300E belt tensioner problems
 

1987 300E - I tried replacing my belt tensioner damper earlier today, but I ran into a problem getting the belt off. I loosened the 19mm bolt several turns, and I unscrewed the tensioning nut until it was completely slack, but I couldn't get the pointer to move more than about 1mm right. It moved slightly, and then stopped, no matter how much I turned the adjustment screw. There was no noticeable change in the belt tension, nor when I tried retightening the adjustment screw. I suspect my belt tensioner is shot, but can anyone explain what might be going on?

thanks,
anthony

If the belt tensioner is shot, the belt will simply have too much slack to begin with. Does yours have this problem?
Mine had so much slack that I did not even have to loosen the tensioner. I just lifted it off the puleys with my fingers and took it out! By the way, the car was running fine even the air conditioning was working fine although the belt was so loose!

I had a similar problem. How old is the tensioner? The problem with mine was that the rubber used to make tension had taken a set. This tensioner is basically two cyilinders with a thick rubber sleeve between them. The outer sleeve is connected to the idler pulley and the inner sleeve is then twisted producing the torgue necessary to tension the belt. What happens over time is that the rubber looses it elasticiticy. So the mechanic tightens it up a bit, then is looses that, an on and on, Soon there is no more adjustment left. You just need a new one. It is a real pain to change it put. Good luck.

had my tensioner and damper changed recently. parts
and labor for $400.00. it is a pain doing it yourself since
there are so many things you have to take apart.

my belt lost its tension and was told the tensioner was shot.
damper was changed for good measure. save yourself the
grief and ask a pro to do it.

PE - am surprised your a/c works despite the loose belt. yeh,
it will work but will cut out, in my case. took me a lot of headache
and research to trace the problem. a/c is suppose to cut out if
there are any distortion in belt cycle, and speed, i.e. loose belt, shot damper. this is the MB safety feature of the single belt technology running most MB cars. Most seasoned mechanics disagree with this feature since there are so many things and procedures to follow to identify the culprit. Hence, most of them suggest to by-pass the system (Klima control).

With the 19mm bolt loose and the adjusting nut completely off the seat, try pressing on the belt with the handle of a hammer or something like that to see if you can force the tensioner to relax. Note that if it is sticking, you might not be able to tighten it again.

Sixto
91 300SE
81 300SD

I do have a problem with my A/C cutting out at idle (and it cuts back in when I get going again), but I don't have a problem with too much slack - it's on pretty securely. I tried pushing on the belt and yanking on the tensioner pulley, but it stayed stuck.

If I move the pointer around, and try tightening the adjustment nut, the pointer WILL move - does that mean something's still working, or does the indicator work separately from the actual tensioning device?

(It's the original tensioner the car came with)

If you have access to the 103 engine manual or CD-Rom, review the section that covers serpentine belt replacement.

Try loosening the alternator and push it toward the engine. You'll gain enough slack to remove the belt. This also comes in handy when trying to reinstall a belt.

Thanks for the advice on loosening the alternator. One more thing - on the 300e's with the one-piece fan shroud, is there any way to remove it without unbolting the fan clutch? With just the fan off, there still isn't enough room between the front of the clutch and the radiator for any side of the fan shroud to slip by. Am I missing something?

anthony

I changed my tensioner and the water pump recently. I did it myself and I could see no way to remove it without taking out the fun clutch. By the way, I had a difficult time taking out the clutch, since it turns when I tried unbolting the main bolt that holds it in place. You will have to find a way to keep it from spinning when trying to loosen the bolt that holds it in place, not an easy thing without special tools, but it can be done with some ingeniouty. It took me quite a few hours.

Various configurations of the M103 have different methods of holding the fan clutch in place. I believe the 300E has a couple of ridges at the 2 o'clock position on the fan stand behind the pulley. Put a hex key (4mm?) or a really stiff piece of wire bent to an L between the ridges and slide it forward to engage one of the holes in the pulley. You'll have to turn the pulley to find a hole. That should hold the pulley while you work on the allen head bolt holding the fan and clutch. If you have stubby fingers, try pliers to get the allen key in place.

My don't-try-this-at-home-kids alternative is to tighten and loosen the fan bolt with the belt tensioned.

Sixto
91 300SE
81 300SD

Thanks, the hex key idea sounds good. I'll give that a shot.

anthony

The fan is actually very easy to remove with the clutch once you fab the correct tool. The engine manual describes the tool and it is made of 3/16" rod, very similar to the heavier coat hangers one might find at one's office. There is a 90 degree bend at the end that catches in a hole in the back of the fan clutch to hold it tight while turning the 5 mm allen bolt in front. Once you get the allen bolt to bust loose, you're there!

Check out the manual for the tool, they are sold by the tool companies for $15 - $20, but it's the easiest tool I ever made.

someguyfromMaryland

The hex socket(allen-type) used to remove the fan/fan clutch assembly is actually an 8mm. The tool with the MB star imprinted on it is made by Stahlwille. Part number on the tool is 10301109.
You might be able to get one at PartsShop. There are places all over the WEB that sell the socket and the ft. long rod bent at one end to secure the pulley from behind.

Thanks to everyone for the tips - I've picked up a tensioner and borrowed some tools from a local mechanic, and I've now run into my next problem with this job! Does anyone have any tips on getting the belt pulleys off? Everytime I try to break loose the bolts on the water pump and power steering pulleys, the pulleys keep turning against the belt (which, I assume, when tensioned is ordinarily sufficient to keep them from moving). I've sprayed some liquid wrench (on the bolts, not the belt), to no avail...

thanks,
anthony

The following has worked for me:

Place a wrench on one of the bolts securing the pulley and turn clockwise as if to tighten the bolt. This will serve as a counter. Now place a wrench on another bolt and loosen it. You'll eventually have all but one bolt loose. Use one of the now loosened bolts as a counter, then loosen the remaining bolt. Any that need to be reloosened may now be done by merely grapsing the pulley from underneath with a firm grip and loosened with a wrench.

