BOTH MY HEADLAMPS BLOWN!

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tris
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BOTH MY HEADLAMPS BLOWN!

Post by tris »

Its strange, i was drivin back from work today in my '93ax debut and radio lost power for about 5 seconds then started working again upon which my headlights both blew at once! Side lights and full beam are fine. I have taken a multimeter reading from the buld connectors reading about 25v, is this ok??? I havent been out to buy some new bulbs yet as i dont want then to blow also.
Am I right in thinking that there was a power surge which caused the radio the go off? If so, y didn't it just blow the fuse in the lighting circuit aposed to the bulb(s) themselves???
Please help, is it ok to fit new bulbs, i need to use the car tonight to pick up a lady!
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AndersDK
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Post by AndersDK »

tris -
If your AX is an early model - there won't be any fuses in the lights circuit.
You're right that this can be caused by a power surge.
From your description by the radio falling out - it seems that somewhere you had a shortcircuit on the +12V side to earth - causing the dead radio. This causes a huge load on the battery and the generator - and the generator tries all it can to compensate - by supplying all possible current via the highest voltage it can supply.
Normal vibrations from the running car then suddenly cured the shortcircuit - and then the generator was at it's maximum output - which takes a couple of seconds to regulate down.
The result is a power surge burning out your headlamp bulbs.
The problem can also be caused by a bad/intermittent connection from generator to battery. If the generator can not "see" the battery voltage - it "thinks" the battery is totally discharged - then again it turns up maximum charge power.
I'd say you should immediately check all cabling running between generator/startermotor/battery - i.e. all the heavy duty cabling involved including the earth cables - as your problem is more than likely to turn up again.
Even the battery pole clamps may be a problem (corrosion/sulphur).
lhm_leak
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Post by lhm_leak »

tris - If you really *have* got 25 volts on the bulb connectors, then you've got major problems with your alternator/regulator! The reading should be around 14.5 volts, give or take a small amount, with the engine running and around 12 with it stopped. If you didn't just misread the meter, get it fixed quickly before practically everything else blows too, and your battery boils dry. As Anders just said, check the wiring carefully, especially for corroded connections in the battery and alternator areas. I once had a Ford Sierra (not the one I've got now) pack up on me because of this, it turned out to be down to the alternator connection - no prior warning, and the fault wasn't even visible, but loosening the nut, cleaning the connection, and retightening cured the fault completely.
If you *do* have fuses in the lighting circuit, I wouldn't expect them to blow under these conditions. A fuse is intended to be the weakest link in an electrical circuit - it's whole purpose in life is to melt ("blow") before the main conductors do - protecting the equipment (i.e. your car) from catching fire. They are not intended to protect equipment from overvoltage damage - in fact, a bulb (even a fairly hefty headlamp one) is an extremely weak link in a circuit because it is already much closer to meltdown during normal operation and doesn't take very much extra current to tip it over the edge...
Stu.
davey
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Post by davey »

The alternator is a constant voltage (max nominal output of 14vDC) variable current device. It only supplies amps as required so if you have low electrical load you get low alt amps even at high revs.
The clue here is your 25 volts. I suspect the diode regulator pack has gone part short circuit within alternator. With engine running measure batt volts. This should not really be above 14vDC. Also take an AC volts reading (shouldnt be more than millivolts)- any substantial reading here will confirm AC leakage thru bust regulator.
Some alternators have 2 outputs - 1 to supply batt and main electrics the other to supply main headlights. So do same checks with main lighting circuit.
If they are high - check alternator out soonest as damage can/will occur due to overvoltage (replacement bulbs wont last long!).
Problem is; if alternator is knackered what caused that? possibly a large short circuit..... good luck.
lhm_leak
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Post by lhm_leak »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Also take an AC volts reading (shouldnt be more than millivolts)- any substantial reading here will confirm AC leakage thru bust regulator.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Good idea, but before doing this, make sure that the meter you are using gives a true AC reading, i.e. that it is capacitively coupled.
To test this, set the meter to read AC volts, and connect it across the battery with the engine stopped. It might "kick", or briefly display strange readings if it's digital, but should read zero afterward. I say this because some older meters will pass the DC component of the signal right through, giving unreliable readings. It's not a problem with more modern kit.
Stu.
davey
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Post by davey »

True enough lhm leak. Use good test gear every day on systems worse than dodgy citroen electrics... Like you say it has to be a true RMS meter to get 'real' reading.
Dont we just love electrical problems - wish i was a tree surgeon or somthing!!
tris
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Post by tris »

thanks very much for your replies, will check the car out tomorrow on the points you mentioned.
merry xmas
tris
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Post by tris »

hello again, was being a plonka and had the multimeter on the wrong setting! Yes voltage at the battery terminals with the engine running is 13-15v. Thinking back to what the time when the bulbs blew. The radio went dead, gave it a gentle tap, that didn't work. So for some reason i turned the ignition off and on while driving thinking back to a problem i had once when the key got stuck half way between on and start. Would that be y the bulbs blew. ie me being stupid?
tris.
tris
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Post by tris »

The alternator has been sprayed from a small leak from the engine, would this cause the regulator to play up?
tris
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Post by tris »

(engine oil)
davey
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Post by davey »

Tris .... turning ignition off whilst driving! For starters that puts electrical system in completely abnormal state (if engine already running when turning back on) so anything goes; not to mention small things like your steering lock could have engaged!
Oil in alternator ... fuel oil certainly eats carbon brushes (in your alternator) for breakfast and engine oil will easily cause contammination and premature failure of brushes. If oil definately in there you will have to strip it down pronto and clean otherwise it will fail if contamination is excessive (sooty gunk will begin to show around the alternator housing).
Enjoy.
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