Now there are somethings on a w126 that people mistake as a problem or a minor problem that looks major. Now if you take it to some dealership or a mechanic with dollar igns in his/her eyes, then they will try to scare you. Here are some common things.


The rear vent always blow cold air:This is normal as it is a fresh air vent. The heat is suppose to come from under the deats according to the air circulation diagram. To prevent this interfering with heater of rear passengers, close during the winter.

Light bulb indicator on but all bulbs are working-Contrary to popular belief that indicator isn ot checking whether or not a bulb is working but the voltage. A bulb that is out will return a no voltage and trip the light. A bulb that is under that socket rating will not return enough voltage and indicator light. A bulb returning too much voltage will trip trip indicator light. So if all lights are working, before you go tearing into the dash, check to make sure all the bulbs are proper setup or ignore the yellow light.

Shocks-If they are leaking, replace. Bounce test is not too good of an indicator for these cars as they are not the made to bounce like the wallowy Cadillacs. The ride probably will be gone way before that shows. Blisteins are most popular and usual choice to use.

Trunk leaks- This is a pre buy check spot. Before they try to sell you a whole new rear end for half your house, check the simple stuff. The leak can come from the anttennae seal, which is pretty easy to replace or the tail light seals. These things are rubber and in heat tend to dry out after a while. And sometimes it can be the rear widow. Rear window might be a iffy proprosition as there might be more rust than meets the eye. But if the area is small and no rust through spots, fix the leak first then the rust. No point in fixing the rust spots without fixing the leak. Sounds like common sense but know many shops do just that. They dont want to bother finding the leak but charge you an arm and ear to fix the rust spot.

Eletronic gremlins seem to be attacking multiple systems- Before you go out and start throwing modules and wires trying to fix the problem, check the fuses. They can be bought for dollars or less and are easier to change than a tire. Be surprised how many mechanics will basically rebuild your eletronic seats charge you for it only to find out it was only a fuse in the first place. That is a blatent scam but so many get away with it.


Front end clunk- A mechanic's special. They will try to sell you everything under the sun as if your whole front end is shot. Many times it's not the case and usually unless your car been in a wreck or have actually damaged the metal components themselves, then 90% of the time there is no point in spending $300-500 per component. The bushings are usually replaceable. Some of them are a bugger to get out but they replaceable and will save you lots of cash that didnt need to be spent. And usually a front end clunk is usually the tie rod ends or guide rod bushings. Both are relatively cheap to replace and can bring back that new feeling all over again.

Rear end clunk over bumps- Sway bars usually.

Rear end clunk switching gears- usually Differential. Many people confuse the two. And before you go and replace a whole differential, try changing the fluid. If there are metal pieces i nthe old fluid, then it might be too late. If not, you might be just in time. Many forget to change that fluid. I know W126s that have 240,000 on the dash and the owner give a deer in the headlight look when asked about the last differential fluid change. These things are sturdy and last a while and take quite a bit of neglect but this is something you dont want going out on ya while ya driving.

Vibrations coming from middle of car usually at low speeds- I'm not talking about the little shudders from unbalanced tires or unround rims. I'm talking about the type of vibrations that feels like someone is shaking your tranny as you drive. This can be a weak flex disk. Get this fixed asap or you'll find yourself buying more than a flex disk. Flex disk is not too expensive but the labor can be. So check flex disk. If it has crakcs in it, missing bolts, or distorted, time to replace. Front one and back.

W126 wont stay in park, back up lights wont come on (even though bulbs are good) and or starts up with the tranny in dirve-Before that mechanic start giving you lip service about expesive this and that and whole tranny replacement, check the neutral safety switch. Not and expensive part to buy and not too hard to replace. It's best you replace the switch link bushings and other rubber parts down there too to make things line up right.


Tires dont balance right, vibrating, wont align etc- This car is not made for cheap tires. Michelins are you safe but can get pricey. You can experiment with different brands as I found many non michelin brands while I was on tour in Europe that worked very well but not many of those was DOT certified. Also the person aligning your car might not know what the hell they doing. Even if they aligned thousands of fords/chevys/dodges. This is a Mercedes and less room for error in this department. Once you find a guy that have it just right with your tires, remember that guy. I remember I had michelins on my 560 at one point. The guy who did the work did an excellent job. But when I had continetals, vibrations (Michelins was not avail. for that car in mid nowhere Missouri) came upon and that guy didnt know what to do with the balancing issue. Took it to a friend of his and back to normal.

Seat/window switches work only sometimes- The contacts is probably dirty. Clean the contacts and they should work good as new usually.

Slow sunroof- Before blaming the motor or cable, get some good sunroof grease and it can make a hell of a difference. I know I might get blasted for this but actually I noticed the problem first on my BMW and went to the dealership and got some grease for it. Surpringly it was pretty cheap. It made world of a difference on the BMW so I figured hell, if this Sunroof works that fast on this car I wonder what will happen on the Mercedes. I tried it and it worked wonders on it. I didnt even realize it was slow until it was fast again.

