If you’ve a Transit, for example a MK6 (2000.5 -2006.5 ) that has a boost sensor mounted on the intercooler as opposed to bulkhead mounted one, you’ll have found out that the limiter in the ECU program itself isn’t known so one option is to fit a zener ziode on the boost sensor circuit to limit what boost the ECU actually see’s. Its a trick, & a good one at that. (If you’ve a bulkhead mounted boost sensor just disconnect the silicon pipe that feeds it! weirdly that’s enough ! )
The theory is this – the zener passes any required voltage level up to the desired amount & bleeds off to ground any excess voltage. Let’s say on idle the return signal voltage is 2V, max boost before Ecu starts limiting is 4.4V, with a 4.4V zener in circuit any excess voltage between 4.4V & 5V is fed to ground so the Ecu only sees a max of 4.4V so fuel isn’t limited & boost & fuel continues to rise.
You’ve got 3 wires on your boost sensor-
Signal In (5V)
Signal Return (less than 5V, peaking towards 5V at max boost)
Try a 4.3V 1/4W or 4.7V 1/4W Zener or measure the voltage at which once breached, the ECU cuts the fuel, the choose a zener of a value that has a breakdown voltage at that level. In reality the effectiveness of the circuit isn’t so clear cut, & a resistor is usually used to limit current in zener circuits to make the bleed off ( breakdown voltage) more accurate & consistent. In reality, trying a few different zeners is practical. You’re attempting to limit a voltage range of between 4-5V so get some in that range & experiment.
The side with the line on the zener needs to be connected to Signal Return & the other to the Ground wire. Let me know your results & comments by posting below & i’ll update the post for more model specific details.