My '86 300SDL still has it's factory-issue 70A alternator that it left Sindelfingen with 31 years ago. I've not and still don't have issues with the battery running down, but while tinkering with something else earlier, I noticed some trouble signs.

With the engine idling and everything electrical shut off in the car, charging voltage is 13.50V. Revving the engine will not change this, 13.50V is the max charging voltage it ever produces.

Turn the A/C on and the blower on high, the charging voltage drops below 13.25V, and once the aux fan kicks on, it's to 12.75 and dropping quickly. It finally bottoms out around 12.25V at idle. Revving the engine above 800 RPM will bring it back to 13.50V, but at idle with the A/C doing its thing, down it goes. Turn the headlights on and it's into the 11's.

So what is the issue here? Regulator? Alternator going south? Alternator too small? Since it was built this way, I'd expect the alternator to be sized properly for the vehicle load.

The 13.50V maximum output certainly raises an eyebrow towards voltage regulator issues, however I'd expect there to still be problems even if the RPM were raised. Since it seems capable of returning to 13.5V with the RPM elevated, I'm inclined to think that the alternator is low on capacity as well. Is that even possible?

I don't care to waste my money on a voltage regulator for a 31 year old alternator, I'd rather just replace the entire unit at this point. My thoughts being that if the alternator output is questionable anyway, the money spent on a voltage regulator could have been applied towards a new alternator.

Are there any reliable vendors of rebuilt 70A alternators, or is this a case where it's worth it to pony up for a 90A version? I don't run anything that the car didn't come from the factory with, no subwoofers or anything else that draws any sort of power. I'm uninterested in the 115A version with clutched pulley, so please don't suggest that route.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]