1991–1997 Audi A6 C4 (4A) Service & Repair Manual PDF

Description

1991–1997 Audi A6 C4 (4A, 4A2, 4A5) Service and Repair Manual PDF

Two things on a C4 will hurt you if you do not know they are there.

The first is Procon-ten — a mechanical safety system that runs steel cables from the engine to the steering column and the seatbelts, held under permanent tension. Pull an engine or a gearbox on an early car without understanding it, and you release that tension in a way you will not enjoy.

The second is the central hydraulic system. On these cars the power steering and the brake servo share one pump, one reservoir and a pressure accumulator. It does not take brake fluid. It does not take ordinary power steering fluid. Put the wrong thing in, and you destroy every seal in the system.

Neither of these appears in a generic manual. Both are covered here.

What the A6 C4 is

The C4 platform arrived in 1990 as the Audi 100. In 1994 Audi facelifted it and renamed it the A6 — same car, same type code, new badge. That is why documentation for this generation covers both names, and why a “1991–1997 A6 C4” set necessarily includes the 100 years.

It was sold as a saloon (4A2) and an Avant estate (4A5), with front-wheel drive or quattro. It is a fully galvanised body, which is why so many are still on the road when their contemporaries have long since dissolved.

Note that C4 quattro is a Torsen centre differential — a permanent, mechanical all-wheel-drive system. There is no Haldex here, no pump, no controller and no filter service. It is a gearbox, not a computer.

Procon-ten — read before removing an engine

Fitted to earlier C4 cars before airbags took over, Procon-ten is a purely mechanical occupant protection system. Steel cables link the engine block to the steering column and the front seatbelt reels. In a frontal impact, the engine is pushed rearward, the cables tighten, the steering wheel is pulled away from the driver and the belts are tensioned — all without a single sensor or explosive charge.

It is clever, it works, and it is permanently under tension.

The documentation covers the cable routing, the tensioning specification, the correct method of releasing and re-securing the system, and the procedure required whenever the engine or gearbox is removed. If your car has Procon-ten and you are about to lift the engine out, read this first.

The central hydraulic system

The C4 uses a single tandem pump feeding both the power steering rack and — on many variants — the brake servo, via a pressure accumulator.

Three things follow from that:

• The system takes Pentosin CHF hydraulic fluid, not brake fluid and not conventional PAS fluid. Using the wrong one swells and destroys the seals throughout the system.
• A failing pressure accumulator means a hard brake pedal and heavy steering, and it presents as a brake fault when the cause is hydraulic.
• Leaks show up in places that make no sense until you understand that steering and braking share a circuit.

Pump, reservoir, accumulator, fluid specification, bleeding and pressure testing are all documented.

Engines covered

Every engine in the range is belt-driven and every one is an interference design. There is no chain-driven exception, and a snapped belt ends the engine on all of them.

The 2.5 TDI inline-five deserves particular attention: its injection pump timing must be set correctly after any belt work, and getting it wrong produces a running engine that quietly destroys itself.

Transmissions and quattro

012 / 01A five-speed manual — front-wheel drive and quattro variants, with different internals and different fluid specifications. Clutch, dual-mass flywheel, linkage and adjustment.

Four-speed automatic — fluid procedure, solenoid diagnosis, and the adaptation reset that resolves a surprising number of apparent gearbox faults before anything is stripped down.

Torsen centre differential (quattro) — the mechanical torque-sensing differential, plus the rear final drive and the locking rear differential where fitted. Operation, inspection and fluid.

Electrical and diagnostics

Full wiring diagrams for saloon and Avant, connector views and pinouts, the pneumatic central locking system, the climate control head, ABS module identification, immobiliser where fitted, and fault code tables.

Diagnostics on the C4 are pre-OBD-II on early cars and K-line on later ones. Blink codes, the correct interface and the reading procedure are all covered — because plugging a modern OBD dongle into a 1993 A6 will get you precisely nowhere.

Known A6 C4 faults this documentation walks you through

• Instrument cluster pixel failure — the dot-matrix display fades line by line until the mileage, warnings and trip data are unreadable. Cluster removal and the repair route.
• Timing belt on every engine — all interference designs. Interval, tensioner, water pump and locking positions for each family.
• 2.8 V6 30v cam chain tensioners — a start-up rattle that worsens over time. Not the timing belt, and constantly mistaken for it.
• 2.5 TDI injection pump timing — must be reset after belt work. An engine that runs is not proof that it was done right.
• Central hydraulic system failure — hard brake pedal, heavy steering, or leaks. Pump, accumulator and the Pentosin fluid specification that most people get wrong.
• Pneumatic central locking — the vacuum pump runs constantly or the doors will not lock. Leak testing the system rather than replacing the pump on suspicion.
• Climate control head failure — buttons dead, display blank, or fan stuck. Diagnosis and the internal fault codes the unit will tell you if you ask it correctly.
• Plastic-impeller water pump — the impeller disintegrates and the engine overheats. Always replaced with the timing belt.
• Distributor cap and rotor on the earlier petrol engines — misfire and poor starting.
• ABS control module faults — warning light diagnosis and module identification.
• Wiring loom degradation — insulation hardens and cracks with age, particularly in the engine bay. Loom repair rather than chasing symptoms.
• Rear wheel bearings and suspension bushes — the noises and the alignment consequences.

Format and delivery

Supplied as a set of searchable PDF files covering the C4 platform — saloon (4A2) and Avant (4A5), front-wheel drive and quattro, including the Audi 100 years before the A6 rename. Readable and printable on any device: laptop, tablet, phone, or the workshop PC. No software to install, and no internet connection needed once the files are saved.

Select your year at checkout. After payment, your download link is sent within 24 hours.

Frequently asked questions

My car is badged Audi 100, not A6. Is it the same thing?

Yes. The C4 launched as the 100 in 1990 and was renamed the A6 in 1994 — same platform, same type code, same car underneath. This documentation covers both.

What is Procon-ten and do I need to care?

You need to care if you are removing the engine or gearbox. It is a mechanical safety system using steel cables held under permanent tension between the engine and the steering column and seatbelts. The release, tensioning and re-securing procedure is documented. Do not improvise with it.

My steering is heavy and the brake pedal has gone hard. Are they related?

Almost certainly. On the C4 the power steering and the brake servo share one hydraulic circuit, one pump and a pressure accumulator. When the accumulator fails, you get both symptoms together. Diagnosis and pressure testing are covered.

What fluid does the hydraulic system take?

Pentosin CHF — not brake fluid, not conventional power steering fluid. Using the wrong fluid swells and destroys every seal in the system, and the repair bill is considerable. The correct specification is given.

Is the 2.5 TDI inline-five covered?

Yes, including the injection pump timing procedure that must be carried out after any timing belt work. This is the step people skip, and the engine will run afterwards regardless — right up until it does not.

Does the C4 quattro need a Haldex service?

No. The C4 uses a mechanical Torsen centre differential. There is no pump, no controller and no filter. It is a permanent all-wheel-drive system and it is serviced as a gearbox, not as an electronic component.

Will this work with VCDS?

Partially — later C4 cars support K-line diagnostics and will talk to VCDS with the right cable. Earlier cars are pre-OBD-II and use blink codes. Both routes are documented, because a modern CAN-only adapter will tell you nothing on either.