Ok, here are the clues:

- fans always on, constantly
- AC doesn't blow cold air, it worked fine before the swap
- both water temp senders changed
- according to stock gauge, no overheating
- fuses and relays are good and wiring hasn't been messed with

Any ideas?

OK, I have the opposite of your problem. I switched the water temp sensor today, which is a plug that is red, there is something next to it. Is that the second water temp sender?

Originally posted by AntonioG
Ok, here are the clues:

- fans always on, constantly
- AC doesn't blow cold air, it worked fine before the swap
- both water temp senders changed
- according to stock gauge, no overheating
- fuses and relays are good and wiring hasn't been messed with

Any ideas?

Your fans are turned on by the ECU as a self defense for the engine. It does it because it's not getting critical info that it's expecting, so it defaults by turning on the fans to keep it cool, and I would expect (but not sure) it also retards the timing some.

The gauge uses a different sending unit from the ECU. The ECU uses the sending unit with the red plastic connector. The gauge uses the sending unit with the single spade connector.

Check grounds, check connections, etc. Check the ECU for codes.

Your AC probably got boogered during the swap and is now out of refridgerant. Either that or your compressor just isn't turning on. Check those connections as well.

Oh yeah, one major thing to do......

Clark and Mike have told me on a number of occasions that quite often when there seems to be problems related to the ECU, simply disconnecting the ECU from the harness, and then carefully reconnect the ECU. Do this first.

Geo,

Thanks for the tips. I knew about the ECU self-defense measure, but have been through the system systematically and haven't found anything glaringly wrong - maybe I need a second set of eyes to look at it ;).

I'll try the ECU replug and play method and see if that works.

Thanks!

Originally posted by Chris Colburn
OK, I have the opposite of your problem. I switched the water temp sensor today, which is a plug that is red, there is something next to it. Is that the second water temp sender?

The red sensor is the water temp to the ECU, the black, smaller one to the right of it goes to the gauge, which is pretty much a cold-ok-too hot indicator.

I just changed both of the sensors on the DET to US spec ones since I have heard the JDM one behave a little differently. How, I do not know. The red ECU sender is new while I reused the gauge sender.

I have both fans always on with my DET. But i posted this before, when i'm cruising at aruond 70mph for about 20miles the temp needle goes from about 40% to about 60% and really puts me on edge. Could this sensor (the JDM one) be the reason for this? The gauge showing hot while it's really cool, i'm planning on purchasing a water temp gauge for a better reading.

Ok, update.

I did the Geo trick of unplugging and replugging the ECU. Nothing. Fans still on all the time.

I re-re-checked relays, fuses, connections. Everything is connected. The compressor doesn't kick on when I turn on AC. The harness is connected.

I have no idea what is wrong.

Is it possible to run a known good DE ECU at idle to see if it may be an ECU problem? Am I missing some ground somewhere? I don't think so, sinced I traced all wires.

Any insight appreciated, thanks!

If you haven't replace both sensor and sending unit, do would do so and get both from a nissan dealer. I did a swap to a j spec engine and could not get my fan to turn on when my gauge was reading hot. both senors were after market. After replacing with factory it correct the problem. The problem could be same with both of you .The sensor may not be at the right spec.

I did buy a new water temp sensor from a Nissan dealer and put it in. However, this new sensor has a blue plug while the old one had a red plug. The fans still stayed on with the red plug anyways.

Is there a difference between the blue and red water temp sensors?

You changed the temp sensor , but did you change the temp sending unit? That's the little one beside it. as for the colors, it does not matter,at least that's what a dealer told me.

That little one next the temp sender is for the stock gauge. The ECU ignores that one.

Good point, but when I did my swap I left the sending unit that came with my motor and it was reading that my engine was over heating when it wasn't, same thing happen with the aftermarket sender when I replaced it with factory it read normal, so what maybe reading normal may not really be normal.

Besides that, did you double check your connection/wires to the sensor for a short or open? If it not reading the sensor it may put the ecu into a failsafe mode, just as if the connector itself was unplugged making the fan run continously. I mean there could be a bad connection or a crimped wire and not even realize it!?!?!

Originally posted by ODonHP
You changed the temp sensor , but did you change the temp sending unit?

Semantics. Sending unit, sensor.... same thing.

If you want to be technical, the FSM calls one the Thermal Transmitter, and the other a Temperature Sensor.

Originally posted by AntonioG
Geo,

Thanks for the tips. I knew about the ECU self-defense measure, but have been through the system systematically and haven't found anything glaringly wrong - maybe I need a second set of eyes to look at it ;).

I'll try the ECU replug and play method and see if that works.

Thanks!

It might be worth spending the $65-75 for a Consult diagnostic. Have you checked the ECU codes?

Could be the knock sensor or water temp sensor/sending unit/whatever.

Originally posted by SE-Rious B14
I have both fans always on with my DET. But i posted this before, when i'm cruising at aruond 70mph for about 20miles the temp needle goes from about 40% to about 60% and really puts me on edge. Could this sensor (the JDM one) be the reason for this? The gauge showing hot while it's really cool, i'm planning on purchasing a water temp gauge for a better reading.

The JDM gauge sensor is much more sensitive than the USM sensor. I still have the JDM sensor in my car. Before I installed a double row radiator it would do exactly what yours is doing. Now it reads cold whenever I'm moving.

Change it out.

Originally posted by AntonioG
I did buy a new water temp sensor from a Nissan dealer and put it in. However, this new sensor has a blue plug while the old one had a red plug. The fans still stayed on with the red plug anyways.

Is there a difference between the blue and red water temp sensors?

I don't know for certain, but that would be my guess.

Originally posted by Geo91SER


Semantics. Sending unit, sensor.... same thing.

If you want to be technical, the FSM calls one the Thermal Transmitter, and the other a Temperature Sensor.


Not to argue, but most of the mechanics call it sensor when its sends a signal to the ecu and sending unit when its going to the gauge. But as you stated later people/companies have a different name for the same thing.

As for hooking up a scanner to the ecu, that is a good idea. I was thinking the same thing just before going to bed last night. If not, putting the ecu into diagnosic mode sometimes will still give you a code.

Or you can try unplugging a temp sensor harness off a known good car and see if it behaves like yours Antonio. I think there are a few nearby :D

I say disconnect and reconnect all harnesses and use Luis good temp 'sensor"/ thermal heat transmitter/flux capacitor. have him take it off his car =P .

GEO what double row radiator are you running?


Larry

Originally posted by Low612
GEO what double row radiator are you running?


I cannot remember the brand. I found it at a local radiator shop. Plastic tanks with aluminum core. I paid something like $150 for it.

They don't have one for the SE-R though. :(