1986 Carrara short circuit.

[Oneredspyder]

Hello, I own a 86 , 911 with 90,000 mi, all original. I have developed a short that kills the battery in a couple of weeks. The battery is new and has been checked out and is fine, it will take a full charge. Everything electrical works on the car and there are no blown fuses. The car starts right up on a full charge, no dash lights come on.What is it? I have checked all the cables near the battery and they are not shorting. I now take the ground cable off the battery. When I touch the cable back to the battery there is a big spark, however the car will start right up.
Fools me.

[adias]

Could it be a bad alternator rectifier diode?

Perhaps you could start pulling fuses until that spark vanishes.

[86 911 Targa]

Originally Posted by Oneredspyder

Hello, I own a 86 , 911 with 90,000 mi, all original. I have developed a short that kills the battery in a couple of weeks. The battery is new and has been checked out and is fine, it will take a full charge. Everything electrical works on the car and there are no blown fuses. The car starts right up on a full charge, no dash lights come on.What is it? I have checked all the cables near the battery and they are not shorting. I now take the ground cable off the battery. When I touch the cable back to the battery there is a big spark, however the car will start right up.
Fools me.

In order to fault isolate, you will need a multi meter
to conduct a battery static current draw test.

The typical draw is 100-150 mA

Disconnect the negative battery cable,
and place the meter from the negative battery
post to the disconnected cable.

If the static current draw exceed 400mA,
leave the meter attached, and pull fuses one at a time
until the draw does not exceed 400mA.

pm me as needed.

Best,

Gerry

[gomezoneill]

There was a similar post like this the other day same year car same problem. One response was a guy found out that his window control relay was stuck closed because of a door switch was stuck closed. When that happens the relay stays in the closed position so you can roll up the windows with the key out of the ignition. Most likely not the problem but worth checking.

[86 911 Targa]

Originally Posted by gomezoneill

There was a similar post like this the other day same year car same problem. One response was a guy found out that his window control relay was stuck closed because of a door switch was stuck closed. When that happens the relay stays in the closed position so you can roll up the windows with the key out of the ignition. Most likely not the problem but worth checking.

^^^^^
Great comment!!!!

And a cheap fix.

Gerry

[crustychief]

You say no dash lights? I think that circuit is not fused, I know it was not in my 79 SC. Gerry has a fix for that.

[Tremelune]

Regardless of the reason, I would recommend keeping a tender on the car if it won't be driven for weeks at a time. I got nine years out of my last battery. They pay for themselves quickly:

https://www.amazon.com/Battery-Tender-021-0123-Junior-Charger/dp/B000CITK8S

[jferrante]

Originally Posted by 86 911 Targa

In order to fault isolate, you will need a multi meter
to conduct a battery static current draw test.

The typical draw is 100-150 mA

Disconnect the negative battery cable,
and place the meter from the negative battery
post to the disconnected cable.

If the static current draw exceed 400mA,
leave the meter attached, and pull fuses one at a time
until the draw does not exceed 400mA.

pm me as needed.

Best,

Gerry

+10000
this is a general skill you need to know on any car

[Oneredspyder]

OK, thanks for all the responses, I am going to get to work on the multimeter suggestion. Rather than pull all the fuses I hooked my multimeter up to each fuse terminal with the battery hooked up and the car off. These all had full power: fuse #2 A/C, electric seats, 3 Headlamp washing system, 4 clock, radio, interior lights. 5 warning light. 6 Fuel pump. Should these have power with the car off? I realize that 4 and 5 should, but was surprised at the others. I will pull each one and check to see how much drain there is at each fuse block number and get back to you. Thanks again.

[86 911 Targa]

^^^^^
Per my factory manual for the M/Y 1986.

There is a direct connection from the battery (6mm red wire)
which places a constant 12V to the locations mentioned in your post.

(fuse #! which is bridged to other fuses).

These voltages are switched as needed by the ignition switch.

pm me as needed.

Best,

Gerry

[Cevan]

You should just follow the procedure Gerry mentioned in post #3. There's no shortcuts here.

[darrin]

Originally Posted by Cevan

You should just follow the procedure Gerry mentioned in post #3. There's no shortcuts here.

ALMOST -- since I had the same door switch problem, start with easy -- do your interior (dome) lights come on when you open your driver's door? What about passenger door?

If the interior dome lights don't work when you open your driver's door, your power window circuit will remain active and draw enough current to kill your battery after a week. SO, if you find this to be the case, start by cleaning/fixing the door switch (or try opening the passenger door, if it still causes the interior lights to illuminate, before putting your car to bed) and see if this fixes the problem first.

Dirty door switches seem to be a common problem with these cars -- best to start with a potential shortcut here, as if I recall, the traditional diagnostic technique outlined above won't clearly point to this being the problem. Of course, if your interior dome lights work fine and your power windows stop working after the driver's door is opened, this isn't your problem.

