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Old 12-28-2003, 08:57 PM
Registered User
Join Date: Feb 2001
Location: Midwest, USA
Posts: 171
1980 450SEL Journey

I drove my 'new' baby around the block today. I've had the car for about 6 months. I have not spent much time working on her and this is the first time I've driven it anywhere. This car is gorgeous. It has 86,000 miles and belonged to a good friend since new. I'm the second owner. Obviously it was not driven much over the years and that is the main problem.

The car was towed to my house on a flatbed after a series of events.

The saga began when the car died on the road, a roadside shop jumpstarted it frying numerous electrical components and nearly every single bulb. It was taken to a Mercedes Dealer 4 hours from me who was unable to figure out what was wrong with it. All they said was, "this car needs work!".

It has many new electrical components, a new alternator, and a new 'sketchy' battery, as well as new bulbs in most lights. The owner gave up on the dealer and asked if it could be driven home. They said it was running really lean and adjusted it to get home. The owner puttered it to about 30 miles from here and at the gas station (perhaps the fourth fill in four hours), could not start it. So, it was towed to my home.

The next day, I went out and it started right up, but the smell of fuel was nauseating to say the least.

What I've done or know so far:

First, the Air shut-off valve was sitting in the trunk. I opened it up and found a broken diaphragm (replaced). The aspirator valve was still installed, but the top had been ripped off of it (replaced). The air injection pipe had a rusty hole in it (jb weld). System now intact (many thanks to Phil from Mercedes Shop who found me the only air shut-off valve in the country!) All of this made me feel better and it did seem to make a slight difference.

Next, we are still pouring out fuel. I start to fiddle with the idle and mixture which is foreign to me at this point since I'm a diesel freak. Nothing seems to help and the car at one point actually 'farts' the blackest soot (when before I had this white cloud! woops!)you have ever seen all over its twin '79 300SD. So much so, that you can't read the license plate. I move the diesel out of harms way.

I decide to check the timing. I can't get a reading on number one at all? I know my light is working because I checked each wire and got a strobe light on 5 out of 8! I sort of made that test up, but it let me know that 3 cylinders were not firing.

I ordered wires, distributor and a rotor from Phil (and plugs which have yet to be installed). Now, all 8 cylinders are firing. I also ordered an oxygen sensor since that was my main thing to replace from the start. The accelaration had a dead spot and when checking the timing finally, it was set for TDC with vacuum. My 1980 has to be at 5 degrees before which is different from the rest I guess. I adjusted that to 5 and took her for a spin. She did well.

Suddenly, this car is running pretty well. I adjusted the mixture and the idle a little at a time so that it feels (and smells) right. That's the Volkswagen owner in me! I have bought the Sears $30 multimeter that Larry Bible suggests so I can check the on-off ratio, but I still want to replace the oxygen sensor. I was unable to get a very reliable reading on the number 3 pin and ground. I'm not comfortable with it anyway until I replace the oxygen sensor. I'm not sure it is working at all. Could the car be adjusted at this point to run without the oxygen sensor? Because that's how I feel it is running.

When trying to do that, it appeared that perhaps the oxygen sensor had been welded in by the lame muffler shop that had replaced the exhaust. I was there and I remember feeling sorry for the car. The middle muffler hangs so low. I'm actually going to take it to another muffler place to have them replace the oxygen sensor with the new one, even if that means welding it in again. With all of my mini-mite, I could not budge it. If I keep this car and get it running right, I'll get her a new exhaust for her 24th birthday.

What is still wrong:

I still have numerous bulbs burned out and no tach on the instrument cluster. I'd love some advice on why the tach does not work. I don't think the cigarette butt trick works for this one. I've replaced the fuse to no avail.

I have no heat, yet my third little auxilliary heat pump to rebuild I guess!

The worst problem at this point is that the alternator, though new, does not seem to be working. I'm going to pull it and have it tested and if it is okay, look for any energy draws. I can't understand that one. I'm very worried about electrical components. When the car is first started, the auxilliary cooling fan spins very, very slowly. It's below freezing and this fan is wanting to go! weird.

I'm afraid to drive this car and that makes me sad. If it were a diesel, I'd have no fear.

I'll have to get used to this gas thing because it is the prettiest car I've ever seen!

If you've read this long post, thank you! If you have anything to add, I'd love it. I think the 1980 450SEL is a one of a kind, so it is hard to find out much about it here. I think the emissions were changed in 1980, and the car was changed completely the following year.

What's the story? Any other 1980 450SEL owners out there? This car is too beautiful to believe. I won't give up.

