940 with charging problem

[GRT]

Hello all
I have a 1997 940 2.3 that's displaying the well documented problem with 'no charge and ABS light on'. Scenario is:
Other night dim ignition/charge light came on, got brighter as the headlights got dimmer and we stopped.
Battery recharged/tested & is good. Also swapped out for another known to be good - no difference.
Tested alternator - apparently no charge. This alternator looked to be the original - grabbed a recon unit and fitted it today.
Still getting ign/ABS/SRS lights up on start. No charge. Initially revving up to around 2000 rpm would extinguish all the lights whilst holding revs up - drop back to tickover and it/s back to ign/ABS/SRS lights back on (and no charge, just battery voltage at alternator).
Now even that is not happening - on engine start a fair few lights stay on briefly then I'm left with IGN/ABS/SRS at whatever revs.
If I have ign on but engine off and disconnect the spade terminal from the back of the alternator then the IGN light will go out (along with a fair few of the others).
Discounting the possibility that the recon alternator is u/s then what else could be the problem?
Thanks

[john.wigley]

Are you certain that the recon alternator is good, 'GRT'? Have you tested it or substituted it for a known good one from another car? It certainly sounds like the alternator to me. It is not unknown for replacement units to be faulty; we once got through three in very short order on my wife's 744 some years ago.

Regards, John.

[Rversteeg]

When I had the same charging failure indications as you are having, one of the brushes in the alternator was worn. I replaced the brushes and initally the alternator did not work either. It needed some running in of the new brushes before it actually started charging.
Actually the alternator needs the "no charge" light in the instrument cluster to start up charging. The 12V feed runs through the light and is grounded in the alternator. Once the alternator start charging, the ground is lifted to the charging voltage and the light will go out. So if disconnected the alternator will not charge at all.

[RollingThunder]

If you're happy that the recon alternator is from a trusted source and is definitely good, I'd go back to basics and check the wiring in the circuit. ie battery leads, all earth leads including the one from the alternator to the engine block and the one from the block/gearbox to the bulkhead. Check the cleanliness of the connection, the tightness/security of the connection, and also check the cables for breaks at the connectors - this is sometimes given away by blackening of the insulation where the break is arcing. Yes I learned that trick through experience - that was on a V70R!

I had some charging issues a while ago and also fitted a recon alternator (ouch - didn't expect them to be that expensive!), after which I found that the blue earth lead between the alternator and engine block was getting hot, so I replaced that with a home made super heavy duty cable. I can't show you a pic as I've stopped my website for the time being - but it was battery lead spec

Since then everything has been fine

[Laird Scooby]

Originally Posted by GRT

Hello all
I have a 1997 940 2.3 that's displaying the well documented problem with 'no charge and ABS light on'. Scenario is:
Other night dim ignition/charge light came on, got brighter as the headlights got dimmer and we stopped.
Battery recharged/tested & is good. Also swapped out for another known to be good - no difference.
Tested alternator - apparently no charge. This alternator looked to be the original - grabbed a recon unit and fitted it today.
Still getting ign/ABS/SRS lights up on start. No charge. Initially revving up to around 2000 rpm would extinguish all the lights whilst holding revs up - drop back to tickover and it/s back to ign/ABS/SRS lights back on (and no charge, just battery voltage at alternator).
Now even that is not happening - on engine start a fair few lights stay on briefly then I'm left with IGN/ABS/SRS at whatever revs.
If I have ign on but engine off and disconnect the spade terminal from the back of the alternator then the IGN light will go out (along with a fair few of the others).
Discounting the possibility that the recon alternator is u/s then what else could be the problem?
Thanks

As mentioned above ^^^^^ check the earth strap/lead between alternator body and engine block - this needs to be PERFECT (not just "looks alright to me") or the problems you describe towards the end of your post are a very high probability.
As for disconnecting the warning light wire with just ignition on and the charge/ABS/other warning lights going off, that is correct. All those lights should light with no charge, it's a thing to check their function and also provide enough current for the alternator rotor to start charging. Other manufacturers use a single 3W bulb, Volvo use several smaller bulbs wired through diodes to give the same current flow through the bulbs and a function test.

This actually proves the w/light cct and gives a clue to the earth strap being the cause of the problem. As the alternator starts charging, if the earth strap is resistive because of a fault in it, this resistance will become hot and the resistance will increase as a result - this will reduce the output of the alternator and hence give rise to the charge warning light coming on again.

[GRT]

Hello All
Thanks for all the input - I've done everything that has been suggested/I can think of and I'm left with the 99% certainty that the recon alternator is actually not working.
It's going back and I'll source another (anybody got a good source for the Bosch type alternators??)

Thank for your help
Graeme

[griston64]

Originally Posted by GRT

Hello All
Thanks for all the input - I've done everything that has been suggested/I can think of and I'm left with the 99% certainty that the recon alternator is actually not working.
It's going back and I'll source another (anybody got a good source for the Bosch type alternators??)

Thank for your help
Graeme

Where are you based ?

[GRT]

Originally Posted by griston64

Where are you based ?

West Wales near Aberystwyth (car parts desert)

[griston64]

Originally Posted by GRT

West Wales near Aberystwyth (car parts desert)

Bit far from Glasgow

I have two spare but not sure if one works