ZF 5 HP-24A Teardown: Step 1: Preparation

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Before dismantling the transmission, we cleaned it externally, while taking care not to get the cleaning fluid into any openings that would have introduced it to the inside of the transmission.

The previous owner had already drained most of the transmission fluid, otherwise we’d have made a point of doing that, too.

To do the work, we needed a very clean and dust-free work-space, as shown above. The floor has been painted white; the dark spots are scuffed paint, not dirt.

Our transmission jack is a $99 special from Harbor Freight but it did the trick.

We also had a variety of hand tools ready. We also bought the official ZF repair manual for this transmission, and downloaded the free-of-charge ZF spare parts catalog from the ZF website.  Sadly, we own none of the ZF tools as referenced in the repair manual, but we were happy to improvise.

ZF 5 HP-24A Inputs and Outputs

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The concept of this computer-controlled 5-speed Quattro Tiptronic automatic transmission as a whole is pretty darn overwhelming, but now that my tech & I have dismantled one, we feel much less intimidated. This article covers what we have figured out so far.

I’m basically a software development geek, so I tend to think in terms of input, process and output.

Inputs to the transmission as a whole:

  • Analog direct current from the transmission control module to the solenoids
  • Transmission fluid from the radiator
  • Rotating force from the engine
  • Shift lever
  • Analog direct current to the speed sensors
  • Analog direct current to the temperature sensor
  • Counter-force at the physical mounting points to hold the transmission in position

Outputs from the transmission as a whole:

  • Current-draw feedback to the transmission control module from the solenoids
  • Transmission fluid to the radiator
  • Rotating force to the rear drive shaft
  • Rotating force to the front drive shafts
  • Analog direct current from the speed sensors
  • Analog direct current from the temperature sensor
  • Force on mounting points that hold the transmission in position

As to the processes … it’s complicated. So let’s worry about them later, and worry about how the inputs relate to the outputs.

Analog direct current to and from the solenoids

The transmission control module is horribly complex, and it chats with the rest of the car using a complex serial bus network protocol, plus analog direct current, all via a massive 88-pin plug. Fortunately, the way it chats with the solenoids inside the transmission is relatively simple: analog direct current.

The wiring bundle from the transmission control module ends up at a round plug with more than half a dozen, but less than a dozen, electrical contacts. From there, a wiring bundle runs inside the transmission, along the valve body, until it reaches the solenoids. These are all arranged in a row. The wiring branches off to each of the solenoids. Two or three wires run to each solenoid.  The wires are different colors. One of the colors is purple. The purple wire is probably a ground wire, common to all the solenoids, since each of the solenoids has at least one purple wire running to it.

Some of the solenoids are simple on/off solenoids whereas others can be completely on, completely off, or somewhere in between. This is probably to enable the complex and subtle timing of the transmission being able to shift while the car is accelerating, yet without flat-lining the accelerating curve. This would also explain why, if the transmission control module goes haywire such as when it gets wet, it sends unintentionally destructive commands to the transmission.

The transmission has no intelligence built-in. When the transmission control module tells a solenoid to energize, then it’ll energize, regardless of the consequences.

Presumably, the transmission control module can sense when a solenoid isn’t firing, due to the current draw characteristics.

Transmission fluid to and from the radiator

Two black steel lines run connect the transmission and the radiator. One line is for fluid flow from the radiator; one is to the radiator. As to which is which, the clever ZF folks have stamped the directions on the casing at the appropriate position.

Rotating force from the engine and to the drive shafts

The torque converter is bolted to the engine, and its outside casing spins at the speed of the engine and with whatever torque the engine puts into the movement.

This rotational movement and torque comes out of the transmission via the two front drive-shafts and the rear driveshaft.

Shift lever

Regardless of how the shift lever chats with the rest of the car, the shift lever is mechanically attached to the transmission and as it moves, mechanical things inside the transmission move accordingly.

