Not-So-Catchy Name

I was raised German and it was my first language, although I wasn’t born in Germany.  One of the things one tends to take for granted in German culture is the practicality of everyday things.

For example, in German culture, if you had an ice-cream factory in Mannheim, your company would probably be called the Mannheim Ice-cream Factory.  If your product became very popular then parents might talk about buying their kids a MIF, meaning your product, the one from the Mannheim Ice-cream Factory. The underlying long name would still be the basis, and your ice-cream would be a mouthful in more ways than one.

And so, if you made engines in two locations, Augsburg and Nuernberg, then you might call your business the Engine Works at Augsburg and Nuernberg. In German that would be Maschinenwerke Augsburg-Nuernberg which explains big diesel truck driving around with MAN on their radiator grilles.

If you were located in Bavaria instead, and you made motors, then you might call your business the Bavarian Motor Works. In German that would be Bayerische Motoren Werke, which explains passenger cars driving around with BMW badges.

As a second-to-last example, if you were located in Baden, and you had a soda and aniline factory, you might call it the Baden Soda and Aniline Factory, which in German is Badische Soda und Anilin Fabrik, which is the full name for the now-massive German chemical company BASF.

And so lastly, if you were located in Friedrichshafen, and you had a gear (or to be precise, since Germans are precise: gear-wheel) factory, you might call it the Gear Wheel Factory at Friedrichshafen, which in German is Zahnradfabrik Friedrichshafen, with the acronym ZF. Hence the name.

Products

Amongst many other things, ZF makes high-quality automobile transmissions such as used by Alfa Romeo, Audi, BMW, Jaguar, Land Rover, Maserati, Peugeot, Porsche, Volvo and Volkswagen — high-end automobile manufacturers.

As an example, the ZF automatic 4-speed transmission named ZF 4 HP-22 is used in the BMW 318i, 325, 528e, 533i, 535i, 633 CSi, 635 CSi, 733i, 735i and probably other BMWs too, yet it’s also used in the Volvo 740 series. I know that for a fact because when the ZF 4 HP-22 in my BMW 633 CSi failed, I went to a local junkyard and pulled one out of a Volvo 740, and it’s been living happily in my BMW for the last ten years or more.  It was also used in the Peugeot 505 and Maserati Biturbo – the latter a very powerful car and yet while being strong, that transmission is so light that a slender blonde girl (me) can, and has, personally pick one up such as when I carried it into a repair shop.

At the time this transmission came out, 1984 or so, most other transmissions were still 3-speed automatics, and car buyers were always deciding whether to go with the convenience of an automatic or the better power and fuel economy of a manual transmission. The ZF 4 HP-22 gave them both.

Not just does it have four forward speeds (hence the name) but it also has a lock-up torque converter, which works like a 5th overdrive gear so that (at cruising speeds) this particular automatic has the same low power loss and fuel economy as a manual transmission.

Several of my BMWs have this transmission and although they will eventually die when neglected or some muffler shop hangs the exhaust directly from the transmission using a heat-conducting steel rod (guess how I know this) they live a long and happy life when serviced with anything resembling half of a reasonable maintenance schedule.

So, now I own an Audi A6 with the ZF 5HP-24A, which deserves an entire essay as to its technical merits.

For now, I’ll just say that the world is a much better place thanks to ZF.

I have owned several BMWs with ZF automatic transmissions, and much as I like these transmissions, they don’t last forever — especially if the previous owner neglected to service them. Whenever I buy a used BMW, I like to immediately service the timing belt (if any), and the automatic transmission.  As to replacing the timing belt, if that hasn’t snapped yet, then it’s as if all past sins of maintenance neglect for that part have been forgiven and things are 100% OK again. As to the transmission, not so. Even the best transmission service in the world won’t fix problems caused by past neglect.

On several of my cars with such transmissions, they were ostensibly in good health but eventually failed, and in my experience, the failure has always been gradual. I assume it’s due to an internal component mechanically disintegrating or wearing out, and the debris or sludge in the bottom of the transmission oil pan tends to support that hypothesis.

By contrast, if an electronically controlled transmission fails suddenly, without any prior warning such as slippage or juddering, I would suspect an electrical problem. That’s exactly what happened to at least one of our Audi A6 project cars.  So, that’s where I’m looking next.

On the ZF 5HP-24A, the bad components that can cause electrical problems are:

• Transmission control module (TCM)
• Cable from the TCM to the transmission
• Cable from there into and inside the transmission
• Solenoids

I have already had three different TCMs in our project car, and none of them have fixed the problem. It’s possible that they are all bad, but I’m going to shelve that hypothesis for a while, and focus on the alternatives.

The TCM has a built-in output test for the solenoids. If any or all of these fail, we can’t as yet rule out a problematic cable, either, but isolating the problem beyond that point becomes a lot more viable. So, that’s the next test.

We have just acquired another 2000 Audi A6 project car, with the 4.2 V8 engine and the ZF 5HP-24A transmission.

As told to me, the story is: The previous owner was driving the car to work one day, and noticed no symptoms of abnormal behavior until he stopped at an intersection. As he tried to accelerate away, the transmission slipped and then the car went into limp-home mode.  It has been misbehaving ever since.

I found it interesting that there was no warning. If the failure was due to clutch slippage then normally I’d expect that to be a very gradual and very noticeable thing that gradually gets worse and worse.

I have tried to immerse myself more as to the differences in Audi transmission control modules a.k.a . transmission control units, specific to the Audi A6 Quattro or A8 cars of the 1997-2005 era, those fitted with the ZF 5HP-24A transmission. I don’t have any brilliant insights yet, but I’m learning. And, I like to have my data first-hand, so there I was, last night, with a stack of half a dozen different Audi transmission control modules, trying them in our two different Audi project cars.

