Can anyone give me some pointers to sort out the following electrics problem with my UK '98 740 (D24T)?

When ignition is switched on only the Glowplug, Oil, Parking brake and Safety Belt lights come on full strength. ABS light comes on and then goes off.

Remaining lights - Battery, Bulb Sensor, Washers etc are showing only a very faint glow (maybe Battery is slightly brighter than the others).

Engine starts fine and ABS light comes on/goes off immediately (ABS is working OK). Battery light etc remain glowing, but not brightly.

Voltage across battery with engine running is only around 12v.

So I'm assuming 'no charge'.

This problem occurred intermittently a month ago and the garage diagnosed failed Alternator, so it was replaced. Battery also replaced. Everything has been fine up to today when the above problem re-occurred.

The only other snippet of information I can give is that with the ignition on, and the red wire that goes to the D+ terminal at the Alternator disconnected it is showing 7.5v - which seems strange to me, I'd expect 12v.

Does it look as though I've got another blown Alternator or could it be some bad earth somewhere. Is it possible to check the actual output of the Alternator ( I have got a digital AVO meter, but not much experience in electrics!)

/// This sounds like a ground problem as one of the other posts suggested. This can be a bad connection at any joint in the primary, battery charging circuit. Since the lights are dim at first switch on, they should all light before you crank, it is probably there.

With an assistant watching the lights use a heavy wire to jump the cables one at the time. From battery negative post to engine ground, would be my first choice. Then bypass the system wire from the positive post and follow by testing the connections and wires to the alternator. You are looking for a bad connection inside the lead/brass battery post clamp. You cannot see the condition of that connection.

You can measure the voltage drop while the car is cranked and then while running. Any wire that shows a drop of more than one or two tenths of a volt is suspect.

Bear in mind that your meter may be off, test it. A battery reading in the morning after overnight rest should be 12.66 volts, 12 volts is half charged. If you have charged the battery burn off surface charge by using the headlights for a few minutes, then read voltage.

The system must be loaded to force the alternator to charge especially with a new battery. After cranking the charge time may be very short. Turn on lights, radio, AC to force charging voltage.

Thanks for suggestions.

See later post, but certainly the wires to the positive battery terminal look a bit 'tired'. Am thinking about re-making them with a new post if I can find one suitable for a home fix ie. Big hole and chunky bolts :-)).

Give priority to advise posted here by others more knowledgeable than I am particularly as regards electricity and diagnostics, but I will share my experience with you. I had similar problems with a 760 turbo (1986). Diagnosis from specialized shop was bad alternator to be rebuilt, come tomorrow. That night I loosed the bolt that holds the ground (negative) cable from the battery to engine block and sprayed plenty of dry wire fluid on the connection and tightened the bolt. That solved the problem. In view of my new found prowess, I became ambitious and cleaned and sprayed that ground and both battery terminal connections. I also sprayed some WD40 inside alternator, particularly around brushes. Since then, the system is charging perfectly and better than it ever did during the many years I have had the car. Needless to say, I did not have the alternator rebuilt. I did change the engine wiring harness, but that was weeks before all of this happened.

Rafael,

I reckon that's worth a go!

Thanks for the input.

Rafael,

I reckon that's worth a go!

Thanks for the input.

Power (12v+) from the ign,travels through the bulb, and gets a ground in the alt. When the alt starts charging, the D+ terminal becomes 12v+, and since two positives will not light a bulb, it goes out. If you unplug the D+ terminal, and then turn the key on, and measure voltage at the wire, it will be closer to 12v. If you have 12v power on the large wire on the alt, and a signal from the bulb, that is all you need to make the alt work. You might try an old rebuilders trick, and give the alt a good thump with a small hammer, or metal rod, while the car is running (carefully), the light may go out, and the alt may work, although it still will need attention. Good luck! MK

Mark, Thanks for the reply and for explaining how the dash lights should go out - I was wondering how that was actually achieved.

Now if I only have 7.5v on the wire with ign on...does that mean I'm suffering from the 740 wiring syndrome......?

Will try your suggestions tomorrow - starting the Tank up at midnight (Local) won't please the nieghbours :-)

Cheers

you groung that wire, the bulb will light up brightly. Not really important, even 7.5 or less voltage would turn on the alt anyway. If the light goes out (no longer dim as you described) when it is unplugged, and the key is on, your problem is most likely in the alt. Let me know what you find. MK

Mark,

Filled with optimism I removed the D+ wire, grounded it and switched on fully expecting the lights to 'light' - they didn't. Absolutely no difference between the D+ wire connected or unconnected :-(

Decided to repair to kitchen for a caffine + nicotine boost and a think (well it was early in the morning). No cigarettes :-((

Drove down to the local, bought said items and switched ignition back on.

Guess what - all lights correctly lit :-O

Sped back home and out with AVO. Performed test as above - exactly same result on grounding D+ wire - no lights. Replaced wire - lights. WTF is going on!

At this point I noticed a single black wire 'T'd into the D+ wire about 1' away from Alternator. Followed it back to a relay and wires behind glove box. I think its part of the Volvo remote and Alarm - car has movement sensors inside - a legacy from the previous owner's time in Italy.

Now I'm wondering if this is confusing the issue - but even so, surely if the D+ wire has 7.5v and the lights do light if its attached, then they should also light if its grounded away from the Alternator.

Also searched the Archives and came across a suggested test from Alternator case to Ground - should be < 0.5v apparently. If greater, then it denotes a bad ground. Mine is 1.02v.

Although the top mount of the alternator is rubber bushed, the bottom (Adjustment bracket) is un-damped and attached via bolt to engine block, so I can't see how to get a better ground than that.

Well, as everything is OK at the moment, I guess my action should be to fit an Ammeter/Volt meter and hope whatever it was has self corrected (but I somehow doubt it!).

Doncha just love 740 electrics :-))

Thanks all for input so far.

Cheers

or a delay relay. I dont know why they have to make it so complicated, hope it fixed itself, but time will tell. When I had my auto electric shop, I had all the diagrams, and could have helped more, but alas, they were sold with the business.MK

...of course it is an '89 NOT 98 (I wish)