This is fairly repeatable and completely frustrating.
When it gets cold outside, below 45f or so, the instrument cluster is dead and the battery does not charge.
The gauges and warning lights don't come on at all, except for the overdrive off light is on (even though the overdrive is still on. And when you check the system voltage it's in the low 12v range (not charging).
The turn signal indicators work fine and the cluster lighting is fine.

Now after several minutes (5-20 depending on how cold it is) of "warming up" it will just fix it self and the gauges come to life, and the system starts charging.
Sometimes it seems to be fixed by turning the ignition off and back on, or flipping a switch, or something. But that could just be timing and me beating on everything so I can get to work.

If I rev the engine fairly high 3-4k the alternator warning light comes on and nothing else changes.

The fuses are fine and once it fixes itself, everything's fine till the next morning when it's cold again.

I've tried disconnecting and reconnecting the gauge clusters connections with no luck. (oh and i don't know if it's supposed to be connected or if it's related but I did notice a yellow wire(no stripes) behind the cluster that isn't connected)
Any help would be great.

The problem is in the circuit board of the cluster. There are good write ups on how to fix in the FAQs for the 700 series. Turbobricks also has a good right up as well. The best way to check for your problems is to remove the cluster when cold and use an ohm meter to check for any open circuits. Do one pin at a time and then mark the pin once checked to the end of circuit. I have found that most of the breaks happen to the left and right of the speedo. If you do not find any breaks, then I would resolder all the joints at crossovers and pins. Good luck and post with your findings. Pauli

I think you're right Pauli.

To expand on that (cold contracts!) in the 240 context, here's a drawing to show how the alternator exciter voltage gets from the ignition switch to the alternator. There's a "flex fuse" on the instrument cluster held by two screws. A screw can be loose from a former attempt at servicing the odometer (even though it needn't be messed with) or the pins on the speedometer gauge may be incorrectly seated in their sockets. Sometimes the whole problem is in the contact force at the L-shaped plug for the speedometer.

In case we are wrong, and it is the ignition switch (red/black wire), I'd expect some wiggling of the key to affect it.






--
Art Benstein near Baltimore

A truly happy person is one who can enjoy the scenery on a detour.

I had this same scenario on my 1990 245. I fiddled with wire behind the gauge with the ignition on and was finally able to pin-point the problem to the "L shaped connector" Art spoke of in this thread. I finessed the contacts to grip the circuit board tighter and then used epoxy to secure the anti-tug cover on this connector. I'm not sure how that cover is supposed to come off but I can see that if someone gingerly pulled the cluster from it's home without first disconnecting it, damage could occur to the circuit board it attaches to.

I have an intermittent 'no charging' problem that I'm beginning to think is related to this - although it doesn't seem to be weather related. I've changed the alternator, brushes, checked the connections, grounds etc.

Intermittently, and its seems to be after driving the car for about 1/2hr the alternator appears to stop charging. All of the cluster warning lights come on dimly including the battery warning light. When this occurs the alternator stops charging. Sometimes just turning of the car and restarting fixes is (although not always).

I did end up removing my instrument cluster several times about 3 months ago in order to fix my odometer as well as clock (capacitors) and ended up ultimately installing a tach. Ended up taking the cluster in and out many times cos I couldn't get the new odometer gear to work consistently (in the end due to a microscopic piece of plastic in the gears). So I'm wondering if all of this somehow upset some of the connections required for the alternator exciter current. I also replaced one of the warning lamps - not sure if it was the battery one.

I also fiddled with the L connector while the engine was running and it seemed to have an effect. Only for the problem to reappear. The problem is so intermittent as to be very annoying and hard to diagnose!

So would it be good for me to perhaps re-solder all of the connections in the pictures? Also may try swapping some of the warming bulbs around since I can't help feel that the fact I replaced one may be more than a coincidence. Other ideas to narrow it down?

Other than this the car is running better than ever before!