Yes, two. We now have two 2000 Audi A6 project cars with the 4.2 engine and the ZF 5 HP-24A transmission, and the 4B0927156BS transmission control module. Both are failing in exactly the same way: in “drive” the transmission slips, and then there’s a a jarring “thump” and the car goes into “limp-home” mode, and the instrument panel indicator reflects this.

Swapping the two transmission control computers didn’t fix the problem in either car.

In each of these two cars, we have also tried two transmission control modules with part number 4B0927156CT … meaning basically the same part but a variation. On the blue car, the car immediately goes into limp-home mode, so the car treats that as very different from the 4B0927156BS part. On the silver car, the car treats the 4B0927156CT part the same as if it were the 4B0927156BS part. The car tries to go, then slips, then collapses into limp-home mode. I’m VERY surprised that the same transmission control unit is treated differently in two cars of the same model year.

As I understand the Europaparts explanation of the part numbering convention used by the VW-Audi Group, the first three characters (i.e., “4B0”) are the model for which this part was first introduced.  According to the CDIF/3 website, “4B0” is the C5 Audi A6, made from 1997 through 2005. So, okay, that transmission control unit was first made for the C5 Audi A6. But, might something else work too?

The 4th digit, “9,” means it’s electrical. The next two digits are the sub-group. “27” is presumably “control module.” The next three digits, “156,” identify this particular type of control module, probably the transmission control module for the ZF Tiptronic automatic transmission.

Lastly, the letters (“BS” or “CT”) are a variant or modification code. So visually the “BS” part is the same as the “CT” part but they are not internally identical, nor always interchangeable.

On a hunch, I visited a premium automotive junkyard, and bought half a dozen transmission control modules for the ZF Tiptronic automatic transmission. They all have the core part number of “925156” though they are different as to the first three characters, and then also the letters at the end.

I found many contemporary VW Passat models that seems similar to the C5 Audi A6, and so I found their transmission control modules in the same location (under the carpet in the front passenger foot-well), and so I removed and bought some of these also.

We spent maybe an hour swapping transmission control modules into our project cars, and the only ones that have so far worked as such are the ones with letter codes “BS” or “CT.”

Much as I love Audis, I also own several BMW E30 cars, and I sell parts for them too. My most popular item? Cylinder heads for the BMW M20 engine.

This engine design uses a timing belt and it’s an interference-fit engine, meaning: a particular valve will try to occupy the same space that the piston does, unless the camshaft is doing its job. If the camshaft doesn’t do so, then the valves impact the piston. I’ve seen the resultant damage range from not-great to downright horrible, such as valves being broken off and mashed up in the cylinder, finally making a hole and then bouncing around the crankcase, doing more damage.

All it takes for this all to happen is for the timing belt to fail. This has never yet happened to me in one of my own M20-engined BMWs because as soon as I own the car, I tend to change the timing belt.

However, a failed timing belt HAS happened to me on my 1983 Volvo 240 Turbo. Engine damage: zero. Reason: the engine is a non-interference design. Even with the camshaft not doing its job at all, the valves never impact the pistons. After replacing the timing belt, the Volvo was good to go again.

The failure of a timing belt has, to my knowledge, consistently been the result of gross neglect on the part of an owner who either knew better or should have known better.

So now when it comes to the Audi A6 rocket-ship, is the 40-valve V8 4.2 engine an interference-fit design or not? In typical automobile-forum tradition, a great many people voice their clueless opinions on the Web (including by rudely lecturing on the need for regular timing belt replacement) when someone asks the question: is this an interference-fit engine?

I prefer to get my facts first-hand. So, I walked to a dead Audi 40-valve V8 4.2 engine that I’d bought inexpensively, for analysis, a few weeks ago, and I inspected it. The cylinder heads were already off.

One valve was literally bent out of shape. Valve-shaped bright marks were obvious to see on the tops of many pistons, and the impact zone was similarly visible by there being bright metal on the sides of some of the valves. Did the valves impact the piston? Yes. Is this an interference-fit engine? Yes.

This issue came into sharp relevance yesterday, because someone in the SF Bay area was advertising his 2001 Audi A6 with the 40-valve V8 4.2 engine for $375. The engine had failed. Personally, I thought the car would be a good deal, engine or no engine. I contacted the seller but received no reply and shortly afterwards, the ad was deleted. Someone else had presumably bought the car.

Sadly I am not convinced the buyer will be happy. In the ad, the seller seemed to exude an air of being irritated and said he didn’t want to answer many questions. He did, however, mention that the car needed a new timing belt, which he’d already bought, and he indicated or implied that the new owner needed simply to install the new timing belt and all would be well. However, that’s an unlikely scenario. More likely the engine would start right up and run after the timing belt replacement, but at some point soon the damaged valves would take their toll. The hot air rushing past the unwelcome gap in the valve seats would cause burned valves pretty quickly. A clueless owner might not notice the car eventually missing on one or two of the most-affected cylinders especially since the loss of power would be gradual and there are so many cylinders.

So, from here on, I resolve to always change the timing belt on any Audi I own that has the 40-valve V8 4.2 engine. And before I buy one, I plan to diligently do a compression test, first.

Oddly, all the damaged A6 40-valve V8 4.2 Audis that I’ve seen advertised, or in junkyards with engine damage were 2001 cars, and all the damaged such cars with transmission failure were 2000 cars. Maybe a coincidence, maybe not.