Perplexed. My left headlamp is out, but full beam works. The lamp tests good and the multimeter tells me there are ±12v at the terminal with the lamp removed from the 3 prong connector, but as soon as I plug the lamp in, there is no voltage at those 3 prongs! I am confused (and a fixit ticket looming!)

Any thoughts?
Cheers

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97 RAM2500 Cummins affair. Big longass lifted biodiesel thing. Too big. Too spendy - got rid of it. Sure could pull some weight though.

Grounds? Corrosion? It sounds as if there is voltage potential but a severe current limitting situation (causes voltage drop hence voltage with a meter as the meter sources no current)

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1982 300SD 180K, rebuilt engine
1973 450SLC Megasquirt
1990 Volvo 780 - 273k
1993 Volvo 240 Wagon - Scrap yard slumber

http://www.fuelly.com/sig-us/44619.png

Thanks for the response.
The contacts seem OK (impressively robust wiring) but how do you check for ground issues?

Forgot to mention the lights were intermittent before going out - switching randomly from left headlamp bad, to right full beam bad! Right (full) is fine now that left (dipped) won't play.

(I don't expect these to be related but I have no hazard lights and the indicator relay seems a little temperamental)

Thanks again

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97 RAM2500 Cummins affair. Big longass lifted biodiesel thing. Too big. Too spendy - got rid of it. Sure could pull some weight though.

Are you testing a sealed beam or a "H4 bulb"?

I'm assuming that you have looked "back down the harness" right? Broken wires, bad grounds, etc...

Have you swapped bulbs (sides, to rule out the bulb)?

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AJ

1985 300D (SOLD)

yeah, checked sealed bulbs and wires, but they disappear into that big sheathed bundle and don't seem easily traceable

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97 RAM2500 Cummins affair. Big longass lifted biodiesel thing. Too big. Too spendy - got rid of it. Sure could pull some weight though.

Faulty relays/connections to the relays? Not sure where these are on a 123. Did you check all of your fuses (spin and check for corrosion) With your flasher going I really suspect ground issues. I do not have a 123 schematic or I would send one. Good luck!

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1982 300SD 180K, rebuilt engine
1973 450SLC Megasquirt
1990 Volvo 780 - 273k
1993 Volvo 240 Wagon - Scrap yard slumber

http://www.fuelly.com/sig-us/44619.png

Check Fuse 13. Also let us know if the left fog light is out. F13 is dedicated to the left low beam, and the left fog light uses the same ground. The circuitry above these points is common to both left and right, so that leaves these as the most likely source of the problem

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

>>as soon as I plug the lamp in, there is no voltage at those 3 prongs!

This is the true situation, and as others have written, it indicates that there is a high resistance somewhere else in the circuit.

Leaving absolutely everything plugged in, and the lamp switched on;

A) put the positive lead of the multimeter on battery +ve, and the negative lead on the live terminal on the back of the bulb - what voltage drop do you read?

then

B) put the positive lead of the multimeter on the earth terminal on the back of the bulb, and the negative terminal of the battery - what voltage drop do you read?

If you read a large voltage, say 12v, on test A, the fault is on the live side, between the battery and the bulb - most likely the fuse as mentioned above.

If the large voltage is found on test B, then, you have an earth fault.


The idea of the test is that when the circuit is running, there will only be a small voltage drop across a good length of wire - as soon as there's any bad connection, the voltage drop will increase.

Well - after all that testing and head scratching it was the fuse which was not blown, but the end contacts had worn away allowing a voltage reading, but not enough current to power the lamp.
Seems obvious now in retrospect, though I'd not have imagined an individual fuse per lamp.

I am grateful to find the support and direction I get from you all and understand a little more about the basics of voltage drop, current and resistance in the process. Cheers!

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97 RAM2500 Cummins affair. Big longass lifted biodiesel thing. Too big. Too spendy - got rid of it. Sure could pull some weight though.

"Nuff said

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily