The wastegate control modulator (also referred to as a boost modulator or boost valve) allows the engine ECU to control the turbo boost pressure. This improves engine response massively. These are not fitted as standard to Td5 Defenders, and often bypassed on the Discovery Td5 as it is thought by some not to be required. However it does increase engine response and is worthwhile keeping. Another benefit is that the wastegate rod length can be set purely for preload, rather than trying to control the actual pressure which can result in wastegate creep. Pierburg units are very reliable and will give many years of use.
Fitment is straightforward. The Td5 boost modulator is fixed with one bolt and has three pipes coming off, one going to the intake, one to the intercooler pipework and one to the turbo wastegate, plus one electrical connector. The operation of the boost modulator can be checked with a Nanocom.