Instrument Binnacle Lights - bad earth? - where?

[eeedelli]

Hi folks,

I've lost all night lighting on my instrument binnacle and centre console etc. Warning lights etc. still seem ok. (I've tried turning the brightness control just in case it was a dirty wiper on the pot but to no avail... and that wouldn't kill the centre console anyway ... though it's so long since I paid attention to that bit in the dark that the bulbs may have all long blown there anyway, to be honest, but the instrument cluster certainly went last night. OK on the way to the shops, nothing on returning.)

The fuse must be ok as the sidelights/numberplate lights are (allegedly) fed from that one too and are ok. The lighting switch seems unlikely to be the culprit as I notice that the radio buttons do light up when I switch the side lights on, so it looks as though the positive 12v side is likely to be ok.

Looking at the circuit diagram for my model (pre-facelift 88/89 740GL), it appears that they all go back to a common 0v point somewhere. My guess is that the screw has corroded.

Does anyone know where it is?

Thanks,
David.

[bob12]

On the 740 that's likely to be a VDO cluster.

Your problem is not uncommon and is usually a dying or dead lighting rheostat (dimmer). Of course make sure it hasn't got turned down to minimum.

Check to see if the other lights such as the cig lighter and heater controls are working or not. If not that also points to the dimmer. The fact that the radio lights are also off suggests dimmer/rheostat dead.

Bob.

[eeedelli]

Hi Bob,

I'm pretty sure it is a VDO cluster but I think you may have mis-read the info I listed.

As mentioned - Yes, I did try turning the pot up and down to check and, no, the radio lights are not dead. They still come on when the sidelights are switched on.

I don't think the centre console lights are controlled by the dimmer are they? I thought it was just the binnacle lights.

Ta,
D.

[bob12]

It's a good few years ago that I had to deal with this problem and I may have forgotten whether or not the radio lights dimmed. The radio I had fitted at that time was the original CR-702.

If you look at the dimmer on the white backboard you will undoubtedly find a large heat stain above it. It heats quite dramatically and in my view after a long span of time it fries the innards! I solved the problem with a working second hand dimmer that I do not use on much dim to avoid stressing the internals too much.

If it's any help I can assure you on my 1990 745 GL with a VDO that the dimmer controls the 3 cluster/dial lighting bulbs; the heater control, cig lighter, and all the switch light bulbs. I have just been outside and checked. It doesn't control the radio lights and may never have done so but, I have a upgraded head unit.

I would have assumed that if all the other warning lights and dials are working that the problem is not associated with a bad earth connection.

I do not know where the earth block is as I have not ever had to search for it. However I do know there is a large earth block behind the drivers right hand side lower leg knee level panel.

[Laird Scooby]

If memory serves correctly, there's an earth point on the bulkhead to the left of the clutch pedal but there's also one behind the central electrical unit behind the ashtray - i may be mixing the two locations up in my head.

Invest in some new bulbs as well so when you've found the problem you and renew any dead ones as you go.

Considering the centre console illumination has gone out, i'd go for bulbs first and also check the earth point.

Do you have a Haynes Manual for it?

[eeedelli]

Yes I've got a Haynes, thanks.

Those sound like good starting points and will save me a lot of time faffing about searching!

According to the Haynes, the common earthing point looks to be fairly close to the centre console lamps and all those plus the return wire to the binnacle seem to go to it, so I'll have a look around there first and see what I can find.

If that doesn't produce any joy then I might have to take the binnacle out and check that I'm getting voltage and earth return continuity on to the relevant connector ... if I can work out which one that is (and the relevant pins) and then trace through to the rheostat as I think that would be the most likely next thing to look at. I was hoping I wouldn't have to do that. (Is it a fairly standard sized wirewound pot, Bob - or a weird Volvo 'special' that will undoubtedly be unavailable now?)

Thanks.

[Laird Scooby]

Is it automatic? If so, does the shifter selector gate light up? That isn't fed through the panel rheostat. Fairly sure the radio is fed from the sidelight circuit pre-rheostat as well.

Wouldn't be surprised to learn it was all bulb failures causing your Lucas moment.

http://mez.co.uk/lucas.html

Whatever the rheostat is, it's only going to be a few Ohms, daresay you could buy a new pattern one from Rock Auto but i think (not sure though) that they are NLA from Volvo these days and if not, they'll be £OUCH!!!

[eeedelli]

LoL! Well if it's not a Lucas moment, it must be the fault of Sainsbury's - Instruments all lit up nicely when I drove there last night. All black when I tried to drive home. (Unlikely to have all blown simultaneously while I was in the store!)

No, it's a manual, so no clues there I'm afraid. Ironically, my 940 is auto but the bulbs have gone in that selector and I haven't got round to delving. (Mind you, it'll be a while before I can drive that again - head gasket went a few weeks ago, which is why I'm back in the 740, even though that needs some work doing too. At least, I hope it's just the gasket, not the head - one exhaust port had been 'ticking' under load for quite a while - I thought that was just an exhaust manifold port gasket. Eeek!)

If I have to pull the binnacle out, I'll see if I can trace a similar part from RS or CPC for the rheostat. If it's not a bog-standard electronics component, that's going to make things tricky. Probably NLA, as you say. Might have to substitute with a pair of resistors of reasonable wattage.

Incidentally, I wonder whether you can get LEDs that are just a straightforward plug-in replacement for those panel lamps - they're a pain in the neck to replace. (Never can remember which type they are.) LEDs should solve that problem for the future.

[Laird Scooby]

Originally Posted by eeedelli

Incidentally, I wonder whether you can get LEDs that are just a straightforward plug-in replacement for those panel lamps - they're a pain in the neck to replace. (Never can remember which type they are.) LEDs should solve that problem for the future.

You mean like that? The answer is yes.

By the way, why bother with a pair of resistors? Just link it out on the early 740 if memory serves.

[eeedelli]

Oooh! That looks good! Can you remember which type/size of base they are and where you got them? (Wow - you have a working fuel gauge too! ;-) )

The reason for resistors is that it would make the lamps last longer. I suppose if bulbs were substituted with LEDs then that would be less of a problem, though LEDs can get over-driven too, of course.

Although those LEDs must have a resistor built into them to limit the current to the correct value so as to work on a supply of 12V (as the LED forward voltage would normally be around 1.2V), the instrument bulbs appear to be fed directly rather than via the voltage regulator, so may well be getting around 14V when the engine's running, which would shorten their lives if the built-in resistor was calculated for 12V (i.e. 17% over-driven).