Diagnose this problem please

unkel

The problem:

Today the car is idling in P (park) no problem, the engine is fully warmed up. But as soon as I put it in R (reverse) the engine switches off. Tried it 3 times.

The background:

My '96 BMW 735 automatic (141k miles) has last been regularly driven on a daily basis in November of last year. Since then it has been parked up. The petrol in the tank was put in last November. I regularly (every two to three weeks) start the engine, let it warm up and reverse up and down the drive and run the aircon. I did so last night and everything was fine. The battery in the car is completely dead, I jumpstart the car from another car with a running engine. The other cars battery is not strong enough for the 7-series, so I use a battery pack as well. The car starts first time every time with this combination

The cause: ???

crosstownk

Let the car idle for while - maybe an hour. Rev the engine every so often to stop the build up of condensation in the exhaust. It might settle down after a while. Consider replacing the dud battery.

Connection to a diagnostic tool would hopefully glean a lot more info.

sundodger5

Engine switches off or cuts out. The new battery idea is not a bad one unless you can get a charger. if you can leave it on charge for a min of 12 hours and then see does it hold its charge.

unkel

sundodger5 wrote: »

unless you can get a charger. if you can leave it on charge for a min of 12 hours and then see does it hold its charge.

The battery is gone. It doesn't hold any charge no matter how long I charge it or what way I charge it. But the battery was gone last night and the few previous times I drove the car on the driveway and I didn't have this problem. Is it likely to be battery related? I thought that once the engine was running, you don't need the battery, just the alternator? But maybe I'm wrong there? The alternator is fine BTW.

crosstownk

There may be an internal fault in the battery which can lead to weird problems. You may have a buckled/faulty cell in the battery. Change it. No point in attempting a diagnosis with a fault present in something as fundamental as a battery.

You are correct - the battery is not really necessary once the engine is running but a faulty battery in the circuit may induce all sorts of oddball faults.

aujopimur

a car alternator will not function if battery is completely distcharged or dead

Mr.Diagnostic

Its all a bit too vague but possibly idle adaptions lost due to power supply interruption. Vac leak is also possible.

sundodger5

if the battery is gone then you have no power reserve (maybe the wrong word) you may be getting enough into/out of the battery to enable the car to idle. go for a gear and the autobox says more revs please but the battery can not give the power to sustain ignition.
a diesel does not need to have power for ignition but you have a petrol.

sogood

unkel wrote: »

The problem:

Today the car is idling in P (park) no problem, the engine is fully warmed up. But as soon as I put it in R (reverse) the engine switches off. Tried it 3 times.

The background:

My '96 BMW 735 automatic (141k miles) has last been regularly driven on a daily basis in November of last year. Since then it has been parked up. The petrol in the tank was put in last November. I regularly (every two to three weeks) start the engine, let it warm up and reverse up and down the drive and run the aircon. I did so last night and everything was fine. The battery in the car is completely dead, I jumpstart the car from another car with a running engine. The other cars battery is not strong enough for the 7-series, so I use a battery pack as well. The car starts first time every time with this combination

The cause: ???

A bit of a long shot and moving away from the bad battery scenario, but could the problem be related to the start inhibitor switch that prevents the car from being started in gear? So if the switch is dodgy, could it work in reverse, killing the engine when reverse is engaged?

Row

I'd say you need to monitor what voltage your getting from your battery
when.....
1)engine switched off...should be getting around 12-12.5 volts
2)engine runing...the alternator should be pushing out about 14-14.5 volts.

just thinking out loud here....but..
if the engine is cutting out under battery load it may related to....
1)reverse lights bringing it down (shortcircuit).....
2)proximity switches shorting out in the autobox when reverse (R)
is selected...
I guess "process of illumination" will have to be introduced...:eek:
best of luck with it......

Mr.Diagnostic

I think Row might have hit the nail on the head there. Often with a dead and the car running on the alternator a slight electricial load can kill the engine. It might be just managing with the brake lights but the extra load of the reverse lights may be too much.
Try putting it in reverse with the jump leads still connected.

FWI, regular jump starting of these is not a good idea. Buy a battery.

VanhireBoys

+1
I was just about to suggest that keeping the jumpleads connected as you select reverse.
If it dies then its the inhibitor switch
If it stays going its the battery/charging system

Make sure you keep the brake on as well :eek:

unkel

sundodger5 wrote: »

if the battery is gone then you have no power reserve (maybe the wrong word)

This makes logical sense as the reason why you do need a battery with some charge in it (and not just an alternator once the engine is running). The alternator only provides a steady charge, not a peak charge.

Anyway, just dropped in a new battery and everything is perfectly fine

Thanks all for your help!

Gary ITR

Is it sold? or are you keeping it?

L-M

unkel wrote: »

This makes logical sense as the reason why you do need a battery with some charge in it (and not just an alternator once the engine is running). The alternator only provides a steady charge, not a peak charge.

Anyway, just dropped in a new battery and everything is perfectly fine

Thanks all for your help!

Happy days then!

+1 on what Onkle said.

unkel

Onkle wrote: »

Is it sold? or are you keeping it?

It might be swapped...

unkel

Car gone. I'm delighted it's going to a good home

peasant

unkel wrote: »

Car gone. I'm delighted it's going to a good home

Hooray for a quick sale !:D