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Old 02-09-2014, 09:56 PM
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bad battery, OVP or ignition switch? 1986 300SDL

Hi all, my '86 SDL has come up with a strange symptom; about a month ago it didn't crank after sitting at my parents for some weeks, so I jumped it (had no choice) and took it for a long drive (home). It's been fine since and I take it for a long drive at least once a week when I go visit my folks...drove it around town yesterday and then tried to start it today...and nothing.

I cheated and tried to jump it again since it's on the street. Starter initially clicked then all power disappeared from the dash cluster. I checked all fuses and the fuse on the OVP relay; all of them look good. If I pull out the OVP and put it back in the dash lights come on and starter clicks - then back to the same symptom where there is no power going to the cluster with key on. Same thing if I disconned and reconnect the battery (-) terminal. At one point during this, I hooked a voltmeter to battery and it gave me an extremely low reading of <2v which didn't seem to make sense...after disconnecting it briefly, I tested it again and it seemed closer to normal with a reading around 12v; however, I didn't get a reading of amps.

I am not sure if my jumping attempt fried the OVP, or if this is a bad connection in the ingition switch, or quite possibly a bad battery. Appreciate any thoughts...thanks!

Edit: I took the battery out - will take it to AutoZone during lunch tomorrow to get it tested so I can rule out a bad battery.
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Last edited by ruchase; 02-10-2014 at 01:44 AM.
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Old 02-10-2014, 06:04 PM
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Thought I'd report back since this may be useful to someone else in the future.

The strange fluctuation I mentioned regarding the battery voltage turned out to be a bad main ground. It occurred to me something similar had happened to me several years back on my 300sd. I cleaned all the terminals and removed the main ground and gave it a thorough cleaning. Installed the fully charged battery (trickle charged it during the night) and she fired right up. As a preventative maintenance, I will clean the engine/body ground and any other obvious ground straps that I come across.

This might be a good bi-annual preventative maintence for all those that park on the street.
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Old 02-10-2014, 07:36 PM
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Great

Quote:
Originally Posted by ruchase View Post
Thought I'd report back since this may be useful to someone else in the future.

The strange fluctuation I mentioned regarding the battery voltage turned out to be a bad main ground.

It occurred to me something similar had happened to me several years back on my 300sd.

I cleaned all the terminals and removed the main ground and gave it a thorough cleaning.

Installed the fully charged battery (trickle charged it during the night) and she fired right up.

As a preventative maintenance, I will clean the engine/body ground and any other obvious ground straps that I come across.

This might be a good bi-annual preventative maintenance for all those that park on the street.
Good to know it was a corroded body connection of the battery ground.

Simple and easy to fix..

.
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Old 02-10-2014, 08:17 PM
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Thanks Roy...I was seriously stumped by the symptoms when I first tried to diagnose it; however, it all makes sense now that a bad main ground would cause the erratic voltage readings.

I'm storing it in my garage for a few days just to make sure the issue doesn't reappear. After that I'll clean all the chassis ground connections and return it to the street since it kind of takes up the entire garage space!
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Old 02-10-2014, 10:58 PM
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Put a dab of grease on the connections before bolting them down again. When you clean old connections usually the applied plating on the metal has been sacrificed or is partially removed with the cleaning. A little di electric paste or just heavy grease squashed in there usully means you will not be going back there anytime.

I do something like this with all my connections when taking electrical systems apart for usually other reasons..
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Old 02-11-2014, 01:05 AM
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Quote:
Originally Posted by barry12345 View Post
Put a dab of grease on the connections before bolting them down again. When you clean old connections usually the applied plating on the metal has been sacrificed or is partially removed with the cleaning. A little di electric paste or just heavy grease squashed in there usully means you will not be going back there anytime.

I do something like this with all my connections when taking electrical systems apart for usually other reasons..
Will do - thanks!
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