[Flynn]

Hello all, I've been lurking on this forum for years but today I'm getting stuck in.

Quick bit of background to me I suppose - I've been a Land Rover man for a long time, I've had a 90, 2 110's, a Range Rover Classic that I sold when they were still worth nothing (it seemed like the only option I had at the time!) a 200Tdi Discovery, one other P38 and a TDV8 L322. I am now working through the issues on my current vehicle, a 2001 4.0 HSE P38.

I bought the car dirt cheap knowing it had a few issues but fundamentally it was a solid, well specced car. This brake bulb message has been present since I took ownership. I have fixed almost everything else that was wrong and now I have got to this item on my list!

So, everytime I cycle the ignition, with the first press of the brake pedal, I get a BEEP and a message on the dash that reads 'LH Brake Bulb Failure'.

First check - Do the brake light bulbs work correctly - Yes, they are all operational.
Second check - Quick visual check of the harness and connector block on the LH side light cluster - bit of a mess as there is an aftermarket park distance control box fitted which has an earth connection tapped in to the earth wire for the LH light cluster. Other than that nothing obviously wrong.
Third check - Swap bulbs side to side - No change. Swap bulbs for a brand new pair I had in my tool box - No change.
Fourth check - Thorough clean of the bulb holders and connectors, making sure connections are firm - No issues observed prior to cleaning and after cleaning there was no change.
Fifth check - Notice I have a genuine LR towbar with electrics, dig out a tow socket checking tool, find everything works fine. To rule it out though, I find the the harness plug in the boot by the RH rear cluster that connects the towbar electrical connector to the rest of the harness and unplug it. - No change.
Sixth check (starting to swear now) - Remove boot carpet and trim panels to expose rear light wiring harness on both sides of vehicle, inspect it for any damage, make sure the earth connections to the body are good. Get annoyed just looking at the park distance control box and remove the tapped in earth wire (will tidy all this up later as the park distance thing actually worked really well but it looks like it was installed in a rush!) - Can't see anything wrong and makes no change, still get the LH brake bulb failure message.
Seventh check - check current draw on the left and the right brake bulb wiring (my thinking here is that the failure message is brought on by a difference in current draw on the LH and RH brake bulb circuits - please correct me if I am wrong - I am not a trained 12V elec and in no way an expert on the BeCM, just a general car mechanic that has an interest!) - LH is 1.58A, RH is 1.56A - would such a small difference bring the message on?
Eighth check - Turn my attention to the BeCM, find the connectors and the pins responsible for supplying power to each bulb and just have a quick visual check for any damage/corrosion etc. - nothing to report and no change.

So, that brings me to where I am now, still with a false LH brake bulb failure message, do any of you have any ideas what I could try next to reach a diagnosis?

Any help appreciated.

Many thanks.

 

[95classiclwb]

the bulb out message is triggered by lack of resistance across the burning filament of the bulb. in my opinion if all works well, I would consider installing a resistor of bulb value as a test. if it works then I may consider leaving in situ, the only draw back you will never be advised of bulb failure again.

the other solution may be repairing the possible failure of the becm circuit, which does not recognize if the bulb has failed or not.

 

[Bolt]

+1 on 95classiclwb's response.
Just wanted to complement you on excellent fault finding write up.
I would say that if the issue was with anything but the Becm detection circuit, then you would have found it....... The only thing you have missed is completely removing ALL the wires from the park control box, as current loops can still occur even without an earth and are a bugger to track down. If it was installed poorly, it could also have been installed incompetently (Read that as "Wrong") and could be somehow confusing the Becm.
If not, You are lucky that Marty is over there in your neck of the woods!

Possibly the admins would consider adding your troubleshooting write up to the electrical sticky, as it is not only a good guide to any light bulb issues, it is a good logical progression to any kind of fault finding. (Adding in the bit about removing the boodged Park warning system, of course)
Good luck!

 

[Flynn]

Hi and thanks for the replies.

95classiclwb, with nothing to lose I'll certainly give it a go! Last time I did any sort of electrical engineering was high school (which seems like such a distant memory now!) so help me out here, what size resistor should I try? I'm desperately trying to recall my physics lessons where I might've learned this... V=IR seems familiar so R=V/I?? 12V/1.56A??... 7.69, call it 8ohms?? As I say, help!

Bolt, thanks - Earlier today I removed the only other wire from that park distance control box and unfortunately, no change to bulb failure message. Whilst I have pretty much all of the boot trim out I will find somewhere tidier to install the box, whoever installed it just sort of stuffed it behind the left hand rear light cluster, took a power supply from the reverse light wiring in the tailgate and an earth from the LH rear tail lights.

In better news, I decided to have a stab at fixing my drivers heated seat function which hasn't worked in all the time I've owned this car, I had taken the seat out for better access to the BeCM anyway so with it just sat there I got to work, I've found the seat back heating element is broken so I'll get that repaired and back together and then I'll have atleast one thing fixed from this weekend!! Just in time for summer too :???:

Thanks again.

 

[95classiclwb]

Flynn, normally I take a new bulb and measure resistance and based on such I locate the resistor needed. on my P38 I had similar issue, it turned out to be the caravan (trailer in the us) harness was some how defective, I ended disconnecting from the inside connector next to the tail-light.
currently, I am not towing thus don't have a need for the connector. I have not traced the failure for needed repairs. for now my brake light out message is resolved. just another hunch.

 

[Flynn]

Hello again,

95classiclwb, I tried disconnecting the tow bar electrics from that same plug but to no avail. I've tried alsorts wired in series with the LH brake bulb but the message remains. I assume this confirms a fault with the BeCM? Anyone reading this with good knowledge of the BeCM please get in touch!

On a more positive, the rear trim panels of the car and the drivers seat got the cleaning of their lives whilst they are out, they're like brand new!

 

[95classiclwb]

there is a member from the uk, Marty-Nz. he fiddles with BECM among other things, I know I'm being general as he can be geographically opposite from your location but he is on your side of the pond.

 

[marty_nz]

I'm based in Swindon, but do a lot of BECM bits via mail order.

It could be an issue on the power board of the BECM - maybe there's a bit of moisture that has been in there and now a bit of surface corrosion which is causing there to be just enough of a connection to the error warning circuit for that particular lamp. Or maybe the MOSFET (solid state switch) for that output has gone faulty and reports a problem when there isn't one.

I'm away until the weekend, but happy to take a look at some point for you if nothing in the back end of the vehicle seems to sort it!

Drop me a PM on here, or email me directly on [email protected] if it's easier, and we can work something out. Where in the UK are you based?

 

[Flynn]

We have a successful conclusion!

Sent my BCM to Marty_nz who found and replaced a faulty resistor. All sorted now, no more beep.

Thank you all for your help!

 

[95classiclwb]

wonderful thanks for the update, this could help other members in the future.