Brake Controller Installation Warnings

Brake Controller Installation Warnings

Yet another iffy brake installation this time on a fellow A'vanners Toyota RAV-4. You can clearly see the problem in the photo on the right above. Here we have the in-line fuse supplied with a Tekonsha Voyager brake controller (absolutely top product in our view) using 6mm auto cable. Look at the blue crimped connector and you can clearly see that the cable size has been reduced to 3mm. This is EXTREMELY bad practice as the run from the battery to the A'van axle can be almost 7 metres depending on vehicle type. The photo above left on the same vehicle shows a 4mm red wire connected to the battery. Whilst everything still works this also is bad practice in our view. Also on this same vehicle the self resetting relay has been placed in the earth line. Yes folks a "professional" installation by a very large company - let's hope this was a one off.

Bob Eustace

UPDATE 27th June 2002

A self-reset is primarily there to guard connecting cable against overheating and catching fire in the event of a short circuit along the cable to the appliance (in this case brakes).

It will also protect that cable, and the appliance from further damage, in the event of a short circuit, or some major overload in that appliance (this is better performed by a separate fuse). All this is in my book.

Where the chassis or other connected metal is at earth potential, the reset must be in the positive lead and as close the the battery as possible. Then a short circuit to earth anywhere between that battery and the appliance (or within that appliance) will cause the breaker to trip and thus protect the cable.

If the breaker were between the appliance and earth then full current will flow through the positive cable if that cable shorts to earth upstream of the breaker - or a short circuit develops within the appliance. Either occurrence may burn out that cable - with a very real risk of fire.

Remember that the A'van chassis is at earth potential relative to the towing vehicle battery because there is a metallic connection via the towball and socket - (even if, as in an A'van) the A'van chassis is not used as a massive negative lead.

The power for a brake circuit is taken from the vehicle battery, hence the feed cable to the brakes is live with respect to any part of the towing vehicle and coupled-up A'van chassis and any metal work (electrically) connected to them.

So the reset should be in the positive lead. If its in the negative lead there is an alternative parallel path available via the chassis. A short anywhere along the cable to the brakes will cause that (now unprotected) cable to overheat and probably cause a fire.

The A'van chassis is also likely to be at earth potential relative to the A'van battery unless there is total isolation between the towing vehicle and A'van charging and battery systems (as for example where the A'van is totally solar-powered). This can readily be checked by coupling the A'van to the towing vehicle and starting the engine. There should be no voltage when measured between the A'van battery positive terminal to A'van chassis.

In other words, the A'van chassis is likely (almost certain) to provide an earth return to the system battery/s. It's just that the designer has decided not to rely on it.

The exception to the reset in the negative line is of course those mostly pre-50's vehicles that had a positive earth (i.e. where it was the negative lead that was 'live'). Also those very rare vehicles, such as some Mercedes, where the entire electrical system is isolated from chassis earth (there, the reset may be in either lead).

Collyn Rivers - www.caravanandmotorhomebooks.com