No BOOST with CRUISE ON

[Scooby]

I have begun making the 96 1500 6.5TD pickup into a truck I want to keep. I am in the process of installing a pillar gage set, and have temporarily hooked the boost gage on the side of my ashtray to see how it works till I get the pillar in. I tapped the side of the intake pipe just after the flex coupling. When I drove the truck this afternoon, I noticed when I set the cruise control, the boost went to zero and stayed there. I am still using factory boost control (soon to be changed!!), so I am only seeing about 5 psi cruising at 60 mph. Set cruise, boost goes to 0. Leave cruise set, accelerate to 70, boost is still 0. Switch cruise off while holding accererator and speed at 70, boost jumps back up to about 5. I have done this numerous times today, and have no idea why the boost disappears with the cruise control on and set. Any ideas ??

[Salemone]

I think the boost controller uses vacuum. It probably fails safe. (no vacuum wastegate blows
open) Marginal vacuum pump or leak in cruise
diaphram (if it has one) or electrical problem.

[Polar Diesel]

the cruise control in these new trucks doesen't require vacuum

[gmctd]

Does it manually accelerate to 70 in cruise with same power as with cruise off?

Cruise on, push the pedal to the floor to 70.
Drop back to same speed, switch cruise off, pedal to the floor to 70.

[Scooby]

The cruise has to be 'set' for the boost to disappear. If I set it at say 55, boost is zero. If I rapidly accelerate to 70, I will see boost briefly during acceleration, but once I reach 70 and keep it there with the pedal, boost goes to zero. If iI then reach down and click the cruise OFF, (it is still SET at 55), while I am maintaining 70 with the pedal, the boost immediately jumps up and stays around 5 psi. I am planning to do away with the factory boost control and go to Turbo master anyway, which is why I am installing the boost gage. I dont notice a difference during acceleration, as the boost does seem to come up then. It is during steady throttle operation I see no boost when the cruise is set. :confused:

[gmctd]

No-to-little boost would seem to indicate wastegate is bypassing under cruise, but you would see low power and heavy black smoke under acceleration.

Cruise, ideally, should be minimum fuel consumption, minimal power - the 6.5L runs well normally aspirated.

Dunno - may take a Tech-II to determine the answer.

The mechanical boost controller should tell the tale, as well.

And, your boost is digital, a function of the PCM and electronic inj pump.

[Scooby]

I was talking to Bill Heath about the Turbomaster, and i mentioned my no boost with cruise engaged. He said GM has many different ECM programs running these trucks, and some of them do turn boost down to like 1/2 (.5) psi with cruise on. I guess I have that program. There's another reason to go with the Turbomaster, and do away with factory boost control.

[MTTwister]

Scooby - given that FL is fairly flat - but if you use cruise and encounter a hill that requires more power, or a downshift, does the boost increase?
Interesting way to program a truck, if ya ask me..
( I'm about to take mine on the road, I'll keep an eye on the boost under cruise. Been along time since my cruise was reliable, pre IP swap).

[Scooby]

The boost does increase if I push the accelerator, but as soon as I hold the speed steady, boost goes to zero (or darn near.). If I switch the cruise off and hold the same speed with the accelerator, boost is up around 5 .