RPM drop when step on brake in P/N

[DrX]

Anyone else see this? It will almost stall if you step on the brake in P/N. Truck is equipped with hydroboost brakes.

[Taz]

I take this does not happen when in gear ?

Try adding some airflow to B4338 in P/N. If this doesn't correct the issue to your satisfaction, try adding a little airflow to B4307 in P/N.

The OEM systems had a third wire on the alternator plug - which fed the OEM supplemental brake system (if so equipped). This would create a greater load on the alternator / engine.

Not sure how your setup is configured - but hopefully a little extra airflow resolves the issue.

[DrX]

I take this does not happen when in gear ?

Try adding some airflow to B4338 in P/N. If this doesn't correct the issue to your satisfaction, try adding a little airflow to B4307 in P/N.

The OEM systems had a third wire on the alternator plug - which fed the OEM supplemental brake system (if so equipped). This would create a greater load on the alternator / engine.

Not sure how your setup is configured - but hopefully a little extra airflow resolves the issue.

I have not noticed this in gear, or at least it is not as noticeable.

I believe I have played with B4338 in the past for this issue, but will try it again. Right now B4307 seems to be close to where it needs to be to get average idle spark close to 0 with decent startup and idle. Although it gets complicated with running SD, on a scaled tune, with 2 throttle bodies. Logged DYNAIR is about 1/4 of the values in B4307. It would be nice to know exactly what calculations the PCM executes with the values in 4307.

The 3rd wire from the alternator does not connect to anything on hydroboost vehicles as there is no vacuum pump. It has no mate on the inline connector where it terminates.

[joecar]

Anyone else see this? It will almost stall if you step on the brake in P/N. Truck is equipped with hydroboost brakes.Does the hydroboost system have its own electric pump, or does it use a belt driven pump (does it share the power steering pump...?)...?

[DrX]

Does the hydroboost system have its own electric pump, or does it use a belt driven pump (does it share the power steering pump...?)...?

Yes, it shares the power steering pump.

[DrX]

I revamped my firmware code for the secondary throttle controller. The faster update rate should now allow the secondary throttle to better track the very small fluctuations in the primary throttle position at idle. Will find out if that helps at all in a few months when the snow is gone.

[EagleMark]

Is it a Hydroboost conversion? These put a big load on the stock pump at idle... the hydroboost pump that came on the vehicles compensated for this, higher idle pressure I beleive?

[DrX]

The Hydroboost is OE on this vehicle.

[DrX]

Finally getting the truck out again after a very long, very cold winter here. Did a little testing yesterday.........

I notice an RPM drop just spinning the steering wheel in P/N, more if I crank it all the way, but no stall as I can cause by pushing hard on the brake.

I set the P/N and In Gear Desired airflow tables and the Desired Idle Speeds for both to exactly the same values. If I step on the brake in Gear, there is a very slight RPM drop but it does not stall, whereas in P/N it will stall unless I apply the brake very gradually. Seems to be the opposite of what you would expect.