Not to be quick to judge or anything, but when there’s a dead gunshot victim on the floor, and the person’s spouse has just found out about an illicit affair, and has a history of jealousy and violence, and is still holding the gun which is still smoking and the ballistics match, and the ink isn’t even dry yet on the large life insurance policy on the deceased …. then, yes, this person is the most likely suspect.

That’s what I’m trying to find as to our 2000 Audi A6 4.2 project cars slipping in first gear. There are many components, but which one is the most likely suspect to be causing this symptom?

To recap either of our Audi project cars’ transmission issues:

• In reverse gear, the car is quick off the line and shows no sign of juddering or problems.
• In first gear, the transmission slips, as if it’s mostly in neutral plus a little bit of inching forward, and then with a bang and a jolt, it engages and goes into limp-home mode. Or perhaps it goes into limp-home mode and that causes the jolt. When this happens, the error code in the transmission control module shows a ratio mismatch, as in it analyzes the transmission output speed as compared to the input speed, and if there’s a mismatch, as in: it’s slipping, that triggers a fault code to be stored, and limp-home mode to be engaged. Interesting. So maybe first gear doesn’t eventually engage at all. Maybe the jolt is due to limp-home mode (i.e., 4th gear) engaging.
• Neutral works just fine.
• So now when I study the truth tables for these three gears, I see that all three of them work by having the pressure regulation valves 1, 2, 3 and 5 (not 4) energized. So, something else must differentiate them, and meanwhile I can rule out all five of the pressure regulation valves and their solenoids as being suspect.
• That leaves the three solenoid valves.
• For reverse, solenoid 2 (not 1, not 3) is energized.
• For neutral, solenoids 1 and 3 (not 2) are energized.
• That’s puzzling to me. That covers all three of these solenoids in some way, and doesn’t enable me to suspect any of them. So on to door number three.
• For 1st gear, solenoid 1 (not 2 or 3) is energized. So the difference between 1st gear and neutral is solenoid number 3 being energized or not. For neutral it’s supposed to be energized. For 1st gear, not. So, what if instead of being “not” it’s “kinda sorta?” Then first gear would behave somewhat like first gear, meaning the car would move forward, but first gear would also act a lot like neutral, which means: slippage.  So channeling the spirit of Sherlock Holmes, I conclude that a kinda-firing-when-it-shouldn’t solenoid number 3 could cause this, or a valve that’s sticking and behaving as if it’s kinda sorta closed when it should be fully closed.
• Mechanically, the big difference as to 1st gear is that the freewheel gear comes into play, so the transmission probably energizes the freewheel when the valves for 1st gear are in the correct on/off combination.

I feel good about this. It is all starting to make sense to me now.

To see how different these two transmissions are, I suppose I could go read and compare the ZF parts lists for the ZF 5HP-24 and the ZF 5HP-24A, the former being for RWD cars, and the latter being the variant used for the Audi Quattro.

However, news of a big-picture mismatch would mean I needn’t bother. So far, such news hasn’t been forthcoming.

The Jaguar XK8 uses the ZF 5HP-24 and the contemporary Audi A6 with the 4.2 liter V8 uses the ZF 5HP-24A.  I’ve been comparing their technical documentation as to the truth tables for the various valves and how they affect the various internal brakes and clutches. My conclusion: identical.

So, worst case, if my Audi has a bad solenoid and a new one would break the bank, or if I just wanted to learn more, then I could hypothetically go buy an old ZF 5HP-24 and strip it for parts. Finding that is a lot easier than finding the ZF 5HP-24A version as in the Quattro, and if the internals are the same except for the 4WD aspect, then yay!

Not that Jaguar XK8 cars litter the junkyards locally, but BMW E38 cars do, and this was the transmission of choice for the 735i and 740i cars.

My life just got a lot easier.

The beginning of my first day at school involved me standing alone and crying pitifully, feeling overwhelmed and scared. Then, another little girl, Sarie Joubert, came over, took my hand and led me away, comforting me and saying it’s all going to be OK.

