When the first 190SL rolled off the assembly line in 1955 it used a distributor from what appears to be a 180 or 190 sedan. This distributor was fazed out after only 900 cars were built in favour of the mechanicaladvance only units which were used until the end of production in 1963. In all about 7 different distributors were used. While the basic function remained the same, many of the parts both inside and out side of the distributor changed through out production. Octane rating compensator adjustments from inside the car to simple dial adjustments were all used to good effect as well as advance curve changes on the 928 engine cars. Poor engine performance is often blamed on the carburetors ( as well it should) but often it's the ignition system that is also to blame. Much has been written on these subjects and the technical manual gives reference to this. However, I'd like to take you through the rebuilding and tuning of the Bosch distributor used on your 190SL and familiarize you the owner with some of the finer points of operation. . Installing ignition points and setting timing is discussed at length in the 190 sedan workshop manual - I'll leave you to those sources. Visual and functional testing: There are a variety of methods to determine if you have a functional distributor. Take the cap off and move the rotor forward and then release it which should cause it to spring back into its' original position. No movement or very stiff movement indicates a seized or sticking cam which will need to be freed up before full engine performance can be realized. Oddly , this one of the few internal parts which was used in all of these distributors so if you find one that's worn or damaged in any way it's easily replaceable with one from a similar unit. Often the springs will be weak and the rotor may spring back but not all the way. An easy way to tell if you have this problem is to drive your car until it's fully warmed up and then bring it to an idle. Shut the engine off and then restart it right away several times without moving the throttle. Note the changes in idle speed and record them for comparison. If you have a wide variance in idle speeds while driving that changes to one steady speed after you stop and start the engine this is an indication of weak return springs. When you shut the engine off the weakened springs pull the flyweights back into position which is something they can't do while the engine is running at higher speeds . As soon as you throttle up the springs are unable to hold the flyweights in and the idle speed increases due to timing advance. While annoying idle speed fluctuations can be traced to worn throttle linkage parts it's just as likely to be a small spring inside your distributor. Move the shaft in a radial or sideways direction to see if there's any movement. Very small amounts are OK but it should not move at all or a loss of dwell angle is likely at higher RPMs along with a resulting timing advance loss. The most likely problem is worn bearings or the main shaft is worn especially at the top bushing. The only way to repair this is to completely disassemble the distributor and replace the bushings which are available in a bronze alloy material instead of the original steel ones. Inspect the end play of the main shaft running through the whole unit after removing the distributor from the engine. It should be just loose enough to turn freely without binding yet have no noticeable end movement. Small compensating washers are available in .1 , .2 and .3 mm , the placement of which will be covered while doing a full rebuild. There are also a number of small shims under the breaker plate to keep the rotor stabilized at higher speeds. Push and pull on the cam to see how much play is present. Only a small amount is needed but it should be considerably more than the end play requirements of the main shaft as a too tight condition with cause the distributor to make a high pitched squeaking noise while running. It's very important to have free play in the movement of the flyweights or they will not work at all. Open the oiler at the base of the main housing. Oh, it's seized shut? That's not a good sign, as it indicates that the main shaft was probably never oiled resulting in bearing wear and eventual failure. The remaining parts such as points, rotor and condenser should be replaced during a tune up, or in this case, a much needed rebuild. Disassembly: Rebuilding of your distributor isn't difficult but there are a number of points to consider before you attempt. Do you have the proper tools? Is your distributor correct for your car and can it be rebuilt? ills needed to do the job? Are you interested in learning more about your car? If you answered yes to all of the above , let's begin. Removal First thing is to disconnect the battery. Remove the distributor cap and plan on replacing it if it's old. Same goes for the spark plugs and ignition wires. More on that later. On the early cars you will need to undo the octane rating compensator cable. There's a number of small parts at the cable end - don't loose any of them. Remove the wire that goes to the ignition coil with a 8 mm end wrench and the ground strap that goes to the cylinder head. On early models you will need to remove a grub screw below the distributor which holds the aluminium casting from sliding out of the engine block. On later models an allen screw placed underneath and behind the distributor will need to be removed. Remove the aluminium housing from the bottom of the distributor. A metal clamp with an allen screw used to tighten it will be at the bottom edge of the housing which must be removed. Place this housing in cleaning solvent while you continue to remove pieces. There are about 80 pieces which you will need to remove, clean, replace and reset before you're done. Distributor break down: I usually start by removal of the condenser and then the long threaded screw that goes through the housing. A whole set of small washers , insulators and nuts go with the screw. Remove the points and set them aside. Never discard anything until you have everything back together. You may need a part for reference or comparison , or you may drop one of several small parts down a crack in the floor and wished you saved it. Remove the bails or clips that hold the