Serious Oscillation -- '72 280se 4.5 - PeachParts Mercedes-Benz Forum

kpb · #1 12-14-2003, 02:03 PM

Ok folks, another in my litany of questions. With the collective wisdom of all here combined with my seven days of owenership, many of my issues have been resolved or, at least, I have a clue where to head.
YET, the oscillation I noticed from 40-50mph when testing the car, has now inexplicably moved itself to commence at about 20 or 25mph and seems to be consistent through acceleration. If I lighten up on the throttle and cruise at steady speed, it mostly goes away. I also cannot make it appear during deceleration, it only rears its ugly head under power.
I put the rear end on ramps and crawled all around because it felt like drivetrain stuff to me but I found nothing indicative of such. The oscillation is through the chassis and is defintiely not coming thorugh the steering wheel like it would with a bad front alignment or balance. Could it be sloppy/broken motor or subframe mounts letting the assemblies jiggle about under acceleration?? Is there something I should check at the rear?
BTW all the wheels are tight. I recently learned on my '65 Mustang that sometimes it really is the simple thing to look at first when I was ready to redo ball joints or stabilizer bushings only to find FIVE (!) loose lugs nuts on the LF wheel which I have never had the hubcap off of before in over seven years of ownership.
Thanx for the advice...

psfred · #2 12-14-2003, 03:00 PM

Pull the distributor and clean the trigger points. They get oily and cause intermittant loss of fuel injection, makes the car shake longitudinally, feels like driveshaft or wheel vibration sometimes. The trigger points are held in place by two screws in the base of the dizzy, just pull out. Spray them down with brake parts cleaner or carb cleaner without methylene chloride to avoid melting the plastic covers. Rinse out the dizzy, too, so you won't oil them up immediately again. If you can get the plastic covers off (have a care, they are not available as replacements), you can close the points on a strip of paper to wipe any carbon off. No abrasives, they are platinum plated and you will wipe the platinum off.

Take a look and the engine mounts -- if they have not been replaced, they will be squashed down (the center steel block should be higher than the rubber, not vice versa). If dodgy, replace them and the tranny mount. May help.

Peter

kpb · #3 12-14-2003, 05:45 PM

C-R-A-P!! Every time I do something on this beast, it seems I make another task for myself fixing something else!!

Thanx for the info Peter. Ironically, not one-half hour ago, I was so-o-o happy that my three-door 280 is now a four-door, and the fix did NOT result in the destruction of any parts, I decided to stretch my mechanical prowess and clean up under the distributor cap. Shined up the rotor, cleaned off the post in the cap, and (gasp) used a fine abrasive strip on the points. As you must expect, and much to my chagrin, I put it back together and found the auto will not start!!

Now with Peter's info in hand (belatedly), I suspect I ground off the platinum on the points. (Platinum points -- whodathought?) I just figured I jostled the points enuf to throw off the gap, which by the way appears miniscule. The engine spins freely but does not come close to catching.

Do you think this is the problem? But for my oscillation, the car seemed to be running quite well until I stuck my ham-fisted appendages under the hood. Woe is me...

psfred · #4 12-14-2003, 08:00 PM

Get some clean paper (no coatings, no wax) or thin cardboard. Pop the distributor off, open the points by hand, and slip the paper or card between the points. Let them close and pull the paper or card about 1/8", open the points, and then remove paper or card. You can spray them with carb cleaner or brake cleaner too. This will remove, hopefully, the bits of abrasive on the contact surfaces that are holding them open. Remember, these are only switch points, no current, so they won't self clean.

Don't pull the paper out all the way, it's very very likely to pull fibers off the end and that will keep the points open.

You may need new points if you cannot get them to contact. Point gap is around 0.030", but Benz only specifies 30 degrees dwell.

Peter

kpb · #5 12-15-2003, 07:44 AM

Thanx Peter. In rereading our mini-thread here and looking at some online parts, I realized that I mistook your first reference to "trigger points" to mean the old-style electrical points directly under the rotor. Now I see there are separate trigger points, I guess on the dizzy shaft? I see how that could cause the oscillation, but now, since the "regular" points apparently are not platinum and I apparently did nothing harmful in a light sanding to clean them, why will not the auto start? I do not think I changed the gap (other than by sanding slightly the point surfaces) and it appears way less than .30, so my sanding actually should have helped increase it to book setting, no?

Further, when I dig into this trigger point issue, I must mark the dizzy when pulling it, so as to have a reference for reinstall, right? I've only pulled a distributor once before, 29 years ago and I did not know to mark it so, was it a PITA to remediate? Oh yes...

Since then I have had a "hands-off" policy to these items.
Any and all info will be gladly digested.

