Bulb fail warning light

When I brake the bulb fail warning light momentarily flashes, and I mean
momentarily, just for a millisecond. This only happens if I haven't touched
the brakes for a minute or two. The brake lights work perfectly as do all
the other sensed lights. I was wondering if there could be a slight lag on
one of the bulbs.

Is it a problem or a "characteristic" (what you call a fault when you can't
diagnose it). I can live with it, unless it is a warning of impending doom.
I don't even notice it unless I happen to be looking at the instrument
panel.

Thanks

Bill

 

Should have said!!

 

I have a 89 740 (UK)

I do like this sensor, but it can be a right pain.
The sensor works on the resistance of the filament of the bulb, and is
obviously "paired up" with the other bulb on the opposite side of the
vehicle.

I have known it to light continuously, because the two of the bulbs in my
brake lights were made by different manufactures and had differing
resistances.

You need to check that all the connections to the bulb and bulb holders are
clean.

Or perhaps replace both stop lamp bulbs.

hope this helps,

Steve.

 

About every six months the right rear bulb starts to developed a poor ground
connection. This is on a 86 740.

 

I had this on my 740's, most common cause I have found, apart from
mismatched bulbs, is oxidisation on the bulb contacts, or on the holder
contacts. Make sure everything is clean and buy bulbs in pairs.

Stuart.

 

This specific glitch is usually the third taillight - it is gimmicked into
the system and is the one blub that is on both circuits.

Either that or a loose solder joint in the taillights.

 

Thanks for that, but can you tell me what you mean by the "third taillight".
I have red rears for the dark, brake lights, reversing lights, one fog lamp
and my number plate light. It is a term that we don't use this side of the
pond.

Thanks

Bill

 

Bill,

I did not see anywhere where you stated the model.

I had that fault develop on a '99 V70 T5 about a month after I bought
it. The vehicle was under a years warranty so I took it back three
times and they never found the fault. They changed the bulbs, checked
the connectors, which were all like brand new, checked the earth runs
and changed the sensor pack.

The sensor pack is a switch relay with reed type connectors which
should switch simultaneously and only fail to do so if there is a
variation of amperage drawn through either coil due to a fault or
mis-matched bulbs.

The usual cause is a bulb on the way out and it's resistance has gone
up\down realtive to the other and the cure is to change both. Mine was
due to none of the standard reasons I have seen on any Volvo boards
and I just resigned myself to living with a momentary flash of the
bulb out lamp which had no pattern, other than foot brake operation. I
had a career in electronics and would have loved to have nailed the
problem with a multi-channel memory scope but other factors in my life
were more important<g>.

The fault, which I always suspected was a floating voltage (lack of
lock down on stability of the power rails) ended after a battery
change 6 months after I bought the car. Some batteries die very
slowly, as did mine, but many just fail out of the blue. I feel sure
that many of the Volvo electrical gremlins are due to battery related
issues and saw in the last month of mine, onboard computer zeroing,
clock going to 00.00 and three Electronic Throttle alarms. I put the
biggest, best 4 year warranty battery on and have never seen a problem
since.

Liam

 

He means the highmount or center brakelight, it's mounted in the lower
middle of the rear window, IIRC it wasn't included unil '86, so if you have
a very early '86 it may actually be an '85 model, that light being one of
very few differences.

 

I realised that ommision as soon as I posted my intitial query, and put in
in the post "Sorry, '89 740GL".

Reading yours and the other replies, perhaps I'll live with it until
something else goes.

Thanks

Bill

 

It probably just means that the bulbs don't have *exactly* the same current
draw
when first lit, but the difference isn't enough to trigger a constant warning
from the
circuit. Knowing Volvos, though, it could also be a bare or loose wire,
somewhere...


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