85 560 SL mystery for the "real" techs - PeachParts Mercedes-Benz Forum

psfred · #1 10-12-2004, 11:58 PM

My friend Hans has a 560 SL (I believe he said it was an 85) with a very strange problem.

It runs at 20 degrees ATDC at idle, and naturally is rather severely defficient in power and the exhaust manifolds are overheating.

Engine control and ignition computers have both been replaced with new ones, fuel distributor is now OK (more on that later), good ignition, etc. O2 sensor is OK, mixture appears to be OK, but with the timing that late it's rather hard to set.

The car itself has a strange history. Seems the PO had this problem, couldn't solve it, and had a carb installed, alonth with a different distributor. Supposedly ran perfectly for two weeks, the problem (lousy preformance) re-appeared. When Hans got it, the injection system had been re-installed, but with a used fuel distributor that was bad.

It will occasionally work fine cold, but once it gets warm, it goes back to very late ignition timing. EHA current will go to 30 mA when cold, but will never go over 2mA on acceleration once warmed up.

Temperature sensors are, I belive, OK.

According to the two most recent owners, the engine/transmission have not been separated, ruling out the possiblity that the flywheel was installed a bolt off (possible, if difficult).

I suspect the crank position sensor myself, as that is the only thing I can think of that would delay the spark that much. Normally, they work or are inert, but I'm wondering if the signal from the pickup is strange (large skew on upswing rather than a nearly vertical volgate rise) resulting in delayed igntion triggering. Rpm signal from the computer appears to be correct.

Any ideas? MB Hotline is as stumped as Hans, no one has ever heard of this particluar condition.

Peter

mpolli · #2 10-14-2004, 12:26 PM

Hi psfred,

I can't believe this has gone over a day with 55 reads and no response. Well you are asking for "real tech's" but you didn't say what kind so I will chime in since I am a "real" electronic bench tech with 6+ years experience troubleshooting industrial control systems to component level. So there is my resume. I have no experience with your car. My nearest experience is with my 86 Supra which has a L-Jetronic based system. First question sounds dumb but here goes: Don't you set the timing by turning the distributor?

Anyway, in terms of the crank sensor, do you know if it is Hall or VRM type? If you suspect the signal coming out you could look at it with a scope. But the trouble could also be in th box that reads the signal. Could be a bad solder joint or component on that board. There is the approach of just change out the cam sensor but that might be quite a bit of work. As a bench tech my approach might be different than yours. Another thread mentioned that Mercedes uses lead free solder which causes bad solder joints. On the one hand this is not actually true, lead free solder can make perfectly good solder joints. On the other hand cold solder joints do exist and I just fixed one in my 98 C230. And bad connections of all types, and expecially cold solder joints can be very temperature sensitive. In my world connections were the source of all evil and solder joints are one type of connection. Just some ideas to maybe get this going somewhere.

Mike

porkface · #3 10-14-2004, 09:33 PM

ok, i'll bite. even though i'm in over my head. i can't see the car.

first thing i would do is pull the valve covers and 0 the crank pulley. cams lined up? dist pointing to 1? if the auxillary shaft slipped a tooth or 2, or the dist is off a tooth, best i can do. otherwise, i'd bet on the ezl unit or the flywheel has been off. keep us posted, chuck.

Gilly · #4 10-14-2004, 09:48 PM

I'm not sure but I don't think there was an 85 560SL, so double check what car it is.
I's double check also what you're saying about the TDC sensor (it is NOT a crank position sensor, it only senses TDC). It should be located on the front cover, not the flywheel on a 117. So causes for the timing being off, (besides an electronic fault) would include an incorrect front cover or harmonic balancer.
Of course one possiblity of a engine that runs worse as it warms up is a bad coil.

psfred · #5 10-14-2004, 11:10 PM

Gilly:

All I really know about the car is that it's black, it's a 560 SL, and Hans has been pulling what hair he has left out (although he's now to the point he HAS to find out what the problem is just out of curiosity).

When we talked about it, I asked him what could delay the spark if everything was working correctly otherwise (new engine computer, new ignition switching unit). I was thinking slow coil, bad switching unit (but that has been replaced with a new one), bad crank sensor. The crank sensor is supposed to be on the flywheel (I'm sorry this is such a fishing expedition, I'll check in and get better info this week), and the Hotline asked if the flywheel had been removed since getting it back on one bolt out of position would be just about exactly 20 degrees.

If, however, the sensor is on the balancer, I'd bet the balancer has slipped (but how could that show late timing?).

I'll tell him you suspect the coil (as I do -- too much resistance could cause the spark to be late due to slow field collaspe -- wouldn't it be worse at high rpm?).

To answer the distributor question, timing is not set on these engines with complete electronic control, it's done by the computer. The dizzy just gets the spark to the correct cylinder.

Test equipment is working properly, shows the right data on Hans' 560 SEC.

Thanks!

