Rear discs overheating

[John B.]

2008 Rx400h. 

 

Both rear discs are getting really hot after quite short drives. Front discs are cold and car rolls very easily so brakes not binding. Offside hotter than nearside. No warning lights. You can feel the heat from outside the wheel after an 6 mile easy drive with no hard braking. 

 

Anyone any ideas?  Might have started after 12v Battery change. 

 

John B. 

[katabrontes]

Is the parking brake jammed or did you forget to release it?

[Herbie]

Sticky caliper pins?

[John B.]

Parking brake is off and I can roll the car easily. Just had both bottom caliper pins replaced by Lexus and there's no hint of brakes tight on discs on my drive. 

I'm wondering whether the electronic brakes can missaply the rears only??

[Herbie]

2 hours ago, John B. said:

I'm wondering whether the electronic brakes can missaply the rears only??

I doubt it as they aren't electronic brakes as such. Regen braking is done by the electric motor(s) actually turning in the opposite direction and working as generators and there is no physical braking involved until the car drops below a certain speed (3mph comes to mind but I may be wrong) or if the hybrid Battery is fully charged and can accept no more.

I think you may need to get Techstream on it to interrogate the ECUs and see if there's any codes stored.

[John B.]

Thanks Herbie. There must be some electronic control of the hydraulic brakes to manage the switch between regenerative and friction braking but it's hard to find a decent description of how it's done. I suspect that it's done via the ABS/VSC.

I've booked the car back into Lexus for them to have a look but as they're 40 miles away it's going to be a struggle to get it to them without causing damage - the OS disc gets that hot. 

I'll let the thread know what happens. I've got a good code reader so may see if anything is logged and/or try disconnecting the Battery again to see if anything resets - clutching at straws.  

[NemesisUK]

The braking force distribution is controlled by an ECU, so in theory it could possibly miss-apply the brakes but I doubt it would fail that way. Rear brakes account for very little actual braking (disregarding regenerative braking) I think as little as 20%.

Lexus-Tech.eu has all the tech docs on how the system works, this is the doc for my 2019 RC, just for example

 

Brake Control System.pdf

[i-s]

Assuming a regular single-piston sliding caliper then if the pins are free, have you checked the pad itself?

I had a similar issue with the front brake on my Nissan Leaf - pins were free and easy, but the brake was heating up (and brakes on the Leaf NEVER got hot as regen did 95% of the work). The inside pad was totally stuck in the carrier because the pad ears had corroded and swelled, jamming it in there. I got it out, cleaned up the edges of the pad ears, cleaned the stainless shims, cleaned the carrier and put it all back together again having checked that the pad could now move freely. That solved the problem. 

[John B.]

According to garage they cleaned up the calipers but they may have only swapped out the bottom pins.  Checked codes today - none.

[John B.]

Garage say that pads seized and now ok. Took them a couple of hours to sort and as bottom caliper pins were replaced a few weeks ago and calipers cleaned up unsure why the pads seized so quickly. 

Also unsure how car could roll so freely yet pads heat discs so much. 

Anyway sorted foc other than taking 1/2 day traveling and waiting for work to be done. 

 

Thanks for suggestions, etc.  Prize goes to i-s. 

[i-s]

I've seen it on a few vehicles. 

Typically it's because they try to get a pretty tight tolerance with the pad ears because if they are looser then you get click/clack noises when coming on/off the brakes (or when changing direction from forwards to reverse) as the pad moves in the caliper. Those tolerances are fine when new, but as the carrier builds up some corrosion/muck and the pads (where the backing is usually cheap mild steel that will rust easily once the paint goes through) corrode (and so they swell) then the whole thing binds. Garages, especially main dealers, are much more likely to want to replace something than spend time disassembling and cleaning it. 

[Barry14UK]

Glad you got to cause of problem.  It's really a matter of degree as to how much the discs and pads were affected and whether these should be replaced which can be necessary in some cases.  Many years ago I had a Fiat 2300 saloon.  I bought another for spares and towed it home for perhaps 10 or so miles.  I don't know whether my friend in the towed car kept his foot on the brake pedal most of the time to help keep the tow rope taut  or if the rear brakes were seized in the calipers but when we got to my house I found the rear discs (rotors) were glowing red on the towed car.  I examined these in due course and there was plenty of material on the pads but they were cooked rather like charcoal and of course beyond further use as were the rotors.