Aftermarket sounds systems- If any monkey try to bolt an amp to the rear of the trunk in your car, run and take your car with you. They obvious never installed anything or nor have a clue about the layout of the W126. The gas tank is right behind that metal wall and most screws will go through that. With gasoline, you're asking for a firework show at you expense. If the front dash tweeters are working fine, then leave them alone. They add a good tweeter quality to go with the bass. Too many people take those out and add mids which muddle the sound quality. If you just have to add mids, do so in the rear deck or doors even, but me I left the original speakers in there (thinking they would blow soon) but they stuck in there and made the whole sound nice and clear. The subs of aftermarket added the bass to go with the relative tame mids and tweeters of the stock speakers. They sound a lot better than the current 300 I have with the same set up.

Mercedes wheels- If you have alloys, the silver alloys, treat them as you would treat your paint job. I dont care what the label on those wheel cleaners say in Autozone. I bet those labels say "keep off painted surfaces of you car." Guess what, those wheels area painted surface even with clearcoat and everything. If you spay harsh chemicals on them, what would happen to your paint job on the car surface will happen to them wheels. And those wheels are becoming expensive to replace. If you wouldnt spray it on your car, dont spray it on those wheels. If you have chrome, then treat it as chrome. But dont worry before someone says that you have to replace your faded, peeling scuffed wheels for an arm leg and an ear or two, they are pretty simple to get repainted, just like a car.

Hood pad- Dont rip the old one off and place the new one off or you'll be doing it all over agian very soon. Most of the work will be getting the old stuff off the hood and use a 3M glue. It's a relatively simple job but one that you want to get right the first time or dont be surprised to open your hood and find it laying on top of your engine. SOme places try to charge a lot for labor for this procedure, but seriously, if you can do it yourself, you should do it yourself. Even if they spit this "special glue" nonsense the glue is not that hard to get and dont cost that much as they might try to lead you to believe. Even a friend or two might help with this job and save much.

Battery- If your battery looks like it belongs in a Toyota Corolla, then it's probably wrong. Usually the battery that goes in this car look like it belongs in a large truck. This is especially important in the 560 as it has a large engine to crank and you might find yourself with a no start and a mechanic trying to talk you into getting a brand new alternator. Check for proper battery first.

ATF fluid under car- Before you assume it's the tranny and tell a mechanic to fix it, check the atf cooling lines that connects to the radiator. These usually leak before anything else. Also check around the tranny pan seal is another common place before investing in a rebuilt tranny only to find the leak has not gone away.

Radiator- First off use the yellow stuff. Secondly, the original radiators mostly came without the reinforced neck which tends to break after a few ages and even quicker with the wrong type of coolant. The breaking of this can result in rapid loss of coolant. If you temp guage is rapidly rising and looking like it will over heat, DO NOT try to make it that extra 20 miles. Over heating these blocks can rapidly ruin the engine head andy ou have a can of worms on your hands. 300 towing bill or 3000 engine rebuild. Your choice. If the radiator breaks, try to get the upgraded reinforced neck radiator. It's the one with a metal ring in the neck.

Coolant loss and overflow- First check the over flow tank cap. Look at the gasket, if it is shot, this might be your problem. When the car heats up, the pressure suppose to raise the boiling point of the coolant. That exspansion tank cap gasket maintains that pressure so make sure you get the right one for your car. The gasket nor the cap itself is not exspensive at all.

Wiping blades- Dont forget the rubber on those things are replaceable and dont have to replace the whole entire unit each time.

Window motor work but window dont move up but will move down or slides down freely when driving- This is usually a $5 bugger of a piece of plastic that is on the sliding jaw. These things can split in two and the sliding jaw cannot get a grip to roll the window up and down causing it to come out of it's place. Before your mechanic start throwing the words "Window regulator" at ya, check that first and it's real easy to replace but you will have to take the door panel off and have some patience putting it in. Getting the broke one out is usually the easy part. There is rumor of a metal piece that replaces it but I have not personally seen it and not sure how good it works. To me the plastic ones work just fine but like any good plastic will break eventually.

Rear seats- On the 420, 300, 350, 280; easy to do. Find the two tabs on the bottom edges of the seats. Puch those and slide the bottom towards the front and it should come off. Guide the middle seat belt around it. This will reveal the screws securing the back portion of the back seat. take them screws off and push up first and then pull forward. There are tabs you dont want to break. The seat back should come off pretty easy. If there is lots of tension, that probably means there is a tab that is still stuck. On the 500 and 560 with electric rear seat is a bit more complicated and never personally removed those seats. There are other post online that will explain it to you.

Rough idle that can be tracked down- Intake manifold rubber might be leaking. Most overlooked portion of the engine. Know people who rebuilt half there engine but forgot to check those parts.