Analog direct current to and from the speed sensors

The transmission has a speed sensor by the torque converter, another one approximately central to the transmission, another one attached to the driver side front drive shaft, and another one in or by the tail housing.

Each sensor independently generates pulses along a simple to-and-from wiring circuit that runs from the transmission to the transmission control module and back. The speed of the pulses show the speed of the rotation, probably due to a Hall effect switch at each speed sensor. This data is presumably useful to the transmission control module such as to calculate whether or not excessive slippage is occurring.

Analog direct current to and from the temperature sensor

The transmission has a transmission fluid temperature sensor inside it. The sensor is probably a heat-sensitive rheostat, meaning its electrical resistance varies with its temperature. Analog voltage goes in one side, and there’s a voltage drop out the other side. The size of the voltage drop depends on the electrical resistance of the sensor, which in turn depends on the temperature. A simple to-and-from wiring circuit runs from the sensor to the transmission control module and back.

In-balance forces at the physical mounting points

This transmission is a heavy beast. I’d guesstimate it at 350 pounds or so. Keeping it in position as the Audi goes bouncing around on a road, stopping, accelerating, leaning, pitching … it takes some impressive counter-force to keep the transmission in position, not least when maybe 300 HP is being pumped through it.

The main mounting surface is from the housing to the engine. With the exception of the tail shaft, the entire housing is one piece of cast aluminum. Perhaps the tail-shaft varies depending on the variation of front-to-rear differential that’s used.

There is, by implication of the one-piece casting, no separate bell-housing, and the Audi 4.2 V8 is the only engine that the ZF 5 HP-24A bolts up to, with some massive bolts.

Other 4WD vehicles such as the BMW X5 also use this basic transmission: the ZF 5 HP-24 but not the ZF 5 HP-24A. The “A” variant has Quattro 4WD technology built-in; the “non-A” variant is only RWD, and an external mechanism (perhaps a transfer case) enables the 4WD functionality on the BMW X5. Because the “non-A” variant is used with various BMW engines, and also Jaguar engines, it presumably has a removable bell-housing.

Attached to each side of the transmission is a transmission mount, that in turn attaches to the body of the car.


The various processes inside the transmission enable the desirable outputs using the available inputs.


ZF 5 HP 24-A Teardown Fun

My assistant and I had planned to have a normal eight-hour workday. It was, after all, a Saturday, and besides, the Saturday before Christmas. Problem is, we were in the process of dismantling the ZF 5 HP-24A transmission out of a 2001 Audi A6 Quattro, the one with the 4.2 liter rocket-ship engine 40-valve engine.

And yet by the time that Hawaiians would say “Pau Hana” or Zulus would say “Chaila” (quitting time) … the work was just way too interesting to want to stop. So we enjoyed a nice snack and kept on working. My assistant used to be in the US Army and although she’s sweet as pie, she’s also hard as nails, and if she’s impressed by anything, she normally doesn’t let on.  And yet, finally, the quality of the engineering behind this transmission melted her heart. She kept exclaiming how cool this or that item was, as the brilliance of the design became ever more apparent. And, of course, I was in full agreement.

Quite some time after midnight, it occurred to us that maybe it’s time to finally stop working, and so we finally dust-proofed everything and we left.

However, as the events were winding down, one of us made the mistake of noticing that the clutch A drum of the transmission looks a lot like the torch of the Statue of Liberty. And, of course, there’s always time for silly pictures (with no lack of patriotism implied).

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Audi A6 3.0 Transmission Control Modules

Our analysis yesterday continued with inspecting the transmission control module on an Audi A6 3.0 non-turbo, build month 11/2002 hence a 2003 model. The part number was 4B0 927 156 FE.

Next, we inspected the transmission control module on an Audi A6 3.0 Turbo, and no, I wasn’t smart enough to record the build month. My eBahn software tells is had to have been a 2002, 2003 or 2004 model. The part number was 4B0 927 156 FF.

