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Sorry for the length of this -- trying to be as detailed as possible...
I recently acquired an '87 740 Turbo with 128K miles and am having an issue with an electrical problem. Now, I know what you all are already thinking, so let me give you a run down on this issue as I think it is pretty specific and someone most likely knows the cause.
I live in Denver, Co. USA where the winter nights can be quite cold. When I crank the 740 it starts right up. This time of year it can be anywhere from -10 to 35 degrees F. After starting everything seems fine until you notice that certain parts of the instrument cluster aren't working. The fuel gauge, volt meter, tach and odometer are all dead. Checking the battery voltage shows 11 volts, so the alternator is not charging.
If I drive the car for 5 minutes or so, the alternator light goes bright red. Stopping to check the battery voltage it now shows 13 volts. So the alternator has kicked in but the alternator light is still glowing and the instruments I mentioned above are still not working.
5 more minutes and the alternator light goes out, the instruments come to life and the shift (overdrive) light comes on. Sometimes the instruments come to life, the red alternator light goes dim and and shift light comes on dim. When this happens, pushing the OD button usually turns off both lights.
Once the car is warm everything works fine. The only exception is that the light bulb warning light (yellow in dash) comes on whenever the lights are turned on. All of the bulbs are fine and in working order.
If I start the car cold and turn on the lights the yellow light bulb warning light does not come on until the instruments start to work.
Here's what I've done so far. I've replaced the overdrive relay (automatic), fuel pump relay, J relay and K relay. This car has a melted #16 fuse slot, so I put in an inline fuse for the heater fan and re-connected the hot line for the other appliances that share the #16 hot. I have a good fuse pannel that I plan to install one of these warmer days. I've also replaced the heater fan, although this is most likely not related.
The engine harness has not been replaced on this car. It appears to have some deterioration to the brown (ground) wires on top of the intake manifold. I have cut some of the outside sheiths open and inspected the wire insulation and have not found deteoration on these wires, although I know it may exist where I haven't looked. I am looking for a replacement harness but as of yet haven't found a recycled one and I'm not spending $ at the stealer for a new one.
It seems to me that this may be an old, sticking relay that has not been replaced but I can't be sure. This isn't my first car with electrical problems, so I'm not too upset with this as it doesn't seem to be dangerous.
Let me know what you think and thanks in advance for your suggestions and for reading this long description.
Pete
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