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Does this sound like bad ground wire?

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?

Reply to
Greg Schwall
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Battery is cactus - what you are reading at 100% is just the surface charge - if, after applying a load, it drops to 50% it means the battery can't carry a load.

Reply to
wherwithal

Sometimes it is the obvious solution. After letting it sit overnight, the battery read 11.7 V. It was right at three years old, so I replaced it and the car fired right up. I had previously failed to jump start it with my chargers "Jump Start" feature, but it might not have had the amps for the task.

Guess this is normal when a chemical engineer figures being able to explain electron orbitals makes him an electrician .

Thanks to everyone who gave input.

Reply to
Greg Schwall

Not the battery unfortunately. Got the same symptoms today with a new battery. Immediately after getting home I read 10 volts across the terminals with the engine running. After turning on the fan and lights this dropped to

9 volts in five minutes. Looks like I get to call around and see who has a voltage regulator in stock.

Reply to
Greg Schwall

Before doing that, it might be in your best interest to have the alternator checked out. It may not be producing any charge at all.

Reply to
murphy

Good reason to buy batteries from Sears. They check out the whole system before sticking in one of their batteries - just be sure to get the one with the longest guarantee.

mcbrue under the bridge in the trailer down by the river

96 S420
Reply to
MCBRUE

I second that notion. An alternator typically has three circuits and three diodes and produces between 15 and 16 volts when operating. You show 10 volts, which is 2/3 of the operating voltage, ergo, one of the circuits is dead.

Reply to
Kenneth Fanyo

Sounds like a dead cell in the battery. Check alternator voltage at the alternator connections, not the battery.

Reply to
fred_emmerich

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