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Good Body Mechanics and your Battery

It was 10 degrees Fahrenheit here recently (that’s about minus 12 Celsius) and tonight it was warmer but not much, and my 2000 Audi A6 with the 4.2 V8 didn’t much like starting. The engine turned over very slowly the first second or so. I was VERY glad that I’d installed a massive battery that just barely fitted in the battery compartment. Anything smaller and I might well have had a no-start condition.

However, I later needed to remove the battery to move my BMW around (and its battery needs to be charged, so I’m using the Audi’s). Removing the Audi battery risks a lower-back injury, but then my assistant had a clever idea. The batteries I buy (and that, I hope, you buy) have a handle at the top, and putting a broomstick handle through it makes it easy for two people to lift out.

Transmission Up/Downgrade

My 1989 BMW 325i has a ZF 4 HP-22 automatic transmission, and it behaves perfectly. I can find good, used ones easily for about $100, and the transmission weighs less than 70 pounds. I’m a girl and even so, I have personally, physically picked one up.

My 2000 Audi A6 with the 4.2 V8 has the ZF 5 HP-24A automatic transmission, and it is wonderfully high-tech, but it has some very puzzling problems. Finding a good, used one means spending $1,800 or so, and the thing is massive and weighs almost 320 pounds. I’ve personally removed one, and it scares me.

It’s sort of like the movies where person A is in love with person B, who is glamorous but difficult. Eventually person A becomes less and less enthused and finally checks out, and chooses person C: someone less glamorous, but more practical — and nicer.

You can probably see where I’m going with this.

I’m fantasizing about replacing my ZF 5HP-24A with a ZF 4 HP-22.  And yes, the former is an AWD transmission and the latter is RWD, so I’d have a Quattro without superpowers. Still, it’d be a lot better than it being dead in my driveway. And besides, I live in Nevada on mostly dry, flat, level ground. Do I really need AWD? No. Maybe I could just remove the front half-shafts and call it good?

Originally I’d fantasized about the ZF 4 HP-24 as used in the 300 HP BMW 750iL, but that also uses electronically controlled shifting and I need a vacation from electrical issues. A nice, simple  transmission with hydraulically operated shifting would be wonderful. The most complicated electrical component should be the back-up light switch.

Power comes in the front, power comes out the back, a shift lever controls the basic direction the thing is supposed to be going, and that is that. Beautiful and simple. I miss that.

Sadly, the Audi’s torque would probably tear the ZF 4 HP-22 apart. The torque rating for the Audi engine is: 300 foot-pounds or so, wow.  The most torquey engine used with the ZF 4 HP-22 is, as far as I can tell, The Jaguar XJS 4.2 liter engine, with 236 foot-pounds.

Next, I started wondering what life would be like if I took the Chrysler A727 transmission out of my old Jeep and stuck it into the Audi. How viable would that be?

Wikipedia says:

The … 1962 … A727 … was initially referred to in consumer-oriented publications as the “TorqueFlite 8” … Compared to the early cast-iron transmissions, many and various internal improvements were featured … A727 Torqueflites became — and remain — wildly popular for drag racing, off roading, and monster truck applications because of their controllability, reliability, ease/cheapness of repair and brute strength.

The Chrysler A727 transmission:

  • Could certainly handle the torque and power that the Audi engine would generate.
  • Weighs about half of the weight of the ZF 5 HP-24A transmission.
  • Electronic components: zero.
  • Has affordable parts. A master rebuild kit is priced at maybe $150, and many mechanics can rebuild one. By contrast, just the solenoid kit for the Audi is priced at more than $500.
  • Doesn’t need expensive ATF.

Suddenly, the idea has become VERY tempting. So, what would be hurdles, aside from getting my hands on a Chrysler transmission (which is easy; I own three of them)? I’d have to…

  • Mate a mechanical torque converter to the Audi engine and yet end up using the same ring gear so that the Audi starter still works.
  • Mate the Audi engine to the Chrysler transmission using a custom-made bell-housing.
  • Mate the rear of the Chrysler transmission to the Audi drive shaft, or have a custom one made. No problem; our local drive shaft shop rocks.
  • Deal with the back-up light wiring
  • Deal with shift lever issues
  • Deal with shift lever console issues, such as no gate for the Tiptronic.  At least one model of the contemporary Passat has a non-Tiptronic transmission so I might use a center console from that on the A6.

