Tech Article: Audi C5 A6 Secondary Air Injection System Design

This article discusses the Secondary Air Injection (SAI) system for the Audi C5 A6. We wrote this focused on the 4.2 V8 model as made from 1999 to 2004, with the main focus being on how the system was designed, and why. For the 2.7 Turbo, the same principles apply plus it’s more complex yet. If you’re not here for the technical analysis but would rather buy a used unit from us, guaranteed to work and fit, then please select the link below:

The information herein is based on our own 4.2 V8 cars.

When a car with an internal combustion engine is at normal operating temperature, it’s very clean emissions-wise. The catalytic converter is part of the reason why. It changes the chemical composition of the exhaust gas to something that’s nicer to breathe.

However, when the engine is cold, it needs a more-rich fuel mixture so as to run, and when the catalytic converter is cold then it doesn’t work as well as when it’s at normal operating temperature. So, that’s a double whammy that make the exhaust gases less clean, when the engine is cold.

The folks at Audi used a clever design on the C5 Audi A6 to solve this problem. When the engine is cold, they artificially pump vast amounts of air into the exhaust system, using a high-volume blower (as in, a fan that pumps air). This helps the catalytic converter work better until it reaches normal operating temperature. This system is called “secondary air injection” a.k.a. “SAI.”

When the SAI system doesn’t work right, sensors in the car pick up on this and then the car turns on the “check engine” light in the instrument cluster, warning the driver that an emission-related process has failed.

To find out the specific reason behind the check engine light coming on in our project car, I connected my MS Windows laptop PC to the Audi and asked, “What’s wrong?” It replied: “My SAI is messed up” … essentially. For the dialog, I needed a special cable. One end plug into the Audi and the other end plugs into any Universal Serial Bus (USB) port in my laptop PC. I also needed some special software. I got both from a company called Ross-Tech. They’re beyond brilliant.

In its simplest form, the design of a SAI would involve the engine management computer sensing that the engine is cold and then turning on a blower. The blower would simply suck air from wherever it can get it, dirty or otherwise, and pump it into the exhaust system anywhere upstream of the catalytic converter.

There are several problems with this simplified design.

1. Pumping dirty air (e.g., with road dust) through the blower and into the catalytic converter would shorten the life span of both.
2. The blower would be very noisy, sounding like a vacuum cleaner on steroids.
3. The blower uses a lot of current so it’d need a thick set of wires to and from the computer, and a big, strong switch.
4. Once the engine reaches normal operating temperature, then the process would reverse, and air from the exhaust would escape out of the blower, which would destroy it, make a huge noise and pollute more.
5. If there were a problem in the wiring, the computer could easily be damaged.

So, our design would need to be improved:

1. The air would have to be sucked out of the clean air flow downstream of the air filter.
2. The blower would have to be located below the car and as far forward as reasonably possible, away from the passengers. Below the air filter happens to be just such a place, yay!
3. Instead of running heavy-duty wires from the blower to the computer, we would use a relay to handle the switching. That means we can have light duty wires from the computer turn on a little switch inside the relay which in turn, um, turns on the big switch. That’s what a relay is: a little switch that turns on the big switch.
4. We need some sort of valve that opens when the SAI system is active (such as when the engine is cold) and closes when it’s not active (such as when the engine reaches normal operating temperature)
5. A heavy-duty fuse would protect the computer in case of a wiring malfunction


Instead of one valve, we should have two, one for each bank (a.k.a. half) of the V8 engine — because it has two separate exhaust systems, one for each bank.

The computer would turn on the SAI system electrically. However, the valves are near the exhaust gas so they’ll get hot. Heat doesn’t play well with electrical components. So a vacuum-actuated valve system would be more reliable.

Better would be if the electrical signal would turn on a component that’s placed far away from the valves. The component would allow vacuum to reach the valves via a vacuum hose that runs to each valve. When there’s vacuum in the hose, the valve opens. This type component is called a solenoid.

… and that described the the SAI system on the Audi C5 A6, so now we know why it was designed as it is. Clever!


