Over the last week or so, I've noticed something that I can reliably reproduce.

I'm in a 2000 automatic 4wd. I've had the truck for about 2 months now. I don't remember noticing this issue until about a week ago, but it's also possible (likely) I wasn't paying attention to it.

Anyway, with O/D Off, at about 1500 RPMs and 35 MPH, I'll experience a momentary rev drop/rebound when I apply brakes.

Here's the full picture:

Recent work (within last year, previous owner and myself):
- new radiator
- new valve cover gaskets
- new timing belt
- new battery
- new water pump
- new driveshaft
- new cat
- recent oil change
- power steering fluid exchange
- new starter
- new MAF sensor
- new spark plugs
- new coil boots
- new pcv valve
- new fuel filter
- cleaned throttle bottle/IAC
- fixed various vacuum leaks (one that thew a P0171 and another that prevented my 4WD from engaging)

Things that don't seem to matter (this happens either way):
- Coolant temp
- ATF temp
- ECT norm/pwr

Pre-conditions:
- O/D Off
- 3rd gear
- Cruising about 35 mph
- RPMs about 1500

Steps to reproduce:
- Apply brakes to slow down to about 30 mph

Symptom:
- The moment I brake, the engine quickly drops to about 900 RPM before correcting and quickly climbing back up to about 1200 RPM.

Torque converter?
Any thoughts? Something about locking/unlocking/re-locking the torque converter? since i'm in O/D Off, then brake, but not enough for a downshift? Trans fluid is at the correct level, looks pretty nice and red. I'll be adding a Magnefine filter shortly, doing a drain & fill, and adding a B&M external cooler in series. Have not tested this with 4wd engaged.

Also, is this something to be worried about?

Thanks!!

No idea why.

But why drive at 35mph with OD off?

I only turn OD off when climbing or I need to pass another vehicle at highway speeds, it does not help my vehicle's performance at 35mph. I can't find another time to use it that it makes a difference but hey who knows....

Thanks for the reply.

Why OD off at 35mph? No good reason. Just experimenting and learning as I go. I've been really enjoying fixing up this truck, and this forum has been massively helpful.

Torque converter locking/unlocking under those specific conditions would be my best guess

The torque converter is not locked at 35mph, OD on or off.

It does have something to do with the torque converter going from loaded under power to coasting. I mostly notice mine doing it in 4th/OD going from cruising to coasting.

Not 100% sure it is normal, but it hasn't caused any problems with my truck in the last ~300k miles...

-Charlie

Do you know what mph the torque converter will lock up with overdrive off?

I’d also be interested in the answer to this.

Depending on the load my torque converter will lock up with overdrive off at around 36 mph

Officially from the Toyota manual (OD on) (for V6 4WD):

It doesn't have a spec for when OD is off, but in my experience it is about 5mph lower and will hold for higher throttle openings.

-Charlie

Double check your brake booster vacuum hoses for cracks or loose fitment. Do you also get too soft of a brake or too firm of a brake at the same time?

Mine revs down but never down that low. More like to around 1200 RPM's at the lowest.

Hmmm great question. I’ll check for brake feel during this maneuver. I want to say I get a soft brake feel, but I’ll test it out.

All my vacuum hoses are pretty worn out, being 21 years old.. I recently ordered this silicone hose kit and will be replacing all my hoses soon.

Only when you hit the brakes the drop/rebound happens?
What about just cruising along then taking foot off the gas pedal, no rpm drop/rebound happens?

Yep. Only if I use the brakes. Doesn’t happen if I let of the gas as far as I remember. I will double check that scenario though.

I mostly notice it after I crest a hill and begin the descent, but traffic ahead prevents me from continuing at the same speed, so I push the brake to slow down a little.

All day today I kept O/D On, and did not experience this at all.

Update: was just able to test. Brake feel is normal.

Update: confirmed that it only happens in this very narrow set of circumstances, only when I hit the brakes. Letting off the gas does not reproduce the issue.