Hello. Need a hand. Have 2000 Civic DX with temp gauge not working. Coolant fan turns on when warm. Heater works. Very odd, when I turn the ignition to the on position the needle moves slightly DOWNWARD but stays well below the C mark whenever on - even when driving around. Need a hand please.

 

The sensor (sender) is screwed into the end of the cylinder head underneath the distributor toward the back of the engine. It has one wire. Make sure the wire is securely plugged in to the sensor. If you ground the wire and turn the key on, the gauge should go all the way up toward H. Do not leave the key on and let it slam at H. If that works, probably the sensor is bad.

 

I don't remember if its screwed on but if you unplug it and drive around and that needle still stays down like you said it does than you have signal coming in from somewhere to that plug.

 

Thanks SO MUCH for the update. Now it gets interesting. I have now done the following to try to get it to register anything:

1. Tried troubleshooting the pigtail, the wire was frayed thought that was it so went to LKQ to get the following:
a. Picked up 3 pigtails for the temperature sensor.
b. Bought 3 replacement dashboards (2-DX and 1-LX).
c. Bought 1 temperature sensor from Auto parts store - Inter-motor WT5009.
c. Bought 2 each temperature sensors from the HONDA parts store - Sensor Assembly, Water Temperature (TW5) (Matsushita) - Honda (37870-PJ7-003).

This an attempt at elegance in finding the problem, but, to also have a sledgehammer to get this done and fixed.

2. Now for the process steps
a. Pulled the battery and reset everything overnight.
b. Replaced the pigtail. Soldered, capped, and then heat treated seal. Drove the car until radiator fan kicked on. Temp gauge is still dead.
c. Pulled dash, replaced dash with each of the three dashed from LKQ. They all have the same exact behavior. Temp gauge in all three dashes same behavior. Needle pushes ever so slightly down when the key is in the RUN position.
d. Replaced temp sensor with parts store unit - still needle moves ever so slightly down.
e. Replaced temp sensor with HONDA temp sensor - still needle moves ever so slightly down.

3. Then pulled every fuse both under the hood and in the engine bay to check them. None blown.

Car appears to drive ok except mileage is down slightly. It thinks it is cold I guess so putting more fuel into it. This is very challenging as I have tried many things here with no good result. I guess now I am looking for wiring schematics for the temp sensor so I can manually trace the wires? At this point I have NO CLUE what to do next and I think it is probably not good to drive this car around like it is being driven now. Any help here is so very much appreciated.

 

The wiring is very simple. The gauge head has three terminals that are screws through the back of the cluster. One is power, one ground, and one the wire to the sender. The power comes from a bus on the printed circuit plastic that also feeds the tach, speedometer, gas gauge, and most of the warning lights.

The sensor is a resistance to ground that reduces as temperature rises. Cold it is several k ohms. With engine warmed up it should be about 80 ohms. This is why grounding the sensor wire should drive the gauge to full hot. With it open it stays at cold.

Test continuity from the sensor plug under the hood back to the cluster plug.

 

You have confused the sensor signal and the instrument cluster gauge combination. The ECU gets the engine's temperature from the TWO WIRE plug and sensor on the end of the cylinder head. The sensor part number is as you have posted above (37870-PJ7-003). The problem is that this has absolutely NOTHING to do with the gauge !!! The single wire sensor just aft (towards the firewall) from the one you have been messing with provides the signal for the temperature gauge in the dash. That Honda part number is 37750-PH2-014. It is a single wire connection and the wire color should be yellow with a red stripe.