You know Steve, you are quite a knowledgeable person on the Volvo. At least from what I have read. Anyway, I would love to answer that question if I can. I am just very surprised that you asked this question. I'd like to explain why first.

Well, I have known about belt tension gauges for some time. However, I do not know of any local mechanics, either gas stations or specialty shops that use this type of tool. In fact, I am usually laughed at when I mention it. Even my cousin, an experienced DIY'er laughs at this. Yet these tools are made by some very competent tool mfrs., like Burroughs, Kent Moore, and OTC for instance. So why don't most mechanics sue them. I don't know, but I do know that one person's thumb pressure (like a small female), can be much different from another's (a 200 lb. male mechanic), so I got tired of guessing about whether the tension was correct fiddling with them all the time and just got a quality gauge from a A to Z Wholesale, a local industrial supplier.

It is a sweet gem of a gauge and costs under $100. It does V belts and up to five rib serpentine belts. But the really nice thing is that it has a dial that you just read and it tells you when the tension is correct. The gauge is factory calibrated to within 10% accuracy, the most you can hope for without spending a few hundred on a "belt-specific" gauge (the type used on vehicle assembly lines), and they only give you about three % more accuracy. The cheaper gauges are 20 % accurate.

Regarding your question, I too was trying to find out what the correct tension was before I got the gauge. But it was a hard thing to find out. Believe me, I have spoken to many mfrs. of belts and gauges, like Gates, and they just don't have anything but speculative and vague answers about the correct tension setting. Anyway, 100 lbs. is what I was told was a desirable setting to shoot for. My gauge lists 30 to 70 as too low, 70 to 90 for a used belt, 100 for a belt in between used and new, 110 to 130 for new, and 130 and above as too high.

Interestingly I telephoned Toyota about using this recommended gauge on their serpentine drive belts. They recommended a setting of 175 for a new belt - a very high setting. They claimed that the belt would loosen up. But their recommendation for used belts was the same as that on the gauges markings, so I ignored their advice on their new belts, and used the gauge's dial without any problems.

Boy, I'll never go back to that guessing by the hand deflection method. However, now that I have used this gauge I have a much better sense of what to feel for just in case I ever have to hand set one.

If I were you and I didn't know the correct tension in lbs., I would have it set by a real good expert and then measure it before you start the car measure it cold, and measure it hot, just for your own info. That may be the best recourse you have. That should work well.

Sincerely,
Rob

PS. Let me know if you need the Newton scale measurement equivalents, or anything else. Be patient though, as I don't always check my email on a daily basis.