1985 300D Alternator - double pulley running on one belt?
 

Hey all, first time poster.
I recently changed out my alternator and all my belts in the process. Unfortunately since I haven't been able to find a matching pair to replace the ones that go on the alternator double pulley. What I'm wondering is, it's a 65A alternator, and a small water pump. That doesn't seem like a very high load to warrant two belts, except for redundancy.

Is there any reason I can't run the alternator on a single belt? It keeps throwing one of the two, but the other is on snug and the car still runs. I see some old posts saying that it's better to have both, but others saying you can't buy "matched belts" from suppliers anymore.

Wouldn't it be better to run one belt, rather than overtighten two to compensate for slightly different sizes? I have to imagine the bearings would last longer.
Thanks in advance!

Run on one belt? It will probably start to slip in a short period, I'd be comfortable with one belt in the short term. I'm under the impression matched sets are no longer sold as new belts are now made to exact tolerances.

Good luck!!!

By way of reference, the M110 engine with the same alternator and same water pump uses one belt.

I don't know where you live, but think of glowing it three times to get it to start in a Kansas winter. How much torque will the alternator require once the engine starts? I don't know the answer, but I bet MB did when they decided to spec double belts for the diesel when they only had one belt on the gasser. I woulsn't worry too much about matching belts. Back when they were available, I didn't think they were all that matched. I remember AC belts on a 75 Mopar that I bought as a matched set with the same lot numbers on them and I could watch the labels moving in relation to each other as it was running. And you'd potentially only be over tightening one belt, the long one would have the right tension. If you don't have one, get a Gates Krikit belt tension checker. They're like $20 and the official MB tool.

The alternator input power requirement, and hence torque, is a function of the output power. A 65A alternator is capable of an output of ~850W or 1.14HP. Input power will be approx. 1.25 x output, or 1.4HP. The belt in question is conservatively rated (industrial application) at 2.2HP.

The purpose of dual belts is not redundancy. It's to allow for reduced belt tension.

That could be more of a pulley alignment problem than a belt problem.

In theory, one belt requires twice the tension as two belts.

I agree that running two belts in a car designed to have two is a good idea. The two belt systems take a slightly narrower belt too. If it keeps throwing you may have the fatter belt.

Also worn mounting brackets may be twisting the alternator causing it to flip off.

My recently purchased 84 TDT had a slipping belt on startup. I tightened it and months later it started slipping again slightly. I noticed my 85 300D had a dual pulley so I added a dual pulley to the 84 which I had and problem solved and the belt tension is MUCH less.

The alternator sits low where water could splash up on the belts. That would be my guess why there is 2 belts.

If you run one belt long enough the pullies will not wear evenly and when you put 2 belts on one belt is going to be loose.

The crankshaft pulley sits even lower and is common to all belts. Which means that your guess was not a good one.

Dual belts are used to reduce belt tension. No guessing required.

If the crank pulley and the alternator pully both get wet the situation much worse.

We also know that most of the Cars on the road that used V-belts had only one belt and have worked fine like that. But, normally their alternators are also higher up. We also know many have higher amperage alternators that that have the potential to put more of a load on those single belts.

Thanks for confirming my suspicion. And thanks all. Ended up cutting the other one a couple days after posting, so far daily driving 50 miles for the past couple weeks has been fine, haven't adjusted the tensioner and no slippage yet. Will report back if I regret this.