What most of us fear - a phantom short

[georgem]

Like most of us, I enjoy tinkering on my car and have, over the years acquired a fair amount of mechanical competence - from our 230 and also other cars that I have restored. I say mechanical, please note, and not electrical as I always seem to struggle here, especially finding short circuits.  Well, sadly I have one.  I am continually blowing the fuse that controlls the dash lights, RH side low beam and tail light.

Before I start looking for the needle in the hay stack, has anyone had this problem before or any suggestions on how to track it down. So far I have resisted the temptation to replace the fuse with a fencing nail but I don`t know how long I can hold out. 

BTW, it happened again the other night. Both high beams are ok so I used them and no oncoming traffic objected. I may have to upgrade the bulbs to halogen!

Any help would be appreciated

Cheers

George
230SL Blue - Grey interior
Brisbane

[Peter van Es]

George,

check: http://www.sl113.org/wiki/Electrical/Start and especially the sections on Troubleshooting and Grounding. You'll als find circuit diagrams and connector pin-outs there to help you trouble-shoot.

First question: did anything change -- even if seemingly unrelated to electrical issues -- recently?

I had shorting problems on my left headlight after a rust repair and repaint... what happened is that the connector for the headlight assembly came apart and the mechanic re-assembled it, such that when the headlight was out of the car, everything worked. He swapped two leads however, such that when the headlight was back into the chassis, everything appeared to work. When indicating left however, there were intermittent short circuits which fried the indicator relay. I can tell you, that took a while to find...

So check back: what has changed, what has happened? Is (or was) there ANY nonstandard wiring associated with that fuse?

Peter

[georgem]

Peter,

Thanks for your thoughts. I will follow through. The other thought that crossed my mind was the size of the fuse. I checked the BBB but couldn`t find the fuse size. Anybody have an idea what size fuze #7 should be? (or where I can find the info)?

Cheers

George
230SL

[georgem]

Hmmm, the mystery starts to unravel.......

I checked the tech manual (should have done this 1st up) and the fuse that was blowing was #7, rh tail light, instrument light as well as the rh low beam. BUT the tech manual tells me that the RH low beam should be on fuze # 11.    SO, maybe a PO had incorrenctly wired the fuze box and fuze # 7 can`t carry the intended load PLUS the rh low beam.  I won`t say the problem is solved but it gives me something to shoot at.

Cheers

George
230 SL

[waqas]

So far I have resisted the temptation to replace the fuse with a fencing nail but I don`t know how long I can hold out. 

Please continue to resist, as the damage to wiring or the resulting fire may be much harder to fix.

Can you isolate which accessory causes the short? Does it depend on the load? (i.e. the number of accessories vs. the any one in particular) If the latter, then inspect the ground connection and the wires in the vicinity of the electrical accessory in question. If the short occurs when a certain number of accessories are switched on, regardless of which ones in particular, then it may be what you already suspect: insufficient fuse rating.

In order to eliminate the fuse rating as a culprit:

• Make a list of the group of accessories that cause the fuse to blow.
• Narrow the list down to the ones on that particular circuit (check this by verifying that a given accessory no longer works once the fuse is removed).
• Determine the power rating of each accessory in Watts (e.g. standard European headlight bulb in low-beam is 45W, assuming no upgrades)
• Divide the power rating (in Watts) by the voltage applied (should be 12V, as per the battery) to determine the current draw (in Amperes) of each accessory.
• The sum of these currents (in Amperes) should be less than the fuse rating (also in Amps) for that particular circuit.

Let us know how it goes.

Of course, you should plan on eventually getting the wiring back to how it's supposed to be.