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[aaronjb]

So the M3 broke down last week - it has the SMG-II gearbox and the cog of doom appeared, stranding me in Neutral.

I started doing diagnostics this weekend having bought a litre of BMW hydraulic oil as the reservoir looked low. Sadly on opening it today with the car stone cold all the fluid had reappeared, so it wasn't just low fluid..

Plugged the laptop in and looked at the errors (hydraulic pressure and pump duty cycle) it looks like it's the pump (cheaper to convert it to manual at that point, or set light to it and cook some burgers on it!) but while doing that I bumped into some fun diagnostic trickery.

I pulled up the live values which showed ~60bar of healthy hydraulic pressure after the engine has been started - so the pump definitely primed the system on start. Start changing gears and I have gears, hurrah! But what's that? The stored pressure is falling.. down to ~37bar and at that point it fails to engage gears anymore.

Odd, the pump doesn't fire.. so I try to manually actuate it from the menus only to be told it cannot actuate as the hydraulic pressure reads an impossible '-12bar' - except the live values still say ~37bar.. 

*mutters* damn German engineering

[05omegav6]

Leaky valve in the box somewhere

[aaronjb]

Maybe, but the fluid wasn't actually low (just in the accumulator at the time, I think) - there's fluid there now but the pump only fires once when you start the car, it never fires up again to re-charge the accumulator..

There's a slim chance it's the so-called 'salmon relay'..

[Andy H]

So the M3 broke down last week - it has the SMG-II gearbox and the cog of doom appeared, stranding me in Neutral.

I started doing diagnostics this weekend having bought a litre of BMW hydraulic oil as the reservoir looked low. Sadly on opening it today with the car stone cold all the fluid had reappeared, so it wasn't just low fluid..

Plugged the laptop in and looked at the errors (hydraulic pressure and pump duty cycle) it looks like it's the pump (cheaper to convert it to manual at that point, or set light to it and cook some burgers on it!) but while doing that I bumped into some fun diagnostic trickery.

I pulled up the live values which showed ~60bar of healthy hydraulic pressure after the engine has been started - so the pump definitely primed the system on start. Start changing gears and I have gears, hurrah! But what's that? The stored pressure is falling.. down to ~37bar and at that point it fails to engage gears anymore.

Odd, the pump doesn't fire.. so I try to manually actuate it from the menus only to be told it cannot actuate as the hydraulic pressure reads an impossible '-12bar' - except the live values still say ~37bar.. 

*mutters* damn German engineering

Faulty sensor or wiring perhaps?

I wonder why you are getting two conflicting values are there two sensors? or maybe the sensor is reading intermittently and -12 bar got stored when it last went AWOL and now the ECU is throwing a hissy fit

[henryd]

Is there a filter in there somewhere? ,if it's blocked it may cause the loss of pressure as it draws up the shite in the box sump.

[Andy H]

This looks like it might be useful:

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CC8QFjAA&url=http%3A%2F%2Fmaben.homeip.net%2Fstatic%2Fauto%2FBMW%2F5%2520SMG.pdf&ei=xZ9ZUranCrSU0QWCoYGACw&usg=AFQjCNGT1MwPOHiguEn81AUZA86kqZiFxQ&bvm=bv.53899372,d.d2k&cad=rja

[Andy H]

Corrosion on the pins of the relay crops up as a cause in several threads:
http://forums.m3cutters.co.uk/showthread.php?t=1053

[aaronjb]

Ta fellas - I'll be doing some more diagnosis .. well, next weekend I guess as we're away tomorrow. Too many things to do, not enough weekends to do them in!

It'd be really nice to avoid changing the pump; the choices for that seem to be £800 for a s/hand unit with ~80k on it or £2000 for a brand new one!  At that price, might as well stick a third pedal in it and rip the hydraulics out..

[Andy H]

Ta fellas - I'll be doing some more diagnosis .. well, next weekend I guess as we're away tomorrow. Too many things to do, not enough weekends to do them in!

It'd be really nice to avoid changing the pump; the choices for that seem to be £800 for a s/hand unit with ~80k on it or £2000 for a brand new one!  At that price, might as well stick a third pedal in it and rip the hydraulics out..

At those prices there must be an incentive for the reputable companies to get involved in either sourcing individual components or offering a refurbishment service (BBAreman for example?)

[05omegav6]

I bet that if you stuck a manual box in it that the ecu would refuse to even fire it up rather than the polite 'transmission fault' message that the Omega displays

[cem_devecioglu]

needless to say smg boxes are a headache..  and imo dont try to convert it .. either repair it or sell as it is..


did I say bimmers are not like omegas

ps: buying a second hand m3 probably will be cheaper than a new smg
 

[chrisgixer]

Sounds to me (I know nothing of electrickery and less so its devils) like the pump is fine and its seeing an electrical fault.

...but do remember that ex BMW mechanic I mentioned, if you get stuck.