Temperature gauge stuck near cold

Thanks Danny.

I started with the last two images you provided regarding the coolant temperature gauge test. I was not sure where to look for the ECM/PCM harnesses, and that is less familiar territory.

I believe I have vindicated the temperature gauge and implicated the temperature gauge sending unit.

Here is what I did:

1. Check the fuse:
I checked the No. 9 (7.5A) fuse on the driver’s side under dash fuse. It was fine.

2. Check the temperature gauge:
I found a picture in a Haynes book that identifies which connector is (A) the temperature gauge sending unit and which one is (B) the engine coolant temperature (ECT) sensor for the computer.

I measured the resistance of a jump wire at 14.8 ohms. Note that this is 0.0148 K-Ohms.

With the ignition key switch off, I disconnected the yellow/green wire from the coolant temperature gauge sending unit. I connected the yellow/green wire to vehicle ground using the jump wire.

With the ignition key switch still off, the temperature gauge inside the car remained at the cold end of the scale. I turned the ignition key switch on and the temperature gauge immediately went to hot. I turned the ignition key off after it had been on for only a few seconds. The gauge inside the vehicle appears to be working.

3. Check the temperature gauge sending unit:
The Haynes book indicates that, "With the engine warm (133 degrees F) resistance should be around 137 ohms. When the engine is hot (185 to 212 degrees F), the resistance should drop to 46 to 30 ohms."

I connected the jumper cable between the yellow/green wire and one probe of an Ohm meter. I connected the other Ohm meter probe to vehicle ground. Before starting the engine, I measured 2.487 K-Ohms, or almost 2500 Ohms. The ambient temperature was between 80 and 85 degrees Fahrenheit when these measurements were taken.

I started the engine to let it warm up. When the Ohm meter display got to 0.998 K-Ohms (just below 1000 Ohms), I looked at the (ECT) temperature reported by my OBD II meter, which had been ramping up between 129 and 138 degrees Fahrenheit about the same time. According to the Haynes book, I should have seen something closer to 137 Ohms (0.137 K-Ohms). Even if I adjust the measurement to account for the 14.8 ohms in my jump wire, I cannot get from the measured 998 Ohms to something close to 137 Ohms. This looks like a failed temperature gauge sending unit.

The lowest reading I observed was 214.4 Ohms when the OBD II indicated an ECT between 203 and 206 degrees Fahrenheit. If I subtract the 14.8 Ohms contributed by my jump wire, that gets us to 199.6 Ohms, where the Haynes book indicates we should see something in the range of 46 to 30 Ohms. Again, this looks like a failed temperature gauge sending unit.

How do I find the part number for the temperature gauge sending unit for my car?

My temperature gauge appears to be working the way it used to. I think the problem is solved, but I did learn a few things.

I am generally biased towards getting original parts from the dealer. The nearest dealer's parts department did not answer the phone on 7/4/2018, Happy Fourth! I found a part closer to home. Based on various web searches I determined that a TU121 or a TS-172 part would fit my car, and I found a TU121 about a mile from home, so that is what I have. The original part was removed with a 12 mm deep socket, and the new one installed with a 13 mm deep socket, and the color of the material is not the same for the new and old part. There is little chance that I have the OEM part, but I generally do prefer original OEM parts.

My Hanes book indicated "Use a conductive sealant on the threads (not Teflon tape)." I used a very small amount of Permatex TUNE-UP Dielectric Grease. I wiped much of it off with a paper towel, but there certainly was some in the threads. On one hand, this might help prevent a coolant leak; on the other hand the part being replaced is itself a resistor in the circuit, albeit a special one where resistance varies with temperature. There is no coolant leak and the new part seems to work.

