P38A How are the tails lights powered by the BECM?

MrGorsky · Feb 4, 2017

Evening all,

Does anyone know how the tail lights are powered from the BECM? I've read RAVE on the subject, and whilst it does a good job of telling me what wires go where, there is no real info on the internal workings...

The reason I ask is that I'm looking in to towing my P38 on an A frame, and if I do do that, I need to turn the car in to a "trailer"and make the fitted lights work from the towing car. I think I could power the tail lights, brake lights, fog light, and indicators by just connecting the towing vehicle up to those lights through the trailer socket.

The bit I can't find out about though is, if I'm powering those lights with 12V from the towing vehicle how will my BECM react to having 12V applied to it's outputs (from another vehicle)? Will it just do nothing, or will I blow some internal component and fry the lot.

Also, would putting some diodes in the wires protect it all?

Fergie46 · Feb 4, 2017

Much easier to just get a lighting board on a curly lead and fasten it on the back of the P38 when your towing it. It'll have the number plate from the tow vehicle on it also.
Recovery guys use them all the time.
Hope your towing vehicle is suitably robust, Range Rovers are heavy.

MrGorsky · Feb 4, 2017

That would be much easier, but unfortunately the law says I can only do that if I'm recovering a broken down vehicle.

If I use the P38's wheels, then the law sees the P38 as a "trailer" and any light fitted to it have to work. So I can't use a light board.

The other option would be a car transporter trailer, but they are expensive.

The towing vehicle is suitably robust...

Grrrrrr · Feb 5, 2017

@martyuk might know.

could open up the spare BECM and try to see where the plug goes. Would it need a MOSFET for that sort of amps?

Grrrrrr · Feb 5, 2017

Just thought. Storey Wilson: would he have come across this and put anything on a site?

Saint.V8 · Feb 5, 2017

Get hold of some rear light cluster wiring connections from a scraper or other source......make up a fly lead with suitably long tails to travel the length of the P38 and down to the tow vehicle electrical tow socket.....

When you tow the P38, disconnect the P38 wiring loom to the rear light clusters and substitute your new loom and connect to the tow vehicle.....this will bypass the P38 wiring and be run direct from the tow socket.

Then Robert is your mothers brother!

MrGorsky · Feb 5, 2017

There is a parallel connection to the trailer socket from the output that powers the rear lights, (not the indicators) so I was thinking I could just connect to the trailer socket and power them through that. Backwards if you will.
I'm just not sure how the BECM will react to being powered from its outputs.

I think diodes are the way forward.

MrGorsky · Feb 5, 2017

Saint.V8 said:
Get hold of some rear light cluster wiring connections from a scraper or other source......make up a fly lead with suitably long tails to travel the length of the P38 and down to the tow vehicle electrical tow socket.....

When you tow the P38, disconnect the P38 wiring loom to the rear light clusters and substitute your new loom and connect to the tow vehicle.....this will bypass the P38 wiring and be run direct from the tow socket.

Then Robert is your mothers brother!

That is a really good idea Ant.

Doo · Feb 5, 2017

Ermm, if you fasten a trailer board tot he car,plug it into the towing vehicle, then those are fitted, albeit not permanent.

How do you think trailerboards are fitted? I only ever use a trailerboard hooked onto the car with bungee's and I have never been pulled up for that!

In fact, I was towing a cabin with my van, got pulled over by plod and take to VOSA station. There, they found the fixed fitted lights stop bulbs weren't working (which was funny as they had been when plod stopped me and they agreed this was the case) so I received a prohibition stating they needed to be repaired.

I went into my van, retrieved a lighting board with LED's, fitted it to the cabin and tested it. I told the VOSA bloke, he came out, checked the lights "fitted" were now in fact working, and took off the prohibition (I still have the paperwork).

So, my point, unless the law has changed dramatically in the last year, I say you ARE allowed to attach a trailerboard, as long as it is secure and fully functioning, you should be fine.... After all, if it's on an "A" frame, it is now a trailer

Edit, I wonder are you getting confused with all driven wheels MUST be braked? You eed a hydraulic contrivance for that...

wammers · Feb 5, 2017

It depends how much function the car has. But if you are going to tow it any distance on an A frame remove the rear prop shaft.