There’a a nice C5 Audi A6 being advertised in Walnut Creek, for around $10K. The main attraction is that the car was converted away from the ZF 5 HP-24A to stick shift by someone savvy, who did a nice, clean job. I’ll probably go have to pick their brain as to how to get the Audi wiring to be happy even without the ZF 5 HP-24A.

Imagine the ad for a Tanya-converted car:

  • 2000 Audi A6 with the 4.2 V8
  • Converted to RWD and Chrysler A727 Torqueflite aAutomatic transmission, rebuilt with 10-year warranty.

That should do it. I’m located in Nevada anyway, and Nevada urbanites tend to need a Quattro about as much as they need a high-lift monster truck to drive to and from the grocery store, or ten miles to work, on flat, level, dry ground.

Cleaning the Valve Body

I’ve just read some sage advice from the most savvy BMW tech I know to exist, on the Web; a gentleman who goes by Shogun. And yes, he does live in Japan.  Someone was having issues with his automatic transmission, and Shogun opined:

“Maybe a valve body problem. Have you removed the valve body and cleaned it and checked all the small balls and orifices. And especially the solenoid valves inside valve body!!!!”

That got me thinking. So I poked around on the Web some more, and found a nice article on the website for Transmission Repair Cost guide, titled “Transmission Solenoid: Symptoms & Replacement Cost.”

As I understand the article, the owner serviced the valve body and the automatic transmission is now noticeably happier albeit still not perfect.

The author also makes the point that old, dirty fluid can cause the valves to stick, and since many Audi A6 cars have NEVER had an automatic transmission service, that might well explains the problems on my Audi A6 project cars.

Officially, according to Audi, that transmission doesn’t need service. According to the transmission manufacturer, it does. To me, the latter opinion carries more weight.

Another article on MyAutomaticTransmission.com ties slippage to the same problem:

SYMPTOMS: Transmission slips out of gear intermittently when engine warms up
Internal seals or valves allowing fluid to pass causing pressure loss
REPAIR: Transmission Rebuild or Replace”

That same website also has an article that explains how the same symptoms might be caused by issues that require a rebuild, as well as issues that don’t require a rebuild. The article lists nine things to investigate before giving up hope, and deciding to do a rebuild. These items make a lot of sense to me.

Several of these tie back to the transmission fluid not having been changed, so I have ordered some more transmission fluid, and I plan to do a project car service soon: fluid and filter change.

The Price of Replacement ZF 5 HP-24A Internals

For our two 2000 Audi A6 project cars, the model with the 4.2 V8, I’m fantasizing about replacing the solenoids for the ZF 5 HP-24A. Each car is misbehaving and I haven’t figured out why.

As parts go, the eight solenoids seem to be relatively easy to replace.  Instead, were we to get into replacing clutches etc. then I’m way out of my depth.

New solenoids as a set are … okay, wow, about $500. Not cheap. When I discover something like this, I start to wonder how I can test a used solenoid to see if it’s good beyond the obvious.  I could get an entire used transmission, albeit in unknown condition, for less than the price of a new solenoid kit.

Ditto as to the other internal components. They are pricey.

I also recall reading “DO NOT PUT ANY USED PARTS INTO YOUR ZF TRANSMISSION” so … there’s that. Whoever wrote that probably had some good reasons, maybe for the same reason one shouldn’t use previously used … what’s an example … geez … toilet paper. Or condoms.

Still, given the high price of these new ZF internal parts, I’m tempted.

It’d help a lot to know what I’m doing, which right now isn’t the case.

For example, until recently I worked on the principle of “don’t ever re-use manual transmission clutch components; use new stuff only.”

