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MG MGF Technical - Battery Draining

My 1997 VVC has developed a problem where the battery drains if the car is left standing. Doing some investigation with a meter reveals that with everything off - doors shut and bonnet light disabled a currrent of ~ 1.2 amps is being drawn. On extracting fuses from the fuse box to determine which circuit is causing the drain I find it is caused by the circuit labelled ECU (engine control unit I assume). There are 2 fuses with this label. The one that is suspect is on the left.
Has anybody any ideas on what might be causing this current drain. The car starts and runs OK. The alternator seems to be functioning normally with the correct charging voltage.
I understand the correct reading should be around 100mA.

David Fudge

I beleive there is a noticeable drain on the F's battery even with everything disabled, though TBQH I don't know what would be regarded as a excessive drain. However, it does seem fairly common for the standard battery to slowly fail with such symptoms - the exact same thing happend to me after about 2 years from new. Personally, I would suspect your battery of 'ageing' and being in need of replacement.

Home of the F'ers Gallery, MG Dealer Guide and forthcoming MGF FAQ. :)
Paul Lathwell

Hi David,
some time ago when doing the "delayed window circuit" together with Dieter I was a bit curious about the diff. times it took for MEMS to shut off.(See Dieters Tech. site ) When talking to a technican who really knows the system (Yes - they do exist !)he told me that some MEMīs units had been found faulty in a way that they never shut down , i.e. doing the calculations all the time . On some units a total reset cured the symptom. This is a thing that normally goes undetected, driving the car every day doesnīt drain the battery enough to make it hard to start the next day.. When taking the measurements be sure to wait at least 10-15 min. to be sure that this is the problem. By then a normal MEMS unit would have gone to rest !

Regards , Carl

The best solution I have found which is what the Elise owners use is to buy a solar car battery charger which sits on the dash inside the car a recharges the battery for free. do a 1.8w Exide one for 30 including posting and VAT.

1.2 amps is quite a bit. Just doing the basic math, with a 60amp/hr battery in tip top condition, this will be drained in 50 hours!
I know for sure that my ECU/MEMS is on the upper limit of the power down period at 15 minutes.
As a test, you could stick a meter into the brown/pink wire's connector at the purge valve (in the engine bay).
The area is well documentated on Carl's MGF Site "The delayed power signal article"

Note the time taken following ignition shut off for the meter reading to fall to zero. If it doesn't, then Bobs your uncle (and a builder), you've found your problem. If this is the cause, hopefully a total reset will cure it.

Ask Carl (above) for a link to his site.
Paul Lane

Just seen this thread and am currently having a problem with my battery draining during the week when the car is not used. It first did this about 3 weeks ago and the AA said my battery was defective and sold me a new one. The following week the new one was as flat as a pancake. Out came the AA who said the car was draining 0.27 (amps/ volts?) when everything was off when it should have been draining 0.03. They went through all the fuses pulling them all out one at a time but this seemed to make no difference to the drain. They suggested I go to my local Rover dealer. Unfortunately I have no confidence in them at all. Any ideas? Is this the same problem as is refered to above?

marc hanson


Try a different dealer?....

MG Dealer Guide at........
Paul Lathwell

This is the third dealer Ive dealt with. Theyre rated as excellent (by me I believe) on Paul's website but that was a while ago,; for the last 10 months theyve been s**t. Oh well, on to my fourth dealer in 5 years....
marc hanson


I have a similar problem on my 1998 VVC. Every 6 months the battery dies over about a week. It won't even charge - it is dead! The car has now done 33,000 Kms. The dealler keeps having bright ideas (the last being replacing the leads on the alternator wires!). I get (insist on) a new battery every 6 months - I'm now on my 5th. The only hassle was last time I was "caught out" a distance from home (so the car was trucked from to the dealer under RoverCare which I still subscribe to {wonder why?}).

The car runs perfectly, even with a dead battery. There was an early theory that when I had the seat lower on purchase (They make us big in Aussie) a cable was "crimped" but that was replaced.

Just one of those idiocyncratic joys of owning an MG - and I get a new battery every 6 months!
John Baragwanath

I started this thread so I will provide an update. Following up suggestions on looking at the ECU I monitored the current drain through the ECU circuit and at the same time the state of the "delayed power off circuit". This later stayed on for a good 10 minutes after everything was switched off and then turned off with an audible click. However the current drain through the ECU remained at ~1 amp. I therefore assumed that the ECU required a Hard reset as a previous contributor had suggested.
The Car is now at "Diamonds of Hatfield" - they have not found any problems with any of the electric circuits and report normal current drain measurements. (Ģ65+VAT for NO Fault found!). I suspect an intermittent condition within the ECU and will carry out further investigation when I get the car back.

AS a tip over the last week whilst trying to sort this out I have been removing the 30amp ECU fuse (Far left - facing the windscreen) in the fuse box at night. This has stopped the battery going dead and leaves the alarm circuits, radio etc in play. It is also a lot less messy than disconnecting the battery. Sticking an Ammeter across the fuse terminals with the fuse removed is also a good way of measuring what each circuit is taking in current.

David Fudge

Thanks for the update.
You say Diamonds report "they have not found any problems with any of the electric circuits and report normal current drain measurements".

Have they done the ECU reset, or are they giving you the usual Dealer treatment (bloomin customers, they haven't got a clue, tell 'em something you think they will believe, then they'll go!).

You measured a high load after mems power down (~1A). In "low power mode" (after power down) the ECM sould consume less than 1mA (about the same as an analogue clock on a Ford (?)). This figure from the Workshop Manual.
If you can measure it, why can't your Dealer (although I believe it can be difficult making accurate electrical readings from the crewroom with a mug of tea in your hand).
The F is a fine car if you can match it to a fine Dealer. Unfortunately, a great number of us out here can't!

Keep us posted.
Paul Lane

This thread was discussed between 28/02/2001 and 07/03/2001

MG MGF Technical index

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