350SL US model blues

[JSFmbSL]

Okay so here is the situation. We love our Our 1972 350SL (4.5 L US model). within the last month or so it started inconsistently running as though it was missing on one then several cylinders, cutting in and then out and then fine for awhile. It will start fine and warm up promptly and idle normally. On some short trips, 5 or less miles it will seem fine. Then it will miss then back to normal. It does not matter whether lugging uphill or down, it just does it unexpectedly. Checking the usual culprits, I replaced the spark plugs, spark plug wires, and fuel filter. It has a new (slide) cold start valve, and new breather hoses. One injector is new, all other seals are new. Much other engine work was done previously within a year, including timing chain, guide, tensioner, head gaskets, valves timed an tuned up. It ran fine all summer until early this fall. Any suggestions? Seems like fuelish rather than electronic but I am a novice hoping for some help on this. Thanks in advance.

[page62]

Spark control module???

[Richard Wooldridge]

I'm not familiar with the injection on your engine, but if it is the older electronic fuel injection it most likely is the trigger points, under the distributor plate. Of course, this wouldn't apply if your engine has CIS injection.

[Damian]

It could be your Ignition control module.

Not only do they suffer from the darlington power transistor failing, which usually happens when a spark or coil lead has become open or intermittent.

These also suffer from dry joints as per many other electronic modules in these cars. If you have a soldering iron on hand you can open the unit and remove the PCB and look for dry joints around the board and resolder them.

A little tricky to remove but if you undo all the screws, remove the leads to the resistors that feed through from underneath first, then unsolder the transistor leads and those to the front connector, with some manipulation it should come out fine ( I've done heaps of them).

Pay particular attention the all the red and yellow capacitors, the joints on them may look ok, but if you tug on them many of them pull of the board.
Same as in the cruse control and idle control modules. Best resolder them anyway, as I feel they must have been slightly tarnished berfor they were installed during manufacture.

Good luck

Damian

[JSFmbSL]

Thanks all for your help. My car is an early 1972 US model of the 350 SL (1099th 4.5 L car built) so it does not have much emissions stuff. In my Haynes Automotive Repair Manual for the 450SL I was reading about the trigger points so I suspected checking that next. However, I never could find anything about the Ignition control module in the Haynes manual. Could you tell me where it is physically located so that I might see if it can be inspected? I looked online and the replacement for an Ignition control module is very expensive so if the current one could be repaired that might be practical. Also can anyone suggest a mechanic in south central Michigan or northern Indiana for these early 70s electronic fuel injection 4.5 L engines? Thanks again.

[page62]

I see that *************** has them for $606. Ouch!

I've heard of some people using Crane Fireball spark control units as a replacement. They are designed as an economical replacement for the OE Bosch (and other) units. Two choices, both around $100!!!

http://store.summitracing.com/partdetail.asp?autofilter=1&part=CRN%2D3000%2D0231&N=700+115&autoview=sku
http://store.summitracing.com/partdetail.asp?autofilter=1&part=CRN%2D700%2D0231&N=700+115&autoview=sku

Of course, you want to verify that this is an ignition problem first...

[JSFmbSL]

So where exactly on my car do I find the Ignition control module? Remember it is a 1972 early US model 350 SL (4.5L engine).

[page62]

It might be on the fender wall, near the distributor. Look for wires coming from the distributor to a black box about 4 inches square and 2 inches high.

[alabbasi]

I had similar issues on my W108 280SEL 4.5 which uses the exact engine and FI set up as yours.

You have a very early EFI setup called Bosch D-Jetronic. In some cases over time, it can become very unreliable.

The system is a true EFI setup and there are a couple of known problem areas.

1) Trigger points: These are points that sit on the base of the distributor and is used to fire the injectors as the points trigger the sparks. These can get dirty or faulty. Try to clean them (carefully) before replacing them.
2) MAP sensor - controls the air / fuel ratio. Can act up and cause the system to run either rich or lean.
3) ECU - Brain, also controls the air fuel ratio , can act up and cause the system to run rich or lean. There is a dial on the side of the computer that will adjust this mixture at idle.
4) Fuel pressure regulator: Regulates the fuel pressure.

