Electrical gremlins... Trunk lamp inop.. Can anyone help?

[HFR_Racer]

My first post to this thread wasn't much help so I've armed myself with more information and am asking again for assistance.. Any advice is appreciated.

Root problem: My trunk lamp is inoperative.

Troubleshooting thus far:

1) Isolated and tested the trunk lamp and switch from the nearest two-wire connector (within a foot of the lamp). The bulb and switch function is ok.

2) I printed out the wiring diagram shown on Pelican which I believe is accurate for my 86 Carrera model. The electrical diagram from Pelican shows that the power originates from terminal #11 (trunk fuse panel). This is wrong for an 86 model.

3) I determined that power for the dash clock, glove box, interior lamp and trunk lamp originate from terminal #4 (trunk fuse panel).

4) The wire pair that controls this circuit is a black wire and a brown with white stripes. Continuity exists between terminal #4 and the black wire (at the harness connection, near the trunk lamp). Continuity exists between vehicle ground and the brown w/ white stripe wire.

Here is where the trouble begins....

1) I noticed that terminal #4 is fused at 25 AMPS!! The fuse panel labels shows that it should be fused at 5 amps. (see photo). Someone before me has been playing electrician....



[img]

Am I correct that this should be a 5 amp fuse or does the 5 on the label shown mean something else?

2) When I make the connection from the two-wire harness connector (from the main wiring harness) to the trunk lamp two-wire connector (the wires are the same colors) the connection arc (gets hot quickly) because it is shorting out.

3) Also, I do not agree with my current wiring of the trunk lamp harness from the lamp to the trunk switch (even though it looks factory). The hot (black lead) goes to the lamp providing current to the lamp (this is correct). What I don't feel is correct is that there are two brown wires coming form the lamp housing. One of the brown wires goes to the trunk switch (this is correct) since the lamp creates a voltage drop and the switch is grounded to the trunk panel when the trunk panel is open. The other brown wires goes back through the two-wire connector and back through the wiring harness behind the dash where the "short" exists.

Can someone that has an 86 with a functioning trunk lamp check out their lamp wiring for me and report back? Please... Please.. The wiring looks factory to me, but doesn't seem correct.. If this second brown wire goes back to a main ground somewhere on the vehicle then the trunk lamp would remain on even when the lid was closed.

(I've attached a couple of photos).

[img]
http://forums.pelicanparts.com/uploads6/trunk+lamp+harness_0011128992451.jpg[/img]

Anyone know what the second two-wire connection shown in the photo is for?



In a nutshell... I can make my trunk lamp operative by severing the second brown (ground )wire that leads back to the harness. The issue is that I want to find out why it goes back into the harness and if I have a secondary wiring problem.

Many thanks in advance...

[lagrassa]

This may not be the answer, but on my 70, the trunk light only comes on when at least the parking lights are turned on. It's worth a try.

[HFR_Racer]

Thank you for the input. The trunk lamp connection has power with and without the parking lamps being turned on.

Thank you for responding. Anyone else gonna take a stab at this one?

[450knotOffice]

I can't really add much to the discussion except to say that my '84's trunk lamp comes on anytime I open the trunk.

I wish you well, though.

[NoLift911]

Maybe try to look at it in the reverse of other switches - it is activated in the off position - it should have power all the time... by the way and work when the car is off (not much good otherwise)

I recently removed mine on my 84. The switch or contact switch was sketchy at best and my car only has 33k on it. Make sure everything has a good clean contact so you are not working against yourself. Something telse me you have a short to ground somewhere hence the PO 25 amp fuse in there (Or maybe they didn't have anything else) Check the simple stuff first - resistance on the switch itself and continuity.

The wiring as you have explained it and by the pictures looks correct to me.

-Jeff

[Jim Sims]

Okay, the last wiring diagram I have on hand is for an 1985 Carrera so I'm reading between the lines (traces ). I believe the brown wire with white stripe leaving the trunk light goes through a diode and then to a "clock relay" (a timer relay) for the seat belt warning system. This brown and white stripe wire should go to terminal 86 of the "clock relay" which also connects to one of the belt switch contacts. If someone has "tinkered" with the seat belt switch wiring or there is short in it or in the seat belt buckle switch this could be your problem. Timer relay could be bad too. Fuse should definitely be 5 amp; 25 amp fuse was the knucklehead approach to fixing the problem. Hopefully not too much wiring has been damaged. Let me know what you find. Jim

[Mysterytrain]

Does the car have a factory alarm system installed? My only point of reference is my 78SC. The trunk light comes on whenever the trunk lid is lifted. Power is provided thru black/white wire that is part of the glovebox, clock and interior light circuit coming from fuse 18 (5 amps). The ground is a solid brown that runs from the light housing to the switch and is grounded thru the chassis. Anyway, back to the alarm question..looking at the bentley there is a one page schematic showing the door and hood switches being tied into the alarm system [for an SC]. It shows a Brown/white wire from the trunk switch to the alarm. So there might be two 'ground' connections at the switch in your case. Also, while I'm thinking about electrical circuits..bentley routinely labels some connections NCA ..anyone know what that means?

[fred cook]

I believe the trunk light has a mercury switch built into it. If the switch is bad, even with power at the switch it won't come on. Can you "borrow" another switch to try?

[HFR_Racer]

Mysterytrain:

I do not have a factory alarm installed on my 86 model.

[HFR_Racer]

Furthermore... I took a little bit of time to write this post and document it with photos. It frustrates me that some people feel free to contribute their $0.02 when they obviously haven't read or understood my original post.

I truly do want people to contribute their ideas, but please read my original post and don't cloud the issue at hand.