[86 911 Targa]

Originally Posted by darrin

ALMOST -- since I had the same door switch problem, start with easy -- do your interior (dome) lights come on when you open your driver's door? What about passenger door?

If the interior dome lights don't work when you open your driver's door, your power window circuit will remain active and draw enough current to kill your battery after a week. SO, if you find this to be the case, start by cleaning/fixing the door switch (or try opening the passenger door, if it still causes the interior lights to illuminate, before putting your car to bed) and see if this fixes the problem first.

Dirty door switches seem to be a common problem with these cars -- best to start with a potential shortcut here, as if I recall, the traditional diagnostic technique outlined above won't clearly point to this being the problem. Of course, if your interior dome lights work fine and your power windows stop working after the driver's door is opened, this isn't your problem.

On our '86, opening any door shuts down the P/W CKT.

Gerry

[Oneredspyder]

OK, so I just checked both doors and the dome lights go on and off as they should, as do the electric window lifts. So I do not think that is it. I am having a problem with my multimeter in that it will not read accurately so I cannot check the reading between the negative battery terminal and cable. However I did inadvertidly notice that if I take the key out of the ignition I do not get the big spark that I get when I touch the ground wire to the negative battery terminal. Soooo, without a multimeter, which I will get, is this all my fault for leaving they key in the ignition while the car sits in my garage??? In that by simply inserting the key turns on several circuits, and that was draining my battery? News to me. Or is there something wrong with the ignition switch? Anyway, I can not figure this out definitely until I get my hands on another multimeter. Will get back to you all.

[86 911 Targa]

Originally Posted by Oneredspyder

OK, so I just checked both doors and the dome lights go on and off as they should, as do the electric window lifts. So I do not think that is it. I am having a problem with my multimeter in that it will not read accurately so I cannot check the reading between the negative battery terminal and cable. However I did inadvertidly notice that if I take the key out of the ignition I do not get the big spark that I get when I touch the ground wire to the negative battery terminal. Soooo, without a multimeter, which I will get, is this all my fault for leaving they key in the ignition while the car sits in my garage??? In that by simply inserting the key turns on several circuits, and that was draining my battery? News to me. Or is there something wrong with the ignition switch? Anyway, I can not figure this out definitely until I get my hands on another multimeter. Will get back to you all.

pm sent.

Gerry

[Pstallo]

I had the same problem that was fixed when i replaced the door/dome light switches. I also put a kill switch on the battery. Whenever I'm going to be gone for a while, I just turn the power off.

[Pstallo]

Oh, and if you DO replace the door switch, put a pair of hemostats on the brown wire to hold it. If that bugger falls back into the car, you just added about 3 hours to the job.

[86 911 Targa]

Originally Posted by Pstallo

Oh, and if you DO replace the door switch, put a pair of hemostats on the brown wire to hold it. If that bugger falls back into the car, you just added about 3 hours to the job.

And, also, don't let the wire touch the body,
as the wire always has 12V.

Gerry

[steely]

Originally Posted by 86 911 Targa

And, also, don't let the wire touch the body,
as the wire always has 12V.

Gerry

+1 - and I think they are Brown/White, not just brown.

Originally Posted by Oneredspyder

... In that by simply inserting the key turns on several circuits, and that was draining my battery?...

It could be a small contributor.

A relay in the Seat belt control unit is energized when the key is in the ignition. Not sure if its a function of its just being inserted or if position is involved too (and is too late for me to test).

The Seat belt control unit also plays a role in reading the door switches (when that relay is on), as well as playing a role in triggering the Pwr Window module depending of course what the door switches are up to.

Can anyone confirm?

That being said, I'd encourage you to take Gerry's suggestion.

[Oneredspyder]

OK, somewhat resolved, thanks to all of you. Here is the deal. I obtained another multi-meter and the number 4 fuse : radio, clock, interior lights is the culprit showing 700 milliamps. Interesting the front trunk light switch is the culprit, the brown and white wire was rubbing against the frame of the hood. Having said that none of the 3 switches, the 2 door and the frunk switch work as they should in that sometimes they work and sometimes they do not. I am going to replace them all. Having said that, someone suggested that the window lifts are shut off if the door is open and the dome light is on. That is not the case in my car, they have always gone up with the door open, I have often opened the doors to close the windows quicker and get a better seal when I close the door.
Anyway the only fuse that has ever been replaced and is not original is the number 4 fuse so this bad actor has shown his face before.
Thank you all for your help, and now I have a new multi-meter!
Martin
86 Carrara Targa, owned 12 years, mint 86,000 mi
66 Healey 3000, owned 38 years, restored 37,000 mi
74 Motto Guzzi Eldorado, restoration project.
11 BMW 328i xdrive long roof
gone: 74 Norton 850 Commando, 64 and 63 Healey 3000, 67 and 72 MGB, 58 MGA, 63 XKE,
racing go kart (way too much fun)