Your ECU should be inside the car and accessible from either the driver or passenger side foot well. It looks like big steel rectangle about 12 inch long, 6 inch wide and 2 inch thick.

With my car, it ran like crap because the system was running super rich. This caused gas to mix with the oil and eventually made the car very hard to start.

If you want to diagnose the system try the following:

Check the oil in the dipstick and see if gas is mixing with the oil.
Disconnect the trigger points and then try to crank the car. If it fires briefly (as if it wants to start) then you have gas in your oil and the system is running super rich.

Drain and replace the oil and see how it runs.

You can make a bunch of adjustments on these cars, but the important thing to understand is that if it's running rich, you'll get gas in the oil and as a result you'll need to drain the oil and replace every time you make a change or else it will skew your tests.

In my case, I had a faulty computer (brain). It would run fine for a while and then suddenly turn super rich. Once it did, it would dump so much gas into the system that even when it would go back to normal, there would be so much gas in the oil that the car would continue to run like crap. This made it very hard to diagnose and cost me a lot of money to figure out. A replaced computer from a junk yard car cured the problem.

Cross post your symptoms to the vintage forum, there are a lot of D-Jetronic experts there (including the moderator).

[dieseldiehard]

Originally Posted by page62

I see that *************** has them for $606. Ouch!

I've heard of some people using Crane Fireball spark control units as a replacement. They are designed as an economical replacement for the OE Bosch (and other) units. Two choices, both around $100!!!

http://store.summitracing.com/partdetail.asp?autofilter=1&part=CRN%2D3000%2D0231&N=700+115&autoview=sku
http://store.summitracing.com/partdetail.asp?autofilter=1&part=CRN%2D700%2D0231&N=700+115&autoview=sku

Of course, you want to verify that this is an ignition problem first...

A Crane ignition (optical) module coupled to a MSD ignition will knock your socks off! You will get better idling, better acceleration and long plug life as well as no more ignition "tune ups" The problem in many instances is where to put those modules, they aren't very small. I have relocated the battery to the trunk in order to gain space for the ignition modules, well worth the effort even in a gasser sedan.

[230/8]

JSF:

Your early SL is most likely identical to mine. I suggest a check of the FI trigger points. They are locaed in the bottom of the distributor and are expensive to replace, if you can find them. They may simply be dirty, so try cleaning them before replacement. If it runs crappy on half the cylinders then I suspect the points, each controls half the injectors. Get a manual and check which injectors are being fired by which point set. Your plugs may foul out also giving you a clue.

Your ECU is located under the dash above the passenger's feet. There is a cardboard cover with a flexy part near the outside near the fuse cover. You can bend it down and see the only adjustment possible. There is a knurled knob on the bottom that adjusts idle fuel mixture. It works like a radio volume control...CW to increase fuel and CCW to reduce fuel. The range of adjustment in small, only about 10%. And, this is unlikely to be your problem...just mentioning it for reference. If you get to fiddling with this then I suggest investing $200 in a good gas analyzer. The GasAdjust model is great.

Your spark box is located on the driver side front. Look forward and way below the coil area, under the inner apron...way down almost to the horns. You will see it there sitting exposed to everything, oh those wacky Germans, wild sense of humor. I suspect it is a version I, but there are three possibilities and you will need a manual with pictures to pinpoint which version. But, I suspect this is not your problem.

Check the cold start system and sensors. Sometimes the cold start valve will flood fuel because of a leak or faulty temperature sensor.