So one fine day this summer, my friend (who is a professional mechanic) and I had just removed a BMW manual transmission that I was about to buy when he inspected the clutch. The more he looked, the more he liked. The conversation went somewhat like this:

  • Him: Wow, look as this clutch. Buy this, too. Oooh, and the flywheel.
  • Me: Nah. I don’t ever re-use used clutch stuff, on principle.
  • Him: Wait, this IS just about brand new. The owner of this car must have just had this installed when something else broke on his car. I mean, wow, look at it.
  • Me: I can’t really tell what makes a clutch & flywheel new vs. not.

And so, patiently, he explained it all to me, pointing out various technical details. He convinced me, so now I own an almost-new pressure plate, clutch disc and flywheel for a BMW M20 engine. New, this stuff would have cost me several hundred dollars. Instead, I got the parts, gently used, almost new, for next-to-nothing.

You can probably see where I’m going with this. I’m really enthused to find a way to strip down a ZF transmission, scrutinize (or better yet: test) each part, and then keep the good ones and toss the bad or well-worn ones.

I’m aware that there can be subtleties. For example, often, parts wear as a set, and so if one installs a used camshaft along with used rocker arms, and they didn’t wear into that not-new shape together, then their surfaces won’t touch as a broad plane but more at an angle, and then both parts will wear out way faster. That’s why, when disassembling a cylinder head, it’s crucial to label what went where. And probably a cylinder head is primitive compared to the high precision of ZF transmission internals.

Still … it sure is tempting. I mean, at the very least, the transmission pan and its hardware should be re-usable if it’s a good, used part, without antagonizing the ZF gods. And maybe some speed sensors and cables too. And so on. Somewhere there’s a tipping point, but my guess is that it depends on the relative merits of each part, as opposed to using a broad principle that is unnecessarily dismissive.

Applying false absolutes and buying everything brand new would explain why a transmission rebuild can cost thousands of dollars, which hardly anyone wants to pay since one can buy good A6 cars for less than $3,000 — for the entire car, which explains why I see SO many dead A6 cars in junkyards: once the transmission misbehaves, people give up on the car.

Resurrecting dead ZF transmissions affordably, using good, tested, gently used parts  … that is becoming very tempting to me. I’d be the first beneficiary of his policy. I’d have two healthy Audi A6 project cars.




Making Sense of ZF 5 HP-24 or ZF 5 HP-24A Part Numbers

After much effort, I finally own a ZF 5 HP-24A transmission out of a C5 Audi A6, and it’s in my shop, on a pallet.

I’m itching to take it apart, but there’s a good chance that this is a good one since the donor car was in the junkyard due to engine issues. Sadly, I have a very bad track record of destroying things, the first time I take them apart.  So, I’m thinking I should go find a bad transmission to take apart first.

Problem is, these transmissions are hard to find. The “A” at the end probably stands for “Allrad” as in 4WD, and that’s a rare variant. The RWD variant lacks the “A” hence just ZF 5 HP-24. Those are easy to find. I’m tempted to do that, but I’d like to know how similar the RWD is to the AWD. So I’ve been poking around on the Internet, trying to piece it all together.

One of the interesting challenges of this sort of task is that the information on the Web rarely reconciles, so I need to exercise some good judgment to make sense of it all. Fortunately, I’ve spent enough time lying underneath C5 Audi A6 4.2 V8 cars that I also have a reality-based frame of reference.

Things I’ve concluded to far:

