Help park/Dipped lights & tail lights blowing fuses

Get a list of every component on that particular circuit and visually check them one by one. Wouldn't be surprised if it's a short in one of the tail light assemblies or sockets.

BTW, I'm assuming there is one or more relays for the dipped beams. Remove it (or them) and see if the fuse still blows. If it does you know the problem is elsewhere.

 

The more relays on that circuit that you can remove and connectors that you can disconnect the easier it'll be to isolate, since removing a relay eliminates the components it actuates from the circuit. Assuming the fuse still blows you can focus your attention on the components that are not actuated by a relay. If the fuse quits blowing then you can start reinserting relays until the fuse starts blowing again, at which point you've found the problem area of the circuit. When you have a short in a lighting it's almost always a lamp or a socket as long as the car is in good shape (if the wiring is tatty all bets are off).

Also note that the light switches themselves go wonky but I wouldn't expect that to cause fuses to blow since there shouldn't be a way to create a direct short inside the switch.

Isolating various section of the circuit by removing relays and/or connectors will help expedite the search. For example, if you think it's a problem in the tail lights remove the connectors to both of them and see if the fuse still blows. If it doesn't you know it's in one of the tail lights, so you can just plug them in one at a time to determine which one is the culprit and then diagnose the unit that's bad.

Hope it's something simple. Let us know what you find in hopes it'll help someone else in the future.

 

Blowing both fuses, Steve? This doesn't make sense. There are two fuses for front and rear position lights. One fuse is for the left side (front and rear), the other is for the right side (front and rear). Can you confirm the fuses that are blowing?

I can't see how you can blow both position fuses unless the left and right side are shorted together (even then, I would think that one fuse would blow before the other due to minor differences).

A faulty relay shouldn't cause the fuses to blow (because it's before the position light fuses in the circuit). Power >> Relay >> Fuses >> Lights

Does the Park button on the centre console blow the fuses?

 

ah. ok... so it's more the dipped beam than the tail lights. Although there are separate fuses for the dipped beams, the wiring does run in parallel through the same plug ("F") exiting the fusebox (pins 9 and 10). I would focus on the wiring in and from the fusebox to where the loom splits in two (one going to the left headlight, one going to the right headlight). Sorry I can't be more specific. I haven't followed this harness before. (EDIT) Some of the position light wiring also runs through plug "F".

Has any work been carried out in this area? Has someone put a bolt or screw which is too long in the wrong place, piercing the wiring harness? When did you notice the problem?

A faulty switch shouldn't blow the fuses because the switch only controls the coil of the relay and is independent of the relay output (where the fuses are).

 

Looking at the harness routing on the Ricambi website...

ELECTRICAL SYSTEM -VALID FOR ABS BOSCH AND 355 F1 CARS- - Ricambi America, Inc.

The diagram is probably simplified, but it looks like the harness comes out of the fusebox, runs along the left side of the car, then splits close to the left hand headlight.
I can't see how a short circuit beyond this split would cause both fuses to blow.

 

To be honest I didn't even consider the possibility that you were blowing both fuses since that's a really unlikely scenario. Apologies-- I should have asked. In that case it's almost certainly not something simple like a tail light socket, and the reason for your focus on the relays suddenly becomes apparent.

I'm only familiar with the 348 lighting circuit and have no idea how similar it is to a 355, but will throw this out there in hopes it might help. On the 348 the two circuits are tied together at the center console park light switch, the emergency flasher switch, and *maybe* at the light control module for the ignition key lamp.

I'm not where I could look at the 355 diagram or I would, so all I can do is wish you good luck in finding it for now. May try to look at the diagram later if you haven't found a solution by then. Sorry I can't help more...

 

What turned out to be the problem here?

 

Never insert higher amp fuses. On 348-s, the two 7.5A fuses (3 & 4) are for the headlamp motors only. High and low beams have separate fuses. So, I would look at the motor relays first, relays H and I. Pull them out and see whether the fuses blow or not. If these relays have been replaced recently, check whether they are correct. They should have pins marked as 87 and 87a and should NOT have a diode symbol like ->- between pins 85 and 86. If these relays are the type with diodes, they will blow the 7.5A fuses.

 

Miroljub, why would the low beam fuses blow when the motors, high beams and low beams all have separate relays and fuses?

 

On 348-s, there are only two 7.5A fuses in the luggage compartment relay board and they are for the headlamp motors. The high and low beam fuses are 10A. So, based on the 348 arrangement, and since the KingSteve's 7.5A fuses were blowing, I concluded that the problem is related to the headlamp relays or motors and not to the light bulbs. Just looked at the 355-s fuse chart - the low beam fuses are LH 15A, RH 10A so the blowing 7.5A fuses are not for these. The 355's headlamp motor 7.5A fuses are fuses No. 1, 2 & 3; fuses 13 & 14 are also 7.5A and they are for the right and left parking lights. So, which exactly fuses are blowing?

 

Good point. This points us in a new direction, but is still as confusing. Two new (wrong) relays fitted but not tested? Or perhaps relays swapped from elsewhere for a completely different problem? It might pay KingSteve to check all the part numbers and fuse values in the luggage compartment as per his owners manual.

Not related to this issue, but I've noticed some cars have 15A in both these positions (perhaps the '94 models?). I assume the LH has a higher amperage because it handles both the low beams and the rear fogs.

 

Thanks for the update. Glad it was something simple, even though it might not have been so easy to find.

The relay housings should be electrically isolated from both + and - voltage. They're just covers for the relays and if they have any voltage potential there's something wrong inside the relay.

Sounds like the problem really was in the LED lamps.

 

I remember an old aircraft electrician telling me when I was an apprentice... If there's an usual wiring problem, always go for the last modification

So the fuses were actually for the side lights, not the low beams or motors.

Are the lamps polarised? Before we go shooting the manufacturer, best to check that the installation was correct.

Cheers