The Zener Diode Trick

If you’ve a Transit, for example a MK6 (2000.5 -2006.5 )& have raised boost via adjusting the wastegate, or have fitted a bigger turbo & have a model that has a boost ( MAP) 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 ( cant be raised ) 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 try disconnecting the silicon pipe that feeds it – that works fine on some ECU software versions & results in no boost on others

The theory is this – the zener diode rejects any 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, & at 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 when boost exceeds original max & boost & consequently fuel quantity continues to rise. Without the zener diode fitted , the higher boost will mean higher voltage on the signal return wire than the ECU program allows & fuelling will be reduced ( in order to reduce boost to an “acceptable” level ) & you’ll get a subtle limp mode, negating any gains. 
Many MK6 have a 3 wire MAP ( boost) sensor-

Signal In (5V)

Signal Return (0-5V,  peaking towards 5V at max boost) 

Some MK6 have a 4 wire sensor, so is a TMAP not MAP sensor, the added T meaning temperature

One pin will be “NTC” , this is the thermistor output, for temperature reading. On some Bosch sensors its pin 2 , if you connect a multimeter probe to each pin in turn & other probe to ground, you can work out with engine running which is temp & which is boost by revving it up. A common pinut is as follows but verify yours to be sure

Pin 1 : Sensor Ground

Pin 2 : Temperature Signal “NTC”

Pin 3 : +5v in

Pin 4 : Map Signal Return

Try a 4.3V 1/4W or 4.7V 1/4W / 1/2 w or 1w Zener or measure the voltage at which once breached, the ECU cuts the fuel, then choose a zener of a value that has a breakdown voltage at that level to try as a starting point. Searching for zener diodes can be overwhelming as are the are 1000’s but search for this or similar “300pcs 30 Values (2V~39V) 1/2W 0.5W Assorted Zener Diodes Assortment Kit” (ebay link) & you ‘ll see you can get assortments with common useful values. You might find you need a 4.8V zener & you ‘l have to buy that separately as in reality the effectiveness of the circuit hack isn’t so clear cut, & a foolproof zener circuit includes a resistor to limit current , making the “breakdown voltage” more accurate & consistent but for simplicity, in most cases , just a zener alone will work . You’re attempting to limit a voltage range of between 4-5V so get some in that range ( EG: 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9) & experiment. Its much easier than you think , once you have some measurements & tests done.  If you buy an assortment of zeners & see them labelled as 4V6, thats a 4.6volt type.

Zener Diode

The side with the line on the zener needs to be connected to Signal Return & the other to the Ground wire.

This page has a more detailed explanation of the zener trick

Let me know your results & comments by posting below & i’ll update the post for more model specific details.

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