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Introduction
| 3B System |
ABY Changes
| Fault
Diagnosis
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System Repair
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System Description
The cooling system on the 3B engine has much in common with that on the 20V RR engined UR quattro. They share many components and a cross-flow design for the cylinder head whereby coolant enters the head on the exhaust side, flowing across the head and exiting on the inlet side. Such a scheme ensures efficient and even cooling of the five combustion chambers. The system is pressurised to further improve the cooling capacity of the glycol based coolant. A pressure relief valve is incorporated into the cap on the expansion tank.
What follows
is a rambling description of the cooling system components and how
they interact. In time, a little more structure and sanity will meld
its way thru these pages, but content IS more important than presentation
right ?
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Mechanical Components Coolant
pump - Fixed into the front of the engine block, the
coolant pump is driven by the timing belt to ensure
a continuous flow of coolant around the engine. Audi part number
034-121-004 is common for the 3B engine along with the KV, NM and 7A motors... Handy to know. Note the ABY item is 054-121-004A and that the RS2 item is also different. More on that later... Coolant pumps can also be ordered on an exchange/reconditioned basis with an 'X' suffix. Both of these parts should include the correct gasket (035-121-043 ) for the pump.
Coolant Reservoir - Also known as the expansion tank, the coolant reservoir provides room for the hot coolant to safely expand in this area of the system. The 3B engined Coupe has a different reservoir than the ABY engine. To prevent over-pressurisation of the cooling system, a relief valve is incorporated in the tank cap. This opens at approximately 1.2bar pressure in the event of a severe system blockage. The reservoir cap has part number 443-121-321. Thermostat - Designed to let the engine get up to working temperature as quickly as possible by preventing the flow of coolant to the radiators until it has reached a certain temperature. The thermostat starts to open at 87C and is fully open (with at least 8mm stroke) at 102C. Audi part number is 069-121-113. Note also the O-ring seal is part number N-901-368-02 Main radiator - Located in the left side of the engine bay, this radiator provides the largest contribution to engine cooling. Part number for the 3B is 893-121-251-G and 893-121-251-S for the ABY. Aftermarket parts are readily available. This radiator is assisted by the electrical fan that is mounted directly behind it, but the airflow through this radiator is not sufficient on its own (especially on air conditioned cars which have the condensor mounted in front of the main radiator) for the cooling needs of the S2. There are some known weak spots on this radiator which should be watched for on high mileage vehicles. These are generally found where metal components of the radiator join with plastic ones. Auxiliary radiator - Mounted just behind the front grill, this second radiator provides additional capacity for the coolant system to dissipate heat from the engine. Part number 895-121-251. This radiator is much harder to find from aftermarket suppliers so despite its much smaller size it ends up considerably more expensive than the main radiator. This is doubly frustrating as it is the auxiliary radiator which is generally the first to need replacing. This is on account of the myriad of high speed collisions with insects and stone chips which damage the core of the auxiliary radiator over time. |
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Electrical Components
The description below is based on an attempt to decrypt
the 3B engined S2 schematics found
here
. It applies to non air conditioned cars as I am sure that some
differences apply to cars with AC such as enhanced control logic
and/or dump resistors when the AC is turned on (to activate the
radiator fan on low speed), but the air conditioning electrics should
be thought of as an overlay to the standard cooling system..
