Need Some Help With A 380sl Problem

[Philoprof]

Came home from a 35 mile hwy trip in my high-mileage, 81 380sl last night. Pulled into the garage, turned the defroster off , and the car started misfiring and shaking, as the instrument cluster light dimmed and brightened. In addition, the garage filled up with exhaust fumes.

My first thought was the alternator. But the battery light didn't come on, I didn't notice the headlights dimming, and I restarted the car, after I had turned it off, In addition, it started it again the next morning, and ran perfectly, except for its usual, slightly, rough idle.

The following parts were all replaced, at one time or another:

Recently: Wires, distributor cap, rotor, coil, OVP relay, Idle Control Mod., plugs

Several years ago: Fuel pump, fuel pump relay, fuel filter, accumulator, head gaskets, chain, guides, valve guides, stem seals, etc.

Checked; Ballast resistors meet specs; Idle control valve meets specs and works fine

I suppose that one or more of the above things could have gone bad. And the only other things I can think of, which haven't yet been changed, are the ICU, fuel distributor, pressure regulator, thermo-time switch, and frequency valve --

The car is running perfectly again. But I suspect the problem will appear again, and I don't want to get stranded when it does So, I'm wondering if anyone could tell me what to look for now or when it does happen again.

Thanks very much in advance for any advice anyone can give me on how to deal with the problem.

Bill

[RadioTek]

2 suggestions:

1) Have your charging system checked out. You might be having a problem with your alternator. This could be as little as a loose belt or wire, on up to the alternator itself (tip: have yours rebuilt, it's a lot less than a reman, or new one, and if you find a good shop it can be done in less than a day)

2) How old is your ignition switch? The electrical switch part of it is replacable, and there is a wire from it to the alternator that is often the first contact to goo when it is failing.

Also, how old is the battery, and are the connections good to it?

Just a few wild guesses from over the internet,
Scott

[cushjbc]

Luckily, your poor run condition is not constant. Since it happened with a warm engine at a point where you changed the condition of your climate control system (defroster turned off), you have a place to focus on.

I took a look at the electrical troubleshooting manual for your car. Page 127 covers your Automatic Climate Control. I could not make out what specifically happens when your defroster turns on, but in general, a defroster uses the AC compressor to remove moisture from the air. This changes the load on an idling engine. My car compensates for this with a switchover valve that applies or removes vacuum to the ignition system at the distributor to advance or retard timing.

I would experiment with this condition using the air conditioner or defroster on a warm engine to see if you can isolate the symtoms. I looked at the vacuum system on your car too (article 14-050). There is a switchover valve attached to your distributor (item 43 in the diagram). If you can physically find this valve, verify its operation with a vacuum gauge as you turn the AC or defroster on and off.

If you can get the car to repeat this symptom, verify that you can control the vacuum advance on your distributor by artificially applying and removing vacuum from the distributor. Remove the distributor cap and have a look at the vacuum advance mechanism. I've seen these develop corossion and lock up.

My car is older (D-Jet). Please forgive any tendency to relate that system to yours which I am not familiar with.

[Philoprof]

Thanks very much for the well-thought out replies. I had thought that turning off the defroster was just a coincidence. So I wasn't even going to mention it.

But I'm glad I did because that could have affected the distributor timing via the switch over valve, which I located on the car and in the diagram at: http://www.startekinfo.com/StarTek/outside/11883/PROGRAM/Engine/107/M116_38/15-502.pdf.

However, I haven't been able to duplicate the problem, even though I've been driving around with the defroster on. And putting 12V on the valve made it click. So I'm thinking it may not be the problem after all.

I also tested the alternator, which is putting out 13+ volts, and the connections seem tight. So I can't tell whether that's the problem unless it happens again - though I'm hoping it won't.

I guess the next thing to test is the distributor's vacuum mechanism. But I'm not really sure how to do that, and the car has been running perfectly. So any help you can give me with that would also be appreciated.

Thanks in advance.

[cushjbc]

Find a vacuum pump (any auto parts store). I bought a hand operated one that includes a brake bleeder kit and various fittings. Disconnect the vacuum line that goes to the distributor at the point where it comes out of the switchover valve. Connect your vacuum pump to the line in lieu of the switchover valve.

Remove the distributor cap. Mine is simple; only has two clips, I don't know what Bosch did for hardware in 1981 - you can leave all the ignition wires connected. Evacuate the line with the pump and see if your distributor plate rotates freely. You can visualize what is happening for ignition advance as you do this. If the plate binds, look for corrosion. Do not clean anything in this area with petroleum products.

The pump should hold a partial vacuum to at least 15" HG. If it doesn't, figure out of the leak is in the line or in the distributor diaphragm.

If all is well with your distributor, rebuild it and shift your focus to the switchover valve. You can check the operation of the switchover valve by using a T connection to your vacuum pump. Once this is done, start the engine take readings on your pump gauge with AC / defroster on and off.

At this point, you will understand how the vacuum pump can be used to check all your components. The service manual has detailed steps to test various vacuum systems on our cars.

Best of luck with it - I hope your car continues to run well.