Haynes Motor Museum Mercedes W129 ECU Programming.
This Mercedes W129 project turned out to be one of the more involved jobs we’ve taken on recently. The car arrived to us from the Haynes Motor Museum after another company had attempted to work on the ECU, but unfortunately caused irreparable damage to it. To make matters worse, they refused to return the original ECU to the customer, leaving us with no data to reference, clone, or extract. That meant we had to start completely from scratch.
Without the original ECU, the job became much more complex. Normally, having the original unit allows us to transfer important coding and security information directly to a replacement, which simplifies the process. In this case, we had to manually identify and configure everything from the immobiliser system to the vehicle’s specific programming. Luckily, we still have several used ECU’s in stock.
Firstly, we virginised the ECU. This process wipes any stored data and returns the unit to a factory state, making it ready for reprogramming. It’s a delicate task that requires desolering the HC11 chip on the PCB. After carefully preparing the ECU, we moved on to the programming stage.
Using Mercedes STAR diagnostics, we programmed the virginised ECU to match the W129’s exact specification. This included coding for the immobiliser, fuel system, and other key parameters to ensure proper communication with the rest of the vehicle’s electronic systems. Several rounds of checks and adjustments were needed to get everything aligned correctly as the vehicle also lost it’s coding for several other modules fitted, but once completed the car started up as it should.
It was a great result after a tricky beginning. Despite the missing original ECU and the complications caused by the previous attempt, we bought the vehicle back to life. It’s always satisfying to see a classic like the W129 running again, especially knowing how much effort went into getting it there.
Check out the video below.
