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Discussion Starter · #1 · (Edited)
TempGauge fluctuating-Sensor or Thermostat? Solved!!

I've searched and read all other post pertaining to Thermostat, cooling, heating issues and i just need to narrow this down. Hoping a little DIY will prevent me from taking the whole car in to the shop.
The issue- temp gauge erratic temperature levels.

Here are my symptoms that i've monitored for the past week with the warm and now with cooler weather:

I go to work at early am- cooler weather- start the engine and idle for 30-1 minute at times- and get going. I get the normal heat inside the car, but the tempgauge would start out slow on the first bar-- about 5minute driving to the canyons- the needle jumps to "normal" 190*... as i get on the freeway- average cruising 70-80mph (per se 45 degree outside temp like today)- the needle starts dropping to the 2nd bar then the 1st red bar. I'm still getting the heat however in the car. I would normally slow down at a freeway junction/transition-- and i would notice that the gauge would creep back to 190*, but as soon as i get on a cruising speed after a few minutes, the gauge creeps back down to 120*, then below the 1st red bar on the gauge.
On my way home, same thing, she would show 190* temp, albeit takes a while than normal-but i'm still getting sufficient heat inside the car, and she would do her fluctuation throughout the drive home- at different speeds.. Ultimately, at high cruising speeds- thats when she settles around the 1st red bar on the temp gauge.

Is this the Sensor? or Thermostat related? or could it be something more serious? I would like to get the parts for these and try them out see if it eliminates the temp gauge dance.
By the way, the timing belt/water pump has been changed twice(along w/ thermostat, etc).. Last thermostat/water pump, TB change was at 120k, the car now has 175k. It has the correct coolant g12, and no leaks or possible mechanical/superficial contradictions that i can find in the engine bay.

HELP.

Thanks in advance!

erick
 

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I'd guess the sensor, especially if you have a B5 with the old, bad senor unit. However, it is tough to be sure. Since the sensor is easy and cheap to replace, you might was well try it, and then go from there.

SirWired
 

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I recall reading other posts that state there are actually two temperature sensors. One sensor is for the gauge, and the other is for the engine. So it just may be the gauge sensor. And if not, just make sure to check/change both.
 

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It is the sensor.
even if it isn't the sensor, the sensor is an easy and cheap fix so do that first. The thermostat takes quite a bit of time.
 

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Could be either. Do you have climatronic? If so, as long as coolant stays at 120*F, the climatronic will regulate the flap to provide enough heat so you probably won't notice loss of heat in the cabin. If you have manual a/c then turn temp dial all the way to max. temp and see if you notice any change in air temp out the heater ducts when the temp drops to 120*F or so vs. the 190*F. Usually, when the sensor begins to fail the gauge will fluctuate between 0 and 190*F or some other values in between regardless of engine load and coolant temp.
 

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I recall reading other posts that state there are actually two temperature sensors. One sensor is for the gauge, and the other is for the engine. So it just may be the gauge sensor. And if not, just make sure to check/change both.
There is only one sending unit that houses the two independent coolant temp sensors.
 

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Discussion Starter · #8 ·
Thanks for the response-- I'm going to try out the sensor first.
Just fyi its a 1999 AEB engine- turbo-four tip/ manual HVAC. 176k..ultimately- nothing has gone wrong with cooling/heating/gauges until now.. there's always first for everything i suppose.

Here's another thing that i noticed- i went back to my car for my lunch hour turned the engine on-- set the heat to high (set it at idle for a couple of minutes, drove around the lot (mall, say a mile)- the heat was up in the cabin- but the temp gauge was barely at 2nd bar. I then shut it the engine off, then turn the accesories only- and the needle shot up to 190* (just below) .

i'll put the order in for a sensor today and take it from there then.
 

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Speaking of the temperature sensor:

It's my understanding that the temp. sensor with the BLACK connector is faulty by design, but my car has the temp. sensor with the BLUE connector. Do I still need to replace it with a new sensor with a GREEN connector, or am I exempt from that particular problem?

I also have a 1999 AEB.
 

