Radiator cooling fan NEVER turns on

Two problems, possibly related?
1. The radiator cooling fan NEVER switches on?! Summer, winter, long drive, short drive. Why?? How to fix??
2. The coolant temp gauge is ALMOST ALWAYS down on the absolute floor.
Shouldn’t this coolant temp gauge needs move up a little to 100 or 120 degrees range!!
Question: Why is the coolant temp gauge asleep for a long time until it starts to show the Engine is around 200 F midrange ??

1. Maybe the fans never turn on, because the coolant never gets hot enough, because
2. The thermostat is stuck open?

Have you ever run a Scangauge or other OBD scanner to compare live Temp readings to your analog gauge?

I’d start by flushing your brown coolant, then fixing the leak.

-Todd

Start with the fan itself. Hook 12v to where the brown and one of the red wires go on the fan connector...then the other red wire and brown for the other speed.
The coolant temp sensor in the front of the engine is fallible.

Every time you run more than let's say 20 minutes, the car the temp should be up to 190*F which means that the coolant temp is somewhere between ~170-210*F. It's a very forgiving gauge.
But as mentioned, getting reading of the temp sensor with VCDS will tell you what the ECU is seeing. A Scangauge is probably good enough for that.

You may need one or all of the following...
You may need a new yellow/black or blue temp sensor depending on number of terminals.
You might need a new QUALITY thermostat (not Autozone style).
You may need a new fan.
You may need to back flush the system, and especially the radiator.

JETaah said:

Start with the fan itself. Hook 12v to where the brown and one of the red wires go on the fan connector...then the other red wire and brown for the other speed.
The coolant temp sensor in the front of the engine is fallible.

Every time you run more than let's say 20 minutes, the car the temp should be up to 190*F which means that the coolant temp is somewhere between ~170-210*F. It's a very forgiving gauge.
But as mentioned, getting reading of the temp sensor with VCDS will tell you what the ECU is seeing. A Scangauge is probably good enough for that.

You may need one or all of the following...
You may need a new yellow/black or blue temp sensor depending on number of terminals.
You might need a new QUALITY thermostat (not Autozone style).
You may need a new fan.
You may need to back flush the system, and especially the radiator.

Click to expand...

Actually a Scangauge is better than the cluster and equally as good as seeing the temp in VCDS, because it reports the water temp seen by the ECU, not by the silly gauge that's dampened and never moves at all once it gets to about 190, even if the actual temp rises above that.

WT is one of the four items I see all the time on my Scangauge, voltage, boost and speed are the others.

Scangauge is the best reporting mechanism short of running VCDS constantly, which is impractical anyway.

Steve

Steve Addy said:

Actually a Scangauge is better than the cluster and equally as good as seeing the temp in VCDS, because it reports the water temp seen by the ECU, not by the silly gauge that's dampened and never moves at all once it gets to about 190, even if the actual temp rises above that.

WT is one of the four items I see all the time on my Scangauge, voltage, boost and speed are the others.

Scangauge is the best reporting mechanism short of running VCDS constantly, which is impractical anyway.

Steve

Click to expand...

Good to know.
I have never used one but have seen them in numerous cars that I've worked on.

OK so the leaky heater core is bypassed, now we are no longer losing volumes of coolant water.
Usiing ELM327 Bluetooth scan tool.
Coolant temp starts at 15 C.
After some driving it’s 40 C (104 F).
Max temp with 15 min 2000 rpm driving is 45 C (113 F).
Coolant temp gauge needle is stuck in low range below below 140 F.
Is this normal ???
Fan will NEVER come on, even when you stop and park after medium drive on hot summer day.!’?

Your temps are too low to activate the fans. You need a new thermostat.

Yup.

As a data point, my normal operating temperature on the highway is 80C... if it's a real hot day and I'm climbing a hill I'll get to 85C.

Fans don't come on till around 95C... so I never see 'em, except when idling in traffic on a very hot summer day.

I test 'em every spring and fall by turning on the A/C... but that's generally the only time I hear 'em.

Darn cars are very heat-efficient.

busted thermostats are "supposed to fail OPEN" but i have never seen one that needed replacing that had not failed in the CLOSED position. Depends on how it fails i guess.

