I have a 1986 300SDL, and I was checking my power steering fluid and noticed that I had a A/C mounting bolt missing. I got another bolt and tried to install. I could not get the bolt started, so I checked to make sure the previous one had not broken off inside the hole. Well, it has and now I have a few questions.

1)It is all right to drive the car with just three mounting bolts holding the compressor. I checked the other three bolts and they are all good and tight.

2)What is the best way to remove the broken bolt?

All help is appreciated, especially with warm weather coming. Thanks, Johnny

Snapped bolt
 

Hey johnny. It should be ok. Since the torque and comression of the diesel engines. Things come loose or even break. To keep the cost down. Remove one bolt at a time that holds the compressor. go get some lock tight for bolts. I forget which no# but its the blue cap I think. apply it to the threads one at time and reisntall the remained 3 bolts you should have no problems. Just make sure you get the lock tight. I use it religiously.

hope this helped.





Guido
Factory Tech Mercedes Benz 14 yrs

Johnny, Greetings from NZ.
Hope you haven't started the job of replacing the missing A/C compressor bolt.
I was cleaning around the underside of my 190E and noticed that one A/C compressor mounting bolt was missing. When I attempted to take another one out to measure it's dimensions, it snapped off without any effort! So instead of 4 bolts, I now had 2 holding the compressor in place. The upshot of this tale is that the MB dealer also broke one of the remaining bolts off. (Not their fault, in my opinion, just over-tightened bolts sometime in the past.) The A/C system had to be de-gassed, the compressor removed, and the bolt holes drilled out and rethreaded. Of course the system had to be re-gassed with R134 to complete the job. $600 later I got my car back on the road.
My advice, providing I'm not too late, is to leave the compressor bolted in place with three bolts. It won't go anywhere, nor cause any operational problems. Good luck!

Murray Blackwood

Attn johhny I think if you spray some WD 40 or some teflon penetrate triflow< thats stuff works. Your diesel vibrates alot more compared to the previous reply. Just be careful. Trust me I do this for my living I give the customer the best option. If you dont the others could break or snap off due to vibration.

Thanks for the input. Johnny

These bolts break because of fatigue cracks in the bolts. As stated above the cracks are caused by vibration. They start at a blemish or machine mark or thread at the surface of the bolt and then with each vibration they slowly eat their way across the diameter of the bolt.

If you look at the plain of the crack after a bolt has broken you will be able to see where the crack began (the origin) and then tiny microscopic lines emanating in ever larger arches as the crack progresses through the metal. (Use a microscope or powerful magnifying glass)

If the crack has sufficient time to work its way clear across the shaft of the bolt then the head of the bolt will fall off but if the crack is only partially through the bolt ... chances are that the bolt will snap when removal of a bolt is attempted.

In the case of a 100% fatigue failure the plain of the broken bolt will be smooth and dark from exposure. But in the case of a snapped bolt only a portion of the plain will be smooth and dark while the portion that just snapped will be shiny and rough.

Smiles.

(Former metals lab tech)

PS My advice is to remove the bolts and replace them as soon a possible because eventually the heads will simply fall off.

Soak them with penetrating oil and let them soak for a long time. Then attempt to remove them turning slightly in both on and off directions. If you feel the bolt break lose then you are home free but if it turns quietly with constant resistance then you are twisting the shaft at the crack and the bolt will snap.

A/C bolts missing
 

Johnny
The solution to your problem of the broken A/C compressor bolts, as posted by ejsharp, seems by far the best advice that you could have. I'm very pleased, now, that I had my A/C bolts replaced, as it seems that it could be a MB design/manufacture fault. My car is petrol powered, and has only done 90,000 kilometres, so it is hardly a problem caused by vibration or age.
Murray Blackwood.

Vibration and age alone should not cause a bolt to fail because they are engineered to do the job they are designed for.

Fatigue cracks are caused because of "tired" metal. To know why a bolt fails one has to determine what conditions existed that caused the metal to fatigue (get tired).

One reason is sloppy manufacturing (machining) of the bolt another might be cheap metallurgical properties of the bolt itself. Most often however failure due to cracks is caused by improper torque or such where one bolt is doing the work of two or more. (i.e. One bolt over torqued and others under torqued)

In such cases normal vibrations will weaken the metal of the overworked bolt and the origin of a crack will begin at the weakest point of the area under stress (crack origins usually begin at the surface often at a machine scratch or blemish).

This spot is smaller than the point of a needle but as normal vibrations continue (from the serpentine belt),
the microscopic dot begins to grow with each vibration and eventually works its way through the shaft of the bolt. In other words, whether a car is gasoline powered or a diesel, will have no effect in determining if a generator bolt will fail. Such vibrations are inconsequential to AC bolts. It is the combination of the high frequency vibrations of the serpentine belt along with a defect of design, engineering or assembly that causes such bolts to crack and fail.

Smiles.

[Edited by ejsharp on 05-27-2001 at 12:19 PM]

Johnny, ....get some "easy out", ...measure the tip size, ...get a drill bit close to the size of the easy out tip. Center punch the bolt, ...make sure it's the center. Drill a 1/4 deep hole in the bolt, ....put the easy out in it, ..tap it to make sure it has a tight fit. Turn the easy out counter-clockwise slowly and feel if it is getting loose. Tap it again to ensure a tight fit, ..spray some WD or penetrating oil to lubricate it. Continue doing this until you finally get the broken bolt out. If you're only close to me,...I'll do it for you.

Fatigued A/C bolts
 

ejsharp
The Tech Forum is very fortunate to have expert opinion from people such as yourself. We are richer for the sharing of your knowledge. Thank you. Johnny, I sure hope you can attack the broken bolts with an Easy-Out as outlined by Rudy, without having to remove the compressor. Murray Blackwood.