'75 450SL Idle/Vacuum Help - Page 2 - PeachParts Mercedes-Benz Forum

Tomguy · #16 08-07-2006, 06:24 PM

1 and 5 is 1 group of trigger points.
4 and 8 is another
6 and 3 another
7 and 2 another.

Because your BANKS (groups) are not firing, it's most likely your trigger points. They are NOT the igniton points (visible when removing the distributor cap). They are the points with the wiring harness on them on the left side of the distributor. See #5 here. There are 4 sets of points in here. 2 on the top of the plate, 2 on the bottom. Clean them out. Peter suggests carb cleaner, I suggest carb cleaner followed by an index card run through the point of contact (NOT SANDPAPER!) - this will clean the gunk out and quite possibly restore those 4 cylinders.

If you are really down 4 cyls it must be running like absolute garbage! I'd hardly call that just a lumpy idle!

Edit: Also, test for continuity in them using a multimeter (I assume you have one!) - the pin diagram is probably on that D-Jet manual I sent you a while back but, if not, I can check the spare set I have.

Your trigger points may look like these without the cable or these with it.

350SL4spd · #17 08-07-2006, 07:34 PM

Hi Tom,

I figured out where they were & the points look brand new (they should, the whole distributor is). I will go back & clean them as you instructed, if only to rule it out completely.

To narrow down some of the continuity tests I pulled apart the connector where that part of the distributor ties into the wiring harness (mine is the kind w/ the cable) & the pins were increadibly corroded. I cleaned them up, put it back together & it ran on all 8. Not sure if it was a fix or a conicidence, hence the point cleaning when I'm done here.

As far as running, she was honestly running not that bad on just 4cyl. I was totally shocked to find out she was down 4. It really sounded like just a lumpy idle (or a diesel

).

I just checked the D-Jet book you sent me & I don't see the continuity test for lower part of the distributor. Shoot that one over (mvenn@hotmail.com) or post here.

Thanks!

Tomguy · #18 08-07-2006, 08:01 PM

Although I do not know for certian which pin number fires which injector, I can tell you (at least for the points without the wired plug end)

#12 is the common/ground, all connections must be closed to this bin when the trigger points are at "rest"

With the points removed, EVERY pin should "short" (be closed) with every other pin on there simply because of the fact that they pass from that pin, through the ground, to the other pin. However, it is vital that each pin read closed with respect to pin #12 (14, 21, 12, 22 and 13 as ordered from left to right).

#14 = bottom left. #21 = top left. #12 = center. #22 = top right. #13 = bottom right.

autozen · #19 08-07-2006, 11:04 PM

You have nrver done a compression check and rechecked valve timing. You guys can play your silly games and horse around forever, but until you perform the basics, you are lost. That is what the factory always insists on to diagnose properly.

psfred · #20 08-07-2006, 11:17 PM

You either have two trigger points not making (or breaking) contact, or a bad ECU. You also could have a flaky wiring harness, or a bad ground connection -- typically ALL of the injectors ground at the same point on the firewall (all eight brown wires from the injectors and some other ones, too). Check that spot, a little corrosion there will play merry hell with the injection system!

Most likely the trigger points. I assume you've check the firing order and haven't swapped any wires around (don't laugh, I'm dyslexic and do that stuff all the time, even check twice and still have it wrong!).

Verify that they open and close as installed in the dizzy by pulling it and checking resistance between the contacts in the plug as they open and close -- I don't know which ones are which, as the SL dizzy uses a different connector than the 72 does. I'd remove and flush with brake parts cleaner first.

I would also drag a dollar bill between the ignition points to make sure they are clean -- they crap up badly for some reason, and since there is essentially no current flow (they only switch the transistor box), a little dirt will cause all sorts of trouble.

It will run pretty smoothly as a four cylinder if the right "banks" are out, but it has MUCH more power on 8 cylinders.....

