Cooling System - An introduction

Introduction | 3B System | ABY Changes | Fault Diagnosis | System Repair

S2 Cooling System - Introduction

An Introduction

The cooling system for Audi's I5 20V turbo engine is not the simplest design in automotive history. There are two radiators, two coolant pumps (that work in opposite directions), a multi-speed fan system that works when the engine may or may not be running, plus a plethora of relays, temperature sensors and thermoswitches around the engine bay. Cars fitted with air conditioning have further complexities that interwork with the cooling system. Add to that the general cramped condition of the S2 engine bay and you have the perfect recipe for a number of physical and mental challenges in diagnosing and repairing various fault conditions.

The Basics

The reason that the S2 has two radiators is that Audi felt the single radiator used on many Audi 80/90's of that era did not have enough cooling capacity for the 3B and ABY engines in hot climates. This is also true for other Audi 80/90 models such as the V6 engined cars. The second or 'auxiliary' radiator increases the amount of surface area used to banish heat from the coolant into the atmosphere.

One major down-side of the auxiliary radiator is that it gets in the way of some maintenance jobs at the front of the engine. Many S2 owners in the UK (myself included) have simply removed their auxiliary radiator for this reason and reported no temperature problems. In fact, the irony is that the airflow is so increased by it not being there that my car runs a couple of degrees cooler on the motorway with the auxrad removed. The other advantage is it creates a lot more room at the front of the engine for some maintenance operations.

The coolant (originally a blue coloured glycol based substance as per Audi G11 specification ) is circulated around the engine's waterways and heating system by a mechanical pump in the block which is driven by the timing belt. When the engine is cold, a temperature controlled valve (aka the thermostat) is in a closed position to allow the coolant to reach normal working temperature as quickly as possible. This is achieved by preventing circulation of the coolant beyond the engine block and head in this warming up period. Only when normal operating temperature is reached, does the thermostat allow the coolant to be pumped through the radiators and into the cockpit heater matrix.

As the car is driven, heat is dissipated by the able assistance of airflow over the two radiators. A normal operating temperature for the S2 coolant is about 87-90 degrees celsius. It is completely normal to see lower coolant temperatures when the increased efficiency of fast moving cold air is provided by mother nature. In the event of excessive temperature build up, an electrical fan is used to generate additional airflow through the main radiator. This is normally observed when the car is idling on a hot day for prolonged periods of time.

Hidden Functions

There are a couple of 'hidden' functions on the S2 cooling system that also need to be considered. Many owners are completely unaware of these features - primarily because of the conditions needed to engage them and secondly (one could summise) because some cars may have faulty components used for these hidden feaures.

It is worth pointing out that there is an electrically driven pump which provides coolant flow to the turbocharger under certain conditions. This is engaged AFTER periods of excessive engine temperature to prevent possible damage to the turbo bearings. The 'turbo coolant pump' runs VERY quietly so it is never easy to tell when it is running. Bizarrely the turbo coolant pump (aka TCP) circulates coolant in the opposite direction to that of the mechanical pump. The TCP only operates when the engine is switched OFF as part of an 'after-run' mechanicsm designed to protect the engine from heat soak. During the after-run sequence, the turbo coolant pump runs and the radiator fan is engaged at it's lowest speed to help cool things down.

Another purpose claimed to benefit by the TCP operation is to reduce any excessive coolant bubbling that may occur after a hot-run. Not sure I understand that one, but hey...

This 'after-run' procedure of the radiator fan also occurs on other Audis but many owners have never encountered the phenomenon simply because of the time delay, after stopping the engine, before sufficiently high heat-soak is detected and the after-run cycle is engaged. Most of us are back in the house with the kettle on by the time the fan comes on... Conversely, it freaks some owners out when they notice it for the 1st time and they think the car has gone mad !

Another little known fact is that the S2 ECU (3B, ABY and RS2) actively reduces boost pressure when it is informed of dangerously high coolant temperatures. It does this by turning off the WGFV so that turbo boost falls back to a safe level set by mechanical limits within the wastegate (approx 0.4bar above ambient). Simultaneously, the Autocheck system will indicate a special alert on the instrument cluster warning of excessive temperature. This could be described as a 'pull over now' kind of event :)

Refer to the description of the Multi-Function Temperature Sensor (or MFTS for short) here for more details - this is F76 on the S2 schematics.

NOTE for AAN engined S4 and S6 owners - The MFTS device does NOT provide this boost reduction failsafe in the event of excessive water temperature. I do not know why, as the 3B, 1B, RR engines (to name a few) all preceded the AAN and have this function. Very strange, but true. Thanks again to the fabulous S2 Forum for tasking me with getting to the bottom of that anomolie in the I5 Turbo engined family tree.

As always, the devil is in the detail, and the links at the top of this page will explain the system fully on the 3B and ABY engines. Much of the complexity is in respect to the electrical relays and sensors used to determine when the radiator fan is used and at what speed. A number of diagnostic procedures are outlined as well as some handy hints for system repairs. Further complexity on cars with air conditioning also must be considered.

Coolant Specifications

For reasons not entirely understood by myself, Audi decided to change the coolant specification on all of its cars circa 1993 from one glycol based solution to another. I think the change was to improve the coolant's anti-corrosion properties around the world (due to differing mineral content in water). Fortunately, it is easy to spot the old and new coolants.

The 'old' original coolant as per G11 V8B specification is BLUE in colour.

The 'new' improved coolant as per G12 specification is RED in colour.

Obviously it is unwise to mix the two coolant types... It is specifically 'verboten' in the Audi technical literature. There is no problem with usng G12 coolant in what was originally a G11 system - as long as the old stuff is thoroughly flushed out.

It is worth knowing that G11 is no longer available from Audi (though other stockists may carry it). VW/Audi have introduced a new coolant known as 'G12plus' which is safely miscible with G11 or G12. The new G12plus liquid is pink in colour.