Shortly after I bought my Silverado, it had brake issues. After a month in the shop I got it back and it seemed to work but it wasn't quite the same. There are a couple nagging things that I didn't recall from before its brake failures. I'd appreciate some input on whether the following is "normal".

1. If I start the truck with my foot applying pressure to the brake pedal, the previously firm pedal drops with no resistance for about an inch.

2. After I come to a stop the pedal has a natural firm point that I used to slow the truck. Once stopped, can you push the pedal another inch or so?

3. The hand off between regenerative braking and hydraulic braking requires me to modulate the brake pressure in order to keep from jarring passengers. I'd swear this was nearly transparent when I bought it.

I appreciate any input. I'm working on a list of things for the dealer to check into under warranty.

 

The one thing I remember on the HyHoe was it's dual personality braking ... completely normal under regular driving conditions, bud god forbid I had to make a sudden stop when at low speed, and it always became dash eating time for any passenger.

Number 1 seems like the normal way power brakes engage when the power turns on and the reason they can't really be pressed down with the power off ... but I'm guessing you're familiar with that sequence so you're probably describing something more pronounced.

Number 2 should not happen, at least not in my experience with either my old Hybrid and present non-hybrid truck.

Number 3 does not sound good. First of all, as I understand it, regenerative braking is unrelated to brakes, it's simply reclaimed energy from engine braking ... Basically imagine a manual transmission vehicle reclaiming the energy from the downshifts. To my knowledge that should happen whether the brakes are being used or not and one does not affect the other. Of course my knowledge of these things is limited at best so I might be wrong as to how the process works ... still, I never experienced the condition you describe on the HyHoe.

Hope that helps a little.

 

Xfordguy, there is a pretty sensitive pedal calibration that needs to be done with the service scan tool and NO foot on the pedal. The procedure is spelled out and must be done exactly as written up step by step. The pedal feel upon start with foot resting on it and the feel a few to several seconds after key off will be much different than conventional ABS brakes.

1 and 2 are not out really out of "ordinary".

3 however is troubling. You should be able to set a fixed brake light to moderate pedal position ( and hold it steady )from lets say 35 mph on a flat road with lots of stopping room. Other than some "down-shift" barely perceptable slight disturbances, ( with motor whine "whale speak" sounds ) the braking should be smooth and steady all the way down to zero speed. The hand off bump is usually under conditions where the hybrid generating electricity to store in the battery is the predominant source of stopping. Any mechanical or hydraulic glitch during this can bump you around.

There is a Mode 2 to Mode 1 downshift hand off that involves switching between big clutches while maintaining the near constant "glide slope". If some hydraulic or physical problem happens in this hand off time, the short term slip and then sudden re-apply of the clutch can bump you around quite a bit.

The dealer can use the scan tool to record this event and see if any clutch slipping.

Question and you don't need to answer, it is for your understanding and to help the dealer. Is the required "modulation" with engine on?, Off? Always?

If there is no slip in the downshift hand off, that pedal calibration could still be the culprit.

Good luck

 

H_H, am I in the ballpark with the regenerative braking?

That's what I was once told by a service manager when I inquired if me installing an aftermarket big brake kit (Wilwood) would affect the hybrid components, and he said it wouldn't.

 

Very close there HyHoe but the engine is not doing regenerative braking. The motors are. . The 2 Mode uses "blended" braking. So it can be all electric, all hydraulic, or any combination of both. It all depends on stopping torque and electrical power. Sorry bro but here comes the physics as background.

Assuming a near constant friction coefficient between pads and rotors, the hydraulic pressure applied to the caliper piston will create a brake torque. So X psi will create Y torque.

Now if the caliper piston diameter/area is changed the force acting on the friction coefficient/pad changes. Same psi applied to a different square inchs = different force acting on pads.

Next my favorite. Rotor diameter. I jacked up the 1975 K-5 Blazer and put 3/4 ton axles under it. The front calipers were constant, actually the stock 1/2 ton. But the rotor diameter grew about 2". I installed the 3/4 ton caliper supports and that set the original caliper out at a larger radius. I had much better stopping power with the same original pads, master cyl, and calipers.

More stopping torque at the same pressure because of diameter.

Now the hybrid blends the brake torque. The brake pedal travel sensor, system pressure sensors, and ABS hydraulic pump need to be in harmony to blend with hybrid electric generation. Electric generation is done in the transmission by those two big heavy copper wound permanent (rare earth) magnet motors. Tim Allen moment here Ahh Ahhh Ahh Ahh Ahh .

During braking the brake computer recieves a command from the hybrid asking for a specific brake torque. Pads don't know torque just pressure. So lets say a moderate to hard stop from 45 mph... yellow/pink light. The hybrid, the motors, the brakes, and HV battery computers have a quick conversation about who is going to do what. Few dozen milliseconds max here. If the battery can take the power and the motors can do the torque, the brakes may not even apply pressure to the pads. All electric there bro!

If you push the pedal harder, now the hybrid motors and or battery may not be able to do the whole job or absorb the electrical power. So at that point the brake computer starts to apply hydraulic psi. Blending!

Any time ABS kicks in, or even close, regenerative electrical braking goes off. Stability trumps re-gen.

There will be some folks that get 200,000 miles on factory brakes with these. They are the ones that do "glide slope" long easy stop braking. Now the Type AA guy with the front wheels totally flat black covered in brake pad dust... We know them. He won't get any re-gen and his fuel economy will suck. It is possible to drive these real hard and get very little of the electrical braking benefits.

Sorry to go on so much but too many out there think that the brakes generate the electricity. Same pads, rotors, and calipers as non hybrid. Oh Oh Oh Oreillies.

If the Wilwood kit had pretty much the same friction, rotor diameter, and caliper piston geometry, the hybrid would not catch on to the difference. But if the Wilwood was the same, would it be worth the expense?

So I propose a hybrid rodeo competition. We get a hand held thermosensor (heat scan gun) and we have competitions to see who can have the coolest rotors from a common stopping speed. Next time EV Drag Racing. Like bracket drag but if you start the engine... you break out and lose! We will use Whistle Tips to know!

 

lol, if I remember correctly it was 16' in the front 14' in the back ... so not really the same diameter.

I'm glad I didn't do it cause it seems the service manager at that dealership (unfortunately in many dealerships) knew nothing of the effect a big brake kit would have had on the system. Man, that could have been a pricey failure.

 

so we can install aftermarket pads and rotors without disrupting the regen brakes?

 

so we can install aftermarket pads and rotors without disrupting the regen brakes?

Yes. If I were doing a brake job I would go to my local O'reillys and buy the medium grade pads and their rotors.

 

It turns out #3 was pedal calibration. It feels better, but I haven't spent enough time with it to be sure it fixed everything. I just got it back today.

 

The pedal calibration is "fussy".