Tech Article: Audi C5 A6 Quattro 4.2 V8 Wheels and Components

This article discusses the wheels and components for the Audi C5 A6 with the 4.2 V8, as made from 2000 to 2001, with the main focus being on the styles of rim. If you’re not here for the technical analysis but would rather buy a used part from us, guaranteed to work and fit, then please select the link below:

The information herein is based on my own cars, plus reading, discussion and analysis.

My 2000 Audi A6 Quattro 4.2 V8 cars all came with 17” rims whose style earned them the generally accepted nicknames of “Pie Plate” or “Pie Cutter.” According to the official manual, their specifications are Jx17H2 ET35 5/112. These rims are neither particularly convex nor concave. Their part number is 4B3601025M, and since “4B3” designates (as far as I know) the post-facelift cars.

The center caps for these are part number 4B3601165A, and as far as I know they fit only the “Pie Plate” a.k.a. “Pie Cutter” wheels. Their weakness is that one or more of the tabs in the back breaks off.

Also, Chinese-made fake parts are known to be in circulation.

I have yet to see a 2001 vehicle with such wheels but allegedly they were available on some 2001 Audi A6 Quattro 4.2 V8 cars too.

Not a style that I recall ever seeing in person, yet as per the official parts manual it was available for the 2000-2001 Audi A6 Quattro 4.2 V8: six-spoke rims that, according to the official manual, were only available in 16” size. Their specifications are Jx16H2 ET35 5/112. These rims are arguably slightly convex. Their part number is 4B3601025L. There is a variation with specifications Jx16H2 ET32 5/112 (as in ET32 as opposed to E35). That part number is 4B3601025K.

According to the forum, “ET and Offset both have the same meaning and value. The term ET originates from the German word ‘Einpresstiefe’ which translates as ‘insertion depth’ … Higher offset (ET50 for example), wheels sit closer to car, have chance of hitting brake and suspension components … Lower offset (ET10 for example), wheels sits further out from car, lots of room for components but may poke past fender or impact fender on dips and bumps…”

Presumably the measurement is in millimeters so the ET32 rims sit 3mm further out from the car compared to the ET35 rims.

The center caps for these are part number 8N0-601-165-A, and as far as we can tell they were used on various models of Audis, not just the Audi A6 Quattro 4.2 V8 and not just the A6 either.

According to a source I don’t trust very much, the 4B0-601-025-J was a 15” rim available for the Audi A6 Quattro 4.2 V8. As to specifically the Audi A6 Quattro 4.2 V8 models, I’m not convinced that 15” rims would fit over those large front brakes.

A rim style that I have on my own 2001 Audi A6 Quattro 4.2 V8 and that I’ve seen on a 2003 Audi A6 Quattro 4.2 V8, is a 5-spoke 17” concave rim with a style similar to on the early D2 A8 cars. Part number was 4B3601025P; specifications were Jx17H2 ET35 5/112.

The center cap for these rims has part number 4D0601170A, with the “4D0” implying this part was first used on the D2 A8 – which supports the premise that this A6 rim is based on the A8 rim style.

The non-locking wheel bolts for all of these rims are part number WHT002437, but for the 5-spok rim, as was offered on the D2 A8, the bolts (which were not covered by a center cap, in this style rim) each had a plastic cover, which might have helped for rust and also helped for aesthetics. Problem is that if you were trying to change a tire, these could be a problem unless you happened to have a pick, or the official Audi removal tool: 8D0012244A. In gray, the plastic cover part number was 321601173AZ37 (with the “Z37” being the color code for gray) whereas the back plastic cover part number was simply 321601173A with the color code omitted.

Tech Article: Audi C5 A6 Quattro 4.2 V8 Alternator

The C5 Audi A6 Quattro 4.2 V8 models as well as the S6 all used the same type of 150A alternator, Bosch type AL0797X, Bosch part number 0123520021. Audi has two part numbers for the same essential alternator. The old number is 038903018D. It was superseded by 077903015L.

