Classic cars forum & vehicle restoration.

[Ray White]

I have a battery that reads over 12 volts but when I connect it to the car (MG TC) it only reads 9.3 volts.

I am at a loss as to where the missing 3 volts are going because the engine is earthed with a new heavy duty metal strap to the bulkhead only about a foot from the similar battery earth cable bolted to the steel battery box which forms part of the bulkhead.

The battery cables are both new and tight.

What am I to do please?

[ukdave2002]

Are you are saying that the voltage at the battery drops to 9.3v when connected to the car?

If yes and the battery is good, disconnect it very quickly because it's the sort of volt drop that a load similar to turning the engine over would create, so something could be getting extremely hot!

Otherwise it could be a duff battery.

Dave

[Ray White]

Yes, the battery reading drops from slightly over 12 volts to 9.3 volts when I connect the leads to the battery terminals. Nothing seems to be getting hot so I suppose - despite the reading - that the battery has come to the end of it's life.


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I have given the battery a charge and it seems to have stabilised at 12.6 volts across the terminals with the cables attached.

[Ray White]

I have been unable to trace the battery drain in the P38, so today I have booked a 2 hour session on 10th April with our local auto electrical specialist.


The car is a 2000 model Range Rover Vogue 4.6 petrol engine special edition and while the overall condition is quite good, it will soon need a bit of T.L.C. however until this problem is fixed I expect the car will be quite tricky to sell.

It may go to auction.

[Rusty]

Does it "arc" when you connect the battery?
Does it earth to the engine or the chassis?, if it has a strap running from the engine to the chassis to complete the circuit, I would be looking at a bad connection there, or if someone has done a job like say a clutch, the earth may have been accidentaly left off.

EDIT, Re reading your original post I see you have already checked !!!

[Ray White]

There is a reluctance for anyone in my area to touch these cars. One Land Rover specialist said they stopped taking P38s years ago because - as he put it - "they are trouble". He did suggest that the problem may be corrosion where a computer is attached to the body of the car.

If the problem turns out to be the BECM then it's probably curtains for the car unless I can get another good one (unlikely) cheaply.

I did once have a similar problem on my Triumph GT6 that I traced to a disintegrating fuse holder... but, of course, they are much simpler cars and a joy to work on.

[ukdave2002]

Hi Ray

I think you are going down the correct route getting an electronics specialist involved. I have long since thought that the reason vehicle electronic problems get a bad rap is because we frequently rely on mechanical fitters/ technicians to diagnose electronic faults! You wouldn't expect your local garage to fix a TV or Laptop!

Most vehicle electronic problems are connector or exposed sensor related, the actual electronics rarely goes wrong. If the P38 electronics are a big issue has nobody come up with a cost effective Raspberry Pi based type retrofit solution? much in the same way that MegaSquirt has with EFI.

[Ray White]

[ukdave2002]

[Ray White]

[Vintage Fly Guy]

If you suspect an ECU type issue then it might be worth having a word with this company: ATC Drivetrain UK Ltd, CANNOCK WOOD INDUSTRIAL ESTATE, Cannock Wood St, Cannock WS12 0PL. Unfortunately, their website seems to be down at the moment (perhaps they're updating it or something?) www.atcdrivetrain.co.uk but a Google search should find a phone number for them.

I don't know if they do ECU work for the P38 model, but have used them a couple of times to test the 14 CUX ECUs on Range Rover Classics and have found them to be very honest (both tested OK and did not need repair) and not out of the way price wise. I've also had them refurbish and 'future proof' a spare ECU for my RRC, which came with lifetime warranty if it ever does fail in the future. They apparently also do repairs on LCD instrument displays on classic cars (such as those on the Vauxhall Astra, etc.). So a handy firm to know.

[Ray White]

I will see how it goes with the guys I am booked in with. If they can't find the leak your suggestion might be the answer. They are about an hour from me so that's 5 gallons of petrol (about 30 quid) to add on; assuming they can do it while I wait.

[ukdave2002]

Having spent 40 years (yes I am old and remember the 80's ) fixing electronic stuff and to be clear my hands on ended about 30 years ago!, however managing it gives a broader visibility, I would say that the average component failure cost is <£1....the time to diagnose is the real cost, hence you need to involve the correct people!

Dave

[Ray White]

Possible locations include the tailgate and central locking.

It could also be connected with the electric seat adjustment.

Any interior lights.

Indeed, anything that can be operated without the ignition switched on.

[Ray White]

It took the electrician a full two hours but Carl has found the source of the battery drain. He removed all the fuses (about 50) and checked to no avail. He then noticed two extraneous fuses tucked down beside the battery which were for the LPG conversion; a 2 amp fuse showed a significant drain .

To be honest, I had completely forgotten about the LPG. There is a faulty module in the system that was causing the problem. I cannot get LPG anymore so it's disablement is not a problem.

I am greatly relieved but 150 quid down.

I would not have found the leak as I had been looking in the wrong place.