coolant temp sensor replace, w/o coolant drain

frugality

Top Post Dawg
Joined
Sep 19, 2003
Location
Spring Lake, Michigan
TDI
none, 2016 GTI
This is just an FYI post for anyone replacing a coolant temperature sensor.

Long story short, my 2000 Golf TDI developed a problem where the temp gage needle wouldn't go up to the center 190 mark. Occasionally it was a little shy, and a few times it was a full two hashmarks low on the gage. I suspected a bad thermostat, because I've heard that VW thermostats are designed to fail in the 'open' position, and therefore err on the side of being too cool. However, in doing some searching here, I found that the temperature sensor may be the problem. Seems that the sensors were more prone to failure than thermostats are.

One suggestion came up on a thread that you didn't have to drain the coolant -- just pull the old sensor and be prepared to plug in the new one quickly as coolant starts to come out. This worked O.K., but I want to post a warning. I pulled the old one, and quickly jammed in the new one, only to find that the old one that was now in my hand didn't have the O-ring on it. It had stayed inside the casting, and I'd jammed the new one in with its new O-ring, so now there were 2 O-rings in there. So I had to pull it out again and fix things, and meanwhile I dumped a bunch of coolant. Still, with all that, I didn't lose enough coolant to have to top up right away. The level went from close to the sensor on the reservoir down to just above the hatched 'minimum' area.

The basic procedure:
* Let the engine cool. I waited 2 hours from being hot, and the coolant was bathwater warm. (and my hands did get bathed)
* Remove engine cover and accordion hose between MAF sensor and intake downpipe.
* Open the cap on the coolant reservoir to release any pressure in the system. Then close it again.
* Unplug the old sensor. This was kind of a bugger. I ended up getting it loose by putting a small screwdriver into the area where the release tab is, and then twisting the screwdriver to push on the tab. Having a look-see with a small mirror beforehand is helpful.
* Plug your new sensor into the wire harness.
* Remove the plastic 'C' clip that retains the old sensor. Be carful not to drop it, because it's plastic and you can't fish it out with a magnet.
* Hug your engine. With one hand you'll remove the old sensor, and with the other hand you'll plug in the new sensor, as coolant starts to pour out. You may have to fish out your old O-ring like I did, in which case you'll loose a couple cups of coolant.
* Replace plastic C-clip.
* Check and add coolant if necessary.
* Reinstall accordion hose.
* Start engine and check for leaks.
* Pat yourself on the back.

I'll have to watch for the next few days, but this seems to have fixed the problem. Twice now the temp gage goes right up to 190 and stays there. It's nice to have a quick, cheap fix.

Also as a side note, my 2000 Golf was made in 12/1999, so it should have had the blue, square temperature sensor. Later models have a rounded green sensor. However, mine was black, and was rounded -- sort of a D-shape. Maybe mine was different because my car was German-made.

My local dealer quoted $34 for the sensor, which they had in stock. I got mine for $14+shipping at tdiparts.com. I also got an extra O-ring for $1, and that was a good idea. I lost one down below the engine when I fumbled around removing the new sensor to get the old O-ring out.

***Edit to add removal/installation of accordion hose to procedure.
 
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Harvieux

Vendor , w/Business number
Joined
Aug 15, 1998
Location
Whittier,CA-USA
TDI
06 A5 Pkg.2 w/navi & ASEP
Dean, Most of what you post here in this procedure is correct. One thing I would change is as follows: Pull the plastic horseshoe clip which holds the sensor. Note how the sensor is currently positioned. Usually the flat side of the sensor is facing down and the rounded side is up. Have the new sensor with o-ring attached and then pull out the old sensor with the electrical connecter still attached to it. Be sure the old o-ring is removed (as you found out the hard way) and quickly replace with the new sensor and o-ring in the correct position referenced above. Snap the horseshoe clip in and you can now relax while removing the electrical plug from the old sensor because you now have much better access to it without having to struggle removing it off the old sensor as you also discovered. Later!
 

Harvieux

Vendor , w/Business number
Joined
Aug 15, 1998
Location
Whittier,CA-USA
TDI
06 A5 Pkg.2 w/navi & ASEP
BTW, The older 1Z, AHU, and early ALH had a square electrical plug for the CTS however, the above procedure of removeing the CTS with the elecrtical connector plugged in would still be applicable with the early ALH's. Later!
 

frugality

Top Post Dawg
Joined
Sep 19, 2003
Location
Spring Lake, Michigan
TDI
none, 2016 GTI
Good thought on removing the sensor first before the electrical connection. In an old thread, someone else suggested removing the connector and plugging the new sensor in, so it's tethered down there and you don't lose it during the swap. You'd just drop the old one, if anything. That made sense, too.

