Hydractive ECU problems

XM
Hydractive

The ECU operates three electrovalves to switch the suspension between hard (default) and soft modes. There are three parts involved in this operation:

  • the coil in the electromagnetic valve
  • the solid-state relay in the ECU (this works much like a traditional electromagnetic relay but has no moving part, so it is much more reliable)
  • a so-called freewheeling diode to protect this semiconductor relay (built directly into the valve beside the coil in later models)

Thanks to the solid-state relay, this setup is much more reliable than a traditional relay could have been. The SSR also has thermal self-protection, so in case of overheating, it simply shuts down temporarily and as soon as it returns to normal operating temperatures, it resumes operation. Still, there is a possible avenue for failure: the protecting diode. While working, this diodes does its job. However, when it breaks, it will start to hinder the correct operation of the Hydractive system.

With a failing diode, the operating tension on the coil reduces, resulting in erratic behavior. Even if the ECU thinks it switches to soft mode, the system might remain in hard mode. The solid-state relays can overheat often and while they have thermal protection, this is problematic. First, during their shutdown, there will be no switching to soft mode. Second, frequent overheating quickly leads to the relays failing permanently.

So, when the SSRs are still strong but the diode (one or more) broken, it's important to replace the second. As noted above, the diodes are built directly into the valve on later models, thus replacing the valve renews the diode as well. However, there is really no need to replace the valves only because of the diode. When the diode fails open-circuited, it makes no difference any more, so adding a new diode into the circuit will simply tak its role.

The easiest way to determine the sanity of the diode is to remove the valve from the circuit and measuring resistance over the coil. Do the measurement in both directions (swapping the meter probes): if the resistance is (nearly) identical, the diode went South. If the resistance is largely different, the diode is still OK.