108 Electrical Gremlin

[JMela]

My '68 280s has a recurring electrical short that I've been trying to chase down for years. On startup (seems to happen with some predictability when car has sat for awhile), car will kill when any pedal, signal or gearshift is touched. I then have to cycle the ignition many times and then car will work normally. During these cycles I turn the key halfway, generator light comes on in speedo gauge, I activate an electrical item (signal, brake pedal, wipers), generator light goes off/ gas gauge needle goes to zero, turn off key and repeat, over and over.

Battery, alternator and voltage regulator are all recent. I've cleaned the grounds under the hood. After reading up on grounding on the sl113.org page (http://www.sl113.org/wiki/Electrical/Ground) perhaps the ground on the inner dash will have to be next on the list.

Thought I'd seek the collective wisdom of the group and see if you guy had any ideas. Thanks!

[wbain5280]

Yes, you must check all the grounds and make sure they are clean especially under the dash. Make sure the ignition switch contacys are tight.

Additionally, I'm going to clean all the grounds in my 220SE and tie them all together in a long U shape to a single ground point under the hood. I'm also planning on moving the battery ground cable to the ground strap that ties the engine to the chassis or run an additional cable from the ground strap to the battery ground. I don't like all that current traveling from that ground strap on the chassis to the battery ground on the chassis.

[Pooka]

Replace the flat grounding strap that runs from your engine to the body. It sounds as if it has built up too much resistance internally.

Cleaning the contacts here will not do the trick as the resistance is not at the point of contact. It is inside the grounding strap.

[250 Coupe]

A quick check for bad grounds is to connect a test light in parallel with the suspect ground.

As an example, clip the test lights alligator clip to the engine block and touch the probe to the battery post. Be sure you are on the post and not the terminal. Have someone turn on a high draw item such as the high beams or attempt to start the engine. The test light should not glow at all.

You can also use a voltmeter the same way. Set it on DC volts with a range that will handle 12 volts. You should not see much over a few millivolts with headlights and no more than 1 volt with the starter.

Michael

[JMela]

Thanks, all good ideas here. Interestingly (and perhaps indicative of source of problem) the headlights/ hazards do not cause this short to occur. Hitting the brake pedal does.