This post has been compiled from the misc posts on here (mostly by Mr. cheesewire) and the tons of good info is in this PDF (I presume from the Bentley manual?) - http://volkswagen.msk.ru/index.php?p=page18_cr but please note it's for the North American-spec AAA engines, not the rest-of-world ABV engines.
Hopefully it's definitive, but if it's not let me know and I'll amend it :)
So, first up:
I believe that only early VRs have a 2-stage fan setup.
Possibly all and certainly all late VRs have a 3-stage fan setup.
Essentially, if you've got 3 temp senders in your thermostat housing, you have 3-stages. If you have AC or an auto, you'll always have a 3-stage fan setup regardless of age.
Fan switching temperatures while the engine is running
These are approximate as it depends on the age of the senders and how crusty they are. VAG-specified values and part numbers are listed further down.
Controlled by the temp sender in the rad, directly to the fans
Switch On: 92-97 C (198-207 F)
Switch Off: 84-91 C (183-196 F)
Controlled by the temp sender in the rad, combined with info from the yellow temp sender via the fan controller
Switch On: 99-105 C (210-221 F)
Switch Off: 91-98 C (196-208 F)
Controlled by the black coolant temp sender, via the fan controller
Not listed in the guide above, but it kicks in at around 110-115 C.
For non-AC cars, stage 3 should never come on under normal conditions, if it does, you have a problem.
The coolant warning light is actually a level sensor (those 2 metal spikes in the header tank are what the plug is for and measure yes/no on coolant level). There is no over-temp warning other than the gauge going up/big cloud of steam coming out of the engine bay :lol:
The main fan control temp sender is screwed into the radiator. This is a 3-pin brass jobber and has 2 different pin layouts depending on if you have early/late VR. It looks like early ones are all 3 pins in a row, late ones are 3 pins in a triangle shape.
One pin (not sure what colour, probably brown) is earth.
Pin 2 (thick red) is stage 1 output and is full-current to the fan. It has the following switching temps:
Switch On: 92-97 C (198-207 F) - officially rated at 95 C
Switch Off: 84-91 C (183-196 F) - officially rated at 84 C
Pin 3 (thin red/black) is stage 2 relay-switched output, which has the following switching temps:
Switch On: 99-105 C (210-221 F) - officially rated at 102 C
Switch Off: 91-98 C (196-208 F) - officially rated at 91 C
There are also holes for another 3 temp senders in the thermostat housing. There will be 2 or 3 senders plugged into them as thusly (from left to right looking at the front of the engine):
Yellow, 4-pin plug. VAG part number 701 919 369 D (about £20, plus O-ring N 903 168 02)
Coolant gauge, fan control unit and aux water pump (always there)
Switches on: 101-107 C (214-225 F)
Switches off: 94-100 C (201-212 F)
Blue, 2-pin plug. VAG part number 025 906 041 A (about £8, plus O-ring N 903 168 02)
ECU (always there)
Black or brown
Black, 2-pin plug. VAG part number 357 919 369 F (about £6, plus O-ring N 903 168 02)
For a late-spec 3 stage fan control (or auto gearbox for some reason) and switches on at approx. 112 C
Brown, 4-pin plug. VAG part number 357 919 369 E (about £8, plus O-ring N 903 168 02)
Only if you have AC. This is (apparently) a 3-stage sender (i.e. you get 5 stages in total! :lol: ) - VAG part number 357 919 369 E
Don't quite understand the numbers in ETKA for this, but needless to stay, the first of the stages also switches on at 112C
General temperature readings
Normal ranges of temps vary hugely depending on how hot it is out, how hard you're driving and how long you've been sitting in traffic. Generally speaking, you don't want the oil to go much over 110. Up to 118 is probably ok, but not for the entire journey. The engine isn't really properly warmed up until the oil hits 80ish.
Coolant temperatures tend to fluctuate much more depending on speed and/or sitting in traffic. Again, generally speaking you want the temp gauge to read in the middle of the gauge (98 degrees) when you're cruising. If you're pushing along hard or sitting in traffic, it will usually creep up to just over 104 or so. Much over 110 is not good as the pressure really starts to build up in the system and it's a likely indication that your fans may not be working.
