This is the 4B0927156BS Audi-Bosch transmission control module for the ZF 5HP-24A, an electronically controlled 5-Speed Tiptronic transmission built by ZF, and installed in 1999-2004 Audi A6 Quattro cars with the 4.2 V8.
To buy the part, the price is $400 plus shipping.
Specific Part Number 4B0927156BS
The 1999-2000 variation of the transmission control module has a part number that ends with the letter code BS. I verified that, by personally inspecting two of our 2000 Audi A6 Quattro 4.2 V8 project cars. These have the “ECF” variation of the transmission. You can order a good, guaranteed unit with part number 4B0927156BS from our C5A6parts.com website here.
Many VW-Audi transmission control modules of that era share the root part number of 4B0927156; the last two letters are, however, critical. For example, in a 2000 car that expects 4B0927156BS, we installed a transmission control module with part number 4B0927156CT normally used in the 2001 car. The transmission refused to communicate with the car. It immediately put the car in limp-home mode . When we re-installed the correct transmission control module with part number 4B0927156BS, the transmission functioned normally again.
This isn’t completely surprising to us, because one of the known differences between the “ECF” and “FBC” variants is a difference in the valve body wiring, so no doubt that affects the transmission control module compatibility too.
The control comes from a part generally referred to as the transmission control computer (a.k.a. TCC) or transmission control unit (a.k.a. TCU) or transmission control module (a.k.a. TCM). These are all synonyms. Because the Audi documentation uses the term “transmission control module” that’s the synonym we’re using here, too.
On the C5 Audi A6, the transmission control module is under the front carpet, just in front of the seat, on the passenger side, inside a plastic protective box.
Audi Component Name
Audi refers to the transmission control module as component number J217.
Typical Causes of Damage
We are based in Nevada where sometimes, in jest, we say that our State tree is the telephone pole. In many parts of Nevada, trees are uncommon and most days it doesn’t rain. So, not so much from personal experience, but from what I’ve read, frequent rains combined with an accumulation of dead leaves can cause the plenum a.k.a. cowl drains to clog, and cause the transmission control module to get wet and thus malfunction.
Value in Troubleshooting
When troubleshooting the ZF 5HP-24A transmission, it’s a good idea to start with the transmission control module — not that it’s a likely cause of failure but rather that it’s so easy to swap out, and eliminate as a factor. Here are some example stories to make the point.
- I’ve spoken with the owner of an Audi A6 Quattro 4.2 V8 whose transmission had failed. He bought a used unit off eBay and paid to have it installed, and that didn’t work. He tried again with another transmission he’d picked up somewhere The most recent time we spoke, he was $4,500 into the project, parts and labor, and his car still wasn’t driving..
- I’ve also seen a YouTube clip in which someone tells the story of an Audi owner with a similar issue on the same type of transmission. He had the transmission removed, rebuilt and replaced — but the problem persisted. Then. he replaced the transmission with a good used one, and the problem nevertheless persisted.
In both of these cases, the problem might well not have been with the transmission but rather with the transmission control module.
Indeed, an acquaintance of mine had an Audi A6 Quattro 4.2 V8 whose transmission was misbehaving. He replaced the transmission control module and immediately, all was well again.
As to the ZF 5HP-24A transmission, the pressure regulator in the valve body should, after 100,000 miles or so, be renewed, lest it allow a fluid pressure spike that blows out the rim of the clutch “A” drum, necessitating a transmission removal and repair. This is a classic failure pattern for this type of transmission. After this happens, the clutch “A” drum slips, hence the transmission slips in 1st, 2nd, 3rd and 4th. The transmission control computer then senses this and limits forward motion to 5th gear. If the transmission in your Audi is showing these symptoms then yes, the disabling problem is quite likely an internal mechanical failure.
However, in all other cases — and perhaps in all cases, period — I would start by ruling out the transmission control module as a root cause.
A Simple Test
One simple test that I like to perform is to completely remove the transmission control module from the car. I make sure the ignition key is in the “off” position before I do so. In the past I’ve even disconnected the battery but I’ve gradually become brave enough to not do that, too.
With the transmission control module removed (or completely unplugged, anyway) then with the engine restarted, the gearshift position display is gone on the instrument panel. In forward gears, 5th is the only gear available, and it should work nicely including you eventually being able to get the car up to freeway speeds, though starting out from a dead stop in 5th gear means you have to take it slow, as in not rev the engine and thus burn the remaining good clutches. With the transmission control module removed, park, reverse and neutral should all work perfectly — so if you’re having an issue in reverse gear, for example, even with the transmission control module removed, then you can pretty much rule the transmission control module out, as the root cause of that.
Driving without a Transmission Control Module
As to the consequences of driving with the transmission control module removed: one of our project cars, a blue 2000 Audi A6 Quattro 4.2 V8, has been driving like this for more than a year now. So if it’s bad for the transmission, it’s not obviously so, anyway.
Preventing the Replacement from Failing
If you know that you drowned the transmission control module in your car, and there is not much doubt as to it being damaged, then you might simply buy a good, guaranteed, used unit from us. You would, I hope, also make sure the area is bone dry where you’ll be installing the replacement transmission control module, and you would have resolved any root causes such as clogged plenum a.k.a. cowl drain tubes.
Other Vendors’ Guarantees
For some vendors, the guarantee means “if it doesn’t work, send it back” but on the premise that you’re going to be using this for troubleshooting, we would like you to have a high level of confidence. Some vendors claim they test these units but then when we asked hard question such as “how, precisely” then we were unimpressed by what we heard. One gentleman named or described a process that doesn’t test the transmission control module at all, just the car.
I visited a local transmission specialist who told me he couldn’t do an on-site test of my car’s TCM. He’d have to ship it to an outside vendor, either one in LA or one in Florida.
Our Guarantee — Video Proof
I’ve decided that our test and guarantee will consist of the following, for each individual unit you buy. As you pay, you provide your name and address. We put that on a label and affix the label to a box. Then, we take the relevant car out for a drive, with a passenger videotaping the instrument cluster. Without a break in the filming, the car will be driven forward in all 5 gears, plus reverse. We will then stop the car, turn the key off, remove the transmission control module from the car, and show the serial number. Then, we show the box with your name and address on it. Finally, we put the transmission control module in the box. We send you a link to the video, and you then receive that part, in that box.
Because this is a high-value item, and people can fry or damage electronics, or they can play it fast and loose by swapping labels or covers, we are wary of being burned. Even though to each transmission control module we add one or more hidden identifiers that will be hard to forge, we don’t guarantee it’ll work in your car and we don’t offer a money-back guarantee, due to the risk factors, including the high comfort level that each unique video provides for the individual serial number of the part that you are buying and receiving.
You can also rent the part. The process works as it does for buying but then you have 30 days in which to use the known-to-be-good transmission control module to help troubleshoot your car and perhaps rule out that the transmission control module is the problem.
If you send the part back, and within 30 days of us sending it out, we receive it back, then we’ll re-install it in our test car. If it still works, you get a $280 refund. In effect, then you’ll have paid $120 to rent a known-to-be-good transmission control module.