Dodgy wiring HELP

[cynic-al]

My P1800 started smoking from behind the steering wheel today, intermittently, on a brief and rare drive to work. The RH indicator has not worked for a while and this may be connected.

I've disconnected the battery to be safe, I am guessing I should get a tow home rather than risk driving it - any thoughts?

I had thought that the fault might be easy to diagnose (replace the melted wire!) is that likely?

TIA

[PaulWphoto]

Finding the melted wire is only the first stage i'm afraid - you must have a short circuit in there somewhere and you need to rectify the fault before you replace the wire. Otherwise it could easily happen again. At least if you can identify the damaged wire it will give you a place to look for the short circuit.

Regards Paul

[cynic-al]

Thanks. Home now at least, courtesy of tow-truck, never though I'd see the day!

[Ron Kwas]

Al;

Ref wiring diagram: http://www.sw-em.com/1800%20Wiring%20Diagram.jpg

P1800s only have 3 fuse total protecting all of their circuits. 1. Battery Power, 2. Ignition Power, and 3. Parking (marker) Lights Power [...ask me not for the logic behind this questionable design...and that's putting it nicely!!]. This does mean that the current rating of the fuses 1 and 2 had to be quite high (35A) to accommodate the normal (expected) current levels. The current rating for Fuse 3 is actually also 35A, which is in fact too high (!...it should be 8-10A) and unfortunately this also means that when a true failure occurs, which the fuse should clear, the fuse may not blow open and clear the fault.

I don't mean to scare you, but from the limited info and symptoms you have given, it sounds like you have a (possibly simple) condition (like a loose terminal) with potentially serious consequences.

Before reconnecting the battery, I'd expend a good amount of effort to try to locate the source of the smoke...including looking at back of dashboard with a good light source (and possibly a mirror) and lightly probing around with an insulated poking tool...also wiping with a cotton swab and sniffing it...once you've located the melted wire or loose terminal, or area damaged by heat, report back with findings. You can also trust the color code to tell you which circuit and load device were involved. I also recommend squaring this issue away before taking any significant drives.

Good Hunting!

PS. It's important when something like this occurs to gather as much associated info...can you associate the smoke with having turned ON any particular load...can you associate the smoke with a gauge (ie fuel, tach) or indicator (Charging Indicator) working or not working or lit or not lit? Were headlights ON at the time? Info like this is often invaluable in locating the problem.

PPS. Again, I don't wish to unnecessarily alarm you, but: Replace those Lucas Electrical Aberration FuseBlocks! See: http://www.sw-em.com/tech_bulletin_3.htm

[cynic-al]

Just had a proper look at this (car not used much), this is the only damage to any wiring I can see (behind the headlight switching):



Can I assume that the wiring has worn through, touched & shorted, heated up and melted the plastic insulation etc....and the only issue is to keep the brown and blue wires apart & insulated? Or is it likely to be more involved. I've no experience of car electrics, just school physics!

[Tail]

Was going to say after all these years it may have just rubbed through, it happened to me on an Astra GTE years ago, smoke everywhere and the loom looked like melted cheese, turned out an ignition wire had been rubbing against a small bracket for a long time then wore through. Looking at the photo though, those wires are burnt from the terminal suggesting something shorted out in there and then it disappears into the braided part of the loom, you may have to take that apart to find out how far it goes,it could be a fair way, can you trace to the far end to see if has gone all the way along....

[mikealder]

See if you can find where those two wires exit the cotton overbraid at the other end, if the cables are showing damage at the other end as well as near the switch the whole harness is probably toast - You need to check this out properly as an electrical fire in a car is very dangerous, more so if you are at speed when occurs - Mike

[Ron Kwas]

Al;

[long post...but it includes explanations...and things like this just can't be answered or explained in "ten words or less"]

It's quite apparent from the picture, that an unusually high current flowed in the Brown and Blue wires (this type of thing is typically caused by a short to chassis)...this caused wires to heat to the point of melting the insulation causing a smokeshow...sometimes worse...so you can actually consider yourself somewhat lucky! [Comment: ...it would have been nice if a fuse in the circuit had likewise heated and blown open to clear the fault before this happened! But this is clearly not the case in this circuit**].

