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Old Sun, Jul-08-2012, 01:11:22 PM   #191
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Default Re: DIY: Resistor Mod To Cure Summertime SMG Blues

Why are folks (a) using a huge 10W resistor, and (b) why are they fitting it in an exposed location where corrosion will cause a failure?

The resistor will have 12V across it, perhaps 14V, to be on the safe side, so even without the temp sensor in series, the power dissipation of a 1K resistor will be 14*14/1000=0.2W. Therefore you can use a very small 1/3W to 1/2W resistor.

The best place to fit this is in the electronics compartment next to the SMG control unit. The white wire mentioned in this thread goes to pin 21 of X53004.

Such a small resistor is ideally fitted there by cutting the white wire, sliding some head shrink sleeving up the wire, soldering the resistor in place, and then sliding the sleeving over the resistor and joints and shrinking it.

This will be a much, much neater fix and totally weather resistant.

My SMG box is playing up, reporting “Engine MIL on at request of SMG” from the DME and “Hydraulic Motor On Time too long” from the SMG unit. This thread offers a plausible explanation for that if the SMG unit sees an over temperature and inhibits pumps operation. After one such incident, with INPA hooked up to the car I reset the errors and then repeatedly changed from 1st to 2nd and back again, watching the hydraulic pressure drop from ~75bar to ~50bar over the course of about 6 gear changes, and then saw it rise again taking about six seconds, hearing the pump run at the same time. Having done this several times the pressure failed to rise, the pressure instead staying at ~50bar. The pressure didn't so much as twitch, and I didn't hear the motor running. After a little while (~15 seconds, but I wasn't timing it) the gearbox warning light came on again reporting the normal fault. A few seconds later the pressure then rose, taking the normal six seconds to reach ~75bar. I had believed that this was being caused by the motor being worn, but I'm now of the opinion that the temperature issue was to blame as this only happens on hot days!

The only thing that makes me doubt this is that previously I'd seen the gear symbol flash, which according to the owners' handbook is the indication that the unit is overheating. This happened on a hot day when driving through Milton Keynes, famous for having lots of roundabouts and therefore requiring constant gear changes. I considered this to be "normal" operation. Why then, should this change behaviour to have the light on constantly? Is this just an example of the SMG control unit's error handling not being thought out properly such that the "normal" over temp condition, co-incident with low accumulator pressure, results in the familiar “Hydraulic Motor On Time too long” error?

Before finding this thread I'd acquired a used SMG unit with a totally shot motor (see HERE) and I'm having the motor rewound. I will also replace the temp sender as that is available from BMW for other applications (even though they don't list it for the SMG pump), part number 13-62-1-709-966. If I can delay having to pay to have the new unit fitted though, I'm all for that! I'm going to wait until my warning light next comes on and then I'll check the reported temperature; if it's high I'll do this mod as an interim fix (and see how much of a temperature reduction the control unit reports) until the car's next in for some work. Given that my car has 136K on it, and seeing the state of the pump which I'm reconditioning, which probably has less miles on it, I'm inclined to replace my whole SMG unit anyway at a convenient point in time.

When I get round to this I'll post photos of the tidier mod in detail.

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