So now I’m sometimes in the same situation again, feeling overwhelmed while trying to figure out the symptoms on two 2000 Audi A6 project cars with the 4.2 liter engine and the ZF 5 HP-24A transmission. The transmission on each car slips in first gear, there’s a jarring thud, and then the car goes into limp-home mode.

One at least one of these two cars, the car was fine as it approached a particular traffic intersection, and when it left the intersection, it was misbehaving, and it’s been doing that ever since. The instant nature of the onset is helpful to know. My best guess is that an electrical component failed, but that’s sort of like saying that the needle I want is in that haystack over there.

Unless the transmission control computer is well-matched to the car, the car’s electronics won’t talk to it. I have plugged in about nine separate transmission control computers, spanning seven different variants, and that didn’t fix the problem.

The plug on the transmission control computer is a massive 88-pin thing. Hoping to be able to analyze the signals, I tried to find some information as to what each pin means, but when I found it, the totality of it was overwhelmingly complex.

The ZF 4 HP-22 transmissions that I know and love have hydraulically controlled operation, and the only electrics on the transmission itself is when it announces that it’s in reverse, so that the car’s two back-up lights can turn on.

For the ZF 5 HP-24A, the electrics seem to be all over the place. I once made the mistake of crawling under the car to study the various places where electrical wiring attaches, and that was overwhelming too.

Not just is the transmission control computer already intimidating by being a computer — but it actually talks to the car’s other computers (ECU, ABS, etc.) across a serial bus network. And then there are wires going directly to the transmission too, and to all sorts of sensors.

The way I understand how an automatic transmission (not the torque converter, just the transmission itself) basically works, is:

1. Mechanically, it uses shafts, gears and clutches to transmit power from the input shaft to the output shaft using, at any one point in time, a particular gear ratio that is one of a small, finite set of gear ratios specific to that transmission.
2. Hydraulically, it uses a high-pressure pump and various channels to guide the fluid.. These channels are centralized in a part called the valve body, where the fluids either open or close valves, and that in turn controls the mechanical components. Various fluid pressure conditions cause various valves to open or close under the desired conditions.

So, as a made-up example, to engage first gear, clutches A, B and D should be activated and then clutches C and E should not be, and to enact this, we want valves 1, 3 and 4 to be open and valves 2, 5 and 6 to be closed. The various conditions and consequences can be diagrammed using a grid that some call a “truth table.”

In electrically controlled transmissions, the valves are opened or closed under electrical control, using (for each valve) a solenoid, which I understand to be the sort of electromechanical device that makes the passenger door’s knob pop up when I pull up the knob on the driver door.

In relatively simple electrically controlled transmissions, a particular solenoid can be either energized or not, and so the valve controlled by that solenoid can be either open or closed.

So, to stay with my made-up example, to engage first gear, you’d energize the solenoids for valves 1, 3 and 4. With that premise, it’s not that complicated. I would measure whether or not any particular solenoid wire has battery voltage when it should, and if it does, great — and if does not, then hey, there’s an electrical problem upstream of the solenoid. And if the solenoid gets power but doesn’t react, it’s a bad solenoid. It’s as Simple as that. It’s sort of like being pregnant or not. There’s not much of a gray area.

By contrast, on the ZF 5 HP-24A transmission, a solenoid can be on, off … or somewhere in between, and there’s not just the one mid-way point, either. So it’s a lot, a lot, a lot more complicated.

Or is it? I finally found a truth table for this transmission, which shows how the neither-on-nor-off conditions are not that common, and besides they don’t affect first gear, anyway.  For first gear (which is where I’m having the problem) the situation is a clean, simple combination of some solenoids being completely off and others being completely on.

I also found a wiring diagram, and I finally took a deep breath and started analyzing it. It turns out to be almost comically simple. The wires from the transmission control computer to the transmission are simple voltage-energized lines, not a serial bus or any sort of weird encoding.  There are maybe eight or so wires. One wire goes to the transmission fluid temperature sending unit, which I understand to be a simple temperature-sensitive rheostat. As for the other wires, each one goes, quite simply, to one solenoid. Nice and simple, really.

