Sticky brakes question - help needed

[Toycruiser90]

My Friends 120 auto has a problem, All 4 brakes are coming on when driving, and obviously over heating very quickly too, switch on brake pedal had to be disconnected and then it is driving normal and brakes also OK except the brake lights and the shift lock doesn't work then which is expected.
Question is even if the switch is faulty why would the brakes come on "on their own". is their a "fail to safe mode" on the 120 or what else could cause this, very strange considering all is then OK when you remove the brake pedal switch, also in "N" with handbrake off the vehicle will roll freely - no brake binding.
- Any ideas on this mind bender ???
Thanks in advance, - Rob
P.S the hot grease smell from the hubs remind of a Morris Minor we had many many years ago, the drums brakes use to seize up and overheat and required a few buckets of water to cool them just to get home, hell where can I find a time machine to go back.

 

[Crispin]

Is the a noise from the braking system? Could the stability control or A-TRAC be kicking in? You would hear it, sounds like the ABS being activated.

Not sure though about removing the brake pedal switch. Not sure how that would make a difference.

 

[Jon Wildsmith]

Any warning lights on the dash?

 

[Toycruiser90]

Hi Crispin

No noise at all. just drives as normal then when all 4 brakes got hot all hell broke loose, smoke from all 4 wheels, I don't really understand how stability systems work exactly but I though they would release the braking to some wheels and actual brake pressure would come from drivers pedal pressure meaning when pedal is released brakes were off completely, I have a new switch on order so will try that, its obviously not full brake pressure or the cruiser would not move at all so you could be right that some system is applying some pressure,
As I said with switch removed drives fine, all brakes are free and work as normal, just no rear brake lights or shift lock as expected, no warning lights on dash etc, don't know if ABS would work if needed though....
Spoke to Mr.T and he has never seen this before,
I think it must be Vacuum is leaking into the Brake Servo through some valve or solenoid when the switch is fitted and vehicle is in drive ??? and does not happen with switch removed,
thanks for reply - Rob

 

[Toycruiser90]

Hi Jon
No warning lights on dash...

 

[Jon Wildsmith]

Are the brake lights on with the switch connected? Have you tested the switch? Just wondering if it is a faulty switch, or the switch is being pressed slightly. I assume a 120 has electronic brake force distribution, maybe it has a slightly positive bias in anticipation of use when the brake switch is pressed, but you'd expect brake lights to be on unless the switch is dual circuit

 

[Toycruiser90]

Switch appears to be working normal, i.e brake lights come on when pedal pressed and go off when released, as I said its a friends cruiser and I didn't have a meter with me at the time, tomorrow I will probably get a chance again to check it again,
Switch has 2 contacts, Mr.T thinks one is for ABS, the other I assume is brake lights, shift lock etc. its driving normal but without brake lights, as switch is removed at present, they only have one car so using it as little as possible, for short journeys etc....

 

[Toycruiser90]

Update and hopefully a conclusion, I got the new pedal switch, checked both switches side by side, there is 2 contacts on the switch, the contact which is closed with the pedal in rest position was failing and sometimes made a high resistance connection, the spring was a lot weaker also. Fitted the new switch and test drove for around 20 miles, braking and stopping every few miles to check for excessive heating, all was well no heating, did not use the brakes for the last 5 miles and when I stopped the discs were cold, told the owner it was OK now but to keep a check on the brakes for a while just in case, I think after the original scare they know what to look out for i.e not being able to drive over 30mph and smoke from all 4 wheels was a big clue.
Hopefully thats the end of it and their isn't some problem waiting to reoccur in the brake servo or elsewhere.

Anyway more importantly I got my first drive in a 120 and absolutely loved it, it so much more refined than my 90, smooth, quiet, effortless, comfortable, I looked at rev counter once and I think it was under 2000 rpm at 50mph, auto gearbox is really good you hardly notice changes, I would really love to have a 120 but after having my 90 now for nearly 12 years I just might need a divorce lawyer to separate us, but you never know what might happen in the new year though .

 

[TonyP]

Glad that solved the problem

The 120 is a much nicer car to drive, but some would argue it has too much electrickery going on. Second hand prices for 120's are coming down, and as soon as there are more 150's available on the second hand market, I think 120 prices will drop again...

may be time to move on from my 120

 

[Jon Wildsmith]

glad it was a nice simple fix another bit of information to file away, that they are dual circuit on that switch.