Fuse keeps blowing...any ideas?
 

I noticed that my left side tail lights were out. Looked a bit futher and discovered that the left side front running lights were out too. It turns out that they are on their own 5A fuse. Yep...it was blown. I replaced the fuse and it popped immediately. All of the bulbs look fine (filiaments are intact). Is there something obvious I should check? I'm not very fluent with electrical systems (that's one reason I sold my Triumph TR6). BTW, all other electrical devices work fine.

Any ideas would be welcomed.

You will be getting some great advice on this bbs of which this response is not, but here goes.
You should definitely check your taillight wiring for a short to the body from the hot lead. You can buy a cheap needle electrical tester which will help you identify which is the hot side (actually, you will need power to use this tester. )
So, maybe pull the light bulb in the rear and replace the fuse. If okay, you short is downstream of the bulb. Then try the running light bulb and replace the rear bulb light (now your tester will work). Continue checking.
Bottom line is you need to hunt for the short. Maybe someone else can give much better advice.

Chris is much too modest. His advice was very sound. Also, SEARS has had some great buys on multitestors lately, and they sell a book for $10 on how to use them that is full of info even an electrical idiot like myself can understand. Warren would consider it a pre-school text...;), but it sure helped me.

If you are working alone, then a buzzer or a meter with an audible tone makes the job a lot easier.
take out fuse
put meter on audible resistive scale, place one lead on side of fuse to the lights, (not the 12v side) the othe lead to ground you will hear the tone. Pull the bulbs one at a time, if the tone dissapears that was your problem.
If the tone is still there GENTLY move the wiring around the lights,listening for an interuption in the tone.
The object is to get the tone to dissapear.
Once you start it will become easy.
Jeff
911T

Mike,

I have a Digital multimeter you can borrow. I might be able to help troubleshoot as well - depending on schedules of course. Give me a call about the meter.

Well, Mark and I just spent the last three hours poking around and blowing fuses. Get this....with ALL the bulbs removed, the fuse still blows the instant the light switch is turned on.

Open for suggestions. I can't bear to take it into a shop and leave it for something like this.

Thanks for your help.

I know I'm an idiot, but did you check the wires around the headlight switch? MAybe the switch or related wiring is jacked up?

Just shooting in the dark.

Mike,

This problem is usually a combination of mud and water leakage into the rear lamp housing. So, my suggestion is to unplug the 6-pin bulkhead connector, and remove the housing from the car. Clean the complete wiring harness with Windex or a similar cleaner, then move to the interior and lamp sockets. You can test the lamp housing with multimeter from the bulkhead connector pins, beforehand, if you wish, but the visual evidence is usually clear enough! When all of the dirty cleaning is done and the short(s) are gone ... rinse all connectors and sockets with 90% isopropyl alcohol. After drying is complete, test the circuits again with bulbs and fuses, and if all test OK, put silicone grease on the bulkhead conne4ctor pins and reassemble.

If the problem isn't the rear lamp, do the same procedure with the front lamp assembly. Good luck!

You are correct in removing the light bulbs to perform your checks. Tracing a short with the bulb in can give some strange results due to the filament resistance.

You should see if the short is in the hot wire, in the bulb assembly it connects to, or possibly in the switch. My guess would be the wire. To isolate the hot wire, completely remove the wire at the fuse and at each bulb assembly.

With the wire completely disconnected at both ends, there should be infinite resistance to ground from either end. Check each end of the wire to ground to make sure you read infinity. If you read zero or some nominal resistance, you have a short or partial short in the insulation.

I hope it is a total short because they are much easier to find. A partial may only show up when the wire is wet.

Personally, I like using an analog meter for tracing shorts better than a digital because you're only interested in zero vs infinity and exact measurement is not necessary. However, a digital will work just fine.

If you detect a ground in the hot wire, start looking for places where the insulation rubs metal and take Warren's advice about cleaning and checking the bulb holder.

If you post what year your car is, I'll take a look at the electrical schematics on this site to see if I can get any other clues to help narrow the search.

Good luck

Mike
 

and neither was Mr Lucas :( ........ and he's only put 3 fuses in my TR4 :eek:

I think the biggest issue with the TR6 was the MFI? and it's Mr Bosch that fixes that :rolleyes:

Wow...great suggestions. I will be busy with this again this evening. I will unplug the bulkhead connector and clean it as Warren suggests (where is it located?...car is an 89).

I suspect one of the bulb sockets as the culprit. The tail light assembly was very clean inside and nothing looked visually wrong. Interestingly, the front side marker socket looked somewhat corroded and I will explore this also. One last question...is the engine compartment light also on this same circuit (fuse)?

Thanks again,

With all of the bulbs removed you have no load on the circuit but it still blows the fuse when the light switch is turned on. This suggests the problem is is in the light switch or the wiring to the bulb sockets. If it is not an internal short in the light switch then you probably have a wire that has chaffed against the chassis after the light switch. With an Ohmmeter it should be easy to find the individual wire from the switch that is shorted. The hard part will be to find the actual chaffed point on the wire.

Sean:

We here in the US can only long for the Petrol-Injected 2.5 TR6 that you lucky Brits drove. Sure, Lucas had the weird loop runner to cool the gas and that was quite a leap of faith, but our Stromberg/SU supplied TR6s were dogs in comparison, sigh. I'd do anything to have my TR4A back again, or a TR6 with sidedraft Webers would work!

Sorry for the OT!
John

Problem solved!!!
 

After more hours than I care to admit, the problem is solved. Yes, it was something simple, but it was not simple finding it. Today I elicited the assistance of an electrical engineer from work. He quickly narrowed it down to the wire harness going to the rear tail light.

This part is important...the schematic in the Bentley manual does not indicate that the engine compartment light is on the same circuit / fuse as the left side running lights. Bottom line....when I had my rear spoiler installed, the body shop reinstalled the engine compartment light and had switched the wires upon reinstallation. So...the hot lead was connected to ground. That makes a heck of a short. Problem is now solved and everything on the car works...including the engine compartment light.

I'm a happy camper....and a tired one!

Thanks to all for your kind assistance. I am forever greatful for the wealth of knowledge and assistance on this board.

http://www.pelicanparts.com/support/smileys/wat3.gif http://www.pelicanparts.com/support/...s/beerchug.gif http://www.pelicanparts.com/support/smileys/clap.gif

Congratualtions. I feel guilty because after I posted to you I noticed the engine compartment light on this schematic but did not post back.:rolleyes:

It's sort of tricky but if you follow the II- 8 back it shows connecting to the left side running lights.

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part3-1.jpg