...that Brown wire supplies Battery Power (meaning UNFUSED!) to the headlight switch by way of a heavy gauge wire (reference wiring diagram at: http://www.sw-em.com/1800%20Wiring%20Diagram.jpg ...so there was plenty of power available to pass in the circuit (and to the fault too!)...and from the wiring diagram, one can see that it was the headlight circuit on which the fault occurred. The Blue wire runs off to the Footswitch. (It is also quite clear that the Headlight switch must have been in the ON position at the time.) Headlight switch itself is unlikely to be the problem...output (Blue) wire being overheated indicates short was downstream...at footswitch or headlights themselves...

Question: Can the smoke be associated with any particular action having to do with the headlight system...such as turning it ON, or pushing floor mounted Hi/Lobeam switch...or having recently replaced a headlight element...or having a connector in the engine compartment pull apart or crumble...or having had an accident where a headlight bucket may have taken a mechanical insult...any such sort of thing? Please report back...it could help in troubleshooting...

Bottom line: You are half way there (and you can consider yourself fairly lucky in that damage is somewhat limited)...you know the circuit on which the problem lies, and are in an excellent position to proceed and locate the precise cause of problem so that it can be permanently corrected.

At this point, I would inspect Footswitch for damage or dislodged wires....if you find nothing seriously amiss there (I do expect that you will find more melted wires...please take precise note of these and report back which ones were melted and any other details), then I would recommend disconnecting the Brown wire from Headlight switch, sleeve any compromised insulation allowing copper to show (I prefer heatshrink sleeving for this but tape would do) then wiring a new fuse (with 10A current rating, maintain heavy wire gauge to fuseholder) in line with the wire and terminal from which wire was removed, then replacing wire on same terminal. This is a long-term fix which you can leave in place, and it is also a troubleshooting aid, because it places a the fuse inline with the problem circuit so that fuse can blow and protect wiring should a short and resulting high current reoccur (and there is a good chance of this happening until you repair the root-cause!)...so you will still have the cause of the problem lurking in the works somewhere, but you can drive with relative confidence knowing that you may blow that new fuse (and you WOULD loose the headlights at that point), but at least it will be unlikely that you'll loose the car to another and maybe total meltdown!!!

"only issue is to keep the brown and blue wires apart & insulated? Or is it likely to be more involved." Keeping those two wires apart is only part of the repair...sure they need to be insulated to prevent an unwanted connection, but their connection was not the cause, but only the result of the problem.

...hope this helps: Good Hunting and Cheers from a beautiful spring day in Connecticut!

** Explanation: Headlights are considered a "Mission Critical Load"...that is they are not fused such that a nuisance blown fuse (which sometimes occurs) would not make the headlights unavailable...designers made the decision that they would rather incur the risks associated with an "unprotected" circuit. Unfortunately, IF a problem develops on that circuit (and thankfully these are rare) the result are precisely as you have experienced! [This is all arguable...I personally would prefer more fuses to less...]

[cynic-al]

Thanks again Ron, looks like you are right, see wiring to foot-switch. I was sure my lights were off at the time though!

[Ron Kwas]

Al;

...no surprise there...the entire length of the Blue wire was carrying the overload current so the entire length got toasty and can be expected to have (luckily only partially!) have melted its insulation...this is obviously a confirmed fact from the remaining evidence...the Blue wire is the incoming power to the footswitch...

...so the next thing to check is which OUTGOING wire carried the overcurrent...referring back to the wiring diagram (fotomkoplare) we can see that the Lobeam outgoing wire is the Blue/Red, and the Hibeam outgoing wires are the two Blue/White (one powers the indicator on dashboard)...from the footswitch...from the picture, I cannot make out heat-damage to any of the outgoing wires, so unless you can ( by closer inspection of those three wires) tell which is also heat-damaged, (which will send us to the next location), the switch itself may have internally developed a short to chassis.

Next Step: Very Carefully Inspect Blue/Red and Blue/White wires at footswitch for heat damage...if you find it, then go to other end of that damaged wire and continue following path of overcurrent to short...if none is apparent, internal short at switch is likely and needs to be confirmed...by disconnecting all wires from footswitch and checking for a short (single Ohms) to chassis from three terminals (especially the terminal Blue wire was on)...

...please report back...can you believe this remote troubleshooting???...it's actually working quite well...at this rate we may be at the root cause by tomorrow...and you'll owe me a pint o' Guiness!

Good Hunting!