Dash temp gauge vs MES scan temp

[SeleMatt]

Thank you JohnL

Your salient points here seem to be the following:

When coolant temperature reaches X° (as 'reported' to the ECU by the coolant temperature sensor, correctly or not) the ECU goes into 'closed loop' mode and will 'un-enrich' the AFR.

I don't know what coolant temperature the ECU is programmed to see as being the X° trigger point to enter closed loop mode, somebody else may know(?).


And from Craig

I think a basic description of how the temp gauge and the ECU determine temps is in order - Bazz??

As I recall, the temp gauge and the ECU source their data from different sensors?? (Never understood this myself, I may be wrong).


Thank you all thus far.

[bazzbazz]

Everything that John has written is quite correct, you're going down rabbit holes chasing things like Lambda sensors and such.

Your 156 JTS only has one sensor for engine temperature, the one in the thermostat. It is a 4 wire sensor, with two temperature sensors built in, one sensor & two of the wires going to the gauge and its warning light, the other sensor & wires going directly to the Engine ECU.

At the back of the engine, against the firewall, there are two large black & purple slider connectors for the wiring harness. May I suggest you try cleaning these as they are notorious for causing connection issues with everything connected to the wiring loom, which does include  the temp sensor.

You need to make sure you don't get distracted, you're trying to work out why the temp gauge and temp readings to the ECU are not correct, and basically there are only two things that are involved here, the Thermostat and Temperature Sensor.

SQUIRREL! 

(That's a reference you'll only understand if you've watched the animation movie "UP") 

[bazzbazz]

One other thing, the Temp Sensor also supplies temperature readings to the Climate Control ECU (Via the Engine ECU).

May I suggest from now on do all testing with the climate control turned off and check to see if/when the low & high speed fans are coming on.

[SeleMatt]

Thank you Bazz.

And yes, I will critically remember to disconnect the battery before playing with the purple harness plugs. 😉

[SeleMatt]

Okay.

So I cleaned the harness plugs and the temp sensor plug very thoroughly with high quality electrical cleaner and tested it all. No change to the previous results. When i got back into the driveway, I forlornly tapped the gauge glass with my thumb and it moved a needle width in the right direction and so several taps later it arrived roughly where it should be.
So it's at least a gauge issue.
I'm also getting hold of a temp gun to point at the thermostat housing to see what temperature it's actually running at.

It's running beautifully like usual; just need to sort this out.

[bazzbazz]

Well if you've taken the key out of the ignition and the gauge is stuck at 55 deg and did not go back to zero then one might assume there is a problem with the gauge.

I hate to tell you so. . . . . 

[SeleMatt]

Right you were Baz.

Now to remove the gauges which appear to come out as a triplet.

[Craig_m67]

Right you were Baz.

Now to remove the gauges which appear to come out as a triplet.

Yes.

If this is a facelift car, try not to break the ashtray door (ha!)

[SeleMatt]

Cheers Craig.

It is a Ti face lift car. I'll try to be careful.