UPDATE, Feb 2008 - This section needs to be updated with my findings since travelling the long, winding and downright expensive road of getting schematics for all Audi S2 and RS2 variants along with several quiet nights in studying the various options for manual air conditioning and full blown climate control and how that interacts with the electrical aspects of the cooling system. The description below remains accurate, but rather confusing without the schematics when trying to get your head around it properly in order to learn something or fix a problem. LInks will be added here to the HVAC section in due course where more info will be provided along with the schematics. One thing thats quite confusing when looking across different model years schematics is that there are mistakes and inconsistencies along the way which makes it difficult sometimes to always refer to the same component reference that works on all S2 variants. Apologies for that, but its hardly my fault :) Many of the relays are shown in the schematics without any notation of their internal structure, so I've had to go on pin nomenclature and experience to get the descriptions thus far. There are some confusing pin numbers used which caused some head scratching (and doubts about how accurate the source material is) but I think its a good first cut. As more mysteries get unveiled the details will be added here in due course... Turbo coolant pump (TCP) - As mentioned in the introduction, this pump provides sufficient circulation of the coolant to keep the bearings of the turbocharger within a sensible temperature range. The TCP is activated, as part of the 'after-run' process that protects the engine after it is stopped after a hot run. This is the phenomenon called 'heat soak' which occurs to a hot engine after it has been shutdown and Audi is one of very few manufactures to actively do anything about it. The turbo coolant pump is part number 034-965-561C . It's location is shown in this photo. ;)
The best way to find it is to follow the hose connected to the top of the main radiator and follow it almost immediately down to the TCP. At the other end of the pump, you can just see the electrical connector which activates it. ;)
The TCP is extremely quiet in operation. If you are very careful it is just possible to detect it's vibration with a hand placed on the pump while it is running. On the schematics, the TCP is notated as V51. It is controlled by J155, the TCP/After-Run relay. It is also possible to hear the TCP running for about five seconds just as the ignition is turned ON. This is a great sanity check for the TCP and much it's control circuitry. One concern I have is that it appears that the TCP may NOT be fused on the 3B engined S2. This does not seem to be the case on the 200TQ or UrS4. Close inspection of the S2 schematics shows a connection from the unfused side of S27 into the TCP motor and control relay. This could just be a mistake on the schematics, but you have been warned ! Further investigation beckons... TCP/After-Run Relay - This relay activates the TCP when the coolant temperature exceeds ~109C for a period of time. In addition it provides the control voltage into the low speed fan relay (J138 on a 1991 3B, or J26 anywhere else) so that it is activated as part of the after-run sequence. Notated as J155 on the schematics, this device resides in position 12 (twelve) of the auxiliary relay carrier (under the dash). The thermoswitch used to trigger the relay is notated as F98. The pin nomenclature for this device is very confusing, but my best guess at how the device works is that when F98 is closed, the two 'output' pins of J155 are switched to earth to close the circuit for the TCP and to trigger the low speed fan relay. It seems plausible that the TCP can be activated independently as this relay is aware if the engine is running via a fused feed (S24) out of the fuel pump relay that is shared with several ECU components. After a quick look at the related info here on SJM's wonderful site it appears my train of thought aligns with that i.e the relay pin numbering is meaningless. One subtlety that is not immediately obvious is that the TCP is only active when the engine is OFF. Apparently it does not run when the engine is running - that's just as well as it pumps coolant in the opposite direction to the mechanical pump - according to SJM (and who am I to argue)... But my concerns on TCP fusing on the S2 remain in force for now ! A timer inside J155 limits the duration of the after-run sequence to a maximum of 10 minutes. This preserves battery life in the event of failure of the thermoswitch F98. Best guess on the part number for the after-run relay is 447 965 571A - to be confirmed. Radiator Fan Motor - Notated as V7 on the schematic, this fan provides important assistance in pulling air thru the main radiator when the need arises. An inoperative fan (or its relay logic) on an S2 is NOT something to take lightly as the engine will be simply unable to protect itself from extremes of high temperature. A prolonged roasting can cause severe engine damage. The 3B radiator fan has three electrical connections by way of the following :
Some S2 models use a similar fusible link on the ground side of the radiator fan wiring as found in the older Audi 100 & 200. This protected the wiring loom from a meltdown scenario in the event of the fan motor seizing and drawing excessive current. It would appear to be unnecessary on the S2 as fuses S20 and S5 protect the 12V feeds from the fan relays. Looking at the schematic closely for the 3B, it appears that S5 was an afterthought so I don't
know if this applies to all S2s. This matter was thoroughly resolved with an additional 60Amp fuse (S42) on later model Audi 80 and S2 vehicles.
