Tyres and Brakes

Hello Rob,
Steven Gibbs here, Director, Bushtracker,

Look, I could be wrong, but I am not a bad electrical Engineer on my own... And this does not sound mechanical as much as electrical as in some electrical interaction with the electronic ABS brakes and your brake controller... ??? Now I could be wrong, but without see it, since you say it is not there when you manually apply the brakes and I am assuming you mean the slide bar, then I think it is electronic in nature... If it was shoes breaking in, or some hard spots in the drum, or some other mechanical thing it would not change much when you applied the slide bar manual over-ride.. Have the ABS idea checked out by an Auto Electrician or have the Brake Controller checked the same...

It would also be conclusive if you had someone else hook up to your van and it was not there....

Kind Regards from the Source at Bushtracker

Rob,
Perhaps I can help. I am a mechanical engineer and in my past crazyness of building and racing Cobras, I did acquire considerable experience with Bosch ABS braking systems. The Bosch ABS brake computer, American made and I suspect most others are only interested in friction gain/loss of each wheel relative to road surface which is sensed by each wheel. This is a very complex issue but from your description of the problem I would not necessarily suspect the ABS brakes since the fault does not occur with the van hitched. On the other hand, it may be what appears to be an intermittent fault due to dirt on the tip of one or more sensors.
The ABS system is shielded from all other electrical systems in the vehicle. The only thing it has in common with the electrical brake controller is that is triggered simultaneously by the microswitch behind the brake pedal.
I personally wouldn't bother with auto electricans, they haven't got a clue when it comes to ABS, you will need an OEM mechanic to diagnose the ABS system with their little black testing box.

THanks to Steve and Dave. I certainly shouldnt be discs/pads/drums etc since both car and van have had recent services and done 1000k trouble free sicne then. And interestingly the rate of the shudder is similar to the ABS pulse, so will get that checked out.

Rob,
I was in Perth a few months back having a look around a 4WD shop (ARB or TJM I think) and got talking to one of the blokes running the show about brake controllers.
He told me that when fitting brake controllers they no longer connect them to wiring adjacent to the brake light switch as a connection to the wrong wire in this location can interfere with ABS.

Instead of going to the brake light switch they now run their installs to one of the rear brake lights and they say they now have no dramas. Not sure if this is related to your problem but some of the more learned people are thinking possibly ABS related so it may be worth a look.

regards, Bill James

Rob,
If for some reason there is interference between ABS and the brake controller due to current running back to the brake pedal microswitch from either the ABS or more likely the brake controller, a diode soldered in the correct direction into the 12V wire will prevent this occurring.