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MG MGF Technical - Unexplained Flat Battery

Over the weekend I suffered the syndrome probably best described as ‘Unexplained Flat Battery’. Car was perfectly OK the previous day but battery had only 9 volts the next morning.
There have been several threads on this in the archives.
I checked out the current drawn by the car while standing with ignition off and bonnet light bulb removed. Meter indicated about 0.9 amps which is really high. The source of the drain was traced by checking the current across each of the fuses in the Under bonnet Fuse box. No 2 showed up as the circuit with the problem. The circuit diagram indicates that this supplies the MEMS unit main relay, the fuel pump, starter relay and the oxygen sensor. Also a connection to the central door locking circuit.
The fuel pump was obviously not running as I couldn’t hear it whining.
The starter was obviously not running.
The oxygen sensor is supplied by the main MEMS relay so could not be on without the MEMS.
This left the door circuit and the main relay of the MEMS
The doors were locked and the current checked. Still the same.
The doors unlocked, still the same so suggests not the door locks.
So only thing was to dig out the MEMS relay unit which is behind the MEMS itself. It’s one of those units in a black plastic cover shaped like a wedge of cheese. It contains four relays. By the time I had dug into the unit to remove the connectors the current drain had disappeared. This happened before I removed any connectors which suggests there was an intermittent fault in the relay unit.
I opened up the relay unit and found that the main relay had very little available movement of the armature. In other words the movement from ‘off’ position to ‘on’ was minuscule. The movement is restricted by the normally open contact which is not used by any circuit. I tweaked the normally open contact so that it allowed a bigger gap and reassembled the unit. No probs so far.
In my past career as an electronics engineer I have come across cheap relays which stick on because of residual magnetism in the core and armature. I can only presume that this was a similar case. If it happens again I will have to design a circuit to check the quiescent current and give an alarm.

My question is – has anyone else experienced this same fault from this relay or am I missing something?
TIA
Bruce
Bruce Caldwell

MEMS doesn't turn off when the ignition is turned off - it may be several minutes later. If the current drain is reasonable after (say) 20 minutes, then maybe you have a battery fault.
One of my other cars had an intermittent battery fault - which I didn't think happened. It failed to start the car one night so I jump started it. Next morning on a 'scope the battery was perfect. About five days later though, same thing happened and I could see one of the cells fizzing. The replacement battery was a mere 26 UKP from an autobits shop

Steve
Steve

Steve,
The MEMS stays on for 30 seconds normally after switch off. If the engine compartment temperature is high this can extend up to 8 minutes I believe (so it can run the fan) but in this case the car was stone cold. Several attempts were made to start the car and then ten minutes later or so I began my measurements so the MEMS should have been off by then.
Battery looked OK with all cells at normal level of acid. All operating OK now, at least it was when I came to work this morning.
I reckon the main MEMS relay was stuck on and when I began tampering with it the vibration was enough to make it drop out.
Bruce
Bruce Caldwell

Bruce,
Will you be Replacing the Relay or Just see how it goes??
Mark.









MARK PERKS

Alright Bruce, have had similar problems with 2 or 3 mgf's in the past and the result was a new mems ecu,due to the fact that it was not powering down after ignition switch off,and i seem to remember 1 of them correcting itself after an ecu self check on the Test book. hope this helps
Dave
dave mgf tech

Mark,
Won't replace the relay yet because all is working correctly at the moment. One glitch by the MEMS or the relay in five years isn't a bad record.

Dave,
I did wonder if the ECU had not powered down after the 30 secs interval. When investigating I first removed the battery negative and measured the drain. So that meant I had totally disconnected the MEMS from the supply for a minute or so. Could it have reset itself in that period? When I reconnected via the ammeter the drain was still there, about 0.9 amps. It only dissappeared when I began knocking the relay unit about when I was struggling to get it out from the back of the MEMS. I did measure across the coil of the relay after I replaced the relay but of course by then the fault had gone. I still reckon the relay had physically stuck on. If it should happen again I will wire in an alarm light to show when the relay is supplying the MEMS with the ignition off. I should then be able to see it go off after 30 secs under normal circumstances.
Thanks for your input. Are you an MGF technician by the way?

Bruce
Bruce Caldwell

Bruce - your problm is EXACTLY the same as the one I had fairly recently, and your diagnosis of the relay pack problem likewise almost certainly correct. Solution: find a breakers, get the relay pack from a dead car, and you're sorted.

One note of warning though: there seem to be several different relay packs available, so be careful. Dieter and I had a lengthy discussion of this quite recently, and the correct part number should be on that thread somewhere.

Alternatively, you can get the pack from MG/R, of course.

There was someone else here who fied theirs by wirig in an aftermarket relay to the pack: the £30-odd for the complete pack didn't seem a worthwhile saving to me though.

HTH

E

PS. I'd do the above before going as far as swapping out the ECU though (no offence Dave)
Ed Clarke

Thanks Ed,
I just reworked the relay and will see how it goes. If it fails again then I will attempt to study the relay in more depth because there is no reason why it shouldn't work properly every time if set up correctly.
Cheers
Bruce
Bruce Caldwell

Bruce, yes i am for a dealer in Bromley,and Ed none taken i'm here to learn too.
dave mgf tech


Hi all,
if You can get hold of a soldering iron it is much cheaper (and probably MUCH more realible !) to just exchange the offending relay of questionably quality with a normal 30 Amp. standard car relay... Using the old relay´s socket and some short pieces of cable to the new relay will solve the problem. Did You see any "spot-welding" at the used contact pair ?

Best Regards , Carl.
Carl

Bruce - there seems to be nothing at all wrong with the relay-pack I removed from my car: all the contacts are fine, and as far as I can tell the board and components are OK. The swap still fixed the problem though.

E
Ed Clarke

I couldn't see any pitting on my relay contacts either but my eyesight is a bit shot! If it happens again I will check the contacts under a microscope. In the mean time I am working on a residual current detector to show me when a current greater than 50 milliamps is being drained from the battery with the ignition off. Another warning light!

Carl,
Problem is that there are four relays in one enclosure. Perhaps it would be best to make a totally new unit with four relays since they all appear to be the same specification!

Bruce
Bruce Caldwell

Be aware that the MEMS stays active for some time after the ignition is switched off: a feature that is used to good effect to modify the wiring to the electric windows so that you can put them up for a while after the engine's off (you'll have to find that one in the archive for yourself - I keep meaning to do it to my car!)

I think its so that the engine bay fans can run if its particularly hot (can't think of any other reason...)

Neil.
Neil

Thanks Neil,
Yes, I was aware of this feature. See earlier in this thread. I wasn't aware that there was a mod to make the windows work after the ignition is switched off. That's a good idea. The number of times I've switched off, taken the key out and then remembered the windows are down!
I will look in the archive.
The detector I am making will show me when the current is over a certain level after the ignition is switched off. This means that I will have to remain near the car until the 30 seconds (or whatever) period is over to observe the warning light. If it remains on then I can perform some tests as to the cause, such as giving the relay unit a wack with a screwdriver to see if the offending relay drops out. I don't intend this detector to be a permanent fixture.
Cheers
Bruce
Bruce Caldwell

Neil,
I found the thread in the archives on Window Relay - Wiring Modification but the address given for the article (by Carl in Sweden I think) is no longer working. Do you have a copy of the details on how to wire in the two diodes for the window mod? Or is there a copy of this mod on Dieter's site?
TIA
Bruce
Bruce Caldwell

Neil,
Cancel distress signal! Just found it on Dieter's site.
Ta
Bruce
Bruce Caldwell

This thread was discussed between 28/08/2001 and 02/09/2001

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