The D-Jet systems basically run on vacuum signals which are translated to electronic impulses by the MAP sensor. Other sensors for throttle position and temperature also feed to the ECU. The vacuum system MUST be leak free in all respects or the EFI system will fail you with poor overall performance and lousy driveability. Check every vacuum line and if you have not already, replace ALL soft vacuum lines to get a tight seal. There is a large line to the MAP sensor, located on the fender apron adjacent to exhaust manifold and just aft of the coil. Replace that line too, or at least clamp each end. The cold start system has some large air hoses into and out of the AAV. Check them and replace as necessary, it is probably necessary. Also, early photos show the cars were equipped with clamps on these hoses. The bean counters eliminated them at some point; I suggest replacing them for tight seal.

Get a good manual, I like the $20 factory CD ROM, and, if you can, check Amazon for Bosch FI books. There are several with good data in them. I suggest getting one of each because they all have something good to offer.

I suspect there are no D-Jet "experts" left alive anymore, and if they are they retired a decade ago. There may be some aging indy shop owners in your area, but for the most part there are only those of us who now own a D-Jet car and must learn to care for them ourselves, and ocassionally offer support based on anecdotal knowledge, which in some cases may be slightly wrong. Some of my advice may be off the mark in some areas sometimes. But, what I do know for sure is the vacuum system must be spot on at all times...you can take that to the bank.

Good luck,

230/8

[vwnate1]

All the above is dead nuts -BUT- there's a trouble spot in the trigger points most overlook in thier haste to rip it all apart .

There's a wire with a big rectangular 5 pin MPC that connects the trigger points to the EFI harness , the MPC is inside a big rubber boot just below the front of the air filter intake snorkel ~ first thing to do is : unplug this and wiggle it as you re-install it , then see if the miss is gone , you can wiggle it whilst the engine is running too as if the connector is dirty/corroded (VERY common these days) the engine will likely begin to run on all 8 cylinders again . if this gets it running smoothly , here's the next step :


Unplug it and look inside , most cars these days no longer have bright , shiny pins & sockets , it is a simple thing to carefully take the MPC apart and _polish_ the male pins (never , EVER sand an electrical contact !) untill they're shiny then use a Q-tip & brass polish in the female sockets , then carefully smear a tiny bit of metallic grease (I use nickle based anit-sieze) on each pin to ensure good contact , the cylinder dropout problem will most likely be solved . do not use Dielectric grease ! you're working with milliamps here so any resistance at all will make the trouble worse .

On my '74 350SLC the femal sockets were tarnished dead black and the Bakelite part of the connector was all crumbly so I got a nice shiny Bakelite part from a junker Mercedes , this same part is used all over the car , up under the dash , in the trunk on the antenna's power connections , it is a simple thing to remove the cover and draw a simple map showing the various pns and what color wire goes into which hole so you don't mix them up during re-assembly . I also pinched the female sockets a tiny bit whilst they were out of the Bakelite block to ensure tight connection .

I've had to do this on two cars in the last month , one was a for sale deal - my buddy thought his engine was Kaputi ~ I wiggled the MPG as the engine missed & skipped , it smoothed right out like glass so he was able to sell the car .

Your ignition module is of all places , underneath the left fender , right behind the headlight ~ you'll have to look way up there to see it but it's there and _rarely_ gives any troubles so don't go running off and buy a new ignition sysytem just yet . your symptoms clearly indicate trigger problems .

Don't be fiddling with the dashpot on the brain box either untill you've 1st. replaced the fuel filter and then checked and adjusted the fuel pressure in the fuel ring (tap it at the cold start valve) to 34 PSI .

As mentioned , the vacuum hoses must be tight to ensure proper operation , the hose from the back of the intake manifold to the MAP sensor is a bugger to get re-attached at the engine end but a pair of long nosed Duck Bill pliers will do the trick nicely . I use the spring typ hose clamps on mine .

I'm no ' expert ' but I have been working on Bosch D-Jetronic F.I. systems since they were new...

[72350sl]

I had the ignition control mod go out on my car. It would turn over but no spark . No voltage to the coil. Your problem does sound more like fuel. Ignition control mod. is not in the engine compartment. It's under the front end drivers side, near front bumper and wheel well. John.