W124 - Lighting goes 'Dull/Bright' on tickover

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300CE

MB Enthusiast
Joined
Apr 25, 2010
Messages
1,474
Location
Sidcup, Kent
Car
1990 300CE
Hi All,

The lighting (interior lights/side lights) on the CE go 'dull/bright/dull/bright' whilst on tickover and if i put my foot on the accelerator to increase the revs it just speeds the process up.

Any ideas as to what this could be?. Wasn't sure if this may be the sign of a failing alternator, something to do with the belt or if it's just a W124 trait!

Many Thanks in advance.

Cheers, Justin.
 
Would appear to indicate a varying supply voltage- easily tested with a simple voltmeter. Possible causes- worn or slipping poly V belt- alternator problems- brushes or the regulator pack-or a battery on its way out. Not normal on a W124
 
If you have variation on lights like yours I would test battery (may come up very weak) and if alternetor making extra work or something wrong with it (one diode cooked) then it might produce not enough power/voltage.
 
Most likely a bad connection between alternator and battery. If the alternator also has a voltage sense wire (i.e. it is battery sensed as opposed to machine sensed) it could be this. This would be a thin wire which is the same colour as the thick wire at the back of the alternator.
Could be OVP relay/fuse connection on OVP relay.
As grober says, could be regulator pack on the alternator.

It could be the battery, but that is unlikely, as presumably the car starts ok/
 
Hi Chaps,

Many thanks for all the advice and info. I think i'll start with the simplest options of checking and cleaning the connections on the alternator as well as testing the battery (no starting problems and she starts on the button).

I've had the car 6 months and it's always done it, although it's more noticeable now it's dark!!

Will keep you informed.
 
I've got a similar issue. My interior and exterior lights get slightly dim on tickover but come up to full power and brightness upon acceleration.

Do you think it could be the alternator / OVP? I've just had a new battery put in so can't be that.

Actually, thinking about it, I get a similar effect on my heaters too. They seem to blow stronger when accelerating.

Any advice appreciated.
Thanks.
 
Shakter - that's different.
You will need to do a voltage test at the battery.
The symptoms you describe are either a low voltage on tickover, rising to the correct voltage at charging speed or a normal voltage at tickover, rising to an excessive voltage at charging speed. You will only be able to identify this with a voltmeter.

Signs of overcharging are bulbs blowing, and an 'eggy' sulphurous smell from the battery (it will also need topping up frequently) - is this the reason you replaced yours?

Correct voltage while charging should be 13.7 - 14.3v
 
Also, when i'm driving i can hear a very faint 'tick' sound (this is without the radio on). On low speed my blower tends to speed up - just slightly - and then slow down again as this thing 'ticks'. It is very faint and the change of speed of the blowers is hardly noticable.

Any idea as to what the ticking thing is?
 
Shakter - that's different.
You will need to do a voltage test at the battery.
The symptoms you describe are either a low voltage on tickover, rising to the correct voltage at charging speed or a normal voltage at tickover, rising to an excessive voltage at charging speed. You will only be able to identify this with a voltmeter.

Signs of overcharging are bulbs blowing, and an 'eggy' sulphurous smell from the battery (it will also need topping up frequently) - is this the reason you replaced yours?

Correct voltage while charging should be 13.7 - 14.3v

Thanks Ted. I'll check this out. I suspect the voltage is too low on tickover as the lights do get quite dim. Presuming it IS this - how would one go about remedying this?

I had to change the battery as I left the car sitting for a while, hence it pretty much died. ust put a new battery in and now all good (bar the voltage issue).

Thanks very much for the advice.
 
OK, either the voltage is low at the battery, or at the 'consumers'.
If it is too low at the battery, chances are it's a dodgy battery (unlikely as it's new) or the battery is getting flat, either due to a continuous discharge, or being left for long periods. A good charge on a charger will sort this out.
If it's ok at the battery, but low at the 'consumers' you should be able to see this. With the engine off, check the voltage across the battery. Now with (say) the headlights on, check the voltage between the headlamp live (yellow or white wire IIRC) and the battery negative. This should show no more than approx .5volt difference. Any more shoild be investigated to find out where the volt drop is.Dont forget that there can also be a volt drom on the earth side. do a similar check between (say) the headlight earth (brown) and the negative. Again this should give no more than .5 volt difference. To put it into perspective, it only takes 1 volt drop in the circuit to lose abount 5-6watts.

Try some of these checks, and see what you come up with.
 

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