85 380 SL: Dwell and Timing - PeachParts Mercedes-Benz Forum

rgp717 · #1 03-27-2010, 09:01 PM

OK, I've searched and searched and I'm getting no-where. For an 85 380 SL, using a dwell meter, what dwell am I trying to achieve? Here's my procedure:

1. warm up engine
2. automatic transmission in Park ... car is idling around 1100 rpm.
3. hook up dwell meter
4. use a hex wrench through the air cleaner, and adjust dwell

but dwell to what? I've read 22 and 30-34. So what should it be? Should I also be reving up the motor to 3000 rgp?

And after that, I pull out my timing light. Yank the distributor vacuum hose and try to set the timing. Again, what am I aiming for? X degrees btdc or atdc?

And after that, I suppose I should go through the entire procedure again, just to verify.

So, mercedes-gurus, please help. Car seems to run ok, but a little chunky on acceleration.

Gilly · #2 03-28-2010, 05:54 AM

Ahhh, I believe it's all electronic?????

Gilly · #3 03-28-2010, 05:56 AM

Uh oh, hex wrench through the air cleaner? That's for mixture, has an anti-tamper ball, and you shouldn't really adjust that thing. Well, you can, but not using any of the meters you've discussed so far!

Gilly

rgp717 · #4 03-28-2010, 08:43 AM

Gilly, thank you for the replies.

1. I don't think this is all electronic ... it's an 85 and it really seems to be a nicer older motor. From what I've been reading, the idle is controlled electronically but I believe I still must set the mixture manually.
2. Hex wrench through the air cleaner: The PO made several adjustments to the idle adjustment, and so have I (so I'm past "don't play with it")

Am I completely on the wrong track? I've read so many posts about using a dwell meter to adjust the idle adjustment through the air cleaner ... only turning 1/8 each attempt, etc. (I want to call it the mixture screw, but I read somewhere that is not the correct terminology)?

ctaylor738 · #5 03-28-2010, 02:26 PM

You are on the wrong track.

The idle speed is electronically controlled and can't be adjusted.

The ignitition is electronic, so there is no dwell to set.

The mixture is also controlled electronically. The basic fuel/air mixture is set with the hex key through the air cleaner. A dwell meter attached to Pin 3 in the diagnostic socket can be used to set the base mixture. The spec is set it around to around 22 degrees on the 8 cylinder scale. This cannot be set correctly by "feel" or "ear."

At times, I have advised people in desperate situations to richen or lean the base mixture to get the car to start or stop belching black smoke or otherwise run decently, by changing the mixture in small increments. But you would never do this to "tune" a running car.

RadioTek · #6 03-28-2010, 02:47 PM

'75 is the last year Mercedes used breaker points in these things. (trust me on this one

) What you are using the dwell meter for is to tune the 'duty cycle' some of the electronic meters set up specifically for automotive use have a setting for that.

Just to clear up any confusion:

Dwell is the number of degrees in the ignition cycle that the points are closed. Duty cycle is a measure of the number of degrees of the engine cycle that the fuel injector is active (open).

I would suggest that you get ahold of a cd-rom or other manual, so you have all the specs at your fingertips. I've found mine literally worth it's weight in gold. The CD's are usually available on E-Bay most easily, and they have all the specs for all the various engines and years.

rgp717 · #7 03-28-2010, 04:21 PM

OK, so "dwell" is used to set base mixture, via hex key through the air cleaner. Sounds like if I get the base mixture in the ballpark, the electronics will take over.

So, this afternoon, I pulled down my trusty Sears Engine Analyzer ($40 new in 1975); haven't used this baby in a while. I put a nail in Pin 3 and clipped my analyzer pickup wire to the nail. Nothing? So, then I put the pickup on the coil, according to the instructions for the meter (at least it looks like a coil to me). The analyzer appears to display dwell, and making changes via the hex key affects the displayed dwell.

However, using this highly suspect method, no amount of hex turn brings the dwell above 17. So, using the hex, I set it to the max dwell (17), with the leanest setting. Reset the timing (TDC, vacuum hose removed/plugged), in Park, approximately 650 rpm.

Immediate improvement: It starts so easy now. And the acceleration doesn't seem so chunky (still not as smooth as I hope, but it looks like a step in the right direction).

So, what do you think? Time to stick a crowbar in my wallet and shell out for a new meter?

FYI: My eBay CD-Rom arrived yesterday, but I find these discussions more helpful. And 'thank you' everybody for your help / advice / information.

Gilly · #8 03-28-2010, 08:41 PM

Where in the system is the vacuum hose connecting??

Gilly

rgp717 · #9 03-28-2010, 09:41 PM

The vacuum hose I disconnect is located on the distributor. I only disconnect it temporarily, while setting the timing; when I disconnect the hose, with transmission in Park, the idle drops from 1100 to 700. Which seems to jive with my 70s auto technology memory.

FYI: I'm just a weekend wrench, with minor diagnostic skill, but fairly good at removing, cleaning and installing what-nots.

ctaylor738 · #10 03-29-2010, 08:37 AM

I'm not sure why your meter couldn't get a dwell reading off of Pin 3. I have used a Sears analyzer of the same vintage, and it has always worked. I have no idea what you were reading off the coil, but it sounds like somehow you got the base mixture close to where it should be.

Another proxy for duty cycle is the voltage between pins 2 and 3 on the diagnostic socket. You should see it varying around 50% of battery voltage.

halsk · #11 03-29-2010, 09:53 AM

To RGP
Are you putting the dwell meter on pins 2 and 3.
Sounds like you just put a probe into 1 terminal - and a nail might not be the best type of conductor?

Gilly · #12 03-29-2010, 09:24 PM

Does a vacuum advance at the distributor sound correct to everyone here? I thought the vac line is supposed to go to the EZL......???

Gilly

ctaylor738 · #13 03-29-2010, 10:25 PM

I thought it was not until the 560SL that the 107 got the EZL.

halsk · #14 03-30-2010, 05:45 AM

The 380sl - 107 engine does have a vacuum advance going to the distributor.
Next question - what's an EZL

strelnik · #15 03-30-2010, 08:58 AM

Quote:

Originally Posted by ctaylor738

I'm not sure why your meter couldn't get a dwell reading off of Pin 3. I have used a Sears analyzer of the same vintage, and it has always worked. I have no idea what you were reading off the coil, but it sounds like somehow you got the base mixture close to where it should be.

Another proxy for duty cycle is the voltage between pins 2 and 3 on the diagnostic socket. You should see it varying around 50% of battery voltage.

Since I have just acquired a 84 380SL and am prepping it to pull it out of storage, where is the diagnostic socket on the R107 models?

All my other Benzes are diesel except a 61 220Sb, so none of the others have a diagnostic socket.
Thanks!