Trailer lights.

[Dave Plowman (News)]

Brother - who lives at the other end of the country - has a BMW E46 330t which was pro fitted with a tow bar when he bought it some 8 years ago. Cost an arm and leg.

The car lights still work as they should, but none of the trailer ones do. It also no longer sounds the buzzer or whatever when you connect it.

There are two boxes that appear to be part of the extra wiring to the trailer sockets - one with fuses. One of them seemed to buzz at one time (relay?) even without the trailer lights connected - but they did work OK then.

Is this a sort of standard aftermarket set-up for a tow bar conversion - and anyone know where I could find a schematic?

The car is near the end of its life, but does still get used for towing the trailer around - he has a newer car for the caravan.

I've offered to look at the boxes (since he says he's checked all the wiring etc) but it would help if I knew exactly what they did.

[dennis]

If its the cheap after market towing kits then all they do is switch the

12V to the appropriate lamps using a "relay" so the car electrics don't notice or blow a fuse.

The really old ones used mechanical relays, more recent may have some electronics.

The only difficult bit is to detect when the indicators are working as it has to make a sound when the indicator lights on the trailer are flashing but not if only the cars indicators are flashing even if the trailer is connected.

So there will be some sort of current sensing device in series with the indicators. Maybe just a relay.

A new box is pretty cheap.

[newshound]

+1,

you won't have wasted much time on the old one before you will have justified that spend.

[Harry Bloomfield]

The mention of a buzzer suggests it is an after-market item, rather than the BMW specific item. Two items suggests one for road lights, one for the supplementary fridge, battery etc., usually a voltage triggered relay, switching on when the alternator begins charging.

The proper BMW item tests all of the trailer lighting and immediately reports faults on the dash, rather than just a buzzer for the indicators.

Nothing working suggests to me that the main 12v feed is absent or fuse has blown. The after-market unit uses relays and a separate 12v feed, to avoid loading up the lighting circuits and causing can-bus faults.

[Harry Bloomfield]

The mention of a buzzer suggests it is an after-market item, rather than the BMW specific item. Two items suggests one for road lights, one for the supplementary fridge, battery etc., usually a voltage triggered relay, switching on when the alternator begins charging.

The proper BMW item tests all of the trailer lighting and immediately reports faults on the dash, rather than just a buzzer for the indicators.

Nothing working suggests to me that the main 12v feed is absent or fuse has blown. The after-market unit uses relays and a separate 12v feed, to avoid loading up the lighting circuits and causing can-bus faults.

[Dave Plowman (News)]

What I sort of expected - not being a towing type, so never had to deal with one.

I'd expect the side light circuit to cope with the extra load since it is a small extra percentage? The brakes lights would be double, so a relay etc for that?

Ah - that's likely what my brother said on the phone.

Thanks Dennis. Vehicle wiring isn't one of my brothers best skills, So unless I can find identical replacements I'll likely try and work out what the old ones did and repair, once they arrive here. My brother doesn't do computers, and trying to get SIL to search for car bits can result in no supper. ;-)

[Dave Plowman (News)]

Oh indeed. But unless I can find identical replacements (which will be easier to do when I see them anyway) it might not be easy for him to fit a different one. And in this case cost is an issue as the car is near the end of its life. So not worth paying a pro to do it.

The Amazon one appears to handle the power side only, not the warning for the indicators. Perhaps that's why the two boxes.

[Chris J Dixon]

There is the complication that the car may well have been designed with sensing for its own bulb failure, so will not be happy if the load differs from what it expects.

Chris

[Ermin]

This sounds like a cheap aftermarket wiring job by whoever fitted the towbar. A proper job would have been to use a supplementary BMW loom which would not have any boxes or whatever to connect to the main car wiring.

It sounds as though either a fuse has blown in the power to the 'boxes' or maybe a relay has gone within the boxes.

As the car is near end of life and he has another already wired up I would suggest he does not waste time or money but simply use the other car for both trailer and caravan.

The only issues he may then have are (1) getting an adaptor if trailer is

7 pin and car 13 pin and these are cheap enough and (2) remebering to degrease the towball after trailer towing if the caravan has a built in stabiliser.

[Harry Bloomfield]

Dave Plowman (News) wrote on 01/10/2018 :

It is a matter of not upsetting the can-bus system. Most cars now use can-bus, to avoid much of the mass of wiring that a modern car would need. The can-bus switches the lights remotely using data and monitors whether they are drawing the expected current remotely. Add extra loads and the system can be upset. The extra load of a relay, would not be noticed.

[Dave Plowman (News)]

Ah - of course.

[Dave Plowman (News)]

Right. Although this model wasn't really designed for towing. Rear suspension was meant to be upgraded too. Did say at the time he'd have done better with a 5 Series Touring which has air suspension and made for the job. But in both cases manuals were rather rare in his part of the world.

Not a cheap job, but the usual cheap wiring. Scotchlock connectors everywhere.

The are a couple of external fuses which have been checked If it uses standard relays, I might well have some 'in stock' - or substitutes easily fitted.

'Other car' is the wife's - and kept for clean things. The trailer is used to take rubbish and junk etc to the tip. And so on. Of which he's been doing a lot recently, and not just for themselves.

Both cars are compatible.

[Dave Plowman (News)]

I'm not sure if this model uses canbus. Although my earlier 5 Series did.

[bert]

In article , "Dave Plowman (News)" writes

Towing kits can be pretty sophisticated these days especially when wired either to euro 11 pin or even 12n+s for a caravan. Mine packed up just within warranty due to water ingress in the socket - the cap hadn't been fitted quite correctly. They will depend on a voltage sensitive relay driven by the alternator output. Might be worth trying to locate and check that. Mine is a rats nest of wires and boxes in what is laughingly called a safe under the passenger seat.

[Brian Gaff]

Also of course, is the actual trailer wiring OK and the connector good and than cables that need to bend unbroken internally.

I'd expect items that flex or are exposed to the elements to be the first places to look. Brian

[Harry Bloomfield]

Dave Plowman (News) explained :

Scotchlock connection never last long, can never be relied upon. The slightest hint of moisture and the connection degrades and likely the wire tapped onto will break. Soldered and sealed is much better.

[Dave Plowman (News)]

Yup - spawn of the devil. ;-)

[bert]

In article , Harry Bloomfield writes

Just the existence of scotchlocks suggest a botched job, Cars have had plug in connectors for trailer wiring for years.

[Harry Bloomfield]

bert pretended :

Some have, some haven't. My car didn't.

It had its ECU's already set up to recognise lighting faults on a trailer, a main 12v feed ready to use and to add an extra few inches to its reverse sensor, for a tow ball, even partially bracketed for the bar, but an extra ECU had to be bought, which needed to be tapped into the existing wiring.

[fred]

ISTR buying a BMW 525 about 15 yreArs ago and it required special BMW bits to enable electrical connections to the tow bar. Local tow bar supplier wouldn't wire up the tow bar for me so I ran without for a couple of years.

BMW electrics cba be quite finnicky. A friend got a 'handyman' to fit a radio to a new 3 series some time back. She never got the electrics functioning correctly again despite the efforts of a very good BMW garage.