'96 940 LPT revs bog down and cut out under electrical load

[bmxJeff]

Hi all, after a little advice please.

My 940 generally drives pretty sweet apart from the 'power cuts' it seems to experience.

Normally several times each drive, there seems to be a dip in power to the entire car which results in lights dimming/stereo turning off and sometimes the engine will cut out if I haven't got my foot on the juice.

The car can also not handle using too many electrical things at once. For example, I had my full beams on, sat nav on/charging, stereo on and cabin fans on and the car began to splutter whilst doing 50-60 mph between 2-3000 rpm. It got worse to the point it was going to die until I pulled over and unplugged everything to give it a breather.

If I leave it to idle there is a severe drop in revs when the rad fan kicks in. It will drop from its usual 900rpm-ish to around 200-300rpm if not die completely. I can also cause a drop in revs just by using a couple of electric windows at the same time.

It also quite often has this 'power cut' when I flash my full beams with my main beams on even moving with my foot on the accelerator.

So my thoughts were bad earth or haggered alternator. Does this scream anything obvious to anybody?

Thanks

[Dirty Rooster]

Replace your Alternator TODAY if you can.
You do not want to be stuck!

I found the brushes for inside my alternator supplied by a local Auto Electric shop on a Carlisle industrial estate ... £17 new. I took in the old unit and the bloke went to find a match.
So that's another option.
Two screws to remove : very very quick.

[Rversteeg]

The idling speed should be adjusted automatically by the CIS valve, located just below the inlet manifold. It is basically an elctrically operated air by-pass valve around the throttling plate. When closed, the throttle should just "leak" enough air for approx. 500 rpm idling, the additional air needed to make the engine tick over at 850-900 rpm should be going through the CIS valve.
If on idling the engine load increases (electrical components kicking in, A/C), the CIS valve is opened a bit further, bringing the idling speed back up again. A short drop in engine speed is normal, but it should recover itself within 1 second or so.
I would suggest to clean the throttle housing internally; to check the basic idling speed of 500 rpm (closing hose to CIS valve); to take out the CIS valve and thoroughly clean it; finally check the adjustment of the end switch on the throttle shaft.

However, a faulty idling control system does not explain the hesitations when the engine is at speed. It could be it's not the extra mechanical load on the engine, but an overall voltage level dropping to a too low value. Have you checked the earthing connection from the battery to the chassis? What's the battery condition? Maybe a voltage reading when driving (Voltmeter connected to eg. sigar lighter)?

[JJKM]

I agree about replacing the brush pack as a precaution ; really simple job and will rule it out. I had very similar problems - £20 later and 5 mins work and was fine!

[Rversteeg]

A faulty alternator on my 940 only caused the dahsboard to look like a christmas tree but had no influence on the engine running. But maybe in combination with a bad battery it will.
Last year I replaced the bushes as one of them was worn. Only indication was a dimly lit charging failure indicator...
I would start with just measuring the system voltage under load before replacing anything.

[Steve940estate]

I've read about some cars having faulty ignition switches that can cause some strange faults, could be worth a look if nothing is wrong with the alternator.

[James_EG_940]

My car will certainly drop revs in idle when you draw a lot of current.

Example: at idle closing more than one electric window at once will drop the revs down by about 100rpm. It doesn't recover until I take my finger off the switch.

I've never noticed any other problems!

[Alasdair1]

If the battery is good I wouldn't think an sporadic alternator fault would affect the car too much to begin with. My alternator belt snapped and I still managed to get home on the battery ok. It might be either a bad earth or broken wire? Give the battery terminals and earth straps a good clean and check for corrosion or damaged wires. Also check the alternator belt tension and condition.
Alasdair

[TonyS9]

Please don't randomly replace big items, you need to diagnose the problem or take it to someone whoe can.

I'd suggest a voltmeter for a start, check the battery voltage with the engine running with minimum load and on high beam load. The voltage should not drop more than 0.3V across any cable at the battery (bad connections can occur in the +,- or both) between low and high load. Any more can be considered a fault, but to be honest these things gradually degrade and a good harness should not drop more than 0.1V. It should be close to 14V, 13V will cause charging problems. The crimp connections can suffer with high resistance as well as chaffing caused shorts. shorts are dangerous and can cause a fire, even with the engine off parked up.

The severity sounds like a short. You need to have it diagonsed before it starts a fire.

[bmxJeff]

Originally Posted by Dirty Rooster

Replace your Alternator TODAY if you can.
You do not want to be stuck!

I found the brushes for inside my alternator supplied by a local Auto Electric shop on a Carlisle industrial estate ... £17 new. I took in the old unit and the bloke went to find a match.
So that's another option.
Two screws to remove : very very quick.

Originally Posted by JJKM

I agree about replacing the brush pack as a precaution ; really simple job and will rule it out. I had very similar problems - £20 later and 5 mins work and was fine!

Originally Posted by Rversteeg

A faulty alternator on my 940 only caused the dahsboard to look like a christmas tree but had no influence on the engine running. But maybe in combination with a bad battery it will.
Last year I replaced the bushes as one of them was worn. Only indication was a dimly lit charging failure indicator...
I would start with just measuring the system voltage under load before replacing anything.

Thanks for the quick replies guys. It's good to know how cheap brushes are.

I'm not going to replace anything before proving the fault, been there, done that and wasted lots of money!

I'm not too worried if worst case scenario happens and I get stranded. I have tow to home cover and am rarely making long journeys in the car at the moment.

See my next post, maybe you can help me make sense of these readings...