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Brakes cut out when switching gears?!


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6 replies to this topic

#1 OFFLINE   hikyuuri

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Posted 09 July 2015 - 10:57 AM

So, I know that in a regular car, is is a BIG no-no, but I do it in my Fusion Hybrid from time to time.

 

First, let me ask does anyone know if the Hybrid drive train actually has a gear that switches to go from R to D?  Or does it just reverse the current through the Traction Motor?  If there is a gear, I will immediately stop what I am doing, but if it is just current, then there is no harm.

 

Now for my issue,

 

When I back out of my driveway in the morning, I don't always wait for the car to stop before going from R to D.  Now, I'm not going SUPER fast, usually under 5MPH when I do, but I have noticed something really concerning.

 

My driveway is rather long, so I get moving when backing out of it (15MPH or so).  When I get out onto the street, I hit the brakes, slowing my vehicle (regen, not service brakes), then switch from R to D (with my foot still on the brakes).  The moment I do this, all brake power goes away, and I continue to roll backwards in D as if the vehicle was in neutral with no brakes.  If I take my foot OFF the brake, I stop quicker than with my foot ON the brake.

 

At my house, this is not a concern as I live at the very end of the street, and I know no one is behind me.  But I have had this happen in a couple situations where it was almost dangerous.  I honestly feel that a vehicle should continue to brake at the same rate, regardless of what gear its in, or which way its going.

 

Anyone else do this and notice something like what I am describing?









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#2 OFFLINE   hikyuuri

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Posted 09 July 2015 - 11:10 AM

UPDATE:

 

So, reading another thread lead to me GrySQL's FAQ (Quiz thread).  While reading, I did confirm that the ICE can't drive in Reverse, that is all electric.  This makes be believe there is no gear switch to go from R to D, so no damage is being done by switching from R to D (or D to R) with vehicle movement at low speeds.

 

I also read this:

 

Creep Mode
The hybrid electric system delivers torque to the wheels to mimic the creep mode normally found on vehicles equipped with an automatic transmission. The TCM commands a predetermined amount of torque to be delivered to the output shafts of the electronically controlled transmission. This torque is delivered from the combination of the internal combustion engine, the traction motor, or the generator motor. The maximum creep speed in forward or reverse direction is about 6 km/h (4 mph). The creep speed may vary slightly if ambient temperature, altitude, relative humidity, engine temperature, or weight of the vehicle changes.

 

Is it possible, that since I have my foot on the brake when switching gears, creep mode gets disabled (so you are not "creeping" and applying the brakes at the same time), but the service brakes are not applied, so the vehicle rolls backwards until I take my foot off the brake, reactivating Creep Mode?

 

If so, this confirms my suspicion of a major safety issue that should be reprogrammed in some way.  If your foot is on the brake, the vehicle should stop.



#3 OFFLINE   hybridbear

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Posted 09 July 2015 - 12:33 PM

When in reverse I don't think there is any regen, only friction brakes. Switching to drive may cause the friction brakes to disengage because the car is going into creep mode and trying to accelerate by limiting the power of the electric motor. This would explain the sensation you feel. Coming to a stop before shifting gears should solve the problem you perceive.


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#4 OFFLINE   hikyuuri

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Posted 09 July 2015 - 12:54 PM

Agreed, coming to a complete stop would solve the issue as there would be no momentum.  However, there are people out there who don't do that.  Its still a concern of why does the vehicle not continue to brake under these conditions?



#5 OFFLINE   Waldo

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Posted 09 July 2015 - 12:58 PM

 

If so, this confirms my suspicion of a major safety issue that should be reprogrammed in some way.  If your foot is on the brake, the vehicle should stop.

 

I don't think it's fair to call it a "major safety issue" when you are clearly not following the instructions in the owner's manual:

 

R (Reverse)
With the gearshift lever in R (Reverse), the
vehicle will move backward. Always come
to a complete stop before shifting into and
out of R (Reverse).
 
 
 

Agreed, coming to a complete stop would solve the issue as there would be no momentum.  However, there are people out there who don't do that.  Its still a concern of why does the vehicle not continue to brake under these conditions?

 

 

At light pressure, the brakes are all controlled by a computer.  A computer that is programmed assuming people follow the simple instructions of always stopping before shifting gears.

 

I'm sure if you stepped on the pedal hard enough you'd engage the hydraulic brake system and the car would stop very quickly.


Edited by Waldo, 09 July 2015 - 01:01 PM.

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#6 OFFLINE   hikyuuri

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Posted 09 July 2015 - 01:09 PM

Out of curiosity (and not starting some sort of argument here, I'm honestly curious), what difference is it if the vehicle is going ~5MPH compared to 0?  I know in a standard transmission, an actual gear shift takes place to go from R to D, so movement can cause these gears to slip/grind (damage).

 

However, in our Hybrids/Energis, reverse is handled by the Traction Motor and the ICE can't provide any thrust, so this suggests the traction motor simply runs backwards to provide reverse.  As such, no gears are actually shifting, and all that is happening is the current through the Traction Motor is being reversed.  Since no gears are actually shifting or changing, there is no risk of slippage or grinding.

 

A practical example of why this would be amazing, especially for those of us in snowy/slippery climates, is if you ever get stuck.  If you could just keep a constant light pressure on the gas pedal (that butter zone where you still have traction, just before the wheel wants to break loose and slip), and just switch "gears" without adjusting the pressure, you could rock the car a lot better than needing to actually take your foot off the gas each time.

 

 

 

Also, I do know that the traction motor is a 3-phase motor, so its not as simple as reversing the "Current" like a DC motor.  Its changing the energizing pattern of the magnets that make it go backwards.

 

 

 

When in reverse I don't think there is any regen, only friction brakes. Switching to drive may cause the friction brakes to disengage because the car is going into creep mode and trying to accelerate by limiting the power of the electric motor. This would explain the sensation you feel. Coming to a stop before shifting gears should solve the problem you perceive.

 

 

I have seen the regen circle show up on the screen when I go in reverse over 15 mph and hit the brake, so I do think it does some regen.  I'll look more closely at the amperage reading next time though to see if it is charging.


Edited by hikyuuri, 09 July 2015 - 01:13 PM.


#7 OFFLINE   Waldo

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Posted 10 July 2015 - 07:09 AM

Out of curiosity (and not starting some sort of argument here, I'm honestly curious), what difference is it if the vehicle is going ~5MPH compared to 0?  I know in a standard transmission, an actual gear shift takes place to go from R to D, so movement can cause these gears to slip/grind (damage).

 

 

Here's one quote from the shop manual that might provide some clues:

 

The ABS module uses the driver input on the brake pedal, wheel speed sensor input and the lateral deceleration messages from the RCM to determine the amount of deceleration the driver is requesting and which brake mode is necessary.

 

If the ABS module is seeing wheel speed sensor data and lateral decel going the wrong way while in D, it is likely going to be confused and not know what to do.  That's likely not a condition that was programmed into the system.  So that's why there's a difference between 0 and 5mph in reverse, because the sensors are not matching what the computer expects, therefore the computer won't continue to send the command to brake, whether regen or friction.


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