I have a 2006 LR3 HSE that has recently been showing 2 faults at startup about half of the time and then they go off.

HDC Fault
Transmission Fault - Traction Reduced

Anyone know what could be causing this? I've read a lot of posts about weak batteries causing similar faults...

 

A weak battery can cause various faults, since data busses start to shut down below 10V. I would at least have your battery tested and if its OK, then I would have it scanned for fault codes before doing anything else. Without some direction (finding out what the vehicle is seeing) its basically a crapshoot.

 

Well it could be just a tail light bulb, but the odds are it is deterioration of the brake light switch - hence replace it. Also if you have installed any non LR spec tail and brake light bulbs, replace them even if they appear to be working with genuine.

Regarding the brake light switch, I have a theory that even if the switch appears to be working just fine, it is responsible for creating spurious signals that lead to the false error messages.

The set of contacts that is normally open operates the brake lights; the other set of contacts that is normally closed sends signals to the ABS module.

I theorize that dust from the deteriorating brake light set of contacts fouls the ABS contacts and leads to intermittent operation of the ABS contacts during normal driving particularly when one is not using the brakes. Since the ABS circuit communicates with the Hill Decent circuitry and that affects the transmission and engine controls, then anything is possible. For the most part, we are fortunate that all we get are false error messages and threatened system shut downs.

My reason for thinking the black dust causes problems with the second set of contacts is that that signal thru that set of contacts is closer to being a millivolt signal than the two or three amps of current powering the brake light filaments. In other words, the dust from the brake light contacts interferes with the flow of signal thru the ABS contacts.

As to why other vehicles do not seem to suffer this malady, well no other vehicle, (except maybe some Jeeps just recently), has a Hill Decent system.

Below is a link to a number of files related to the brake switch and changing it out.

DISCO3.CO.UK Photo Gallery - Brake Light Switch Replacement

 

For me it was the battery. After I replaced it all the trouble alarms and warning went away.