Coolant temperature reading high

Hey guys, I’m having problems with my 92. My coolant temperature gauge reads high (all the way to the top) but it’s not over heating or that hot. It functions as it should once in awhile. I’m thinking it’s the coolant temperature sensor and I bought one to replace it but it doesn’t fit( I bought an aftermarket one) have any of you guys had this problem?


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I believe there are three coolant-temp sensors (gauge, ECU, and fan switch). Did you buy the right one?

@Supreme, probably you are the same person on the FB Owners group....

There are 3 coolant related temperature sensors on our NSX and the one for the temp meter on the gauge is called coolant temperature sender unit.

If the gauge going all the way up when it's cold, you better check the sensor signal wire for a short circuit to the GND.
If there is a short circuit to the GND, you better not go into IGSW P2 On as it could damage the temp gauge.

The sender unit is the one mounted on the water passage below the TH body near the EGR valve.
Not so common but found several failure reports of this sender unit (including mine) on here but as far as I know,
they all went the other way, showing much lower temperature even after fully warmed up (rad fan switches on and off properly) or taking ages to reach the middle/horizontal marker
like when you have stuck opened thermostat.

You can easily check the sender unit with multimeter.
The resistance should be about 50ohm when it's fully warmed up.

Mine was like 150ohm instead and the needle sits about 2nd line from the bottom even after fully warmed up and while cruising.
If I park the car with the engine running at idle rpm, from time to time, the needle gradually goes up to where it should be.

For replacing the sender unit, to do it properly, it will involve the use of liquid gasket and thus, best to be done when the coolant is drained.

The sender unit is the same for all models and it's 37750-PH2-014 but as mentioned above,
it doesn't fail so often so please first check the sensor wire for the short circuit to the GND and the resistance of the existing one.


Kaz

Thank you so much kaz! I will definitely check the sender unit with a multimeter as soon as I get out of work. I also orderedthe same sender unit as well, just in case. I really appreciate your help in this matter and hope to find the solution tonight!


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Hi [MENTION=25737]Kaz-kzukNA1[/MENTION] I finally had the chance to check and you were absolutely correct! The wire for the sending unit is being grounded. Do you know where it is being routed or do you have a wiring diagram so I won’t have to tear apart the unnecessary stuff? Thanks again!!!


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First - acquire a supply of acetaminophen or whatever your preferred analgesic is. Chances are you will be needing it!

Go to the NSX Wiki - Repairing Your NSX section. Towards the bottom of the page you will find links to .pdf versions of the shop manual. Download the complete manual or just the electrical section if it is still available. Starting on page 23-15 of the electrical section of the manual, you will find a listing of every connector in the NSX and the harness it is located in and a diagram showing the approximate location of the connector. The connector for the coolant temp sensor is C109. On the diagram page 23-19, you can spot C109 and you can make an educated guess as to how the wire is routed (because as far as I can tell Honda does not spell that out for you). Reading the crumbs provided by Honda, my guess is that the wire exits the engine harness and connects to the floor harness. On page 23-20, you will see that there are 4 interconnecting plugs between the engine harness and the floor harness. These are located on the right side of the bulk head. I am not aware of pin-out diagrams for these intermediate plugs - that would be great if there were. You will have to check the wires colors and hope that the color assignment for the coolant sensor is unique which would allow you to pick it out of the connectors. You could then test to find whether the ground is in the engine or the floor wiring harness. I think the floor wiring harness runs along the right side of the car, up under the dash on the right side and spreads all over the place under the dash; but, more importantly to two 30 pin connectors C412 and C437 which plug into the back of the dash cluster. The coolant sensor connects to pin B28; but, I don't know whether B28 is on C412 or C437.

Honda has a separate manual called the electrical trouble shooting manual (at least they have one for later cars). That manual has a lot more info on wire routing. I have one for my 2000; but, unfortunately it is locked up in the trunk in a wrapped up car in locked secure storage so not available. If you can lay your hands on the electrical trouble shooting manual for any model year, I would bet that the routing for the coolant sensor did not change during production. That manual may give you some more assistance in tracing the wire. Unfortunately I don't think there are any .pdf versions of that manual available.

Before pulling any of the interior apart to check the routing of the sensor wire, I would give a close inspection of the exposed portions of the engine wiring harness as shown on page 23-19. You can spot connector C109 and make an educated guess as to its likely routing. Have a look at the harness at any point where the harness is under a clamp or comes into contact with anything, looking for chafing in the wiring harness wrap. I would assign a higher probability that the problem is in the engine harness just because that is where the heat and vibration is.

Happy hunting!

[MENTION=26435]Old Guy[/MENTION] wow, I want to thank you so much. I think your advice will really help me in my search for this dang ungrounded wire. Looks like I will have my hands full all weekend. I hope it is an easy solution and it’s in the engine harness. That would make the most sense! I will keep you guys posted. Thanks again so much [MENTION=26435]Old Guy[/MENTION] [MENTION=28830]jwmelvin[/MENTION] and [MENTION=25737]Kaz-kzukNA1[/MENTION]


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[MENTION=34545]Supreme[/MENTION], obvious question but did you look for the short circuit to the GND after disconnecting that single wire socket pushed into the top of the sender unit?
This is just in case the sender unit created short circuit to the GND within its sensor body.

Depending on the country spec, looks like the sender unit is not used on the later model and instead, ECU is creating the virtual signal for the temperature gauge but I'll leave it for now as it's not related to your early model NSX.

For those with the sender unit, obviously, it's on the engine (water passage) and the single wire socket at the sender unit is on the Eng harness.
That goes to the right forward section of the eng bay near the fuel filter and you will see big oval grommet at the firewall.



The wire goes inside the cabin through there and just next to the ECU, you will see 4 connectors (grey and blue) stacked on a metal bracket like in the photo.
It will look different with the later model as there are only 3 connecotrs instead of 4.

The 4 connectors are, 18pin, 20pin, 8pin and 16pin ones.
The sender unit single wire is YEL/GRN colour and on the 18pin conn.
Should be the blue one but you can check the continuity.

From here, I'm afraid it goes all the way through the floor harness to the gauge connector so all you can do is disconnect the 18pin connector to confirm whether the short circuit is within the Eng loom side or the Right side floor harness.

You can download the workshop manual from NSX Wiki.
I think someone on here placed the scanned version of electronics wiring diagram somewhere but I forgot who it was.
May be it was Joe on the FB Owners group???


Kaz

Thankyou [MENTION=25737]Kaz-kzukNA1[/MENTION] I will definitely check out everything you pointed out when I have time this weekend. You guys are really great, with all of y’alls help I’m sure I will be able to pinpoint where the ungrounded wire is. I will keep y’all updated! Thanks again so much. I cannot express my gratitude!


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So I finally check all the wiring, i checked every where you guys mentioned and all the wiring is being grounded out. The only thing it can be is the cluster I suspect. Sigh... took me about 8 hours hahha. Thanks again guys!


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Did you ever get this fixed, having same issue.