step on brake when coasting reduces rpm

[EscaladeDiesel]

While driving my Suburban today I noticed that when I would use the break or tap the break the rpm would reduce from 900 or so to 500. Anyone have any ideas why?

[Beedee]

I noticed the same thing. Had me thinking for awhile and the best that I can come up with is the Hydraulic assist for the brakes drags the engine down a couple a hundred RPM.

[rjschoolcraft]

Mine does this. I thought the same thing about the brake booster, but further experiementation makes me think its something in the PCM dropping idle speed. When sitting still, the phenomenon doesn't occur.

[catmandoo]

my 94 sub does that too.first time i felt it i thought what the hay is going on here,but what i have noticed is you just have to press the pedal enough to say turn on the brake lites,yet not apply the brakes.

[EscaladeDiesel]

I don't seem to remember this occurring before, but I am SO VERY sensitive to every noise since the engine was put back together. I just don't remember it occurring previously. Also, I seem to notice a delay in my throttle response (not turbo lag).I KNOW this delay did not occurr previously, wonder if there is any connection to these two issues?

[Dihrdbowti]

Mine also does this. My voltmeter moves pretty good while doing this. About 1-2v dropped. Seems the worst when the engine has warmed up or at night when all the lights are on. Doesn't seem affect the RPM's when the engine is still cold.

[JohnC]

The PCM keeps the RPM up a little when coasting for emissions. When you apply the brake, the PCM drops to idle so as not to fight the brakes.

[DmaxMaverick]

What you are experiencing is normal, by design. The assumptions regarding emmissions/economy are correct.

On many previous GM vehicles (1982-present), when you let off the go pedal, and/or apply brake pedal pressure, the torque converter will unlock, allowing the engine RPM to drop to torque converter unlock resistance, and the sprag in the tranny can allow engine RPM to drop completely to idle speeds. Earlier models just unlocked the converter, later models allow the engine to drop completely to idle. The ECM controls the process.

The good/bad of it? Good for EPA and economy, bad for us (truck users) because it negates the truck's ability to use compression braking in many cases. The earlier truck programming was supposed to "detect" when you are towing/hauling, and prevent the complete disconnect and allow compression braking, but the converter still unlocks.

The new Allison tranny is different, as it uses a specific map to control torque converter lock and gear selection. It will stay locked when on compression and during braking above ~55 MPH in normal mode, and ~20 MPH when in T/H mode (other factors come into play when in T/H, but don't pertain to this discussion). IMO, this is a real improvement in performance, but some economy may suffer. For me, I would rather have the performance over the little economy gain, as would most here. Some would not.