Boost sensor ????

[R. Spence]

I have a 96 6.5TD in a 1500 silverado and I am not a diesel mechanic. (Totally stock.) I am getting a P0236 Boost Sensor code. Here is where it gets confusing. I installed a tee in the main Vacuum line between the pump and the first solenoid. When cold the vacuum guage acts like everything is normal 20-22" vac Idle (25" driving) and the solenoids all appear to be functioning correctly. When the engine reaches normal operating temp (195) the vacuum guage pegs at 30" and does not move at all. At this time I am getting "some" black smoke and loss of power (boost sensor) but also no passing gear. The tranny unlocks the converter but does not down shift... Does the ECM go into a safe or minimum mode when the boost sensor fails and why/how does the vacuum increase ??? :confused: rspence44@juno.com

[Hubert]

Yes the ECM goes into a safe mode when intake temp too high and in this case when it can't control the boost via the waste gate and it thinks boost is too high.

Vaccum guage between the pump and control solenoid: The vaccum pump pulls always. ECM controls the wastegate via the soleniod bank. If the solenoid closes you'll read the vaccum of the pump which is still pulling on a dead end.

I think.... The ECM knows the boost sensor is bad because the sensor is saying its high so the ECM dumps boost via the wastegate (control solenoid closes and you read max vaccum straight from the pump) and the ECM knows its really dumped because intake temperature is not high and the coolant temperature is not high. Because if boost were too high both of these would also read high. It also senses the vaccum at the solenoid and that system is giving ok here signals.

[R. Spence]

Thanks Herbert!!
The vaccum reading threw me. I am used to gas engines, so this diesel is a new thing, although I have driven this truck 177,000+ miles.
I have found this ('96) is a different critter and full of surprises.. :D