If in doubt - check as I don't yet have a clear view when cars left the factory with the corrected fan wiring in place. SCHEMATICS for your model year, coming here soon ! Low-speed Fan Relay - This relay, notated as J138 in 1991 (and J26 from 1992 onwards) on the schematics, is triggered by the TCP/After-Run relay (J155). When so triggered, it provides a 12V signal into pin 2 of the fan motor, via an external dump resistor (N47) to operate the fan at it's lowest speed. This relay resides in location 3 (three) of the main relay carrier - regardless of model year and S2 variant. High-Speed 1 Fan Relay - Notated as J26 in the 1991 schematic (I told you it was feckin' confusing), this provides the 12V control logic for the radiator fan to operate at speed 'Hi-1'. It is triggered by a thermoswitch notated as F18. The relay's 12V output connects to terminal 2 on the radiator fan. The control side of this relay is fused by S15 (25A) whilst the output is fused by S20 (30A). The latter fuse is shared with the fan's low speed operation but the sticker on the fusebox uses the term 'Fan Speed 1' for this fuse and relay. J26 resides in location 11 (eleven) of the main relay carrier. High-Speed 2 Fan Relay - Notated as J101 in the 1991 schematics, this provides the 12V control logic for the fan to operate at speed 'Hi-2' - the fastest possible. It is triggered by a thermoswitch notated as F54. The relay's 12V output connects to terminal 1 on the radiator fan. The control side of this relay is fused by S15 (25A) whilst the output is fused by S5 (30A). The sticker on the fusebox refers to this fuse and relay as 'Fan Speed 2'. J101 resides in location 6 (six) of the main relay carrier. Thermoswitches & Temperature sensors - This section aims to describe the function and location of each thermoswitch and temperature sensor on the 3B engined S2. F18 / F54 - Fan Thermoswitches - These two thermoswitches are actually combined as one single component ( 191-959-481C). The location of this device is at the bottom of the main radiator. F18 turns ON at ~95C, OFF at ~84C and provides the switch to control the fan on speed 'Hi-1' whilst F54 turns ON at ~ 102C, OFF at ~91C to control the fan on speed 'Hi-2'. F66 - Low coolant switch - This normally open switch resides in the bottom of the coolant reservoir. When the coolant level is low, this switch closes (to ground) and the Autocheck system raises the appropriate alarm on the instrument cluster. F76 - Multifunction temperature sensor (MFTS) - This device also integrates a number of components into one physical part. It's location is underneath the water rail which runs below the inlet manifold. It provides for several functions as outlined below...
MFTS Potted History - F76 was a furiously complicated device at first, but it has since been simplified internally to make it cheaper and more robust. Basically, there are two variants of the device. The early one (made by VDO) was a complex active device with a four-pin layout that required 12V power. The later device is a low-tech passive affair with a three-pin connector as the 12V signal is not required. This later device (034-919-369C) can be retrofitted in the event of failure of an early type 4-pin device as the form-factor of the plug and the signals are entirely compatible. The pinout of F76 is given in the table below long with notes for testing each aspect of the device. Note that F76 is also connected to 0V by nature of its mechanical connection to the earthed water rail -
F98 - TCP Thermoswitch - This device is located alongside
the multifunction temp sensor (F76) on the underside of the water
rail that runs below the inlet manifold. This switch closes at 106-112C
and opens at 94-105C. According to SJM, this device is clever enough
to open if the temperature exceeds 130C in a horrible scenario such
as an engine fire. Best take on the part number is
035-919-369B - this supersedes earlier part numbers. |
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Wiring Diagrams -
COMING SOON...
The schematics presented here are for the 3B engined S2 without air conditioning. I have distilled this onto one page from a number of sources that had the relevant components strewn over several sheets. As always, I cannot guarantee the accuracy of this material, so use it with caution - the radiator fan is a heavy current consumer so you don't want to be getting wires and fuses mixed up in this area ! I'm searching for schematics on the S2 fitted with both types of air conditioning to determine the exact electrical differences. One thing I do know is that when the aircon is switched on, the radiator fan runs at the low speed setting to assist the generation of airflow over the condensor which is mounted in parallel with the main radiator. Fuses - This section lists the fuses salient to the cooling system and defines their correct rating. All of these fuses reside in the main fuse/relay carrier under the bonnet.
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