Outer Limits 4x4 Board

Hey guys,
Got a strange problem for ya.

Had a customer come in today, he reports that when using his HF radio he could feel his electronic caravan breaks activating.

So we had a quick look at it today and found that when the HF radio transmitted at certain frequencies, The red light (indicating that the break controller is being activated) was coming on, and activating the breaks.

So we originally thought that the HF radio signal must be inducing a voltage in to the stop light circuit, therfore activating the controller.

Now this is the strange part, I found that when the controller was activating via the HF radio, when the output voltage control was turned down, the actual output went up
(ie the voltage control on the break unit operated in reverse to how it should)
The unit works fine when activated via the break light circuit.

So from that i can only conclude that the HF signal is affecting the electronics in the break unit and sending it stupid.

Know i don't know much about HF radio, but is there special requirements when fitting these, as far as grounding the aerial or sheilding in order to stop these induced voltages????

We are gonna get the car in and diagnose further next week to try and cure the prob.

steve

Yes, earthing is very important. Modern 4wds can be a pain for HF radio installers, as the HF can interfere with engine management if not done properly.

Heard about a guy in a diesel 'Cruiser who was playing games with a petrol Pajero. Everytime the Paj tried to overtake, as it drew alongside he'd hit the "autotune" button on his HF, and it would scramble the Paj ECU and cause it to lose power.

cool thanks, So is it just the base of the aerial that needs earthing??

Is it causing the pendulum in the brake controller to move?

Perhaps some sort of magnetising effect?

Is the antenna cable shielded? And if so is it grounded properly at one end. How close to the brake controller does the antenna cable go. I am assuming that the antenna is at the rear of the vehical. if so you may need to run the antenna cable down the other side of the car so it is well away from the brake controller

HF sets are renound for causing a number of problems within the car enviroment. The key to minimising interference with the cars equipment is good instalation techniques.
1) Earth the head and control unit well.
2) never run power cables along side the control cables
3) never run the coax and control lines that terminate from the control box to the hf antenna alongside any other cables. If you cant avoid this try and keep as much distance as you can and try to cross at right angles to the cable. this will minimise hf induced problems.
4) Try to run data and high o/p lines ie coax on seperate sides of car.
5) Earthing of the antenna base is cruitial.
6) Good quality coax makes a incredible amount of difference to rf interference. make sure that the sheilding or braid covers the entire outer surace of the dialetric (the white bit).
7) All terminations are soldered and no loose or comprimised connections with crud causing the earth to float at some potential other than what it should be and that is earth.

Hope this helps
Cheers Russ

transistors are pretty sensitive little suckers, the HF radio (and sometimes mobile phones) can cause the transistor to conduct at certain resonant frequencies. It could be causing something on the feedback side of the transistor circuit to have a voltage present causing the reverse braking thing you mentioned, the adjustment is most likely just an adjustable feedback loop and an amplifier transistor somewhere, if the transistor is conducting because of the HF, the adjustment of the feedback loop could do anything!.

best bet is as said above keep cables separate and ensure proper shielding and grounding of the cables. alternatively shield the brake controller somehow, or try a different brand (better brand controller perhaps?), or move it away from the HF transmitter if possible.

You'll probably find the brake unit is picking up RF via its power leads.... ie. acting as an antenna. Try putting a ferrite core on the power lead and see if that makes a difference. You should be able to get a clip-on one from Dick Smith or similar.