2010 XC90 engages brakes when going uphill

uncertain · Post by **uncertain** » 20 Oct 2019, 14:56

Hi, my 2010 XC90 3.2AWD exhibits a very strange symptom:
when going up a very steep hill, it would engage right front and rear left breaks and keeps on applying it until i stop the car and shut down the engine.
Anybody have seen something like this before? I imagine one of the stability sensors had gone bad, but how can I find out which one? I'll check the codes, but i doubt there'd be any as I see no messages on the dash display.
Any help appreciated!
-uncertain

Post by **jimmy57** » 20 Oct 2019, 15:28

I would loosen the master cylinder nuts holding the master cylinder to booster 3/4 turn and repeat and test. Those two wheels are on the same circuit on master cylinder. It is not impossible but highly unlikely that the traction system is applying those two only. It is more likely you have a bad booster and something about that situation is tweaking the booster and it is applying brakes slightly. The traction system relies on wheel speed sensors with a longitudinal sensor in the body sensor cluster under rear center console. The longitudinal sensor is used to apply traction control in situations where the 4 wheel sensors indicate rotation but the longitudinal sesnor does not sense acceleration. If that occurred the response would be reduction in torque by reducing throttle and retarding ignition timing. If the traction system was active you should get the skidding car lamp coming on also.

uncertain · Post by **uncertain** » 20 Oct 2019, 18:00

Thanks @jimmy57!
Wouldn’t the booster engage both circuits as well?
The other peculiarity I have in my car is different brake rotors (from different manufacturers).
Could slight application engage one circuit, but not the other?

Post by **jimmy57** » 20 Oct 2019, 20:34

On many cars with hanging boosters you get one circuit sticking. the rear piston is direct from pushrod out of booster and the front piston is pushed by hydraulic fluid in front of rear piston on tandem master cylinders. This makes for a faint delay in the pressure build on the second circuit. WHen you climb that hill underhood temps from exhaust headers and catalyst will go up and often boosters will fail when hot. I have had many failed boosters on domestic vehicles and a few of the volvos that used the front-rear split system. These have the front brakes on rear (also know as primary) piston and the front (secondary) piston is for rear brakes. These will have sticking front brakes but no rear brake drag. The failure is the pushrod being 0.020-.030 thousandths protruding so it is not a full on apply. Usually it is the result of you applying brakes and the front piston comes back a few thousandths behind fill port and the rear one is that thousandth or two short of uncovering it and if the brakes are hot and the lines are warm then you get thermal expansion of the fluid and the brakes drag.

uncertain · Post by **uncertain** » 20 Oct 2019, 21:45

Thanks a lot, it’s very useful!
I am not sure this theory fits my case though, as in my case it’s not merely a small drag - it’s a pretty strong and consistent application of the breaks. Initially I thought it’s some sort of automatic transmission issue (it felt like it switched to a first gear as the car slowed down and gas would not make it go much faster), so I’d kept on pushing the gas and soon saw the smoke - the two rotors in mentioned became red-hot! It felt like the car deliberately engaged these two breaks. What do you think?

Post by **jimmy57** » 20 Oct 2019, 22:21

I know nothing without testing. Loosening the master cylinder bolts is pretty easy as is using a wrench and losening the hydraulic line on rear connection of master cylinder and then tightening that line back immediately. After doing either one of these then strt engine drop in gear and see that engine makes it roll off when idling, needs to be on level ground for this. When I have had a car have this sticking brake issue due to booster what I just listed will release brakes. The brakes often got just as hot as yours and only on one circuit. for the trction system to apply brakes that hard you would feel it come on suddenly as those systems do not apply brakes over seconds but in thousandths of a second.