How to test coolant temp sensor connector

Anyone? I dont have a wiring diagram yet. Getting a P0118, replaced sensor, still a code. I want to ensure the connector from the loom is still good and not shorted or missing ground.

There are 4 pins on the sensor. Pins 1 and 3 are + and 2 and 4 and -. The reference voltage on pins 1 and 3 should be 5V. The first thing you should do is test and see if you get 5V on those connector terminals.

For your reference, I have simplified the wiring diagram into this easy to use breakout. This represents the entirety of the CTS system. An item of note: Ground 269 is impossible to find. The reason for this is because 269 is "ground location located within the instrument cluster wiring harness". Essentially, someplace in the wiring harness of the car there is a ground wire. That wire leads to a ground point. Ground 269 is not that wire, but rather a splice off that wire. The only way to find it would be to rip off the shielding of the entire harness you see where the wires split. I have had mine to multiple shops/dealerships in the past and none of them have been able to find it, and I have searched extensively here and on vortex and nobody there seems able to dine it either. You are looking at upward of 15-20 hours of work to find out where it is, and you would be better off just ripping out the dash and replacing the entire harness if the issue comes out to be a ground. Luckily, I don't think lack of ground is your issue! But if you do suspect a ground problem at some point, don't waste your time trying to find that location 269..

Also, after checking the voltage on the CTS connector, I would go right for T14a. Take out the battery and it is located to the right of it by the fender wall. A number of people (myself included) have had that melt on them, causing all sorts of issues from dead temperature gauges to inoperable glow plug systems.

Awesome man. Thank you much!

I have 5V on pin 3 but only 0.25 volts on pin 1.

Okay, so you replaced the sensor with no change and have a voltage drop at pin 1. I think the next step is to follow it backwards. Go find the T14a connector to the right of the battery. Disconnect it and see what voltage you get there. While you are there, you can also do resistance test on the other side of the connector to see if the wiring between T14a and the CTS is good. You can also do a resistance test on pin 9 of T14a (should be a white and brown wire..I couldn't make a white line on that diagram..) and see if you have a good connection to ground.

Also, did you clear the code with a scanner after replacing the sensor? There was another thread involving glow plugs a few days ago that was resolved with a guy simply clearing the code..

Oh, almost forgot, did you test for ground on pins 2 and 4?

Pulled the battery and did a visual looked ok. I ran outta time so i put everything back together. Havent started it yet to see if the code cleared. I put in a new tstat, sensor, and coolant, all from vw. I'll check it after work. Thanks for the help so far. If it dont work, i will check as u just described. On a side note, i yanked the intake snorkel off that is before the filter box, the screen in it was full of feathers and the screen had crust on it. Cleaned it up. Might be a good thing for others to check

That is called the snow screen, and it should be cleaned every so often. Let me know how it all goes.

Its a bird screen here! Will do.

After replacing the coolant, temp sensor, and tstat, all is well. Thanks for the help.

Excuse my jumping in here, but I'm having the exact same issue. Changed CTS, GP still dont stay on long enough, unless CTS is unplugged.

I followed the troubleshooting and schematic above, and got the following:

CTS plug:
pin 1- very low voltage
pin 3- 5V

T14a:
open on pin 9 to gnd
continuity on both wires to the CTS plug.
9V on pin 4 on the female side of T14a....

So, is that a bad ground at T14a pin 9? If so, instead of hunting or ripping out the dash like detailed above, why not just spice in a gnd near T14a off of the brown pin 9 wire?

It got dark before I got to figure out the reason that I had 9v at t14a but nothing at the cts, despite showing continuity through the purple wire. bad connection at t14a maybe? I didnt notice anything visually wrong there.

Im pretty sure that terminal 1 doesn’t put out +5v I had the same problem with my car so I tore the whole thing apart as far as tracing it back to the gauge cluster. I ended up cutting the wire to eliminate any kind of voltage drop and still only got .24 volts out of it. I tested my brothers Jetta that works correctly and it was the same 5v on pin 3, .24v on pin 1 the other 2 are grounded. It didn’t make sense to me because the temp gauge works correctly and I figured the glow plugs are controlled by the computer. My glow plugs fire only when the sensor is unplugged and I already replaced the sensor.

smitty_762@yahoo.com said:

My glow plugs fire only when the sensor is unplugged and I already replaced the sensor.

Click to expand...

With VCDS look at Group 001.
Check 1st thing in the AM before you run the car.

It should indicate what the ECU reads for coolant temp.
You GP light may only be on for a second or two with the temps we have now.
Is your car starting OK?

It was hard starting when it was under 30 degrees. Through a long an vigorous process I found that the previous owner must’ve been messing around with the glow plug relay and had it plugged into the wrong socket. So I got that fixed and then I replaced the CTS over the summer. Apparently the auto parts sold me a bad one. Because I bought another and placed the new sensor in a cup of ice water and they fire for 10 seconds at roughly 42 degrees. Just wanted everyone to be aware that pin #1 on the CTS doesn’t put out +5 VDC it’s more like .24VDC because I had the whole car ripped apart trying to find a shirt that didn’t exist and to boot it’s in a Beetle so it was a major pain in the ass. Bentleys wiring diagrams are the worst in my opinion, so it all added to the fun. Sorry to sound like such a Debbie Downer, I just don’t want other people learning the hard way as I did. Just goes to show you new parts aren’t always good, test them before you install them! And don’t always believe what people write online. I do appreciate your help @wonneber

It's good you post what you find for others down the line.
I started a wait for 3 more chimes after the GP light goes out to start the car.
Haven't had the 14 JSW long enough for a real cold weather start.