Bad ECU reading for CTS

Hello,

Jetta TDI 2001, alh.
I'm on the scanner and the cts is unplugged.
The screen shows about 7ºC.
The ecu isn't supposed to indicate -40ºC?

Earlyer, I bypassed the cts with a max 10k ohms potentiometer.
I can't go down for max -10ºC and when I decrease the resistance until 8 ohms (the minimum on the potentiometer),
I can reach about 20ºC but the ecu gradualy goes down to 7ºC.

I suspect the wires between ecu and cts to be in trouble.
Can you help me?

Ugh.... The three Cs of automotive diagnostics:

Complaint

Cause

Correction

Let's start with the first C, before we try and help you with the second. What exactly are you dealing with?

Complain

The glowplugs time on at start isn't long enough.
So the car don't start well.
I checked the glowplugs. They are good.

The car's OBD has glow system monitoring. If you have no glow related DTCs, your glow system is working. You have a hard start problem, not a glow system problem.

A "longer" preglow period may mask an underlying problem, making the engine easier to start, but that is not THE problem.

Poor engine condition, incorrect timing, poor injection bits, loss of fuel prime, could all be hard start issues.

Ok, so the base "problem" is actually "hard starting".

In Quebec right now your car doesn't actually need glow plugs to start... so as long as the lights on for 5-10 seconds the system is probably fine and you are probably needing to look elsewhere for your hard starting issue.

Is the Check Engine light on? Have you been able to scan the system for codes?

The lights on for 1 sec. That's all, unfortunately.
The temp of coolant on the scan is 4ºC and actualy the exterior temp is 3-4ºC.
I made a road test this morning and the coolant rose around 80ºC on the scan.
I think my problem isn't the sensor.
But why the glow plugs still don't heat long enough!
Maybe Oilhammer have a good way.

The CTS sensor is actually two sensors, one that tells the glowplug system what the coolant temperature is, and one that tells the dashboard gauge what the temperature is.

Not uncommon for one side to fail and the other to work fine. The easiest test is to unplug the sensor and see if the glowplug light stays on for 20+ seconds.

If so, it's a safe bet that the sensor is defective on the glowplug side. Luckily they are cheap... make sure to get a new o-ring as well.

That said, you used the word "scan"... if that means you are using a Scangauge then it's showing you the glowplug side of the equation already, and yes you might have a wiring/connector issue instead, although it's hard to figure why the ECU would still see the right temp (via Scangauge)

In terms of software, I think perhaps Oilhammer may have been thinking you have a PD engine, which had multiple versions of both glowplug hardware and glowplug software. It sounds like you have a 2001 Jetta, which is an earlier version of the engine (the ALH) which as far as I know was simple enough to only ever have 12V glow plugs and the original software the ECU came with.

Check your injection/cam timing. Some of the earlier cars had a soft crankshaft timing sprocket and the key would wear and allow the timing to change enough to give a hard cold start. If you have VCDS there is an easy quick timing check.

1 second at 4C is too short a time. Sensor likely, but try this troubleshooting thread-
Glow plugs 101 ***Ver. 2.0*** - TDIClub Forums

4 C is about 39 F.... it was about 45 F this morning for me, and my indicated preglow on my ALH was only about 1 second. No idea what the ACTUAL preglow was (remember, the indicator is just that... an indicator...the ECU's preglow strategy for powering the plugs may not mirror the indicator). And of course, as usual it afterglowed below 2500 RPM for at least the first couple miles, although I always try and keep it at about 2600 RPM to avoid the switching on and off.

All too often, people immediately jump to the glow system for hard starting, and it is rarely the cause. Especially on an ALH.

On a dead cold start, jump start the car. If you find the car fires up better with extra battery juice, you could have a battery issue. I've came across this many times when I was slinging wrenches and loosing 10mm sockets.