Thank you,

[Jim Sims]

"trunk light has a mercury switch" ? Where did you see this? I had some of these lights apart and there's no switch to be seen. The switches I've seen are spring loaded plunger types mounted on the hood hinge and contacting a shelf/spar projecting from the body.

[HFR_Racer]

Jim:

Thank you for your posts.. Your replies have been accurate.. Can you send me the 85 wiring diagram that you referred to in your prior post?

Please please... Is it from the OEM manual of from the Bentley?

[Jim Sims]

I was looking at very poor reproduction of the Porsche factory wiring diagram for a 1985 Carrera found in the back of the Haynes 911 "shop manual". Later this evening, I may be able to borrow copies of the real color factory wiring diagrams for a 1986 Carrera from a set of factory shop manuals. If I do, I'll report back. Cheers, Jim

[HFR_Racer]

Awsome Jim.. I sent my personal email to your mailbox here so you can email me the diagrams later.

The previous owner had an aftermarket cell phone installed and this might be where the elecrical issues began.

[Jim Sims]

The 86 Carrera wiring diagram is the equivalent of 22 pages; most of the sheets are three page foldouts. The schematics are black and white with the wire colors labeled with German wire color abbreviations; it appears the factory shop manual color wiring diagrams stopped with the SC's. First of all the fuses: the wiring diagram numbers the fuse block slightly differently than your fuse box cover. In the picture above, left to right, 1, 2, 3 and then the numbering starts over again with 1 to 10 then 1 to 8 so the wiring diagram 2nd number 1 is the same as what your fuse box cover calls number 4. This 2nd number 1 (or 4) is the fuse in question and should be a 5 amp fuse; five amp fuses are colored coded yellow and are somewhat difficult to obtain. You can order the correct 5 amp fuse through Pelican (Porsche PN 90017300690) for about $1 each.

The brown and white wire after leaving the trunk light passes through a 2 pole (2 terminal) connector (which you found) that also contains the black wire coming from the trunk light. The black wire then goes to the glove box light. The brown and white wire goes to terminal 6 of the "Burglar Alarm" control switch harness (a female connector). In cars equipped with the alarm system this terminal connects to male terminal MK on the alarm control switch. In cars like yours that don't have the alarm system (and therefore have no alarm control switch) this terminal 6 (and the brown and white wire) should just end and should not be connected to anything. The wiring diagram does note however that in non-alarmed equipped cars (like yours) in the alarm control switch harness, terminals 7 and 8 are shorted together and terminals 1 and 4 are shorted together. Shorting contact bridges are used to do this. These are likely correct and okay as one connects to the DME relay and the other to the alternator. If they were wrong you would have lots of other problems. Your trunk light circuit problem is likely elsewhere (more on this later). Note that the alarm control switch harness may be found in the front trunk forward of the instrument panel behind the carpet and "treated cardboard" panel and apparently the vent/defroster blower assembly may have to be removed to get to it. Terminal 2 of the control switch harness also comes from the seat belt buzzer relay system but since you lack an alarm it doesn't do anything in your car. More later.

[Jim Sims]

Back to the possible short location. It appears to me there is a short in the wiring between the two-pole connector (a foot away?) and the trunk lamp. Given that the 2nd brown and white wire (the one that you disconnect to make the lamp work?) is on the ground side of the trunk lamp light bulb (at least that's where the wiring harness indicates it should be) it shouldn't matter if it is grounded somewhere else deep in the wiring harness. Although again according to the wiring diagram it isn't grounded but just ends (as explained above) in a non-alarmed car. However if the 2nd brown and white wire is shorted to the black wire (pinched together or crossing a sharp sheet metal edge together) then there is a short that bypasses the light bulb (so it doesn't get much current and doesn't light) and goes directly to ground whenever the the trunk is opened (switch on hinge closes). This of course will blow the 5 amp fuse also taking out the radio, clock and interior lights. The PO solves the problem by installing a 25 amp fuse in position 1 (4) which can withstand the short circuit current flow to ground whenever the hood is opened; note the shorted circuit has some resistance by virtue of the somewhat bad connection at the short and the fact that the black wire heats up (all the way back to the glove box light and further upstream) and increases in resistance (like a heating element) during the fault. Hopefully not too much damage has been done (trunk wasn't left open a lot). I would look for the above described short and wouldn't reconnect the two-pole connector until you find it (or remove the 25 amp fuse when you connect the the two-pole connector). Cheers, Jim

[450knotOffice]

Furthermore... I took a little bit of time to write this post and document it with photos. It frustrates me that some people feel free to contribute their $0.02 when they obviously haven't read or understood my original post.

Easy there. This comes across as somewhat ungrateful, to say the least.

All of the responses to your question looked sincere, even if some of them might not have helped you at all.

Take them for what they are, attempts to help a fellow Pelican.

[HFR_Racer]

Sorry for the previous tone.. Jim has been very very helpful in solving this electrical issue.

Thanks Jim... I'll check the trunk lamp harness (local to the lamp and hood switch again and report back)... Your correct that my fuse panel numbering goes from 1-3 and then starts over from 1-10 and then 1-8. The label on my fuse panel cover lists the fuses from 1-21 and also (1-3,1-10,1-8). My glove box lamp works without the trunk lamp 2-pole connection made so the black wire must branch off to the glove box lamp prior to entering the 2-pole connection at the trunk lamp. More later....


Thanks again,

HFR_Racer

[Mysterytrain]

sorry I bothered

[Jim Sims]

"My glove box lamp works without the trunk lamp 2-pole connection made so the black wire must branch off to the glove box lamp prior to entering the 2-pole connection at the trunk lamp."

Actually the black wire from the trunk connects to the "hot side" of the glove box light fixture but this has the same circuit effect as you describe above. Jim