Radiator fan + cluster temp gauge fluctuations

[alimo20]

trying to nail down the issue here, and have some ideas - care to provide some suggestions?

initial issue: radiator fans do not come on unless i turn AC on
bonus issue that just appeared: cluster temp gauge drops to 0 or fluctuates between 1/2 and 1/4.

diag info:
- fans work, as when i put on the AC they both spin, high/low etc
- torque app reading temperatures as high as 235F(!!) and it seems to correspond with the running of the car (is low at start up, then increases)
- slight (half a gallon every 6 months) coolant leak from somewhere, i haven't been able to trace aside from leaky fins near the end-caps which eventually seal

work done so far:
- thermostat replaced 10K miles ago
- replaced both coolant temp sensors, though noticed that the lower one's connector doesn't fully engage, can potentially slip out
- replaced some coolant but not a through air bubble flush procedure
- checked relay 219, no obvious water ingress issues

questions:
- where does the ECU get it's temp reading from? i am guessing that is what torque is reading from
- is this pointing to a faulty FCM or wiring from CTS
- what else can i do to diagnose?

[old guy]

The ECU gets its signal from the rear CTS. The rear CTS is actually a dual (4 pin) sensor . It sends one signal to the instrument cluster and the other signal to the ECU.

Have you checked for any DTC's?

Does the engine overheat at idle or when steady state driving? Or under both conditions?

[alimo20]

Hard to tell if it is overheating, as I can't trust the cluster gauge - but prior to the gauge acting up, it was never rising above 1/2 in idle or steady state driving. i think torque reading 235F is way too hot and is an overheat condition, but I can't tell for sure. Also the ambient temp gauge has been regularly reading 12-15 degrees hotter than actual outside temp, due to the engine bay getting so hot.

I am going to run out and re-scan for any stored codes with vcds. Torque has only been reporting an unrelated catalyst converter issue.

[seanf86]

assuming 1.8t? 235F is normal if you're cruising, the B6 thermostat is controlled by the ecm, logging on torque while on the highway I see 110+ degrees C stop and go it comes down. If your dash guage is dancing then its a bad CTS, wiring issue or cluster, I've had a dancing temp guage twice and both times it was the CTS.

After having bad luck with aftermarket sensors I only use genuine CTS

[old guy]

The temperature gauge in the dash display is not a linear instrument. It reaches the 12:00 position at 175°f and as Sean pointed out doesn't move above that setting until you go over 235°f.

Torque Pro is reading the ECU signal and not the display signal. I would be concerned with your statement that the fans only come on when the A/C is activated.

[alimo20]

The temperature gauge in the dash display is not a linear instrument. It reaches the 12:00 position at 175°f and as Sean pointed out doesn't move above that setting until you go over 235°f.

Torque Pro is reading the ECU signal and not the display signal. I would be concerned with your statement that the fans only come on when the A/C is activated.

Last night I pulled codes 2 codes, one for each temperature sensor connectivity. Unfortunately I don't have the exact codes as they are on my other laptop which I wont have access to until tomorrow. While driving yesterday, the gauge read dead center, then dropped to 1/4 and stayed there. Torque pro reported normal temps in the range of 205-220 the entire time.

I agree that the more alarming issue are the fans not coming on unless the AC is on. Given that I just replaced both CTSs (to rule out the easy fix) and still getting codes indicates to me that there is a wiring issue, potentially with the CTS harnesses, ECU or FCM. Could be a dud CTS too. Suggestions on how to troubleshoot? Its odd that I would get codes for BOTH CTSs unless they are wired to the same connection.

[alimo20]

No codes for the temp sensors and the appropriate measuring block from VCDS read 100-105C in light city and highway driving. Higher temps in stop/go traffic, but hasn't seen it exceed 115C.

Temp gauge seems to come to 1/4 and stop there, despite what the coolant temp seems to be reporting normal temps. Separately - fans still do not come on without AC being on..

Am I looking at two different issues here? The fans not coming on is pressing since we are going to start having hotter days again. What is a good way to seeing if it is the FCM without replacing it? Anyway to diagnose faulty cluster pods?