Clock only runs when car running
 

1987 911 clock doesn't run with car off but does with it running. Runs when connected directly to 12 battery. Must be an abnormal voltage drop somewhere in circuit to the clock, Any suggests?

Are you sure it's connected to the right power wire? Do you have a multimeter? I'd check the power wire and see what the voltage is when the car is off.

I have seen those clocks stop working under 12v when the capacitors are going bad.

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I had the same situation and it was the battery ground cable. I would recommend you check the battery ground.

If it runs only when the engine is running then it must have a power connection and a ground connection - their quality, however, may be poor.

More than likely, it's time for new capacitors in the power supply.

There are a couple of threads on this topic. If you can run a solder gun then you can make this mod.

I pull Fuse #4 on all my cars as I don't want the clock draining the battery when they sit parked. I then leave the clocks set to 9:11.

In two of the cars, the clocks never run, as intended. In my earlier '87... The clock then runs when the ignition is on. 🤔 I have yet to get in there and understand why.

I have a car with a similar problem. But if I gently shake the clock with the key off it’ll keep running. Need to take it apart, clean and lube it.

My clock (quartz) periodically will run slow about a couple minutes every few weeks. But as blucille said, mine will resume full ticking (loud) after tapping on the bezel.
Even with a battery-tender, my clock still can run slow and too, likely needs a cleaning or than anything.

Corroded wire terminal might cause this by interrupting current flow if wiring had never been tampered with, maybe clean any oxidation on the fuse or spade terminals will help.
Lots of electrical problems are traced to bad grounding connection due to 40+ years of oil infused cabin air tarnishing the brass. I found it to be the bane of my interior lights.

I had the same problem and solved it with new capacitors. See
https://forums.pelicanparts.com/porsche-911-technical-forum/1121824-1978-911-clock-getting-stuck-ticking-but-not-moving.html#post12359113