rpm change when selecting drive or reverse

elcamino12sec · #1 01-12-2007, 12:13 AM

Just wondring why the rpm goes up when I shift from park to drive or reverse? Is this normal? If not what should I do to fix this issue. 2002 f150 lariat w/4.6 auto, believe 4r70w.

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SlammaJamma · #2 01-12-2007, 01:13 AM

Hrmm, mine Idles higher in park than it does in Drive or Reverse, and Higher than all of that in Neutral. Not sure what your prob could be to be honest.

flareside4life · #3 01-12-2007, 01:31 AM

It's normal function. When your in park or nuetral your engine has no load on it, so it spins more freely. When you put it in gear, the tranny is in gear and the engine is fighting the drag created by the torque convertor slipping while your foots on the brake pedal, similar to not having the clutch completely depressed on a stick.

Nevermind didn't read carefully, that is wierd.

elcamino12sec · #4 01-12-2007, 11:41 AM

Maybe I was not clear, when the truck is in park and then I shift to reverse or to drive the rpm will raise just before the transmission engages.

masterd · #5 01-12-2007, 02:21 PM

If I do it to our 04 F150 right after starting it then drop it into reverse itll rev up a little, but not when its hot and idling.

Bluegrass · #6 01-12-2007, 02:33 PM

You see something most don't recognize or see.
When the transmission selector is moved to a gear it signals the PCM to raise idle so stalling is averted even though you still see a drop in idle, the action still opens the IAC a programmed amount due to the converter loading but not enough to cause heavy creep at a stop sign or light.
The same action happens when A/C and other load sensing is done.
These actions have been incorpoated into the computers for many functions since at least 1985.
Also when cruising, just taping the brake will unlock the converter for a second ot two then lock up again. This is an intended action built in the programs to keep the motor from driving the truck in event panic braking has to be done so total slowing/stopping time is not added to.

elcamino12sec · #7 01-12-2007, 07:12 PM

Thanks bluegrass, I was assuming the same thing you said. I recently purchased this truck with 100,000 miles and just wanted to know if what I noticed was a normal thinig or not.