Above 45mph, normal summer cruise water temp is usually 80 - 84 deg C
Above 40mph, normal Winter cruise temp is 75 - 82 deg C
Although the coolant is primarily water it won't boil at 100 degrees, as it's a pressurised system, so it's not necessarily a bad thing. However, the pressure will obviously build up in the system over these temperatures and put extra strain on what may well be 15+yr old rubber.
If you do get a gush of steam out of the engine bay - pull over as quickly as you can and stop! If you run the engine with no coolant and/or too hot, you're in danger of blowing the head gasket and/or warping the head. Neither of which are fun to replace and certainly a much bigger ball-ache than a bit of rubber pipe!
Temperature reading check
If you think you've got problems, it's always worth checking that the dash readout is actually correct. New senders are only a few quid from VAG and worth swapping if they're the old, crusty originals. Then at least you know you're not chasing phantom faults. The older and crustier they get, the more insulated they are from the actual water and thus, the less accurate they are.
If you plug the car into VAG-COM, it will display the temp that the ECU is reading (i.e. from the blue sender) so you can compare that to the reading on the gauge that's coming from the yellow sender and see if you've got a mis-match. There is of course still the possibility that both senders are crusty and reading wrong... :lol:
1. Take the plug off the rad sender
2. Take the plug off the black temp sender in the stat housing.
3. Switch ignition on.
In the rad sensor plug - red wire is stage 1, thin red/black is stage 2. The other wire is the common connection. Bridge each of the two wires in turn across the common wire and see if both speeds come on. If they do, good.
4. Bridge the black plug with a paper clip or something, does stage 3 come on? Sounds uber loud!
So that's the fan test done. Tick it off as outruled.
The controller stays on for 10 minutes after you switch the car off (regardless of temperature) and will maintain power to the aux water pump. Stage 1 and 2 fans will come on if needed (same temps as when the car is running).
After you turn the ignition off after a long hot run, you should hear the faint whirr (or loud whirr/grinding if it's shagged) of the aux water pump and fan stage 1 or 2 will be on.
If you just turn the ignition on (even if the car is cold), you should hear the aux water pump whirring.
The idea behind the after-run pump is too circulate the water round the heater matrix and rear of the block to prevent localised over heating. Thus, the aux water pump runs regardless, come rain, shine, snow, ice and even if the engine hasn't even run (ignition on, then off).
Stage 1 has direct battery power and will run if it's triggered.
Stage 2 has power maintained to it only during the 10 min after run cycle and will also run only if tiggered.
Stage 3 is ignition only.
Fans/pump after-run fault-checking
You can use the following procedure to find out if your yellow temp sender is the cause of no after-run or not. This does not necessarily prove that the yellow temp sender is working, but it will prove if the circuitry that it controls is working or not.
Turn ignition key on, then off again
Disconnect the 4-pin connector from the yellow temperature sender switch
Bridge contacts B, D (brown/red and brown wire)
The radiator fan and auxiliary coolant pump must come on
If NO, check the fan control unit and/or the aux water pump.
Thermostats and radiators
When the engine is cold, you want to keep as much heat in as possible to get it all warmed up. The standard VR thermostat opens at 80 degrees. When the thermostat opens, it allows water to flow through the radiator to provide additional cooling to try and keep the engine termperature to a reasonable level.
Opens at approximately 80 C (176 F) VAG part number b 075 121 113 D (approx. £20, but £10 from TPS)
Closes at approximately 105 C (221 F)...apparently!
Stroke min. 7.0mm (9/32 in.) - i.e. it's supposed to open by at least 7mm.
Thermostat fault finding
If the thermostat is jammed open (usual state of failure) then your engine will take ages to get up to temp and thus be running rich and using a lot of fuel. The engine doesn't switch over to it's normal running map until it reads 70 degrees on the coolant temperature.
If the thermostat is jammed closed, your engine will overheat very easily and most likely start popping hoses. This is easy to spot and the radiator won't be getting hot if this is the case.
You can take the thermostat out and put it in a pan of water on the hob to test whether it opens or not. It's always a good idea to test new thermostats like this to ensure you haven't got a dud.