  • Audi calls this transmission the 01L. ZF calls it the 5 HP-24A, but it’s the same thing.
  • For the C5 Audi A6 with the 4.2 liter V8, there’s an early variation through 2000, and a later one. The earlier variation has a code of “ECF” on the actual transmission plate, and a ZF part number of 1058 020 015. The later variation has a code of “FUL” on the actual transmission plate, and a ZF part number of 1058 020 031.
  • These are unique to the Audi A6; the ZF 5 HP-24A is also used on the Audi S6 and the Audi A8, but those are different variations.
  • The RWD variation (not used by Audi) is found in the high-end BMW and Jaguar cars built in 1997 through 2001 or so. For BMW, the different models each have a different variation of transmission yet the ZF part number always starts with 1058 000 and then has three digits that vary.  As to “different model” in the preceding sentence, for purposes of ZF part numbers, BMW considers the 740i and 740iL to be the same model, and the 540i Sedan and station wagon too. For Jaguar, it’s possible to find an XK8 and an XJ8 with the same variation in their transmission (i..e, an exact match) as long as both have the same type of engine.
  • The theme whereby the 4WD version uses 1058 000 part numbers and the AWD version uses 1058 020 part numbers seems to stop at the transmission unit level. For the housings, for example, the RWD has part numbers that start with 1058 201 or 1058 301, and the numbers for AWD all start with 1058 201, though of course the last three digits are different.
  • As to the housings and the parts that belong there, I didn’t notice any overlap in the part numbers. In other words, I found no parts common to both the RWD and AWD variants.
  • As to the input shaft parts, yes! Match! Perfect match.
  • As to the oil pan, some minor part numbers matched.
  • As to the oil supply, the pump was different, but some minor parts matched.
  • As to clutch packs, identical. Yay!
  • As to planetary drive groups, identical.
  • As to brake groups, identical.
  • As to the duct plate, the plate itself and the wiring harness were different but many components were identical, down to even the valve orifice sizes. Isn’t THAT interesting?
  • As to the valve body, identical according to one site, not so according to another.
  • As to the solenoids, I found them to be identical. That was very interesting, too. Also: of the three solenoids, the two outside ones have the same part number, which is different than the one in the center.  The solenoid replacement kit part number is also common to the RWD and AWD, both.
  • As to the replacement kits, the fiber clutch kits and steel kits have the same numbers for RWD and AWD, both.
  • The overhaul kits, for everything including the kitchen sink, are not identical for RWD and AWD.

This all makes sense to me. When it comes to the housing, the output aspects, and the oil pan, the RWD and AWD are different. As to internal parts, they’re identical or almost so.

It’s interesting to me how much the solenoid part numbers overlap with the more-common ZF 5 HP-19 variants too.

So, certainly, if I go disassemble a RWD variation I’ll still learn a lot about the AWD variation too.

Energizing the Audi A6 4.2 V8 Transmission Solenoids for the ZF 5 HP-24A

I’m analyzing the transmission functions on my 2000 Audi A6 with the 4.2 V8, so as to isolate a problem that most likely seems to be related to transmission solenoid #3, as far as I can intelligently infer from the symptoms and what I understand about the mechanical and electrical aspects.

So tonight I hooked my laptop PC up to the car and ran the amazing Ross-Tech software that talks to the car’s transmission control module and much else besides.

I like that the software displays the part number of the transmission control module, including the variant, e.g., BS or CT.

The software can run some output tests, by energizing each of the three simple transmission solenoids in turn for as long as I like. If I hear a clicking sound, that’s a good sign that the solenoid is probably opening and closing, and causing the corresponding valve to open and close, which is sort of the whole point behind having a solenoid.

I ran the test, and in all three cases, I heard an ongoing clicking sound. They all work, yay!

I next ran the test for the pressure control solenoids, which (I gather) have a more complex task that just being on or off. A variable amount of current flows through them, depending on what the control module wants the transmission to do.

In each case, I heard a click, which is as good as it gets, with these. Next, I checked the software for error codes … none found. So, no problems found there, either.

Next, I started the car and shifted into the various gears, with an emphasis on neutral, reverse and first, since first is the one with the problem. For neutral, reverse and first, the pressure control valves all seem to be in the same basic mode, and the solenoids are the big issue.

  • For neutral, solenoids 1 and 3 were energized.
  • For reverse, solenoid 2 was energized.
  • For first, solenoid 1 was energized.

That’s all correct too.

Pity … that would have been a relatively easy fix.

Still, I’m impressed that this car has these tests built-in. Very cool.