Peter

Gilly · #6 10-15-2004, 08:34 AM

Oh Boy, got out the disc to double check my suspicions, and I am all wet, it is a crank position sensor, not a TDC sensor, and it's in the bell housing. Assuming it is a 86 or newer 560SL, sometimes people will put a newer/bigger engine badge on the trunk, so be careful as far as what the car really is. Since he is indicating the sensor is on the bellhousing, I think it's safe to assume it's a 560.
On the coil, the typical problem I've seen is they break down (short out) at high temps, the insulation inside turns to goo and the engine runs like crahp once it's hot under the hood. High or low rpms, runs really poor, need to scope the ignition to see it, but in the past I've always managed to come up with a "test coil" to see if it fixes it or not.
Gilly

PS If I would have been correct about the engine having a TDC sensor running off the harmonic balancer, the reason it would screw up the timing is the same as if the flywheel would have been installed 1 bolt off, the harmonic balancer has a small magnetic "plug" installed in the edge of it, and this is what the tdc sensor senses. If the harmonic balancer had slipped, or the sensor changed position for any reason, the timing is directly affected. The harmonic balancer is on the crankshaft with a woodruff key to index it to the crankshaft. You can actually buy woodruff keys with offsets to change the ignition timing. I think mainly Hans has to decide if he's got a timing problem (or other ignition problem) or a mixture problem. It should be easy to determine if the timing is correct or not. No offense intended.
PPS If Hans has a 560SEC, have him try swapping the coil over to the SL as a test!

psfred · #7 10-15-2004, 09:43 PM

Mike:

Four pulses per rotation per the manual, there are four tabs or magnets on the flywheel.

I don't know that the timing is correct cold, only that it runs better!

Peter

stevebfl · #8 10-16-2004, 03:19 PM

Gilly: 86- means from 86 on. That means from 86 to the end of production which contains the 87-89 116/117s and the 90 and 91 models also.

Anyone ever check the ignition portion of the temp sensor? Is the coding plug in place?

psfred · #9 10-16-2004, 03:29 PM

Thanks Steve!

I will pass this on -- I've suspected a sensor or wiring problem, too as the ignition timing is calculated. Very hard for it to be a mechanical failure other than incorrect resistance, etc.

Could the coding plug be bad?

Peter

stevebfl · #10 10-16-2004, 03:37 PM

I have never seen one bad. They are only resistors. It could be posible to tamper with one though.

If the timing is right cold, I would really worry about the temp sensor. Otherwise I might worry about the ignition controller.

psfred · #11 10-16-2004, 03:45 PM

Ignition contoller is new, so I will have Hans take a look at the temp sensor.

These can be a pain -- I've seen some on other cars (volvo) and are physically identical, but internally different, so they won't work when installed -- my sister's 92 Volvo 740 Turbo was like this -- severely overheated when the head gasket blew/radiator failed (don't know the sequence, it was fried when I got there). Temp sensors and thermostat bad, so I snagged one from a junkyard as I didn't have time to get one from the dealer. No go, the proper one had two resistors connected to the body, all the others Volvo used had a single resistor connected across the two wires. Needless to say, the check engine light stayed on a lot until we got the correct one in.

Thanks again!

Peter

droederer · #12 11-22-2004, 04:10 PM

Gentlemen, Thanks for all the advice. We have finally solved the problem.

The flywheel (ring gear) was indeed installed in the wrong position. When I purchased the car, the car was not running. I had asked the previous owner if the transmission had been removed and he said it had not. Now we know he had a memory lapse.
We wanted to eliminate everything we could before we pulled the transmission.

Thanks again I appreciate all the knowledge on the list. We put a lot of parts on this car but now I have a great 86, 560SL that I have just over $6,500 invested in.

Dave
86 560SL

psfred · #13 11-14-2004, 06:15 PM

Finally got to the "root" (or actually, ring gear) of this problem.

I'd always though something was fishy, and sure enough, when Hans finally convinced the owner to go ahead and have the tranny removed and car returned to the shop (Hans no longer does trannies on these cars since he works alone), the flywheel was indeed wront -- it appears that the ring gear was incorrectly installed 120 degrees off.

Ring gear is being R&R'd to reposition and Hans will get the car back shortly to tune up the ignition and fuel system (mostly new by now!).

I guess no one understood that a ring gear replacement requires removing the tranny, as all three owners were quite certain the transmission had never been out.

So this story has a happy ending -- and a caveat for other owners of similar cars: if it runs like crap after a ring gear replacement, the ring gear or slywheel was incorrectly installed! It's not necessary, nor particularly desirable, to install a Holley carb insted of the FI......

Peter

ericgr · #14 11-14-2004, 06:30 PM

Incredible to imagine the work that went into resolving problems that didn't exist (fuel system work, etc)-- such a lesson in the value, and difficulty, of root-cause analysis in such complex mechanical systems. And a lesson why doing something off-the-track (like modifying the fuel system like I think was done here) is not a good practice since it flies in the face of resolving fundamental from-the-factory operation. Returning to fundamentals seems key in problems like this (from the distance my rookie mechanical mind can see and has experienced in much smaller increments vis-a-vis cars, very much the case though with complex computing systems in my experience by analogy).

porkface · #15 11-14-2004, 07:37 PM

i had a feeling about the flywheel. pity. good catch, chuck.