tntfox

I've read hundreds of posts about bad headlight switches, but what I haven't found is the location of the infamous lighting harness. When mine was burning out, I could tap on the dash directly above the radio and the dash (and I assume tail) lights would come back on.

So, my question is this : is the wiring harness related to the headlight switch located in the steering column area or is it under the center part of the dash?

I have a 91 conv in excellent condition and I dont want to trash the interior on my first attempt to fix something. Dealer cost to fix, by the way, is in the $500 range ($380 for parts plus labor).

Thanks

hornbm

Sounds like a short to ground man, probably in that area. Your gonna have to pull the pannel off, and look for a wire thats rubbed through the insulation.

Icemark

The headlight harness is plugged into the head light switch and the turn signal switch. It runs from there to the driver's kick panel/fuse box area.

tntfox

I did at one point smell something (like plastic burning) and a little while later the problem with the dash / tail lights showed up.

Should I be able to see the wiring harness if I remove the dash piece surrounding the instrument panel? Or will I have the remove the instrument panel too?

If I just have a short, I'll splice the wire and tape it in a heartbeat to save $500 and be able to get the thing inspected (and drive at night).

Thanks for the help.

TGF

Icemark

Yes, if you remove the cluster surround/bezel you be able to access the harness and switch.

If the plug or switch is burnt you will need to replace both. Also you should get in and clean all the contacts to every bulb on the exterior of the car. The most common reason for harness melt down is too much resistance in the bulbs because of corrosion or corrosion in the harness at the light switch, or both.

There is also a relay mod, to increase the power handling of the stock switch. It will be found in the 2nd gen archive.

renns

Icemark,

How does an increase in resistance at the bulb cause a higher current draw at the switch? Doesn't make sense to me...

In reviewing the wiring diagram, it looks like a design problem IMO. The high current tail light loads are run through two _adjacent_ pins at the switch connector. The connector contact sizes at the switch end are also significantly smaller than the same contacts at the main harness end. The high-load pins should have been spaced apart on the connector.

I stuck a relay in to fix mine over a year ago, and the problem is gone. It's a 5-minute fix with any spare SPST relay you find laying in your toolbox, assuming the harness and switch are not melted beyond repair.

BTW, I've got a headlight harness on ebay right now, going for 1 dollar, with no reserve. This will save somebody some grief down the road...

http://cgi.ebay.com/ebaymotors/ws/eB...tem=2444188707

tntfox

Thanks Icemark. One last thing ... Will it be visibly evident that either the switch or the harness is fried? I assume that if I can look at either from all angles I may see some burnt wiring but will the switch itself show any signs of damage?

Forgot to ask before, but does the bezel come off easily (with just a few screws?).

Thanks for your help.

TGF

Icemark

#1 the current does not increase with resistance. If it did you would blow the fuse and that would solve the problem and protect the switch/harness.

The additional resistance (much like adding sandstone resistors) simply heats up the connections at its weakest links. The switch connection is the weakest link. The current draw is still the same however.

To back this up, if you have ever seen a switch/harness fail it almost always fails on only one wire of the connection initially. If not corrected, however it will progress to the pins and wires next to the problem one that also carry high loads.

#2 there is only one lead that is for the tail/parking lights at the switch. So unless you are confusing the dash lights with tail lights, or the input to the switch as a tail light wire, your "design flaw" doesn't float.

#3 it is almost unheard of the switch/harness connection failing in moderate climates. Interesting enough they do tend to fail on climates that have high humidity or harsh weather (such as winters with snow).

On that same vein, if you look at the bulb connections on the cars in that have had the light switch/harness fail; they all have been not maintained with conductive grease or dielectric or even a regular bulb replacement (why do people only replace bulbs when they burn out instead of when they meet the end of the useful life? but that is another rant).

If it was truly a design flaw related to plug design then cars in moderate climates would have the same problems, or the problem would have shown up back 15 years ago when the cars were new.

While I do agree that Imansen could have used upgraded wiring and connections for the average consumer, and that Mazda could have spec’d better dielectric for the bulb sockets, the extra relay installation that I have posted before solves the problem, as does properly maintaining your bulbs and sockets.

Icemark

You will see burnt plastic and/or discolored metal.

On the S4 the cluster surround can be taken off without removeing the steering wheel. Then there are two large screws on the bottom of the cluster surround and three above the gauges.

Slide the whole thing forward, reach around and unplug two plugs on each side (if you have cruise- two plugs on the drivers side, one plug on the passengers if you don't have cruise).

Then slide the whole thing up between the steering wheel and the dash (except on S5 cars where you will have to remove the steering wheel).

Turbonut

iTrader: (1)

I can't believe this, I actually can add something to Mark's reply. On S5 vehicles, the steering wheel doesn't need to be removed. Have had my housing off a couple of times and didn't need to remove the steering wheel.

renns

Mark,

An increase in resistance at the socket reduces the current in the circuit. Can't see how that manifests itself as an overheated connector. Is there possibly another mechanism at work? Perhaps the corrosion in the socket is creating an alternate current path, reducing the resistance of the socket/bulb assembly? In any case, climate-correlation is interesting, but I still maintain it's a marginal design at best.

Icemark

No, current stays the same (well close enough to the same and still way under the 8.325 amp draw that a 7.5 amp fuse needs to blow).

That is why the fuse doesn't blow.

Current= amps.

renns

If current stays the same, what is it that is causing heating of the switch connector?

Corrosion at the connector is possible, but likely also due to the increased current draw the incandescent bulbs see in cold weather. Years ago I talked with a manager at GE who said their sales of headlight bulbs went up dramatically in late fall as both usage of the light, and current went up at that time of year.

In any case, it's good to know a $2 relay can fix the problem.