1983 300SD with no recent electrical work done.
Today I am driving home and the radio begins turning off intermittently. The radio's LED display cuts out as well. I then notice that turning the headlight on and off causes the speedometer and tach to jump about 10%. While applying the brakes, both gauges fluctuate wildly. Running with minimal electrical load brings back the radio display.
When I get home and the battery reads 12 volts on the multimeter. When I connect a battery charger it shows near 100%. However the car will not start. The glowplug light responds normally, but after turning the key to start all I get is what sounds like the engine getting halfway to turning over along with a few clicking noises. I connect the charger again and it shows 50%.
All fuses look good, including the glowplug fuse. Checking google I saw several similar problems caused by a bad ground connection. Does this diagnosis sound right?
I cleaned the battery terminals, the battery ground connection, and scraped 20 years of crud off the two ground connections by the radiator, but still could not start the car. The radio has a constant power lead that was connected directly to the positive terminal two years ago with a separate fuse so I disconnected that wire just in case the radio shop rigged something wrong. The service manual shows a bunch of devices connected to a ground behind the center of the instrument panel. How do I access this ground?
Tomorrow I'll clean the two grounds by the rear tire wells and look around for other ground points. Does anyone else have any ideas before I have my baby towed to an electrical shop?