MrGorsky · Feb 5, 2017

I might be confused Doo. Wouldn't be the first time! As I understand it, if lights are "fitted" then they have to work. Therefore the lights would have to work, in addition to the ones on the board.

For the brakes I'd use one of those Brake Buddy things. The brake buddy would also take care of the brake lights come to think of it.

pwood999 · Feb 5, 2017

How far are you towing the P38 ?? If you literally are "recovering" the vehicle a short distance, then you will probably get away with it, including using a trailer board for rear lights. See here. Towing vehicle must have brakes capable of stopping the combination vehicle including trailer. If it's for A-B travel, then recovery rules don't apply, and the combo must meet all the usual trailer regs.

Personally I wouldn't want to tow a P38 using an A-frame. I rented a decent vehicle trailer locally for about £55 when I collected my 2nd P38 from the seller, which was 78 miles each way. Noisy trailer on the way there, but handled the P38 perfectly on the way back.

Fergie46 · Feb 5, 2017

I'm with Doo on this. Rig up a trailer board and go for it. DVSA won't know the car hasn't failed. Wammers certainly has a point, if your cars an auto unless of course you put the transfer box into neutral each time.
Come to think of it, you don't see 4x4's dragged around usually they are clear of the tarmac on another vehicle or trailer.
Or you could simply remove the rear light clusters each time you tow it, gets round the lights problem and it will look like it's sorned.

MrGorsky · Feb 5, 2017

I'm planning on towing it all over the USA and the then Europe. Behind an RV.

You can buy a specific A frame kit for the P38 in the US. I'm just looking for a simple, reliable, and quick way to connect it all together without me having to roll around on the floor, or get spanners out!

Selecting transfer box neutral seems to be the preferred method for disconnecting the gearbox from the wheels.

tarphenry · Feb 5, 2017

What about cutting the supply to the rear light and fitting a plug then before you tow, disconnect the plug and plug in a cable supplied by your RV

MrGorsky · Feb 5, 2017

Saint.V8 said:
Get hold of some rear light cluster wiring connections from a scraper or other source......make up a fly lead with suitably long tails to travel the length of the P38 and down to the tow vehicle electrical tow socket.....

When you tow the P38, disconnect the P38 wiring loom to the rear light clusters and substitute your new loom and connect to the tow vehicle.....this will bypass the P38 wiring and be run direct from the tow socket.

Then Robert is your mothers brother!

tarphenry said:
What about cutting the supply to the rear light and fitting a plug then before you tow, disconnect the plug and plug in a cable supplied by your RV

I think this might be easiest way. All the rear lights are fed from connector C325 on the BECM. If I can find the same type of connector for that, I can just disconnect from the BECM and plug in to a lead I make up being powered from the RV. Seems the neatest way to me, no cutting of the original wiring, and I can put it all back to normal when I don't need it any more.

Thanks for the ideas though chaps. Hive Mind FTW!

Saint.V8 · Feb 5, 2017

tarphenry said:
What about cutting the supply to the rear light and fitting a plug then before you tow, disconnect the plug and plug in a cable supplied by your RV

Thats what I bloody said

MrGorsky · Feb 5, 2017

Sorry Saint. Amended!

I've got an old BECM somewhere I can steal the socket out of. The rest should be a walk in the park right?

Saint.V8 · Feb 5, 2017

Nothing is a walk in the park!

Connecting to the connection of the BeCM is OK....but are ONLY the rear lights fed from that?

I would make connections directly to the lamp clusters via their plugs/sockets that way you know you are only connected to the light cluster and nowt else....

As long as you are sure C325 only feeds the lamps then good job, check and double check....

Your plans sound like fun, start an instagram account and post often.

MrGorsky · Feb 5, 2017

Mostly the rear lights....

The only thing that might worry me is the alarm RF receiver power, but that's probably fine....

P38A How are the tails lights powered by the BECM?

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