13 August 2005

Replacing Audi TT Coolant Temperature Sensor

As I wrote a couple days ago, the Audi TT had evidence of a bad coolant temperature sensor. Today I made the repair and took some pictures along the way. The rest of this post details the steps involved in this fix. Because being litigious is still the new hotness, the following disclaimer applies:

If you follow the instructions you see here for this repair, you could seriously damage or destroy your car, the building it is in, and/or yourself. If you are not comfortable with these possibilities take your car to an authorized, certified, notarized, and super-sized mechanic to get this repair done.

  • Audi TT with bad/flakey coolant temperature sensor

  • New green coolant temperature sensor (part 059-919-501-A)

  • New o-ring seal for above (part N-903-168-02)

  • New plastic retaining clip (part 032-121-142)

  • Flat-bladed screwdriver

  • Phillips-head screwdriver

  • Your own fingers

Audi TT Engine Audi TT Engine

First off, this shouldn’t be attempted if the car has been run at all in the last several hours. Completing this work without getting injured requires opening up the coolant system (which is under pressure and extremely hot if the engine has run recently) and bumping around other dangerously hot parts. For my repair I let the car sit overnight and worked on about noon the next day, more than needed, but I’d just as soon not get burned.

In order, to get to the parts we want to work on we’ll first have to remove the plastic engine cover and the plastic battery cover. There are two Phillips-head screwclips that hold the engine cover in place. They unhook with a half turn counterclockwise. Pull the front of the cover up and forward and set it aside somewhere. The three screws that hold the battery cover on are normal screws. After unscrewing them (and putting them somewhere you won’t lose them), push the cover back and left a little and pull up from the back edge and it should come out easily.

View of the coolant temp. sensor Another view of the part Views of the coolant

temperature sensor still plugged

in and lit with a flashlight

At this point you should be able to see and get to the coolant temperature sensor. It’s a little dark, so for my work I slid a large flashlight into place to illuminate the part and to be able to see what I was doing. The sensor is on the right hand side of the engine block down beneath a few other hoses and an electrical bundle. The pictures should give you a good idea what you’re looking for.

Open and close the radiator reservoir to the left of the engine just to make sure there is no pressure in the system. Better safe than sorry.

The retaining clip The retaining clip (right)

that holds the temperature sensor

in (not the metal hose clamp left)

Now that we can see the part that needs replacing we need to get it out of the engine. To remove the part first we must remove the small plastic clip that holds it in. Take note at this point of the angle at which the sensor sits in place, this’ll be important when we place the new one in. Using a flat-bladed screwdriver it should be straight forward to lever the clip out. Don’t be brokenhearted if the clip snaps in half like mine did, it’s old brittle plastic and that’s why you got a new one. The screwdriver or some needlenose pliers can roust out any leftover bits if yours does snap. Just make sure those bits are out of the way, you don’t want them inside your cars coolant system.

A view with the sensor pulled out The port for the sensor

with the sensor pulled out

Once the clip is off the sensor should pull right out easily. Some coolant will spill out, but not too much. The old O-ring may stay down in the port, you can use your fingers to fish the O-ring out. As with the plastic clip, it’s best to dispose of the O-ring rather than try to reuse it. Not having coolant spewing out of the engine on the road somewhere is worth the $1 part cost.

The sensor connector The sensor connector with

it’s release tab visible

There is enough slack in the cable bundle that you can pull it up to a more reasonable location to work with it. Use a flat-bladed screwdriver to push on the release tab while pulling out the old sensor. It should come out pretty easily.

Now to install the sensor the process just moves in reverse. The new green sensor will only fit into the connector one way and you should be able to feel and hear the snap when it is together.

The new sensor in the connector The new sensor in the connector

Take the new O-ring seal and get it a little wet in the port and then push it down into place with your fingers. It may take a little bit of work getting it pressed all the way down in all the way around. After the O-ring seal is seated you can push the coolant temperature sensor back down into place. Remembering the angle it fit in at is key here.

While holding the sensor down in the right spot at the right angle, the new plastic retaining clip can be snapped back into place to hold the sensor in. The metal end of the sensor should be all the way underneath the clip. It is very important to make sure that this is right. If it isn’t the sensor will blow out of the port (along with a lot of coolant) when the system gets up to temperature. Pull and push on the green plastic (not the cables) to make sure there isn’t any sloppiness in how it is fitting into place.

Now that the sensor is plugged in, pushed in, clipped in and everything looks okay, it’s time to close up the hood and go for a test drive. Don’t stray too far from home in case the sensor didn’t seat correctly. The main goal is to get the engine up to temperature and make sure the coolant system can get up to pressure without blowing the sensor out and spilling coolant all over the road. This appears to be easy as long as the O-ring sealed and the clip is in the right place. Keep an eye out for coolant on the road behind you. If you do have a spectacular coolant blowout, don’t try to drive the car with no coolant, it makes the engine sad. And be mindful of the high temperatures, the normal coolant running temperature is between 80°C and 90°C, that’s 175°F and 195°F. To reiterate, doing anything around the coolant after the engine has been running for a while is ill-advised unless hot liquid burns with ethylene glycol is your idea of a good time.

So there we are, all done. Our TT made a successful 70 mile drive today with it’s new temperature sensor and all is clear and well. Safe travels.


Anonymous said...

Hi , I found it is better to put the O-ring seal on the sensor instead of inserting the seal in the port. Also, to cool down your coolant fast, to allow working on the system : Turn off engine but leave contact on. And let the heater run on it's warmest setting.

Regards ,
Vincent Snijder
The Netherlands

Anonymous said...

Excellent step by step procedure, only suggestion is I placed several absorbent paper towel sheets below the temp sensor to catch any coolant dripping. If you do not tear apart the paper towel and leave the sheets together easy to pull out & dispose.

Anonymous said...

Thanks so much for the step by step instructions. They came in very handy today.

Unknown said...

Thank you very much for the pictures, that sucker was hard to find!

Anonymous said...

This is an excellent post. I really appreciate all the details, plus the additional info that one needs to be carefull of, info usually learned the hard way. I can't appreciate it enough, there's no others words I can think to say. Thank you. I will be performing this tomorrow. I had already located it, but will now feel alot better doing it, as I leared alot of what to not do by reading this post. Thanks again and please continue to put in the effort so many other people may contiue to benefit from your words of wisdom.
Dallas, TX

Unknown said...

Thank you very much! I just changed the sensor and that resolved both low pressure light and the temperature dropping to C. I just want to add one more suggestions: Remove the bundle of wires too by squeezing and pushing down the plastic pin. Otherwise is difficult to insert the plastic retainer back to it's place.