Anyways, 3 main ways the fans wont come on.
As others have stated, t-stat replacement should solve both issues. But its almost as likely that you have a bad coolant temp sending unit.
Another common issue about cooling fans is the fan relay box. Usually it fails in the "wont turn off the fans" mode but some fail flat out.
Fuses and proper grounding can also be an issue. I would start by testing the voltage output during use and the resistance in Ohms on the sending unit. tstat is not that difficult to reach but more of a pain than the sending unit.

Use a clean shopvac to collect all the fluid as it comes out of the hose so it can be reused later, saving you money.

The reason i point to the output censor is that even if the tstat was wide open or fully removed, your needle should move a bit. Ive ran a bit with out a t-stat and i always saw some moment of the gauge but not fully warm.
VW and many other auto makers use a "good enough" approach to the coolant gauge, especially on diesels due to the temp differences and swings. AKA, you can have 130*F or 220*F and that needle wont budge and inch and will stay in the "operating temp" of the colt to hot cycle. Scangauge II is great as it tells you in real time, among other things, exactly what temp your at.
Its for this reason i dont think your t-stat is the main issue.

While driving today I kept the elm327 Bluetooth interface obd2 reader connected to the smart phone app to keep an eye on coolant temp as seen by the engine computer unit.

Coolant Temp rose and stayed in the 115 - 128 F range.

Also, the needle on the coolant temp gauge did rise a very small bit. A few mm. It got up to 3 out of the 4 hash marks jammed over at the left of the gauge. So I guess the sender is prob OK. Matches with last week, about half the coolant leaked out, so the little remaining coolant had double the heat to absorb, and the needle actually got up into the mid range where it’s supposed to be, 180-200F. I pulled over and added tap water ASAP to avoid head warping and head gasket destruction.

If your coolant leaked out, then your sender is sending the wrong temp -- because it's not sitting in coolant anymore. It's only making contact with steam or hot air, which will make it read Low.

turbodieseldyke said:

If your coolant leaked out, then your sender is sending the wrong temp -- because it's not sitting in coolant anymore. It's only making contact with steam or hot air, which will make it read Low.

Click to expand...

HAHA, think again. my scan gauge and my auber gauge both tell a separate story than what your saying. while the stock sender on the hose says 250 the lower probe says 220.
coolant boils differently than water. depending on the concentration, 220 to 230 is about the boiling point.

The only way the stock gauge will read above the normal area it sits in, the coolant needs to boil or get darn close to it.

The only way you get lower temps is if the sender is air locked as in within normal temps but a pocket of air is locked in the top end, TDI's are very hard to air lock as to the location of the res and how its piped.

contacting steam would read more like 250+ and i have seen that when i had issues.

Ok, I had another think, and reached the same conclusion. The sensor has to make contact with fluid -- not steam or hot air -- in order to give an accurate reading.

My head gasket got blown because of this -- because a hose failed while I was at highway speeds, and I had no indication of a problem until I pulled off at my planned exit, and finally saw steam coming from the hood. At first, I even thought it was another car's steam, not my own.

Why did I think there was no problem? Because the coolant light started flashing (no big deal, because it would light up every 6 months, then I'd add a quart, and wait another 6 months). Only this time the temp gauge started to read a hair over Normal for a few minutes -- but then it FELL back down to 160. What does that say to a driver who hasn't experienced this situation? "Temperature = Cool." Why did the needle FALL? Because at that point, I had lost too much coolant to get a reading.


A nasty work schedule kept me driving on that blown gasket for several weeks, and I would see the same thing happen, if I waited too long to pull over and top it off with water -- the temp would rise to 190, 200, 210, 220, and then start to drop off.


Chris, you need to verify your temps somehow. Either pop the hood and feel your upper hose, to see if it "matches" your gauge, or zap the engine block with a laser thermometer. If the engine is hotter than your gauge is telling you, then either you're low on coolant, or your sensor is bad. If when you added water, like you said at 180-200F, and it wasn't boiling like a demon, then you're probably ok.

turbodieseldyke said:

Ok, I had another think, and reached the same conclusion. The sensor has to make contact with fluid -- not steam or hot air -- in order to give an accurate reading.

My head gasket got blown because of this -- because a hose failed while I was at highway speeds, and I had no indication of a problem until I pulled off at my planned exit, and finally saw steam coming from the hood. At first, I even thought it was another car's steam, not my own.

Click to expand...

I had the same issue but scan gauge II immediately showed temps of over 240F

Its the dumb gauge, not the sending unit.

It was the sender, for me. When I knew about the problem, and used the OBD scanner, I could see the temps fall on both the dumb gauge and the OBD.