Code 78

[millsroger]

Does anyone know how much boost triggers the fault?? Is it just an over boost or is it a "difference" number compared to what the ECM thinks it should be?? I read somewhere that it triggers when the ECM tries to lower boost and that does not happen. Can anyone clarify how this works?? Specifically for my sig truck, 94 6.5TD.

 

[NVW]

The way I understand it is the ecm calls for a specific boost, either too low or too high can trigger the code 78. How far off the boost can be? IDK

I think Buddy posted about this before.

 

[THEFERMANATOR]

Code 78 basically means the ECM cannot regulate the boost pressure, or meaning it goes high and low. If it stays high it will trigger a code 61, and if it stays low it can sometimes trigger a 62. Code 78 normally comes from varying boost that is not within the requested range.

 

[JMJNet]

To remedy the is to check the vacuum lines, vacuum pump and turbo wastegate solenoid.
If the first 2 are good, then chances are it is the wastegate solenoid.

Some of us put a TurboMaster type wastegate actuator.
This one will give 78 when the boost is too high but we at least know the turbo is working.

 

[millsroger]

So, a 78 by itself is a boost tracking error, where 61 is hi vs 62 being low. Was really wondering if anyone had numbers for the error---like the ECM wants "X" but got "Y". My sig threw a 78 when I pulled out to pass and had to hold WOT to get around two rigs instead of one. Well over 90 when it threw the code. In doing the basic checks I found the actuator did not move freely end to end when off. Kind of felt spring loaded to the center. Would have caused boost to lag behind what the ECM wanted both ways. Replaced. Cleared code but no reason to repeat circumstances. Fuel is too expensive right now to just go out and waste a bunch.