I used a WiFi ODB II code reader and a cellphone app named ODB Fusion to monitor things during a warm up in the driveway and then a test drive. This is not the same test equipment used in the pictures I previously posted, but the measurements are reported by the car's OBD computer, while the tools I have retrieve and display the data. The car got to around 195 degrees Fahrenheit while idling in the driveway before the test drive. The temperature gauge climbed as the engine warmed up, until it was a little less than half way. During the test drive with the A/C turned on, the car got up to around 205, maybe as high as 210 degrees Fahrenheit, with the temperature gauge only a little higher than half way up the scale. The ODB Fusion app seems to indicate that hot would start at around 220 degrees.

When I parked the car, I stopped the engine but put the key back to the "on" position. I expected the temperature might rise a little, but it actually fell. I started the engine and the temperature gauge jumped up a little bit, which was enough to notice. I also noticed that without the engine running, my battery voltage was reported as 12 volts. With the engine running the battery voltage was reported as 13.6 volts. Since the temperature gauge is an analog circuit, this might explain why it displays a higher temperature as soon as the engine starts and a slightly lower temperature as soon as the engine stops. The thermistor is not changing its resistance that quickly, but the voltage reaching the temperature gauges does change quickly. I am not sure whether the temperature gauge will warn me if the engine starts approaching 220 degrees, but I hope that it would, and hope even more that it never needs to do that.

The attached picture shows the gauge at what I believe was its highest reported value during the test drive.

Given that my car is twenty years old (twenty one in October), I am wondering if that gave the Honda dealer difficulty finding the problem with my temperature gauge. Does Honda still build cars that use both (1) analog temperature sensors and gauges as well as (2) separate engine coolant temperature (ECT) sensors that are fed to the on-board computer? That would reduce manufacturing costs by a little, but savings would be realized over many units produced. In any case, why did the dealer not identify the problem affecting my temperature gauge?

I reviewed a couple of receipts from recent visits to my Honda dealer. I am generally satisfied about doing business with them, but like the rest of us, they are not perfect. I had been having intermittent (difficult to reproduce) trouble with the car not starting. The dealer had the car for a day or two, and reported that they had been able to get the problem to occur. A 5/25/18 receipt indicates that the dealer replaced part 35130-S84-A01 : : Switch, steering. They also replaced: 39730-s84-a02 : immobilizer unit. There is an accompany description that indicates, "found imobilizer receiver not operating correctly and ignition switch had and open in it. Disconnected after market alarm system and replaced electrical portion of ignition switch and also imobilizer unit. As indicated, after asking for my agreement, the dealer disconnected an after-market alarm. If that was the problem, perhaps disconnecting it would be the solution. The 5/25/18 receipt also notes that I declined a recommended service, described as "Temp gauge is inop, sensor shows to be working properly per Honda's, estimate to replace temperature gauge-$305.12 plus tax lb1.5 must be ordered. I think it is fair to conclude that declining was the correct choice.

Unfortunately, soon after that visit, the car again experienced the problem starting. On 6/1/18, the dealer noted, "Could not duplicate complaint, possible coolant temperature sensor reading erratically throwing of PCM or possibly main relay faulty." The 6/1/18 receipt indicates that the dealer installed 37870-pk2-015 : sensor water temp, and 39400-s84-003 : relay assembly, main. The accompanying description indicates "replaced coolant temperature sensor and main relay N/C."" Note that the "N/C" appears to indicate no charge, and in fact the dealer did provide these parts and labor at no charge. The car has not had a problem starting since that time. The dealer solved that problem. I appreciate the no charge visit, and the fact that the problem was solved. Based on web searches regarding problems that others have reported, I suspect it was the main relay that was causing the intermittent difficulty getting the engine to start.

The water temperature sensor that the dealer replaced on 6/1/18 seems to be the one for the ECT. The part I replaced on 7/5/18 was for the analog temperature gauge. I do not know if there ever was an actual problem with the ECT sensor. As I said earlier, I believe I made the correct choice to decline the gauge replacement. I wonder if part of the problem is that my car is so old that mechanics at dealers are simply unfamiliar. It could also be that some of these problems are simply difficult to solve. The dealer did get rid of the intermittent problem getting the engine to start.

I have to express many thanks to Danny L and 2carpros. Com!