Alright I'm out of time, hopefully these tips save someone out there hundreds of dollars and is willing to deposit those savings into my account. Just joking, but hope it does help some people out there.

 

Blackwolf,

Thank you for your little outline. Very helpful!

I do, however have an issue where pressure builds up in my gas tank (1987 420SEL 172000Miles). I put a new cap on and it consists. It is getting so bad that I experience leak out of the gas cap too.

Any suggestions?

 

it's probabaly a bad fuel tank vent system. could be the rear side fuel vent check valvestuck closed or the front one stuck closed.

 

Good list!

My 1991 420SEL (204,000 miles) has experienced a few of them...

I enjoyed the comments on the little teflon window sliders, as I've replaced them on 3 of my four windows, over the years.

Cheap part, but a pain to go through the replacement process...

I was trying to think of any others:

I suppose one would be the "slow engagement of reverse" issue, but that isn't confined to just W126's.

Another might be the little valve that turns on/off with a varying duty cycle to control the flow of hot coolant through the heater. Perhaps ACC issues in general.....

 

so I have a quirk that's hopefully more common than I think.. I have an 85 300SD, young with 192,000 miles. My issue is that I have intermittent heat issues. If I turn it on when the car is first started cold, I will never get heat as long as I keep the car running. If when the engine is warm, I shut off the car, leave it for a minute or two then start it, I get heat. This lasts forever if the car is idling or driven VERY lightly. If however I apply moderate or more throttle, the heat instantly goes away. This car also likes to overheat when idling unless I raise the RPMs even by a little bit.. any ideas?

Im clueless on what could be happening, thanks!

 

You may be talking about a couple of separate issues. In terms of intermittent cabin heat, one thing to check is the little valve that controls how much of the hot engine coolant is being directed to the heater core (the heater core is behind the dash on the front passenger side).

This heater control valve is called the "mono valve", and if you search the forum you will probably find more details. It is located in the engine compartment, up close to the firewall, basically next to the battery (on the inner side of the battery).

The top plate of this valve is held in place via four screws.

The valve basically opens and closes at a particular duty cycle, to control the flow of hot coolant to the heater core. In max. heat mode it is open 100% of the time. In max. cool mode it is closed 100% of the time. But in between it is continuously opening and closing. If you put your finger on the top plate, you can feel this repeated opening and closing. If you change from full cold to full heat, or vice versa, you should be able to feel one little "thump" inside the valve, as it moves from one state to the other.

There is a little rubber diaghram inside this valve that is prone to tearing, and it is also possible for corrosion to eventually make it stick, or intermittently stick. You can buy repair kits for these, which basically let you pull the innards out (from the top), without having to remove the whole part.

One little tip is that you don't want to do the repair when the coolant is hot and pressurized - at the same time you want to leave the radiator cap and coolant reservoir caps on. (If the caps are off, you will get a fair amount of coolant draining out when the top of the mono valve is removed.)

(I am writing this from my memory of having put in the repair kit several years ago, so hopefully I am remembering things right.)

In any case, if you have someone move the temperature dial to various settings, while you feel for the little thumps of the mono valve changing state, you should get a feel for whether it is working. And obviously it is only one of many things that can go wrong.....

As a further aside - if the ACC control unit fails, (or if the fuse for the ACC control unit is blown), the usual symptom is that the system defaults to full heat.

 

Would appreciate any members who could help me out .Can anyone tell me how to take the front bumper off a 1989 420 sel, I just want to know the best way to do it, and if its an easy task, how many bolts i have to take off ect.Thanks Jim

 

After considerable research and a couple years of waiting for just the right deal I have bought an '83 W126 with the 5-cyl td, and have fallen pretty muchly in love with it. I take issue, however, with the automatic climate control which seems to have a mind of it's own. The heater tried to fail me on a recent visit to Minnesota that ironically coincided with their first snowfall. The brushes on the blower are worn out but I managed to jimmy them into a last gasp that got me back to warmer climates. On the return trip the heater would blow hot air for a while, then switch to cold and stay there. Shutting the system off still allowed ice cold air thru the vents although the blower was off. Shutting the car down completely brought back the warm air for 30 min. or so then back to cold. It would only stay hot on max heat, and most of the time that was barely enough to keep it comfortable. Also, the rear vent will not shut off at all. I suspect the fresh air vent is getting stuck open all the way, but I don't know how the system works or how to start troubleshooting... any ideas?

 

Is that the automatic climate control or manual? The manual ones are easy to find problems and fix relatively while the automatic climate control can be a throw a wrench across the garage PITA to find problems get to the problem and fix it.

 

It's definitely the automatic control. I had to read the instructions just to figure out how to use it. I've seen a lot of posts where people had trouble with their mono valve but it sounds like when that dies it goes completely and besides that it fails to max heat so I'm pretty sure that's not the problem. My current theory is that whatever controls the fresh air stream is opening and then getting stuck. Does anyone know how that works?