81 280SL Idle Problem Or...? - PeachParts Mercedes-Benz Forum

WINTS · #1 10-20-2009, 02:52 PM

A little background to start...The 280 has had a vacuum leak since I first bought it, but always started when cold, sometimes after a couple of turns, and always when warm. The idle was around 900-1000rpm. It ran well however, with no hesitancy, or missing.

I recently took it into my shop to get the valves adjusted. That was done, and while there the coil had an intermittant grounding problem that my mechanic said he fixed. The coil was not replaced.

The car drove fine the first 20 miles, but now, some 80 or so miles later has been difficult to start, and has started stalling at idle after about 20 seconds. It will restart after a few tries, but there is hesitancy under 1500 rpm all the time, although above that it seems fine.

As it stalled out on me today, I tried to gently catch the gas to prevent it, but heard a quick sudden rattle, and then a puff of smoke from the coil/air filter area under the hood as it died, with a burning smell. That had me worried...

It started up again fine and I drove home real carefully.

Thanks in advance for any replies...

tecqboy · #2 10-20-2009, 05:29 PM

It's always the last thing you messed with. Go back and look at the coil again. Don't hesitate to call it to your mechanics attention. I'd look for a crack or possible bad wire between the coil and distributor. ( both the HT wire and the points wire ) Once you disturb something, anything is possible.

WINTS · #3 10-21-2009, 02:44 PM

Thanks...

Any idea what the burning smell/smoke could be?

tecqboy · #4 10-21-2009, 04:06 PM

If the hot wire to the coil got shorted or pinched to ground ( chassis ) it would get hot and melt the insulation causing much smoke and odor.

rowdie · #5 10-21-2009, 04:57 PM

Have you checked the ballast resistor?

WINTS · #6 10-21-2009, 07:36 PM

Quote:

Originally Posted by rowdie

Have you checked the ballast resistor?

The connection to the BR appear tight and OK? Is there anything else with the ballast that I should check?

Does anyone have a diagram of what the connections to the coil should be, and where they go?

rowdie · #7 10-21-2009, 08:46 PM

The resistor can fail.

Genbiltstein · #8 10-22-2009, 06:38 AM

The coil simply turns primary ignition voltage into a vary high output secondary voltage.
Take a light tester and ground the clamp of the tester. Run the lead along the coil wire from the coil to the distributor.
While you are at it run the lead along each and every spark plug wire. If the tester lights up then you are experiencing a bad wire, bad spark plug, bad connection.
Take a can of carbuerator cleaner or brake cleaner and spray the ballaster resistors and the contacts clean. Oil, grime and contamination can cause problems in a weak primary ignition being sent to the coil. disconnect them one at a time, clean and reinstall.
If you have a vacuum advance on the distributor then chances are the mechanic didn't fasten the vacuum line securely. Check it. Check the other end and where it goes. On some there is a T fitting so check both ends.

An answer to your post
Here is a simple check you can conduct to determine whether your coil connected correctly negative polarity.
Take a spare spark plug and disconnect A spark plug wire. connect the spark plug and make sure the threads of the plug are grounded.
Start and run the vehicle. Take a pencil and insert it in the gap with the engine running and watch where the flare goes. If it goes toward the plug then the polarity is correct. High tension polarity if wrong will direct the spark the wrong way. It should travel from the L shape of the plug and into the electrode. If the pencil test flares toward the plug then good. If not then change the + and - on the coil.
I have heard mechanics state it doesn't matter how the coil is hooked up whether the lead from the ballister resistor contacts the positive or the negative and vice versa with the distributor. Wrong
The center of the spark plug is the hottest part of the plug and current should ground in that direction. Why? it saves current and as an end result it saves ignition components.

WINTS · #9 10-25-2009, 03:06 PM

Thinking along the same lines as Tecqboy, and knowing it was fine before the work, I finally got around to looking at the coil/distributor today.

On closer inspection I found that the HT lead at the distributor cap appeared to be a fraction higher than it should be. I took the lead off and tried to seat it down, but it still looked short, so I simply swapped it around to the coil.

Fired her up, and straight away she was back into a smooth idle, without any hesitation or stalling. Took a 5 minute drive up the road and it pulled like a champ again with no problems.

Hopefully problem solved...

Thanks to all!