I replace the serp. belt with the timing belt at 70K miles (98 S70 GLT). When is the next time I should change it? 140K miles and change both again? Thanks.

Don't have the manual on hand, but seems the recommended interval for the serp belt is 60K. That said, plenty of people go every 70K with the timing belt, and plenty of people change it more frequently. I've heard of folks doing it at 30K in very hot, dry climates. Ultimately, it's a matter of personal preference/comfort level. If it snaps, it's not as catastrophic as a timing belt - but it's still not good.

I do it at 50K because I live in a dry climate and because it's easy for me to remember. Way I see it, the belt costs ~$20, and it takes ~10 minutes to change. You could even do it while you're waiting for oil to drain when you're changing your oil, and you'll still have time to go enjoy a beverage of choice. (If you take that route, careful about getting oil on the belt.)

If you're up in the 140K mile range, you might want to consider putting in a new tensioner/tensioner pulley and idler pulley, too. Do a search of the archives for all of these procedures...there's been plenty written about how easy it is to change the serp belt.

When they first came out with the white engines the interval was 60K but I think they eventually went away from that. We never really changed them as a routine 60K item at the dealership. Reason being was some of the cars had them changed already and did not need it, some of them were running the original and still did not need it, and all of that stuff was going to be coming apart in 10K for the 70K belt change. So the answer to your question is to monitor that belt as part of routine service and change it out when it needs it. A word of warning though, I have seen more than one of those belts come apart, get into the timing belt area, and cause the T-belt to snap and destroy the motor. So while those serp belts will go a long long way even when it sorry ass shape do not push it too far or you may regret it big time.

Mark

thanks.