as can be seen the proper solution is to replace the trans’s fluid temp sensor. However if you can find a shop with a creative electronics tech they could probably do some wiring surgery to install a known good sensor, but place it outside the transmission. As a test to see if the new sensor consistently registers the correct temperature or not. That could help to figure out where exactly the problem is occurring. It’s possible btw there’s no actual problem with the trans, the wiring, or the sensor and the problem is your particular scanner isn’t communicating with your car’s computer correctly and so it is reading the sensor temperature improperly. Trying the same thing using a GM/Buick scanner is probably the first step. The p0173 code could possibly mean there’s a problem with the digital bus communication among the car’s computer modules, which could cause the temp sensor reading problem too. A fuel trim test using a GM/Buick scan tool is in order for that.

I got a good scanner that will read the “OBD1.5” An ACTRON CP9690.I was able to clear the codes but I keep getting bad temp reads from both the transmission and the coolant sensor. Both want to read 304 degrees although neither is that hot. I finally(somehow) got the trans to register a more likely reading of 140 degrees and the coolant was reading 200 and check engine light turned off. Then they both kicked back on to 300 instantly and check engine light returned…The coolant sensor is brand new. No need to disect the tranny to get to that sensor yet as I think it is just fine now.Is there a common wire that could be causing this? The bad coolant temp reading is causing the fan to kick on early and tripping the check engine light but I feel it is an electrical and not a mechanical issue. I am not very good at electronics,just a DIY old guy and computer problems baffle me.In the old days I could rebuld a carb and replace piston rings but any checks I perform on wiring of a computer will be like having the knowledge of a child Your “digital bus communication” is something I will try to educate myself on,but is just Swahili or some foreign language to me at the moment.

you could, if you are electrically inclined, temporarily run a wire from the two sensors (do it one at a time) to your ECM/computer. You would need a schematic to splice the temporary wires into the existing wires just before the ECM, or trace the existing wires to the ECM. There are ways to do the splice temporary without destroying or cutting existing wires. I could imagine if you had wires with rotten insulation or damaged by rodents that your ECM gets erroneous readings.

Here are some schematics for the TFT and ECT sensors (click for full size). These are for 1996, but I assume 1995 is the same. The sensors are circled. Both sensors get a 5v signal and a ground from the PCM. First thing, before checking/fooling with wiring, would be to check all wiring connectors for dirt and/or corrosion at the sensors and PCM. The sensor grounds connect at a splice (S121) in the harness that should be checked for integrity too. Notice that the ECT sensor is a double unit, the second resistor being for the dash temp gauge (if equipped).

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Thanks for the replies.Electronics is the weakest part of my game but it’s about time I graduated into my big boy pants.this looks like a great place to start.I can read architectural plans better than anyone I know but for some reason never learned how to read electronical schematics. I think I will start by finding that S121 and using the wire colors as a roadmap. There is a common convergence of the problem sensors wiring at that point where the grounds connect.
I can also test the voltage output at the sensors I can reach and maybe I won’t have to replace the PCM if I can find a loose or vermin damaged wire.I found some bird seed behind the air filter so that is possible too.

Ok I am getting completely baffled at this point. I keep getting codes Po118 Po713 and occassionaly Po573 I can erase the codes but they keep coming back even when the car runs just a few minutes or not at all! Sometimes things turn on just from sitting in the seat including the interior fan before I even use the key.There is no way a car parked for 3 days has a 304 degree temperature at both coolant and transaxle on the scanner.Once in a while they show more correct temps of about 180 and 98 The ECM was replaced just 6,000 miles ago.Any ideas at all at this point will be greatly appreciated. A few times after clearing codes my car refuses to start now and I prefer to not get stranded somewhere.

The P0118 and P0713 codes are for Sensor Circuit High Voltage for the ECT and TFT, respectively. That means the PCM is reading a high voltage after the sensor(s), which could mean the sensor(s) has failed shorted, or there is somehow a short between a voltage source and the sensor ground circuit to the PCM. The 304 degree temp reading would also imply very low resistance or a short through the sensor(s) or sensor supply to ground circuit short.

Here are preliminary checks that should be performed for the ECT (TFT is similar):

  1. Poor connection at PCM or at sensor.

    Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.

  2. Damaged harness.

    Inspect the wiring harness for damage. If the harness appears to be OK, observe the ECT display on the scan tool while moving connectors and wiring harnesses related to the ECT sensor. A change in the ECT display will indicate the location of the malfunction.

  3. Skewed or mis-scaled (or shorted) ECT Sensor.

As for the P0173 (or P0573?) codes, they seem unrelated to the temp reading issues. If electrical items are coming on with the key off, that suggests there could be other shorts within the harness(es), or possibly bad relays.

I believe the pcm supplies the 5v reference for both of those sensors

I replaced the ECM and all those bad code reads vanished. The car is running and scanning perfectly now. I used the MEM-CAL from my old ECM in the new unit. It turned out to be a malfunctioning ECM. Engine coolant is 193 degrees and transmission temp is 93 at idle.MIL is stay
ing off now.

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