Electrical Q&A

[keitho64]

Hello,

I am new to the list as well as new to the 911.

I have a 69 911T that I am reviving and I found an electrical gremlin. When the ignition is turned on the fuse (number 1 in the main block) blows after a short period of time. The past owner had this wire removed from the fuse block and connected to the + terminal on the battery with an inline fuse. This is how it is setup now. From what I can tell this is for the Starter and I am suspecting the solenoid.

I should note I am in Chicago and it is snowing so I won't be doing much with this for a while.

I was planning to trace the circuit and look for a short or some other problem. However since the fuse does not blow immediately I do not suspect a short. I am thinking it is more of a problem with something on the ignition circuit. Has anyone experienced a problem similar to this? If so what are some of the common causes?

My suspicions are as follows:

1. Starter Solenoid
2. Fuel Pump (it does energize with the key)
3. Ignition module, coil, etc.
4. Alternator circuit (I have a new voltage regulator going in soon)
5. Any other ideas?

I appreciate any help you can provide.

Thanks

[911pcars]

If I'm looking at the correct wiring diagram, the no. 1 fuse supplies current to the clock. A junction at the clock supplies current to the interior light and trunk light circuits (also to the ign. switch; not sure why as this isn't the main source for ign.). Your malfunction (intermittent short) is happening on this side of the fuse, not the source voltage side (battery, starter, light switch and other direct-connection circuits).

Since the fuse blows only intermittently, I suspect some part of one of the above circuits is not correctly insulated and it touches ground when you drive the car. Try disconnecting one circuit at a time (as close to the fuse box as possible), then drive around and see if it makes any difference. However, there's a possibility the short may be in the wire just a foot away from the fuse box; disconnecting the accessory (e.g. interior light) may not have any positive effect. If you can get ahold of a wiring diagram, you can follow the single wire coming out of the no. 1 fuse position and see where it goes; any point along that circuit path is where the short could be located. Fun huh?

This should get you started. Tell us what you find out.

Sherwood Lee
http://members.rennlist.org/911pcars

[304065]

Sherwood's advice is very good: a blown fuse in #1 should have nothing to do with alternator, starter solenoid, etc. The wiring diagrams you will need to fix this problem are on the main Pelican board or reproduced in the factory manuals, also available from Pelican-- and I would guess that your '69T is closer to the '71 than the early car diagram.

Does your T still have a clock? The reason I ask is that on many RS replicas it's been removed and replaced by an oil pressure light or a fresh air vent. Assuming you have original equipment installed, I would check the connections at the rear of the clock- just pry it out and look to see whether the wires have fallen off. Often when the clock is removed the PO doesn't bother to do anything about the wires behind the dash = short potential.

Now it gets interesting: after the clock the black wire runs to the glovebox light-- is it working? Does it turn on when you flip it toward the seat, and when you flip it forward and open the glovebox door, does it turn on? If not, that could be the source of your problem. I don't know whether you can remove it from the front-- when I checked this area, I had the heater blower removed and checked from the trunk side- but I suspect you can, to change the bulb, and that might be the problem.

Then the black wire goes to the trunk and overhead interior lights. Look forward of the trunk hinge on the passenger side, there are a number of connections there, for the trunk lights, and the wire for the interior lights that runs up inside the a-pillar- sometimes people disconnect the trunk lights there when they are working in the trunk for long periods as to not drain the battery. If not reconnected that female terminal can easily short against the body sheetmetal. You can pull the interior lights out of the headliner, gently, to check and see if the bulbs are blown, or whether or not the connections are bent or corroded in a way that would short to the roof.

What amperage fuse did the PO install? Unfortunately, without seeing your car it's hard to say whether he put some other equipment in line with that circuit (e.g. radio, amp) but if it's uprated from the original (which rating should be in the fuse box cover) that's a sign that there's a higher current consumer installed. Either that or the PO was comfortable driving around with a short somewhere, heating up the wires but not blowing a larger fuse. . . frightening.

My electrical engineer buddies tell me that 99% of the time it's not the WIRES in a circuit that fail (from chafing or physical stress)- it's the CONNECTORS, e.g., they fall off, they corrode. Chances are your short is not in the middle of a wire hidden inside factory pvc loom (or worse, PO-applied electrical tape)- it's probably some obvious connection somewhere.

Good luck: get yourself a good Voltmeter ,a wiring diagram and several cases of dark beer. Fixing electrical problems requires only a limited knowledge of electrical engineering-- your real assets are a good wiring diagram and patience.

John

'71 E (with CONCOURS electrical system after countless hours)
'88 Carrera

[keitho64]

Thanks for the great advice. I do have a complete set of factory manuals so I will start tracing wires. I never gave thought to the beginning of the circuit. I was to focused on the engine compartment. Everything you have pointed out could be a cause of problem(s). I have not done anything with the dash wiring, glove box, clock etc. A lot of this is hanging. I will verify all connections and see where this leads me. The interior of the car is not put together so I will complete that before I try starting it again.

I have my weekend of work cut out.

[911pcars]

Keith,
I know you have pcar potential because I also have a '69T; also had a '62 Monza.

Sherwood
http://members.rennlist.org/911pcars

[keitho64]

I guess it is something about the rear engine air cooled thing. At least now I have one with some RPM potential, unlike the Corvair.

[Doug Zielke]

Keith sez: I guess it is something about the rear engine air cooled thing. At least now I have one with some RPM potential, unlike the Corvair."

Don't knock your little Monza.
My Dad had one for years and loved it. He told me once he really wanted a Porsche, but couldn't afford one, so he bought the Corvair. (He still hates R. Nader to this very day!)