Tomguy · #6 12-15-2003, 10:04 AM

KPB: I usually either just remember where the shaft was or, more commonly, turn the engine to TDC (according to the rotor and the notch on the dist) and then yank it. There should be a thin notch at about 1:00 on the distributor and this coincides with TDC. I use a jumper cable to connect the + terminal of the battery to the 3rd terminal on the wire block behind the EFI contrul unit/below the washer resivoir. (I do it with the dist cap off, take note, your car may start by doing this if you don't - it's hotwiring it!)

kpb · #7 12-15-2003, 10:25 AM

Thanx for the info Tom. Amazingly, after sitting all night under 2" of snow and 20 degress, the car started today. It was wet snow yesterday when I worked on it and the inside of the cap appeared moist notwithstanding my efforts to dry it at reassembly. I think (hope) perchance that was the starting glitch yesterday. It remains apparent to me that I've gotta do a tune-up if for no other reason than to check various settings in light of the miniscule gap on my points. I've got th CD and owner's manual on its way and have been cruising the IPS parts site. Any other tips on regular maintenance parts? And, BTW, $60 list on a frickin' rotor?!?!?

I have never seen such a complex rotor in my life, whatsthedealwiththat?

autozen · #8 12-15-2003, 11:14 AM

That rotor is designed to prevent over revving the engine.

Peter

Litton · #9 12-15-2003, 12:20 PM

Interestng thread but I really did a double take on you comments regarding the rotor. My '72 4.5 just has a regular old one piece redish color rotor. Do one of us have someting odd?

kpb · #10 12-15-2003, 01:58 PM

Hey Litton, have followed with interest your various threads and issues with your "new" ride. As I recall, you are the fellow who, upon taking delivery of your (non-running) prize had to endure the "I told you so" looks from the spousal unit -- Loved your observation -- "Now I have two problems..." Whattahoot. But for the grace of God go I!

Back to the rotor, you are a lucky one. I looked at the IPS site and they have pictures of the two types, henceforth officially known as "Yours/cheap" and "Mine/not cheap."

Litton · #11 12-15-2003, 03:25 PM

Yeh, that was me. However, my wife is now a convert after we took the car to a couple of places this weekend and everytime we stop someone came up and complimented us. Vanity rules

. She especially like the "historic vehicle plates".

These cars are a blast and a real learning experience. I got out of the "car thing" years ago when I was building and racing a rotary engine mazda, and restoring a couple of MGA's and TR3's. Left all my tools, parts, and etc. with my ex. This 108 has recaptured my interest and will make a fine retirement project.

This forum is, without a doubt, a fantastic resource and the folks are wonderfully patient with the oddball and repetitive questions. I have been trying to frame a new question/topic as a "new post" so future searchs are easier to skim for info.

Well, I think it's finally getting warm enough to go out and wash the car. Damn cold weather, I don't think it's going to break 70 today.

Tomguy · #12 12-15-2003, 04:06 PM

kpb: Is this your rotor?

FastLane sells that one for a tad over $30. But I BELIEVE that the normal $8.30 rotor will work on your car - you just won't have a rev limiter. (not really a problem unless you floor your car while in park often

)

Most people here (myself included) have the straight rotor. No overrev protection, hell of a lot cheaper. But you need to think what's cheaper: Cams or a rotor

I take it easy when revving it, so I don't need to worry.

psfred · #13 12-15-2003, 07:11 PM

Rev limiter is more important on a manual tranny car, where it will stay in whatever gear you put it in. The automatic will upshift, even with the kickdown floored.

I doubt you would overrev in top gear unless you wanted to do 130 mph downgrade somewhere, in which case I suspect overreving the engine won't be your major concern!

Peter

kpb · #14 12-16-2003, 08:08 AM

Tomguy, that is my rotor. I presumed the spring contraption had something to do with governing the revs. When it comes time I'm gonna replace it with the traditional rotor since I have an automatic. Thanks to you and Peter for the info.

And Litton, if it's so stinkin' warm out there, why do you have a furnace in your garage??

(this from a guy sitting in 2" of snow and a warm 30 degrees) I'd like to post a picture of my new ride too but I am a technological clod and only have a 35mm and a scanner. I have heard this does not transfer well into pixels, bits, bytes, meg, jpg, bitmap, exe, hard drive, master/slave, resolution, goobleygook, very well.

Back to the topic of this thread, while experimenting with my LR window today I hope to at least eyeball the distributor to figure out Peter's advice on cleaning the trigger points. My oscillation during yesterday's 2-mile spin has progressed to the point where I believe parts may vibrate off the vehicle. Hope one of the suggestions here fixes it. BTW, is there a specific fashion in which to check the engine/tranny mounts or is eyeball inspection suuficient to verify their integrity (bearing in mind I have never seen MB mounts other than when I went over the car pre-purchase.) I know Peter advised me earlier here about squashed rubber -- is there also a jacking or prying technique to use?. THANX ALL

Tomguy · #15 12-16-2003, 10:53 AM

My engine mounts are as flat as can be but don't cause a vibration...

Does the engine run fine when it vibrates, or does it seem to struggle or misfire? Have you checked for bad wheel bearings or loose lug bolts? Is a suspension piece in the front (like the lower A-Arm) cracked?

EDIT: Oh, and I use my scanner as I don't yet have a digital cam (though that should change soon...). It works fine, really. If you can, just set the quality to 150 dpi (any higher will just make it huge) and save as a jpeg with 25-50% compression. If that makes any sense... but it should if you know what master/slave means