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MG MGF Technical - MEMS Problem: Help

Hi. I am in the need of expert advise again.
My F -1997 1.8 MPI, 24K only- has been experiencing some intermittent engine failures -engine stops- but would start again after a few minutes wait. Sent to the garage was kept running for a few hours before the engine stalled and then would not start again. They tell me that the MEMS is not sending signal to the ignition coil. I would like to know if that kind of failures are frequent, and if the MEMS can be fixed or will be necessary to replace the unit.
Mike
Mike V


Hi Mike,
worth checking the following at the garage;
The primary winding of the ignition coil(s) sometimes get short-circuited between a few windings. This makes the ignition coils internal resistance lower and puts bigger strain on the power-switching transistor in the MEMS unit. Regardless of type and manufature these are more or less always power-FETīs.
They behave in a different mode than original transistors. A typical silicon transistor gets too hot and finally dies, the power FET "closes down" when getting too hot BUT usually comes back to life when cooled down! As you can imagine this can give some very irritating behaviour when driving the car,i.e. works fine for a few miles ,then shuts down and after waiting some time for the rescue it is cool enough to be started again !
Resistive measuring on the windings of the coil during gently heating the coil with a warm air gun can possibly tell that the coil is faulty.
Of course there can be some other fault inside the MEMS ,but first sudpect the coil and fastenings for cables etc.

BR, Carl.
Carl Blom

Carl,
As you described the problem -"works fine for a few miles, then shuts down and after waiting some time for the rescue it is cool enough to be started again"-is exactly what has been happening to my F. However at present the trouble is that the engine turns but would not start again. Garage told me that there is no output from the MEMS to the coil (?). I dont know if the MEMS can be fixed or is to be replaced. But a question arise: may you be right, could a faulty ignition coil damage the MEMS again? And if so, wouldnt be better to replace the coil too?. I will appreciate your advise most. Mike.
Mike V.

hi Mike,
usually this kind of "try and error" in replacing parts can be very costly in the end, but on this one it might be worth checking with a new coil.
However,IF it is an internal fault within the MEMS unit the garages usually dont fiddle with them = it is a swap for a new one.
For someone with knowledge in automotive electronics it is not impossible to get a MEMS unit back to life.
Take a look at Dieters Tech. site for more info and see an opend up MEMS unit where the output drivers can be seen around the rim of the box.
A garage with a Test-Book including Tool-Box access can always re-program any MEMS -unit to fit your 1.8i F. A swap without re-programming will NOT work...
Hope you get it fixed with minimum fuzz,
BR, Carl.
Carl Blom

In case it is not clear, Carl is saying that the MEMS unit may be fine, and the problem may lie with the coil. getting (a) second hand coil(s)* is very cheap when compared with a new MEMS/reprogramming

Will
* later cars and VVC cars have two coils, early cars have one coil and a distributor.
Will Munns

Carl, Will.
Garage told me that they had already replaced the coil to no effect as the engine would not start. They also told me that will send the MEMS to a "lab specialized in car computers" to try to fix the problem as a new unit might be very expensive.
You said that "a swap without re-programming will NOT work". Do I get it right that even buying a new MG MEMS will not correct the problem? Mike
Mike V.

Same thing has been happenning to my F although not that often but always after a long run and with the oil temp reaching 120°. After reading your experience I am now quite worried. I wonder why this is not happening more frequently as in the F the electronic control unit is literally cooking at 100°C within the engine bay. I believe that in other cars the electronics are located within the passenger area. And most relevant would be to learn how to detect a failure that will happen only at high temp and later reverse to normal?. Test-Book in my car has been reported as normal so far.
Richard James

Don't know if this helps, but I had a problem very similar to this and it turned out it that one of the wires underneath the enginge management system was caught. The thing finally stopped completely when the wire finally broke.

Whilst I don't ever believe 100% anything a mechanic says, I've had no problems in the last 6 months since they fixed it.

just a though..
Elisabeth

>I wonder why this is not happening more frequently as
>in the F the electronic control unit is literally
>cooking at 100°C within the engine bay.
It isn't 100degC and 100degC is not that hot.

>I believe that in other cars the electronics are
>located within the passenger area
I cant think of any cars where this is the case

>And most relevant would be to learn how to detect a
>failure that will happen only at high temp and later
>reverse to normal?
You can buy freeze spray whick could be applyed to the circuit board to see if this cures your problem, I'm not sure if there is enough slack in the loom to pull the ECU over and into the boot, which would cool it a little.
Will Munns

It would had been a great idea before Will, but it is too late now as apparently something definitely burned inside the MEMS and is no longer a matter of let the thing cold. So can not be of use it at this stage. At present the car would not start at all. Mike
Mike V

Mates, I would like to know what is involved in the reprogramming of a new MEMS 1.9 -may the need arise for my F-. Why is it necessary? Could it be done outside the car or needs to be installed? Could it be done somewhere else if I send them the car code? Is done with the TestBook or needs some other tool?
Thanks. Mike
Mike V

>Why is it necessary?
Just a matter of making sure the MEMS is tied to the 5AS immobalizer

>Could it be done outside the car or needs to be installed?
You need the 5AS unit, which is a bugger to remove, and a 'wireing loom' so much easier to do it in the car

>Could it be done somewhere else if I send them the car code?
I've never heard of it being done without the 5AS unit itself.

>Is done with the TestBook or needs some other tool?
Yes


You could avoid this by buying a Emerald ECU, then you get to connect your OWN PC!!!

Or you could get the MEMS programmed to not need an imobilizer (as yours is an early car), this could be done out of the car. You would keep the alarm functions, just not the passive imobilazation
Will Munns

Without being technical at all, I know someone who experienced the same symptoms on his F. It turned out to be some bad wires behind the radio head unit. Some of them had been damaged by the back of the unit, resulting in the same symptoms as you've described.
Erik

From Will:
>You could avoid this by buying a Emerald ECU, then you get to connect your OWN PC!!!
How much would that cost, compared to a MEMS replacement?
>Or you could get the MEMS programmed to not need an inmobilizer, this could be done out of the car. You would keep the alarm functions, just not the passive imobilization
That sound interesting Will. Would you please send details on the how to...
From Erik:
Yes, I was aware of that possibility and was checked...but unfortunately no. MEMS is gone.
Many thanks indeed mates. Mike
Mike V

Sorry to be insistent but, could this be done?. If so, how?:

>Or you could get the MEMS programmed to not need an immobilizer (as yours is an early car), this could be done out of the car. You would keep the alarm functions, just not the passive imobilazation
Mike V

This thread was discussed between 12/03/2004 and 17/03/2004

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