If you’re not here for the technical analysis but would rather buy a used unit from us, then please select one of the links below. If we’re out of stock on one, choose the other; they’re essentially the same part except for the part number.

To remove the alternator, the serpentine belt has to be freed from the pulley, and then there’s a small bolt at the bottom and a 110mm-long Allen head bolt at the top. The back has three electrical wires. The thick positive wire is attached to the alternator using a 13mm nut. A smaller wire is attached to the alternator using an 8mm nut. Another small wire is attached to the alternator using a spade connector.

When shopping around, it’s a good idea to factor in whether or not the unit you’re considering comes with a front pulley; all of ours do but I noticed that our competition doesn’t always include it.

Another factor is the core charge. Some vendors charge it; we do not.

We also make a point of taking our alternators to the friendly local auto parts store to have them formally tested before we sell them, which is why we’re so comfortable offering a money-back guarantee. When buying a used alternator, including ours, it’s prudent to take that to your friendly local auto parts store for testing, before you install it into your vehicle. That prevents wasted time, and it rules out problems caused by the vehicle.

It might be tempting to consider a knock-off, but we prefer Bosch units, even if used.

It might also be tempting to consider an alternator from a non-V8 model, but they’re not interchangeable.

ZF 5 HP-24A Teardown: Step 18: Miscellaneous

With all the major parts removed, the empty transmission casing still has some minor items attached, such as the vents and positioning dowels.

These are items that we would not normally remove as part of working on this type of transmission.

Our focus during a tear-down is on removing items that:

  1. Have failed, or
  2. Within reason, might:
  • Fail in the next x years
  • Hide pieces of metal that broke off from damaged parts. Such pieces might range in size from tiny to large, but each one poses a potential threat.

ZF 5 HP-24A Teardown: Step 15: Sprag Clutch and F Clutch

After removing the bearing from the rear of the transmission, the rearmost sub-assembly inside the transmission can be loosened and moved forward and out the front of the transmission.

This rearmost sub-assembly is the sprag clutch and F clutch. Together, they form a unit that is removed and installed as a set.

The sprag clutch is also called the freewheel; it is a high-speed one-direction-only ratchet, so that in first gear, when the driver’s foot is removed from the accelerator pedal, the car doesn’t lurch forward due to engine compression braking, but instead coasts along smoothly.

If the sprag clutch fails in one way, and now it always locks up, then this feature no longer works, and the car could lurch forward in first gear.

If the sprag clutch fails in the opposite way, and now it never locks up, then the car would not move forward in any forward gear, though reverse would work just fine.


ZF 5 HP-24A Teardown: Step 14: Rear Axle Output Group

We could not remove the F clutch and freewheel until the rear axle parts were removed. So, that was the next step.

To remove this part, we removed the looooong yellow Torx T-40 bolts at the end of the housing.

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We next focused on the flange to which the drive shaft attaches. That area is filthy with black grease, so after trying to tough it out for a while, we gave up and cleaned all the grease off.

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First, we removed the rearmost part.

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With the flange removed, the rearmost portion of the transmission was a plate. We removed the fasteners around that plate, and then removed the plate.

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With the plate removed, the Torsen differential unit can be lifted out.

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The next step is to remove the large rear housing.

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Sandwiched between that housing and the body of the transmission is another housing, perhaps an inch or two thick. We removed that too.

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That enabled us to plainly see the output shaft from transmission, and an intermediate gear that meshes with that. That gear, in turn, also meshes with a gear on a shaft that sends the power to the front differential.

The output shaft of the transmission has some gears and bearings that slide onto it; we removed them.

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We removed the intermediate gear too. Under that gear is a pressed-in bearing that obscures some fasteners that we needed to remove to free the F clutch and freewheel inside the transmission. Removing this bearing was a huge pain. We destroyed the bearing in the process. We tried various pullers and tools, including two special mini-prybars that we’d bought from Snap-On. Finally, the business end of a massive 30mm wrench managed to work the bearing loose. A replacement bearing cost us about $35 plus shipping.

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