distributor cap on. You will notice that one has a longer shackle than the other which is used for locating the correct cap position. The right placement is next to the condenser or where the long screw goes into the housing. A very small mark on the top rim or edge of the housing which locates number 1 ignition wire is just to the right of this installed part. At this point I usually try and remove the small oiler cup at the side of housing. Sometimes they won't come out very well - two screw drivers placed end to end will usually pop them out. You should see one last slotted screw towards the top of the housing which holds the contact breaker plate in place. Remove this screw and pull the breaker plate upwards using a small screwdriver if needed. You will find several small metal and one fibre shim under the breaker plate. Remove and set them aside. Turn the distributor end for end and inspect the drive end. On the end of the shaft you will see a collar with two lugs or teeth on the end. These teeth fit into the end of the helical drive gear which also runs the fuel and oil pump. The position of this collar is critical and must not be changed from the pre disassembly position. The best thing to do is to mark the shaft and the collar with a small punch or chisel. Place the two marks near each other or in a way that won't confuse you later. After marking is done you can proceed to collar removal. On most units you will find a small wire retaining spring which can be slipped off with a small screw driver. Place the collar over a vise and have someone hold the distributor steady while you drive the pin out with a small punch. They're pressed in tightly so it may not come out easily but stay at it as it MUST come out. The pin is made out of a very hard material so it won't bend or break. Once you have the pin removed push the shaft out the top through the open housing. Often they stick half way out so push it back in and look for any burs at the bottom of the shaft. If removal is too difficult get a small file and lightly smooth off the shaft end which should help out. I have had to drive a few out with a punch so use whatever it takes. You will now be able to see the main shaft with the flyweights on the top. You will notice several small shims at the top and bottom of the main shaft - most of these will be replaced.To remove the flyweights pull the small spring clips or E clips from the pins. The flyweights will lift off after you unhook the springs revealing two small, red fibre washers and then a very thin dampening plate. Be VERY careful with this plate as it's really delicate. Although the rebuild kit contains a new plastic one I prefer the originals as the plastic ones often curl up and cause problems. Hey, you're almost there , just a few more things. At the bottom of the housing you should see 3 small vents. you can remove them for cleaning and painting or leave them installed - the choice is yours. Remove them from the inside using a punch close the same size as the vent or you can crush them. There's a small aluminium tag which states the model type of the distributor you're working on. If you want to paint the unit you should either remove the tag or use several layers of masking tape to protect it. Cut the tape with a razor blade until it's a nice neat oval. Removal of the tag is from inside using a very small punch to knock out two rivets. Only thing left to do is removal of the old bushings but first you need to fish the felt wick out from between the two bushings. Reach into the hole and try and start the felt out the other side. This can be a bit tricky but it should come out all in one piece. On rare occasions I've seen them missing or damaged so you should make a new one or use and old one. The rebuild kit doesn't have one which I find odd. The bushings can be knocked out using a suitable punch and now you should have a box full of small parts and probably no idea of how to put it back together. Cleaning and inspection: Clean the housing, contact plate, flyweights, cam, drive collar and any external parts in solvent and then carefully blow dry. Check the main shaft for wear top and bottom as well as the cam for scoring and pitting. Reassembly: I usually put the housing (which is made of cast iron) in a sand blasting cabinet and give it a complete cleaning which is why I remove every last piece before hand. Another blast with compressed air cleans off any left over sand and dirt. I then paint only the top potion of the casting with black semi gloss enamel paint. Inside the housing and everything below the clamp is NOT painted ever. The vents, clamp, oiler ( some are made of brass) can be either painted silver grey, zinc plated or left as is. Once the paint is dry you can press the 3 vents back into the bottom of the housing and then install the plate with all the Bosch part numbers. If you loose the small brass rivets you can glue it in place with a bit of varathane. Next comes the bushings in the housing.These are best pressed into place and never hit directly on the bushing as this will cause it to deform and the shaft won't go in. You will notice that the new bushings are the same length and that the old ones were two different lengths. Not only were the originals made of steel but the top one was also longer. The new ones are probably better so don't worry - I never have. You can press the bottom bushing into place using a vice or press while the top one is best done using a centring mandrel or at least starting it carefully before pressing it all the way in. Don't go any deeper than the edge of the bushing at either end. Find a drill equal to or smaller than the oil hole that was drilled through the old top bushing. Mount the casting in a drill press and drill a hole through the top bushing using the oilier hole as a guide. This will leave a small ridge or burr inside of the hole that will need to be removed with a small round file or the shaft will either not go through the top bushing or will refuse to turn freely. Try fitting the shaft in upside down as a trial fit and see if it will turn. If it won't even start there may be a burr inside the bottom bushing so lightly dress with the same small round file. If you have a micrometer measure the size of the shaft to see if it's same in the middle and at both ends. You may see