Peter

autozen · #21 08-08-2006, 12:52 AM

Quote:

Originally Posted by autozen

You have nrver done a compression check and rechecked valve timing. You guys can play your silly games and horse around forever, but until you perform the basics, you are lost. That is what the factory always insists on to diagnose properly.

I repeat.

aajf@ipa.net · #22 08-08-2006, 09:29 AM

I just replaced the vacuum lines on my 1975 450 SL and it was fairly simple. Yes, you can get replacement color-coded lines...I got mine from ***************. It solved some of my problems but I still cannot slow the idle down?

350SL4spd · #23 08-08-2006, 10:06 AM

autozen,
Compression test was done not long ago & they checked out fine (only 90k mi). Dist. timing is set to factory spec for diagnostic purposes (as specified earlier) but will be set again later, more for performance than emissions. Cam timing is dead on & has been re-checked half a dozen times (fresh chain, etc). All valves are adjusted properly. This isn't my first time.

aajf@ipa.net,
Thanks for the lead on the the vacuum lines. I always like to replace as close to factory as I can get. As far as slowing down the idle; try Tom's suggestion of adjusting the idle speed screw & the idle mixture screw together. (Though I don't know your specific symptoms).

Tom/Peter,
Thanks for the tips. First thing today I'll be checking the continuity in the distributor connector, chasing/cleaning grounds, cleaning points, & making sure they open. I hope like hell it's not the harness. And I'd rather not buy an ECU, but it's a hell of a lot easyier to swap out than the harness.

Yes, double & triple checked the firing order. I stopped obsessing about it after I decided that the problem was intermittent. At the moment (knock on wood) she's running on all 8 & hopefully all the checking & cleaning today will just make sure she stays that way.

Tom,
I still don't have quite as much vac as I'd like. I tried turning the idle speed screw in and I only got a very small increase (1/2in), so I must have a vac leak somewhere. I'm going to pull the lines to the power locks for now, as the PO removed some of the hardware for them anyway (& who needs power locks in a convertible?). Then maybe I replace all the vacuum lines anyway...

Updates later.

Thanks All!

Tomguy · #24 08-08-2006, 10:46 AM

If turning your air speed screw does not raise vacuum well above 16" at idle, or turning it to close doesn't nearly stall your engine, you have major vac leaks. Start with plugging the line to the central locking system (if that isn't where you have your vac pump hooked up). Have you done injector seals at all? Those hoses to and from the aux air valve can be a source of vac loss if they're old/cruddy. Your aux air valve might be sticking open too. But it sounds more like a vac leak somewhere. The most overlooked leaking area is the tranny modulator line (speaking from experience at least). Mine leaked for at least 6 months before I realized the rubber on the other end of it was shot. Took rough shifting when it fell off one end to realize it.

autozen · #25 08-08-2006, 11:41 AM

Thanks for finally responding to my question about compression an valve timing. There are only two things that come to mind that would cause an intermittent rough running on this system: a poor connection inside the plug that goes to the trigger points or a MAP sensor going South. Unfortunately these items would affect all speeds and not just idle. If you want to substitute an ECU, you can plug one in from any year and model just to test.

350SL4spd · #26 08-08-2006, 12:19 PM

Tom,
Turning the idle speed screw in even a half turn will really effect the idle. A full turn will pretty much stall it out. Little effect on the vacuum however.

I still need to plug the line to the doors & check on the tranny vac line.

I pulled the ECU and looked it over & couldn't find the idle speed screw. (If memory serves me the ECU is the front on the left fender...not sure I could miss it

). Possibley on the '75 it is somewhere else?

A few more vac checks and we go for a drive...

350SL4spd · #27 08-08-2006, 05:06 PM

Got her all back together & road tested. She's running on all 8, sounds good, and pulls nice. (Other than a wild new rear end shake @ about 60mph...

).

I don't know if it's problem solved or just problem post-poned. Let's hope for the former. Nothing to do now but drive her & see if she repeats.

Thanks Tom & Peter! Who knows how long it would have taken me to track down the cams in the bottom of the distributor w/o the heads-up. Too much time w/ mechanical injection, I guess.