When I use L instead of D in city traffic on very cold days, when I am slowing down for an intersection, my vehicle will start to slow down for a second or two then it feels like it slips out of gear, the RPMs rise, then it seems to catch a gear again, the RPMs will drop and then rise again. It fluctuates between 1500 - 3000 RPM. My Ford service advisor says I shouldnt drive in L. They havent been able to find the cause of the problem, if it is a problem. If anyone has any idea what is causing this, I would love to have it fixed. Rik

Hybrid battery SOC close to full...?

Turn on all the electrical loads and see if it remains in "regen", low RPM.

Hmmm... "...very cold days..."

Ford makes use of a technique that reduces the level of regen available the colder it gets near and below freezing OAT. I assume that's a pre-emptive measure to help alleviate accidents resulting from loss of directional control due to the higher possibility of excessive regen braking on the front wheels the closer to freezing the OAT gets.

Seemingly it makes sense for use of that technique on any FWD "regen" hybrid.

The other possibility is that you are actually encountering very slight wheelslip due to the use of too much regen braking for (slippery) road conditions and so what you are seeing is a form of regen "ABS". In the past if regen resulted in wheelslip regen was immediately disabled in favor more evenly distributed, 4 wheel hydraulic braking wherein ABS could be put to use.

There have been LOTS of compliants about wheelslip, often caused by a slight bump in the road (virtually un-noticeable to the drivers) resulting in the INSTANT transition from regen braking to more evenly distributed, 4 wheel, hydraulic braking.

"..very cold days.."

So it would make a LOT of sense if someone has discovered a way to apply ABS techniques to FWD regen braking during coastdowns.

Thx for the reply.
FYI "very cold" is -25 c.
The problem is not limited to extreme cold, it merely acentuates it.
I'm fairly certain that there is no noticable wheel slip, since the roads are bare and dry, and I have studded winter tires, but if it only took a very small amount of slip I can't rule it out.

Rik

Your HV battery is to cold too accept regen from "L" gear in all FEH's '08 and older in that cold of weather. Drive in "D" and it might only do the run-up when applying the brakes. What's happening is the traction motor (MG II) is sending that energy from regen to the generator (MG I) and it spins the engine to burn off the energy the battery will not accept. All normal stuff for your FEH.

GaryG

Ditto what GaryG said.

Short answer: What you're seeing is normal and part of the '08 FEH design.


Long Answer:There are two reasons for what you are describing to occur.


1.) A cold (or very hot) battery is protected from damage by accepting less charge than one within the factory temperature limits.

2.)Eventually the regen sent to the battery will reach the top end of the SOC limit.

In either case, since you're driving in 'L' and essentially requesting more regen than normal, the system will divert the extra regen you're requesting into spinning the engine (assumedly in Fuel-cut mode). I like to think of this as virtual engine braking. If you drive in 'D', you're requesting less regen and the FEH will coast further (or use the friction brakes more) depending on the brake pedal input.

P.S. I personally believe this behavior has NOTHING to do with directional control or wheel slip as wwest continues to push. I've been able to go from 'D' to 'L' to 'D' to 'L' and consistently recreate the issue with a cold battery when it was well above freezing outside although my battery was "cold".

Studded tires are VERY prone to slippage on a bare, hard, surface.

Sounds normal to me also, funny you Ford tech doesn't know this. I'd find another shop
with a hybrid qualified tech

My 08 FEH does the same thing Rangerrik. Its been hovering around -22 to -29c here in Calgary and I notice the same problem. The regen going down some of the hills when the battery is cold just doesn't work; pure braking and no regen until the battery heats up.

One thing I did notice though is that the transmission feels like it is slipping when very cold. I use to drive a manual and it feels like when the clutch started to slip. I start the engine, leave it running for a bit (its flippin cold out there!!) and drive away. I apply the accelerator and it takes a bit before I actually start to accelerate. This issue goes away once the engine and transmission have warmed up.

Thx to everyone for the input. I feel much better about this condition. Yes I'm surprised my Ford Tech isn't aware of this. I don't have many options out here, so I'll check out the other dealer next time.

Rik

When the Traction battery reaches its upper limit of programmed charge the energy of deceleration has to go somewhere. So they programmed a "Negative split" mode where the computer sends the excess energy that would have gone into charging the battery into spinning up the engine.

This is the equivalent of engine braking on a standard transmission. Note that an automatic transmission does this too but is far less apparent with a fluid coupling.

There is nothing in the FEH transaxle that would "slip" like you remarked on. Everything is done with gears & electric machine excitation. The "in & out" kind of feeling you're getting seems a bit odd to me, I haven't seen this at those temps near my place.