'92 730i V8 Auto gearbox probems!

[dave de roxby]

Hi Folks!
Well I've been and gone and done it again! Bought this lovely old 1992 730i V8 auto for peanuts, one principal owner and genuine 70,000 miles, super black leather, what a bargain for £250! Or so I thought. Had been taken in part exchange so I bought it as seen. On picking it up, I brought a new battery with me, having been told the existing one was duff, fitted it and Bingo, it starts up virtually first time. Engine sounds smooth, only noise is a whine from, I think, the power steering. Sets off home, after a couple of miles, engine seems to be stalling, always conks out when you try to idle. But starts again willingly enough. Then suddenly, gearbox shifts into emergency mode, locked in third I think. Managed to limp home. Gutted at having bought another lemon. Left it for the night. Next morning, new battery virtually flat, wont start. Hadn't left any lights on or anything daft. Recharged battery, back to square one. Starts fine but same symptoms after a couple of miles. Now, before I start spending megabucks on diagnostics etc, does this ring bells with any of you experts out there. Would be very grateful to be pointed in the right direction. Such a nice car otherwise.
David

[road_rager]

Alternator not charging for starters? Or else a short somewhere?

[dave de roxby]

Thanks for that. Yes will check alternator but as you suggest I think a short must be present for a new fully charged battery to go flat overnight. Maybe its as simple as that, hope finding it is as simple as saying it. Will go through each fused circuit and check for current flow, yes? Will report back but thanks for now!

[road_rager]

If the alternator is completely shot then your batery would be flattened as you drive quite quickly, but I guess then you would see a light on in the dash for that. Has to be something pretty straight forward I would have thought, as you say checking for continuity is a good place to start. Good luck

[8Tech]

Engine running poorly will cause the trtansmission to lock into failsafe so dont assume a tranny fault quite yet. Stalling on these cars can be the breather at the rear of the inlet manifold or a loose induction hose between the AFM and the throttle body. Both would cause limp-home.
Battery discharging should be straightforward to isolate. Check alternator output first then current draws.

[dave de roxby]

Thanks 8Tech, That sounds like the voice of expeience. Will check out as you suggest. You are so right to say dont jump to conclusions. Back in the sixties, I once replaced the whole back axle of a Thames (pre-Transit type) van only to find it had been a front wheel bearing moaning all the time! Was also going to curse these new-fangled electronic gizmos but just worked out the poor car is fifteen years old anyway! It's me that's stuck in the twin SU era. Where does the time go? Talk about Silver Surfer!

[dave87]

As people have said - it may be something simple. IIRC a blown fuse caused Dad's old e28/34 (can't honestly remember, it was a while ago) 525i to jump into limp home mode - gave him the shock of his life as it was the first BM he'd ever owned.

[dave de roxby]

Again, thanks Dave for the contribution. I'm certainly hoping it's something simple (and cheap), wont be whipping the gearbox off just yet. Afterall, the box seems to work perfectly for a few miles so reckon its got to be something external and probably the electronic systems. As others have mentioned above, the battery symptom obviously points to some significant main electrical system fault so will be looking at that first. Just for a laugh, I telephoned a BMW main dealer for advice - got the reply I expected. Bring it down and we will plug in our diagnostic wonder machine. So what will that cost then? Difficult to say sir, but probably around £250 + vat. Hell, I only paid that for the whole car - don't think so! Well, I am a Yorkshireman.