Then, I read that the solenoids on this type of transmission are not deep inside the transmission, but just below the valve body — and the valve body isn’t all that hard to remove, with the pan off.

So it’s possible to test each solenoid and to have some reasonable clue as to whether or not it’s reacting to being energized or de-energized.

Given their relative accessibility, it might even be possible to replace a solenoid without having to remove and replace the entire massive 350-pound transmission. I’m hopeful.

As to the cable, a.k.a. the bundle of eight or so wires from the transmission control computer to the transmission, it is not that complicated either. If there’s continuity on each wire, end to end, it’s good and if not, it’s bad.

As to the electrical components sprinkled around the transmission, they’re not that complicated either. One is for the position of the control cable, and the rest are for speed sensors. Nice and simple, too.

Suddenly, I feel much better. It’s all going to be OK.

My sister wittily posted on FaceBook a pretend quote of a popular-in-the-US deity: “I never said that” (or words to that effect).

This summer, I was in Missisippi in the time-frame when the Supreme Court had just legalized gay marriage, and I had a hard time reconciling the mellow and benevolent style  of Jesus (as I understand it) to the hateful stuff being said, supposedly in Jesus’s name.

If we could email our deities and get a personalized reply as to the recommended course of action in a given situation, it’d save a lot of bloodshed. A non-believer could still choose to disagree with the deity’s take on things but at least the believers would finally be on the same page.

In automotive transmission culture, the company called ZF is about as close to a deity as it gets. They create amazing and complex stuff out of almost nothing, and they are huge, and they seem almost all-powerful. They are good and wise.

I’ve been a believer in ZF for many happy years now, specifically due to my favorite BMW transmission having been made by ZF.

So now I own a 2000 Audi A6 with the 4.2 engine, and the ZF 5HP-24A transmission. It seems to have a transmission problem, which might or might not actually be caused by the transmission. I bought my mom such an Audi for her birthday too, this summer, and the car also has this issue. On each car I knew this when I bought the car, and the super-low price reflected it.

But, it’d be nice to have both cars working. I don’t have the budget to go spend thousands of dollars on the locally recommended solution, which is to buy a local rebuild and have it installed locally. Besides, I like solving puzzles, and this is a puzzle that’s perplexed me since June, i.e., for five months now, as I tried to figure out what the right thing is to do.

So, I wrote to the deity, last night, and described the situation, and asked for guidance. And guess what? ZF replied!!!

In truly wise fashion, they didn’t as yet throw suggestions at me but instead asked me intelligent questions to help pinpoint the problem. I’m so delighted to be able to converse with them directly!

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.

I lived in Germany as a child, and I was raised as a German girl. German was my first language. So, I have some insights into German culture even though I’ve lived in the US for two decades, am a US citizen, love America and so on.

The whole concept of doing so many of life’s activities in one’s car, as Americans do … is not so much a concept in Germany. In the US there’s a drive-through service for many things, some of which Germans would consider very odd. Along those lines, the concept of drinking vast amounts of coffee and soda while driving is much more a US thing than a German thing.

And so, the US fondness for cup-holders is not something that Germans tend to naturally appreciate. And yet, by the late 1990s Audis needed to have cup-holders and so the Germans indeed made one, as every owner of an Audi C5 A6 knows.

However, the abuse that such a part endures wasn’t anticipated by the Germans, and though I have seen many Audis of that era with hundreds of parts in fine condition, including well-functioning 4.2 liter 40-valve engines, I don’t recall seeing even ONE Audi C5 A6 with a non-damaged cup-holder.

That’s a pity because it’s such a cute design that, if I ever find a few good ones, I plan to retrofit one of them to the center console of my own E30 1989 BMW 325i.

Meanwhile, I’m using the opportunity to learn more about how to remove these parts from the center consoles of Audis. It’s not easy. The cup=holder is integrated in a large black plastic piece that serves as a sort of base or foundation, and I have figured out that I have to remove the center console so as to remove the cup-holder from below … but I have not yet figured out how to do that, in turn.

The puzzling continues …