On mine, the rounded part of the sensor was towards the engine/passenger side, with the clip on the bottom. The later style green sensor fit.
 

kwong7

Veteran Member
Joined
Mar 25, 2004
Location
Southern Caifornia
TDI
2001 Golf GLS TDI / White
I would have to recommend the additional clip. I lost mine to the abyss that is my engine bay. I was really close to snapping the original back in when the tension got the better of me. It popped right up, hit the bottom of the air box and must have landed on the skid plate.

Also, I lost about 1.5 cups of coolant/water. Somehow I was under the impression that it would eventually stop draining. Like a fool, I periodically pulled and pushed by the old sensor to get the flow speed. When I realized that the flow wasn't slowing down after the first cup gushed out, I knew I just had to be quick with the sensor swap.

Getting the old o-ring was not a problem at all, just used my index finger to fling it out. The new o-ring was already on the new sensor and went in without a problem. This swap was long overdue, my Vag-Com hinted that this was faulty about 40K miles ago.
 

lrpavlo

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Joined
Jan 1, 2000
Location
Cocoa FL
TDI
09 Sportwagen DSG, 02 NB Auto
Funny Dean! I did the exact same thing with Meander's car....took me 2 tries before the old Lyme brain realized why that new temp sensor wasn't going to fit!!!

Nice write up Leonard!

Belly pans.....curse and a blessing!!
 

tobsav

Veteran Member
Joined
Jul 27, 2002
Location
NH
TDI
2003 Jetta Wagon
Great post and hopefully will get me thru the process... However, I am not sure this is what I need to be doing! So re-assure me this will help me out!
My temp guage on the dash has stopped reading.... car runs fine, heater makes heat, just no needle movement..... it did budge briefly when I restarted a hot engine after it had been sitting for a half hour or so, but then just snuck back down and sat there.
Am I chasing the right culprit?
cheers
T:confused:
 

kwong7

Veteran Member
Joined
Mar 25, 2004
Location
Southern Caifornia
TDI
2001 Golf GLS TDI / White
Tobsav, do you have access to a Vag-Com interface? Sounds like you should start there because you may have more than one trouble area.
 

tobsav

Veteran Member
Joined
Jul 27, 2002
Location
NH
TDI
2003 Jetta Wagon
yeah.. I can probably dig one up....I haven't generated any cel , and for 14 bucks I figured it was worth a try replacing the sensor!
 

Harvieux

Vendor , w/Business number
Joined
Aug 15, 1998
Location
Whittier,CA-USA
TDI
06 A5 Pkg.2 w/navi & ASEP
tobsav said:
Great post and hopefully will get me thru the process... However, I am not sure this is what I need to be doing! So re-assure me this will help me out!
My temp guage on the dash has stopped reading.... car runs fine, heater makes heat, just no needle movement..... it did budge briefly when I restarted a hot engine after it had been sitting for a half hour or so, but then just snuck back down and sat there.
Am I chasing the right culprit?
cheers
T:confused:
The CTS is dual function. It provides a signal to the ECU and also the guage. I can virtually assure you that your CTS is bad if your guage is reading funky. Later!
 

tobsav

Veteran Member
Joined
Jul 27, 2002
Location
NH
TDI
2003 Jetta Wagon
thanks for the input... my pal has a vag com so i will plug it in for giggles this weekend and see if it throws any codes... I have ordered the replacement sensor anyway!..... I will post what I find.
cheers
 

jasonTDI

TDI GURU Vendor , w/Business number
Joined
Apr 26, 2001
Location
Oregon, WI
TDI
20' RAM 3500 CCLB dually HO/Aisan. 2019 Cherokee 2.0T
Yeah, but if the gauge still is odd it's the thermostat. I see a fair number of failed ones up here. Maybe it's the cold weather....
 

tobsav

Veteran Member
Joined
Jul 27, 2002
Location
NH
TDI
2003 Jetta Wagon
vag com says coolant sensor .... so it seems I ordered the right thang!
 

LNXGUY

Top Post Dawg
Joined
Jan 10, 2004
Location
Barrie, Ont, Canada
TDI
'05 Jetta TDI Wagon
I replaced the sensor thinking it was the problem. It wasn't..

It was the harness (Tracing that back was REAL fun)
 

Harvieux

Vendor , w/Business number
Joined
Aug 15, 1998
Location
Whittier,CA-USA
TDI
06 A5 Pkg.2 w/navi & ASEP
jasonTDI said:
Yeah, but if the gauge still is odd it's the thermostat. I see a fair number of failed ones up here. Maybe it's the cold weather....
Could be the cold weather, Jason. I can say that I never ever had a bad T-stat in the over 10 years of my TDI involvement. I take that back. I did have a piece of WP impellar get lodged in one once. ;) Later!
 

kwong7

Veteran Member
Joined
Mar 25, 2004
Location
Southern Caifornia
TDI
2001 Golf GLS TDI / White
LNXGUY said:
I replaced the sensor thinking it was the problem. It wasn't..