P.S. This car has had a strange "thumping" noise in the rear end since new. Nobody has ever been able to figure it out. It is like some heavy object is falling in the trunk, just very rarely. Any and All advice would be appreciated.

P.P.S. What is the proper way to hook up my dwell/RPM meter to this car? I read that the number three spot and ground was correct, but I get no reading. Since my cluster instrument RPM gauge is not working, I have to wonder?

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Old 12-28-2003, 09:25 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Lots of possibilities, but since you are trying to deal with fuel control, that sounds like a good place to start.

The #3 pin duty-cycle indicates the amount of time the frequency vale is open (leaking off lower chamber pressure). What is most important is that it is leaking. Screw the O2 sensor, until you get the freq valve working. Can you hear or feel the valve buzzing? IT HAS TO BUZZ. With the O2 sensor disconnected you should have a 50/50 duty cycle. You must get that working first.

I think the #3 pin is the same as the ground side of the valve. BTW the #1 cause here of no buzzing is a bad OVP (the first year of OVPs). I think the power for the valve comes from the fuel pump relay and should be hot whenever the fuel pump is working.
I would check it all at the valve. One side is hot the other side grounds out the 50/50 duty cycle (that means that the valve is open 50% of a cycle and closed 50% of the cycle).

Once it is buzzing at 50% adjust the mixture. Even though any idle mix is achievable with with the mixture adjustment the car won't get enough fuel under load unless the freq valve buzzes.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 12-28-2003, 10:21 PM
Registered User
Join Date: Feb 2001
Location: Midwest, USA
Posts: 171
Thank you Steve for the speedy and informative response. I will check the frequency valve the next time I start her up. I'll feel for a buzz, much like the buzz off the auxiliary heat pump which is NOT buzzing.

The car was like night and day in running when I replaced the cap, rotor and wires, still, I don't trust it to go anywhere. I worry about such a catastrophic electrical trauma such as it had.

Yet today when I drove it around the block, it was superb. I have to think in my mind that if the Mercedes Dealer said it needed major work, then my little jaunt would turn into a tow if extended. In the back of my mind, I think they were wrong. But, I won't know for sure until I test out all of the systems. I have the CD, and have this place, the W116 manuals and your input as well.

Thanks Steve!

My new starting point is the alternator which started all of the problems to begin with. If it tests okay, then something is running, shorted, etc., which IS a problem.

I intend to keep posting on updates, though I might be the only 1980 450SEL poster here!

I might just start it up and realize that all problems are solved, wouldn't that be nice?

Steve, could you tell me the proper way to check for RPM's? I have a dwell/RPM meter. Am I supposed to hook it to number 3 and ground? I did that and got no reading at all. It troubled me because my dash tach is not working.

Thanks so much
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Old 12-29-2003, 03:55 AM
wbain5280's Avatar
Registered User
Join Date: May 2001
Location: Northern Va.
Posts: 3,386
The Dwell/Tach is connected either across the points or the coil, depending on the design. The meter will read dwell in degrees and the points must be adjusted accordingly. The rpm setting will display rpm. The car probably has a CDI, Capacitive Discharge Ignition. There is not much current going through the points.

The alternator has a small current going through the field coil, or rotor. Turn the key on and look for the ALT light. It must be on. If not, replace the bulb and also check the small red wire on the back of the alternator. The alt. belt must also be properly tensioned. Check the battery for proper fluid level. The regulator may need to be checked. Use a voltmeter and test fhe voltage at the alternator large red lead. Also check the battery voltage.


Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL


Not part of the in-crowd since 1952.
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Old 12-30-2003, 04:04 PM
Registered User
Join Date: Feb 2001
Location: Midwest, USA
Posts: 171
I have no buzzing from my frequency valve...strange how well the car runs? I can't get a reading at all of duty cycle with frequency valve plugged in-unplugged, I get about 38-42%. I'm not sure what that is saying, but next to check for a bad OVP...thanks Steve and Warren...I tested with O2 unplugged.

Would anyone know WHERE the OVP might be hiding on the 1980 450SEL? I assume I'm looking for something with a fuse on it? Perhaps it is tucked in the passenger footwell?

Edited to say that I found it, and it was in the passenger footwell behind the small vented cover that pops off. It looks quite old. Most other relays had been replaced after the jumpstarting. I think they missed this one.

Edited again to say that I popped the red cover off of the OVP and found the thin wire connecting two pins was broken/melted.

Last edited by '79 Super Turtle; 12-30-2003 at 11:15 PM.
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