an area of wear around where the old bushing was and it will likely be below this area that the shaft will have any problems. If you find it's too large file small amounts away until it will side through the bushing. Once you have the main shaft turning freely remove it and install the felt wick between the two bushings . This is done by rolling the wick into a small tube and then sliding it into the shaft housing. Once in place push it around with a small screw driver until it unrolls and slips into place. Install the oiler cup which needs to have the hole facing upwards by knocking it in with a small hammer. The outer portion should turn freely and when the word OEL is in it's proper orientation the hole must be completely covered so oil won't leak out and dirt enter in. Remove the main shaft so that you can mount the flyweights and set the distributor housing aside until you need it again. Main shaft and related parts Inspect the main shaft once more for distortion and wear and then proceed with the flyweight installation. On the top of the main shaft you will see a square plate with two arms on each side. Inspect these arms carefully as they should be at right angles to the plate and not bent or twisted sideways. Any sideway twisting is caused by using screws that were too long to hold the condenser on. These arms hit the screws and then they are easily bent which makes the springs pop off resulting in no advance. If needed, undo the two screws underneath of the plate and either remove the arms for straightening or replace this part with a new one. Once they're bent they tend to go back into the bent position so don't be too trusting of old worn out parts. You will have noticed that this part has slotted holes under the two screws. These are quite important as this is where you set the start of the advance curve. By moving the arms forward the springs go tighter and the advance starts later. Moving them back starts the advance sooner. Most units are set at or near the end of the slot so that there's tension on the springs at all times. If there's not enough tension on the springs you may have trouble with surging at low speeds and that annoying fluctuating idle speed we talked about earlier. I use a distributor tester to set all of these functions but for those of you without such equipment start at the tight setting. Look at the cam and you should see two small springs hanging from the bottom through two small hooks. Carefully open these hooks just enough to slip the old springs out and the new ones in. Close by pressing them shut with needle nose pliers. Lay the dampening plate on top of the square plate while paying attention to the hole on one side. The larger cut out goes around this hole and must not be covered. The two small red fibre washers go on top of the dampening plate and then the two flyweights. The two weights are different and the heavier portion goes face up while a small protrusion on the bottom is designed to slide against the dampening plate. Before you install the cam push a screw driver through the centre hole and a felt wick should fall out which you can replace with one from the new kit. It's a good idea at this time to slide the cam onto the top of the main shaft to test for radial play. There should be none or you will have dwell angle loss. If there is any play it's likely that the cam is worn and will need to be replaced. If it checks out install the cam which has two small pegs on the bottom that engage into each flyweight and the longer one goes into the hole in the plate. If the flyweights aren't right it won't go together so try switching the flyweights which should work. Hook the springs over the arms and see if the flyweights move back into place without binding. Installing the main shaft Place a small metal shim or two on the main shaft and slide to the bottom followed by a grey fibre washer. The fibre shims always rub against the cast iron housing. Side the shaft through and check for ease of movement. At this point I install the contact breaker plate and points then spin it in my tester just to see when the advance starts. By not installing the bottom drive collar I save time if I need to change the start of advance. My machine is set up so that I can run right from the end of the shaft or the drive collar. Either way it's VERY accurate. It will tell me dwell angle, start of advance, distributor RPM , point of fire which should be 90 degrees and stay there all the way through up to 6,000 RPM , maximum advance and at what RPM it's fully advanced at. I can tell if the springs are too weak , too tight , too loose , worn shaft or bearings, worn cam , bad points and condenser and other unusual findings. In most cases I usually spin the distributor before I do anything to it. If the advance and other readings check out I install the bottom drive collar. Install the grey fibre washer first followed by a few thin metal shims . Look for the assembly marks on the end of the main shaft and drive collar then try to fit the dowel pin. If the collar won't turn remove a shim and try again until it will turn freely. Often times once the pin is fully installed it won't turn at all . Remove another shim if you need to until it spins freely with as little end play as possible. The pin should be installed so that it's evenly spaced on both sides of the shaft. Install the small wire spring in the groove so that it covers the installed pin. Installing contact plate and ignition points Before installing the contact plate slide a few metal washers over the cam and then the large fibre shim underneath last. The contact plate can only be installed in one position so move it until all three screw holes line up. Install the three screws, two with the bails and one by itself. Try turning the main shaft which should still turn freely and also check if the advance moves freely and returns to the rest position. The kit comes with new bails which I always use. Installing these is can be a bit confusing until you study how they go on. Make sure you put the long shackle on the side nearest to the condenser and number one spark plug wire. Next comes the long screw with all the washers and insulating plates. Here's how it goes. from the inside: metal plate with two small hooks on one side square fibre insulator with hole offset to