It was the harness (Tracing that back was REAL fun)
Oh no, I think that might be my issue. I changed the ECT sensor, but Vag Com still indicates the engine coolant temperature isn't warm enough to do the TDI Timing feature. I suspected the wiring, but didn't want to believe it.
 

DanEboy

Veteran Member
Joined
Mar 26, 2003
Location
Commack, NY (Long Island)
TDI
Jetta GLS 2000 Auto
Kwong, before you suspect the harness, does anyone know the resistance needed to indicate full warm (190*)? You could remove the plug and stick the appropriate value resistor onto the terminals for the gauge and see if it indicates properly. If it does, then the thermostat is probably bad.
 

VDubbing

Top Post Dawg
Joined
Aug 12, 2004
Location
Stouffville, Ontario Canada
TDI
2003 Jetta Galactic Blue
this may seem to be a silly question but where does one top up the coolant? Through the resevoir bottle?? Im changing a sensor in the AM and dont want any problems

Thanks
Rob
 

tobsav

Veteran Member
Joined
Jul 27, 2002
Location
NH
TDI
2003 Jetta Wagon
well, got around to putting the new sensor in today. Thanks for the help.
The job took all of 5 minutes. I guess i was lucky. The electrical came of no prob, the c-clip popped right off with a little finger nail poking, out came the old sensor...in went my finger to fish out the old o-ring, in went the new sensor. Baddaboom.
Without the hints on this post I would have broken something I am sure.
thanks again.
 

Sappington

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Jul 10, 2003
Location
Manassas, VA
TDI
2017 Chevy Volt now -- sold '03 Jetta Wagon :-(
Where does that coolant go when it spills out? Do you just let it sit in the skid plate or do you clean it up? I wonder if it would be better to remove the skid plate first and put a oil pan or something down there.
 

mittzlepick

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Mar 18, 2001
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union maine
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2004 jetta wagon (365k)2001 wagon tire burner 6spd 2003 wagon(417k)
unplug it and replug it in or just push on it I called my brother to look at his so I could order it round plug or square green black or blue so we figure it out get it vag it before we put it in and no code it just wanted some love that was 2 months ago g figure at least he paid me for the sensor! the reason he needed to change it was the ecu wouln't let me time it after tb change.

snowsleds make a good antifreeze catch-all
 
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BrianM

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Joined
Feb 9, 2005
Location
Warm Springs, GA
TDI
2001 GLS 5-speed, Loaded
Just an FYI to this.... it MAY NOT be this easy. I sure wish it were, because I now have a driveway full of coolant (and a garden hose running to try and dilute it). I ended up taking off the housing into which the temp sensor fits and finding that it was FULL of corrosion, though only where the old temp sensor lived. It took a good bit of scraping with a screwdriver to get it knocked down, then a fw minutes with a dremmel and a sanding attachment to clean the surface back up to aluminum. The corrosion was inhibiting me from putting the new sensor in far enough to get the clip on.

A key that may tell you that you're in for this kind of Joy? If the clip doesn't come out easily (I broke mine, would have ordered an extra if Peter/TDI Parts had one listed... I'm gonna go email him and make this a suggested "must carry" item). So, a 10 minute job turned into a 3 hour job as I fussed with the whole assembly while it was on the car, then finally gave up and took it off the car. Removal is EASY, 2 10mm bolts and a few hoses, so don't worry if that's required. Just drain your coolant first.. yeah? :D

Cheers, and thanks for the write-up anyway!
 

Harvieux

Vendor , w/Business number
Joined
Aug 15, 1998
Location
Whittier,CA-USA
TDI
06 A5 Pkg.2 w/navi & ASEP
BrianM said:
Just an FYI to this.... it MAY NOT be this easy. I sure wish it were, because I now have a driveway full of coolant (and a garden hose running to try and dilute it). I ended up taking off the housing into which the temp sensor fits and finding that it was FULL of corrosion, though only where the old temp sensor lived. It took a good bit of scraping with a screwdriver to get it knocked down, then a fw minutes with a dremmel and a sanding attachment to clean the surface back up to aluminum. The corrosion was inhibiting me from putting the new sensor in far enough to get the clip on.

A key that may tell you that you're in for this kind of Joy? If the clip doesn't come out easily (I broke mine, would have ordered an extra if Peter/TDI Parts had one listed... I'm gonna go email him and make this a suggested "must carry" item). So, a 10 minute job turned into a 3 hour job as I fussed with the whole assembly while it was on the car, then finally gave up and took it off the car. Removal is EASY, 2 10mm bolts and a few hoses, so don't worry if that's required. Just drain your coolant first.. yeah? :D

Cheers, and thanks for the write-up anyway!
Are you the original owner of this vehicle? If not, I would suspect that the previous owner either added the wrong coolant or added regular tap water to the reservoir at one time or another. G-12 coolant mixed with distilled water will not cake up as you describe. Later!
 
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