More electrical help needed on a 69T

[keitho64]

Some of you may remember my past problems with intermittent electrical woes on my project car. Well it is at it again but now I have it narrowed down.

Here are the links to the past threads if you are interested.

Need help tracing a wire - 69T

Looking for help troubleshooting electrical problem

Due to all the tracing and documenting of wires I have found out my car is the 18th 1969T Karmhan bodied car built. Thus the 69 wiring diagram does not match. In the first thread above I documented all of the circuits.

I ran the car tonight and I have number one fuse that keeps blowing. In my fuse block fuse 1 which feeds the ignition switch. On the output side of the switch, pin 15, it feeds the engine compartment and the gauges. These two wires were tied together at the switch as one. I seperated them in case I had problems again. If I unplug power to the gauges then the fuse does not blow. If I put power to the gauges then the fuse will blow. All of the lights in the gauges will work but the tach and the fuel gauge do not read. The car was not run long enough to test the temp gauge.

My question to the group; Is it common to have a problem with a gauge? I really am beginning to suspect the tach. My plan is to provide power to one gauge at a time until I find the culprit. Any insight you have would be appreciated.

Thanks to everyone for all the help to date.

[911pcars]

Keith,
It sounds like you have narrowed the short circuit to this gauge circuit and to either one of the gauges. Your diagnosis is valid. Continue and let us know. Don't substitute higher capacity fuses. If the fuse still blows with either one isolated, look for a common circuit between the two that could be touching ground.

You can also remove all power from the circuit (disconnect the source wire from the switch; disconnect the battery ground wire to make sure), then connect an ohmmeter to ground and the disconnected source wire. If there's a short in the circuit (we know there is), the ohmmeter will indicate "0" (full continuity = short to ground). Disconnect a gauge from the circuit and/or shake the wire loom and see what happens.

Another technique is to temporarily connect a low wattage light bulb in series with the circuit in question. This provides enough resistance to prevent excessive current flow (melt fuse). If the bulb illuminates (Ign. switch ON), there's a short to ground in the circuit. Disconnect the feed wire at the tach and see if the light goes off. If so, the short is inside the tach circuitry .... and so on.

Hope this helps,
Sherwood Lee
http://members.rennlist.org/911pcars

[keitho64]

Sherwood

Thanks for the ideas. This weekend I will try to isolate gauges. I put an inline fuse off the ignition switch to the gauges and have it hanging in the interior. This way when the fuse blows the car stays running. The challenge is the problem is intermittent so it will take a little time. I am assuming it isn't the clock since it has power all of the time and I have a combination gauge to replace the base unit that came with the T. That will leave me with the speedo, tach and fuel. I am betting odds on the tach. Hopefully next week I will have a firm answer.

All I can say is I have learned a lot about the wiring system in my car.

[Early_S_Man]

Keith,

It sounds like you are getting close!

It may not be 'normal' for a 911 to develop a short in an instrument, but is definitely with the realm of possibilites for 34-year old VDO instruments!

[HawgRyder]

Another trick to cut back on the cost of fuses...solder 2 wires to the back of a tail light bulb...and use it as a fuse.
The bulb will light for regular current and glow very bright for a short.
Now...the clock might be your problem....depending on the guts of the clock...some only grab power about every couple of minutes or so....it uses power to wind a small spring with a solenoid (coil)...and sometimes the coil is shorted to ground or has shorted coils so it draws too much current.
It's the only built in intermittant that I can think of.
Bob

[targa80]

I have not read the previous threads but if the fuse blows immediately when power is applied then it is a dead short in the circuit to the guages. You can test the circuit with a ohm/multimeter. Disconnect the wires from the fuse to the guages and Connect one lead to the wires that go to the guages and the other lead to a good ground. if it is a short you would have zero ohms resistance. disconnect the wires to the guages one at a time until the zero resistance changes to either open or a high value. This will save on fuses and stressing the wiring circuit. If the fuse does not blow right away then it is not a short but a high current draw. In this case use a amp meter installed at the fuse and to the wires to the guages disconnect one wire to the guages at a time until the current drops.

[keitho64]

There is some great advice here!

Sometimes it will act like a dead short and blow the fuse immediately. Other times it seems to take a while. I cannot say for sure if I have one or more problems.

I was not thinking about the clock, but Bob's post interests me so I will look at the clock. Another idea I could try is to pull out the gauges and apply 12v to one at a time via a fused circuit and see what happens.

Targa80's suggestion about using an amp meter is great. I have one that I can include in the circuit and connect one gauge at a time. Maybe this will point me in the right direction.

I bought a bunch of blade fuses in bulk from autozone so it comes out cheaper than the bulbs. I wouldn't mind a small circuit breaker while I am diagnosing the problem.

I have a plan of attack for the weekend project so I will keep you all posted. The good part is I can still enjoy driving the car since I isolated the problem. Well, as much as one can in Chicago in January. I could not resist a quick blast done the local frontage road last.

If any of you have ever had a similiar problem with dash based circuits I would love to hear what was the cause of the problem.

[keitho64]

Last night I spent some more time diagnosing the short. I installed a 10-amp circuit breaker in the wiring to the gauges and started disconnecting gauges. The first step was all gauges connected and when I tapped on the oil pressure/temp/alt gauge the circuit popped. I thought I was on to something but if I tapped on the fuel gauge the same thing happened. Well to make a long story short (bad pun intended) I found the fuel gauge to be the culprit. There is a small connector that powers the fuel gauge right near the top. This was broke and touching the body of the gauge causing the short. I am including a picture of the part that was broke and will take a picture of the gauge this weekend when I pull it out. Once I disconnected the power to the fuel gauge I reconnected all of the other gauges.

Now in the past I thought I found the problem elsewhere but I am 95% confident this was the root cause of the trouble. I am going to leave the circuit breaker connected for a while until I am sure this is the problem.

I am in the market for a fuel/oil level gauge if anyone has one. I am going to take this one apart and see if it can be fixed but I doubt that will work.

Here is the picture of the connector. It isn't the best shot but you may get the idea. This is connected right to the fuel level gauge and provides the 12v source.

[dhoward]

That makes the most sense if was intermittent. Kinda makes me wonder now, sometimes my oil pressure gauge doen't work unless I tap it....