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Dash lights not working. 12V- at Fuse 12.

Discussion in '348/355' started by carwhisperer, Jan 7, 2016.

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  1. carwhisperer

    carwhisperer Formula Junior

    Sep 29, 2014
    426
    Sacramento, CA
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    Brian
    #1 carwhisperer, Jan 7, 2016
    My 355 dash lights are not working nor do my license plates lights. I have tried adjusting the dimmer switch (bottom of the tach, right?) and nothing. I did some searching and saw and read that 7.5 amp fuse 12 in the luggage compartment goes to these two. The fuse was good but I thought I'd try putting 12V+ to both of the fuse connectors to see if the lights would light. Nothing happened on the rear side but when I connected 12V+ to the front side of the fuse connector the fuse in the wire I was using popped. I then measured and found that side is grounded. Neither side should be grounded, right?
     
  2. Qavion

    Qavion F1 World Champ
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    Ian Riddell
    #2 Qavion, Jan 8, 2016
    I don't see why you should be getting a ground on either side of the fuse holder.

    Assuming fuse 12 has been pulled and the lights turned off...

    On the positive side of the fuse, it should be open circuit. The volts from your power source would just go to the de-energised contacts of the relay (relay F on my car). The diagram also shows a branch off to relay G solenoid (optional front fog?). Again, if this is switched off, you should see open circuit.

    On the negative side of the fuse, it's a real rat's nest:
    One branch goes to the front luggage compartment light. The resistance of the bulb should should be evident in this circuit. i.e. no reason for your test fuse to blow. I would check to make sure the wiring to the lamp is ok, though.
    Another branch goes to the centre console switches (for switch legend lighting). Legend lighting in each switch consists of an LED plus a 550ohm resistor in series (there should be no shorts on that side because of the resistors).
    One branch goes to the dimmer switch (bottom of the tacho, as you say) for instrument lighting. I don't see how you could get a short here either (the lamps in the instruments should offer resistance).
    One branch goes to the rear license plates lights (the lamps should offer resistance).
    Another branch goes off to the "[P<]" parking light on light in the dash. I haven't been able to figure out this circuit. Unless I'm going cross-eyed, it looks like there are different voltage sources on either side of the lamp. I can't find the earth for the lamp.

    Are your centre console switches still disconnected? At least you could take these out of the equation if they still are disconnected.

    http://www.ferrarichat.com/forum/348-355-sponsored-bradan/509387-355-wsm-wiring-diagrams-component-list-incomplete.html

    Unfortunately, it's not so easy to isolate different parts of the circuit. It's rare that lamps short internally, so it's not like you could pull the lamps out of each branch, retest and hope to find the problem.

    Anyway, I hope something I've said provides inspiration. If nothing else, you have my sympathies :D

    Cheers
    Ian.
     
  3. carwhisperer

    carwhisperer Formula Junior

    Sep 29, 2014
    426
    Sacramento, CA
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    Brian
    #3 carwhisperer, Jan 8, 2016
    Thanks again for the help Ian. You helped me on that other thread you referenced as well!

    I did have my center console out to try to figure out my parking light switch. The wires/connectors to it are missing. I started down that road when my turn signals weren't working but abandoned it when I got them to work, figuring I could live without the console parking light switch.

    I think I'm going to try energizing the license plate switches, with a fused wire, and see if I can get the instrument lights to light up.

    Thank you again for the feedback. It really helps!
    Brian

    Now that I know the instrument lights are in that same circuit, I realize I shouldn't have given up on so easily and put my console back together.
     
  4. carwhisperer

    carwhisperer Formula Junior

    Sep 29, 2014
    426
    Sacramento, CA
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    Brian
    #4 carwhisperer, Jan 8, 2016
    OK a little more analysis has found that both leads to both license plates are grounded. Furthermore, I can get a weak 12V+ signal to the front side of fuse 12 if I energize what I think is supposed to be the hot side of the license plate lamp wires. Apparently I have a hot wire grounded somewhere in that circuit. I think the most likely culprit is the center console, but I don't relish the thought of cracking that back open. and digging all the way back to the footwell fuse panel.

    As far as "legends lighting", does this mean the lights at the switches themselves? When I was messing with the center console I remember seeing a couple of those come on at one point. Now none of them come on with the headlights switched on and the key on. Should they?
     
  5. Qavion

    Qavion F1 World Champ
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    Ian Riddell
    #5 Qavion, Jan 9, 2016
    Yes, LEDs provide the backlighting for the symbols (legends) on the switches. There may be two light sources in the Hazard switch. My car's Hazard switch has an LED for backlighting (with ignition and lights on) and a standard miniature lamp for flashing when the switch is in use (with or without ignition/lights). I assume this is standard. I found a shorted diode in my Hazard switch, but it didn't affect the other lights.

    Definitely. If they were lit up at some point, perhaps the wires in the console were moved into a position where they weren't shorted. Or perhaps you had some part of the circuit disconnected (removing the shorted item)?

    Unfortunately, there are a lot of variables. The problem could be anywhere.... shorts in plugs, shorts in switches, chafed wiring, etc... There are very few quick disconnect points on this car to help isolate circuits. I see lots of potential for future problems on these cars. e.g. the self tapping screws used for mounting the instrument cowling seems to go straight into wiring harnesses if you're not careful with how you position your harnesses. The way the passenger footplate jams up hard against the wiring looms is a worry, too. The wiring looms which run either side of the engine compartment seem to run the gauntlet of self tapping screws, too. By the way, did you notice which way the license plate wiring runs (left or right of the engine?)
     
  6. carwhisperer

    carwhisperer Formula Junior

    Sep 29, 2014
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    Brian
    #6 carwhisperer, Jan 9, 2016
    Yes, it goes to the US passenger side of the engine. I traced it from the license plate, up to where it goes into a larger loom, then up to the vicinity of the oil tank. I couldn't see any obvious problems with it up to that point.

    To be clear, are you saying you are pretty certain the hot side of the switch LED's are in the same circuit as the instrument lights?
     
  7. carwhisperer

    carwhisperer Formula Junior

    Sep 29, 2014
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    Brian
    #7 carwhisperer, Jan 9, 2016
    Progress! The connector to my cigarette lighter was also suspect. I disconnected it and I have dash lights and license plate lights without blowing a fuse! So either the cig lighter is also part of that circuit or someone tapped into it on my car.

    Thanks for the help. I'll just leave the cig lighter unplugged for now and fix it later. Thanks so much for the help.
     
  8. Qavion

    Qavion F1 World Champ
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    Ian Riddell
    #8 Qavion, Jan 9, 2016
    Ah.. good find. The cigarette lighter illumination is also in the console button backlighting circuit. It was on another diagram. I missed it.

    The power from fuse 12 goes to splice 30050. From here it is distributed to the car in 4 branches. One branch goes to the instrument lighting, one goes to the luggage compartment light, one goes to the [P<] parking light annunciator in the instrument cowling and the final branch goes to the passenger footwell fuse/relay panel where it splits in two (licence plate lights and console button lighting). I see now that various splices distribute the console lighting power to the console buttons, console switches, and cigarette lighter. This is lighting power only. Power for the switch contacts in the buttons comes from elsewhere (also, the high power for the heater circuit of the cigarette lighter).

    Anyway, this is definitely progress :D

    Thanks for the tip on the licence plate harness routing.

    Cheers
    Ian
     

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