Fan controller repair
Apparently, the fan controller units (the box of relays in front of the washer fluid bottle). There's a couple of diodes inside that can come unsoldered from the PCB, but can be reattached with some soldering action.
Always refill your coolant system with the bright pink (hurrah!) G12+ coolant. Older G11 and G12 coolants shouldn't be used and certainly not mixed together as they'll clag up the coolant system. If possible, flush out the coolant system with clean water from a hose before re-filling.
The VR coolant system capacity is about 9 litres and the concentrations you should use are on the label of the G12+ bottles. However, for UK use where the temperature doesn't really get down all that cold, 3 litres (2 bottles) of G12+ will suffice and just fill the rest with water.
Main water pump
This is the main engine water pump and is on the left side of the engine, driven by the serpentine belt. VAG part number for a new pump with gasket: b021121004X
It's also worth getting the 3 new allen-head bolts that hold the pump into the block. VAG part number: N90221803 (x3)
Annd while you're there, why not get some new allen-head bolts that hold the pulley to the pump too. VAG part number: N90544202 (x3)
Aux water pump
This is the electric pump by the right side the engine. Brace yourself, it's about £120! VAG part number: b251965561B
Aux water pump rubber ring mounting thingies. VAG part number: 035959209E
This is also an uber-expensive part (circa £100). It lives in the sump and is driven by a shaft that comes down from inside the block. VAG part number: 021115105B
The pump is held in with two allen-head bolts. VAG part numbers: N90355902 (x2)
Coming from the oil pump is a pipe that feeds the oil back into the engine. This has a gasket for it, which appears to be obsolete and is held on with another 2 allen-head bolts. VAG part number: N10227803 (x2)
As the sump needs to come off to get at it, you're best replacing the sump gasket too. This is quite expensive (circa £35) and does change depending on car vintage. Early cars use VAG part number b021103609. Later cars use VAG part number b021103609B. No, I don't know what the difference is :(
Lastly, if you're really keen, you can replace all 26 of the bolts holding the sump on as well. Again, this changes depending on if you have early or late engine, but changes at the same time the sump gasket does. I'd imagine as long as you've got the same screws as gasket, then you'll be ok. Early cars use VAG part number +N90008401 (x26) and are M6x14mm. Later cars use VAG part number: +N90423402 (x26) and are M6x17mm and self-locking.
Low temp parts
You can get lower temperature thermostats and radiator fan switches. These are just parts from other VWs, so don't go paying silly prices for them. Personally, I'd avoid the lower-temp thermostats as they can make the engine run too cool. The VR6's cold running map doesn't switch off until 70 C water temp, so if your Neuspeed low sender is opening at 70 C instead of the normal 80 C, you may stay in cold running mode for a bit longer than usual...and the blue temp sender has quite a big influence on the overall fuelling ;-)
The lower temp rad switch makes sense though - the low-temp fan switches (the one that goes into the radiator) - all are from a T4 Transporter:
Option 1 - VAG part number 701 959 481
This will only fit early VRs with the 3 pins all in a row, so check first!
On at: 87 C
Off at: 76 C
On at: 93 C
Off at: 82 C
Option 2 - VAG part number 251 959 481 K
Same as the above, but for later Transporters... thus it might have the triangular-shaped pin layout. Maybe.
Sadly, there don't seem to be any lower-temp versions of the magic Stage 3 fan sender, but it's not too hard to wire stage 3 to come on at the same time as stage 2.
Wiring up a dash-switch for stage 3
Don't forget the quality "wire up fan stage 3 to a dash switch" mod ;-) An hour with a bit of wire and a switch will do wonders for your summer traffic temps. You can bridge the red/black stage 2 wire and also the black stage 3 wires with a 2 way switch and manually start them in the car.
Lastly, Mr. Cheesewire has also discovered you only need one, yes ONE, 11" SPAL fan to keep the water at 95 deg constantly in traffic, even when it's 27 deg C outside, like it was last night when he was stuck on the A12 for an hour waiting for an accident to clear...
Cold ECU map
Lambda closed loop engages when water temp reachs 70 deg. That's why VRs always feel quicker when they're warming up because they're in open loop and running rich :D