Here are some tips on alternator belt tensioning on 123 and 126 chassis diesels using a special bolt with a rod welded to it at the 90 degree angle. The procedure described is for a 123 chassis (85 300DT in this case).
1. Because of limited access you are better off removing the air intake duct, the air cleaner, and the air cleaner to turbo plastic elbow. Also, removing the EGR valve corrugated stainless steel tube will make the job much easier.
2. Do not turn the tensioning bolt rod nut until you have loosened all bolts to permit the alternator to pivot freely!!!!!!!
3. Get under the car with a long 17 mm box end wrench and loosen the bottom pivot point bolt. Sometimes this may be rather difficult if the bolt is tight, hence a long wrench helps. Because of proximity of the A/C hose bracket, there does not appear to be enough room for a socket/breaker bar, unless you loosen the hose bracket and get it out o the way.
4. I made a special tool for holding the tensioning bolt head stationary while loosening the 17 mm nut on the other side of the bolt. I took a 1/2 inch drive 17 mm socket and cut a slot in it with a metal cutting circular saw, so it would clear the welded rod. I held the adjusting bolt head stationary while loosening the nut on the other side which was very tight. If the nut is tight and you do not use a special tool to hold the adjusting bolt head stationary, you will bend the adjusting bolt rod!!!! It is not very strong to withstand the torque.
5. There is one more 17 mm nut to loosen. It is the adjusting bracket mount bolt which must be loose to allow free pivoting of the alternator adjusting bolt int he bracket slot. Do not attempt to loosen the nut on the front of the bracket (closest to the radiator) because it is welded to the bracket!!!!!! Instead you must loosen the head of the bolt on the back side of the bracket. The access is rather frustrating - once again not enough room for a socket, so use a long 17 mm box end wrench.
6. After loosening all three, you can now turn the alternator tensioning bolt rod nut and the alternator should pivot freely.
Not a user friendly adjusting system, in fact it appears to be designed so you damage or break things unless you know exactly what you are doing. The price of a new adjusting bolt and bracket is astronomical - the cheapest price on a new bolt is about $ 50 and a new bracket from the dealer will run about $ 150. The bracket will bend also if you keep turning the adjuster bolt nut without loosening all of the bolts mentioned above.
I recommend you use good quality belts such as Continental or Gates to avoid having to tension them frequently. Cheap Auto Zone belts will require just that as they will stretch constantly.
were you able to get it done without buying parts?
I actually bent the rod on the adjusting bolt trying to loosen the nut, but I was able to straighten it out. After that I made a special slotted socket to hold the bolt head in place and this worked fine. After I found out what the prices for the replacement bolt and bracket were, it was an easy alternative.
The problems one runs into are overtightened fasteners on these done by former owners or their mechanics. In my case it was the adjusting bolt, in your case it was the bottom pivot bolt.
If you make a slotted socket like I did, the slot must be cut in a certain location, otherwise the socket will not fit. Mark the location of the slot before you cut it. I used a foot long 1/2 drive extension to hold the socket during cutting.
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