one side long piece of paper with hole in one end from the outside: round fibre insulating plate with hole in the centre two flat metal washers lock washer nut lock washer nut Install in the above order but do not tighten. While installing the points slip the spring for the points between the metal plate and the fibre insulator. Make sure that no metal parts are touching anywhere and then install the condenser on the outside of the housing. Turn the main shaft and check that it turns freely. If it stops against something it's very likely that one of the screws holding the condenser on are too long and will need to be replaced with a shorter one. Using the wrong parts can cause the spring tension arm to bend as discussed earlier in this text. The wire leading from the condenser should be placed over the long screw and between the two flat washers on the outside of the distributor. Tighten the nut and check with an ohm meter for shorts to ground. Adjust if necessary. Testing the distributor I use a SUN distributor tester as part of my rebuild process. After mounting the distributor in the testing machine I set the rotation to clockwise and the dwell angle to 4 cylinders. I like to run the distributor slowly at about 400 RPM to set the point gap. The 190SL uses a 50 degree dwell angle with a 3 degrees loss being allowable over the entire RPM range. Once I have the points set at 50 degrees dwell I increase the RPM and note where the advance curve starts. I like this to start around 900 - 1,100 RPM and be fully advanced by 3,000 RPM. Small corrections can be made to the start of advance by slightly bending the spring tension arms in to lower the start of advance or out to increase - you can access the arms through openings on each side of the breaker plate. You only have a few hundred RPM to work with using this method, anything more requires a complete disassembly to move the spring tension arms in their slotted holes. Once I have the start of advance set I check the full advance RPM. Almost every distributor Bosch made back in then had a different advance curve. Some are straight mechanical like the 190 and 300 SL's while other's have vacuum advance or retard. Roughly 30 degrees at or near 3,000 RPM is acceptable - it doesn't have to be exact. What you want is smooth function without any flat spots and the same return each time. After I have all of this set up I check dwell angle at high RPM. Most people don't drive their cars flat out all day long but I still check how high I can spin the distributor until I get point float. This a term used to indicate the speed at which the ignition points open and close so quickly that they start to vibrate or bounce and cease to function. A worn main shaft, worn points or a worn cam will cause this to happen at a lower RPM. A good 190SL distributor should spin up to about 6,000 RPM - well above maximum engine speed. If the dwell angle starts to drop over 4,000 RPM this can also be caused by worn bearings or main shaft. I usually increase dwell angle to about 52 degrees to compensate for this. This closes the point gap slightly and results in less pressure against the cam which reduces deflection. Sometimes you just have to make compromises so it will work for you. Installing the distributor Slide the clamp for setting timing if proved over the distributor bottom housing and then slide the assembly into the aluminium housing. Those units that don't have this clamp slide directly into the housing and are held in place by a small grub screw. Most units have a thick shim that goes under the arm on the aluminium housing so don't forget to place it there. An allen type screw holds the whole assembly on place and is also part of the head bolt arraignment. Attach the distributor wire coming from the negative side of the coil to the screw going through the distributor body using a nut supplied in the rebuild kit. Install the rotor, cap, coil wire and spark plug wires making sure that the wires are on correctly. If you set the engine to top dead centre ( TDC ) the distributor should have slid into place with the rotor pointing to number one on the cap and distributor housing which is roughly at the 5 o'clock position. If it's 180 degrees out or at the 10 o'clock position you probably didn't install the drive collar correctly on the bottom of the distributor. This is the time to pull the distributor back out, remove the drive collar and install it correctly. If the marks you made line up with each other ( you did make those marks I hope ) then you may have the engine to TDC but on the exhaust stroke. Check the timing marks on the front pulley and make sure both lobes on the cam shaft point up. You can see this by removing the valve cover or looking through the oil fill hole after removing the cap. Once you have the distributor timed to the engine try starting the car. You may find that the engine runs poorly or too fast. If the timing is advanced too much it may seem like battery is dead so try moving the distributor clockwise to retard it a bit. If the engine back fires or won't start pull the coil wire out of the centre of the distributor cap and place it near a metal part of the engine. If you have a good spark while spinning the engine it's likely out of time. After sorting out the timing you should be able to get the engine to start. Having an already running engine aids in this quest as a complete rebuild can have any number of things that might stop it from starting. Finish by setting the timing according to the factory guides. Most cars are set a 9 degrees BTDC but you may need to set at a lower place if the engine pings. 190SL engines need high octane but will run on regular if you don't push them or the compression is low. I find that most of them tend to diesel when you shut them off more than they tend to ping. Yours could do both or neither. Drive the car and test for performance gains. The engine should pull hard right up to your shift point and idle smoothly, or as smoothly as any other wear will permit. Idle speed should be steady and with new plug wires, spark plugs and a hotter coil you should see noticeable improvements. The last thing is to drive your car. Sitting around unused is the hardest miles it will ever do. Go have fun! You deserve it after all the hard work you just did. Dan Caron Sl Barn