Mike: thanks for all the help. That last bit of advice helped quite a bit, though I still found that loosening the last bolt on all of the pulleys was much harder than the first two or three.

Anyways, thanks to all of the advice here and in the archives, a few skinned knuckles, and about 10 hours of labor (quite a bit more than the three or four my mechanic figured it'd take), I did manage to get everything off and back on, hopefully without screwing something else up. Here's the procedure that ended up working for me (after some backtracking and head scratching):

1. Remove the fan shroud and place over the engine fan. A long-handled screwdriver is useful for prying the two metal clips off.

2. Unbolt and remove fan/clutch assembly. Look behind the pulley for a slot, rotate the fan/pulley until one of the pulley holes lines up with the slot, insert a rod-like tool into the slot until it engages into a hole. 8mm socket/allen wrench/MB tool takes off the single center fan bolt. According to information found in the archives, after removing fan/clutch assembly, keep fan clutch in a VERTICAL position...

3. Remove fan shroud (on the early 300Es, the fan must come off first... also, takes a minute or two to figure out how to sneak it past the hoses and oil cooler lines.)

3. Break loose the bolts on the engine fan pulley, the water pump pulley, and the power steering pump pulley (very little clearance on that last one). Bolt sizes were 10mm, 13mm, 13mm respectively, I think. Mike's tip on using two wrenches works well, though the last ones to be loosened still gave me problems. Also, don't forget to use the fan pulley holder at this stage - I did, and I regretted it after losing some skin.

4. Loosen the 19mm bolt securing the belt tensioner (right below the water pump pulley) several turns.

5. Unscrew the 13mm tensioning nut (right next to the power steering filter).

6. After first making note of the belt layout, remove serpentine belt. It took me a lot of effort to yank mine off, since steps 4 and 5 did little to slacken the belt.

7. Remove the fan, water pump, and power steering pulleys. The fan pulley was wedged pretty tightly onto the bearing, but it does come off.

8. Unbolt the top of the tensioner damper (13mm bolt and nut).

9. Loosen the 11mm (I think) bolt at the bottom of the tensioner damper.

10. Remove the 19mm securing bolt.

11. Remove the three 13mm bolts securing the Y-shaped bracket to the front of the tensioner. (Here is where I have a hard time seeing how someone could do this job alone - the bottom right bolt has a nut on the other end that seems to only be accessible from the bottom. It takes some finding after crawling underneath with a flashlight, and I can't see how anyone could unscrew the bolt and the nut at the same time.)

12. Remove the Y-bracket.

12. Work the tensioner + damper out from behind various bearings and accessory components.

13. Make note of the orientation of the tensioning nut/screw assembly, remove, check for stripped threads, reinstall. (Now would be a good time to do some engine component cleaning, if so desired.)

14. Install tensioner onto tensioning nut assembly. Since I had a hard time getting the bolt hole in the tensioner and the assembly behind it to line up, I would recommend temporarily threading in the 19mm bolt to check alignment. Also, the tensioning nut should be in the most "untensioned" position (otherwise, you'll have an even harder time getting the holes to line up, among other things).

15. Install tensioner damper.

16. Apply some RTV silicone sealant to the left-most Y-bracket bolt hole and the underside of the corresponding bolt head.

17. Remove 19mm securing bolt, transfer plastic pointer from old tensioner to new tensioner, install Y-bracket.

18. Screw in 19mm securing bolt (not all the way).

19. Attach belt pulleys. Don't forget to use the fan pulley holder! Also, when I was trying to push the fan pulley back onto the bearing bracket, I placed an aluminum block across the pulley (a thin piece of wood would work too) and used a wrench to hammer it back on.

20. Install new belt. I'm not sure what the best way is to do this - I found that the new tensioner didn't leave enough slack to loop a fresh, unstretched belt around the pulleys. I tried the alternator trick (remove bottom bolt, loosen top bolt. rotate alternator up and to the side), but found that, after attaching the belt, the belt tension pulled the alternator forward enough that it no longer fit between the engine brackets. I eventually had to enlist additional brute force to yank the belt over the last pulley.

21. Tighten tensioning nut until the pointer reaches the tip of the triangle or the belt tension is to your satisfaction. (I ran into an issue where the tensioning nut ran out of travel before the pointer reached the triangle tip! I backed off the tensioning nut a turn or two, then secured the whole assembly..)

22. Tighten 19mm securing bolt (75Nm).

23. Tighten pulley bolts.

24. Install engine fan, 45nM torque I think.

25. Install fan shroud. On the single-piece shrouds like mine, I find it works best to fit the top tab into the slot, then get underneath and slip the bottom two tabs into their respective slots. The bottom tabs appear to be slightly shorter than the top one.

26. Insert shroud clips, and you're done!

Hopefully, this is fairly accurate. Let me know if I missed something, because that means I misinstalled in on my car!

Thanks for all the help,

anthony

Well, I thought the job was done, but apparently not.

Starting up the car today, I noticed a severe rocking motion coming from the engine. After opening the hood and taking off the fan, the motion attenuated somewhat, but was still noticeable. Apparently the power steering pump pulley isn't moving in a uniform circle. Is it possible I installed it incorrectly? I notice that there are some weights attached to one side of the pulley...