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^ funny, I was just wondering the same thing, the blue / green sensor depends on your VIN. Pre 3B-X-491580 have the 078 919 501 B (blue) and post 3B-X-491581 have the 059 919 501 A (green) sensor.

My VIN is 3B-X-424190 and I have the blue one, this will be fine since it's not the faulty black sensor.
 

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it is my understanding that green replaces black and that blue replaces blue. The connectors are different so if you did a blue instead of a green or visa versa you would also have to splice the harness. ECS has some picture up so you can take a look.
 

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Vag-Com it to see if you get any error messages regarding
the temp. sensor. Or just replace it anyway. Those things go bad
so often it should become part of scheduled maintenance.

I would probably just replace the temp. sensor at this point.
Besides, my A4 exhibted the exact same problems and it
did indeed turn out to be the sensor. If it's not the sensor
then.. it's your thermostat going and you'll need to replace it.
 

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Vag-Com it to see if you get any error messages regarding
the temp. sensor. Or just replace it anyway. Those things go bad
so often it should become part of scheduled maintenance.

I would probably just replace the temp. sensor at this point.
Besides, my A4 exhibted the exact same problems and it
did indeed turn out to be the sensor. If it's not the sensor
then.. it's your thermostat going and you'll need to replace it.
They go bad that often, eh? Even the non-black ones?
 

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Not to hijack, but is normal operating temperature always 190-degrees? This is my first VW... if I'm driving around town, my 1998 AEB stick shows 190* but on the highway it falls back to the first mark to the left of 190 and stays there...

My previous car (GM) would get to its mid-point of 220* and stay there regardless. I thought an engine not running "at temp" usually meant lower gas mileage. Should I replace my blue sensor? Or is 180* (or whatever the mark below 190 is) okay?

THANKS ... learn something new on here every day =)
 

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Not to hijack, but is normal operating temperature always 190-degrees? This is my first VW... if I'm driving around town, my 1998 AEB stick shows 190* but on the highway it falls back to the first mark to the left of 190 and stays there...

My previous car (GM) would get to its mid-point of 220* and stay there regardless. I thought an engine not running "at temp" usually meant lower gas mileage. Should I replace my blue sensor? Or is 180* (or whatever the mark below 190 is) okay?

THANKS ... learn something new on here every day =)
The temp gauge is basically a cool looking idiot light. If after the intial warmup, the needle moves at all away from 190, you got something wrong.
 

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They go bad that often, eh? Even the non-black ones?
Yeah, they all go... even the new (updated?) green ( & black?) ones go bad
too, just not nearly as often as the blue ones.
 

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Discussion Starter · #18 ·
THANKS AGAIN ALL FOR THE RESPONSE... i'm just waiting to get the Sensor and i'll let you all know how it goes.


e
 

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Discussion Starter · #19 ·
***********update****************
Got the sensor today - I went ahead and purchased it from VW (along with a new left wheelwell liner, and new W8 belly shield) Igave the sales person my last 8 Vin number and he gave me the Green Sensor. After i got the car to cool down after getting home, i removed the sensor (beleive it or not, i wasnt sure where it was located, but i was able to figure it out (behind the engine , near the firewall) Took the ''blue'' connector (which was a perfect fit to the new sensor) and removed the retaining clip and popped the old one out. OOPS!! i should have placed a towel underneath because a good amount of coolant spilled out. The old sensor was Blue, and the whole metal nipple was tarnished badly to dullness. Popped the new one in, placed the retaining clip back and Turned the key to on, then shut down, then turn it on again (yah its a weird thing i do after working on my car disconnecting stuff, Its sorta like a manual reset) turned the engine on, and let her idle for a minute, then took off for a 4 mile drive. The temp gauge creaped up to 190* in a short time and stayed there through the duration of the drive. I also drove out 30minutes later to drive out to BestBuy (8 miles rt) and the temp gauge creaped to 190* like normal and no more fluctuation!!!
Thanks to all!!!
 

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Good. When mine went out on my sedan, the temp would drop all the way to cold.

BTW, the thermostat is pretty easy to replace in the AEB/ATW.
 
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