In a 1989 420SEL 320K KM, the voltage regulator on the alternator keeps blowing and consequently the battery is not being charged. Put in 2 new alternators in the last 2 months. Also a new starter 3 months ago. Blown the 3rd regulator now, which did not even last a day.

How to diagnose and find the problem?

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1989 420 SEL

Finding german regulators is hard.Make sure battery does not have a short,or low on water.Check wiring,and your ground strap too.Stock ground straps are so tiny,I always add one to all cars.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

As said above, check the ground strap from engine to chassis. It should be free of corrosion and tight. Same story with the negative strap from the chassis to the battery. Make sure your battery terminals are tight, free of corrosion, and you don't have any green corrosion on any exposed wiring.

Moving on from there, check all connections on the alternator itself. It is not uncommon for some of the "push fit" terminals to work their way loose and cause problems.

Since you have new alternators and battery, claim warranty on both and install a new battery AND alternator together. I had a friend in college with a 90s GM truck that kept taking out batteries and alternators like clockwork. The battery failed and cooked the alternator. The cooked alternator killed the battery. The new battery died so the alternator was replaced. Shortly afterwards the battery got killed. Repeat above cycle 2 more times before he finally replaced both. New battery at that point lasted 5 years.

The point here is not to throw parts at the problem, but if you have a recurring issue, rule out the "usual suspects" first, then take care of the rest. Sometimes one failure can affect another and mask the actual problem.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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

How do you know the regulator has failed?

What is the charging voltage when a new / rebuilt alternator is first installed?

Does the charging light turn on when the key is on but engine not running?

Check all the parts in the charging system .It might have the wrong alternator fitted to start with .And is the serpentine belt going the right way around alternator?They are not all the same directional rotation... Drop another earth strap on to the alternator bracket .And check the fuses on the top of the OVP relay if it as one fitted . Someone may have put up rated fuses in it . This little part is to stop over loads in the electrics .

How do you know the regulator is blown????..

It seems the voltage regulator blows when the AC or heater is turned on. I've now replaced the alternator 3 times (re-built each time). In this latest alternator I've put in a new Bosch voltage regulator. I also replaced the battery. Also the starter was replaced about 6 months ago.

I know it is blown because the battery is not charging, and the voltage at the battery, when the car is running is 13.8V. I replaced the regulator, and when the car is running, the voltage at the battery now is 14.1-14.3 volts.

There is no serpentine belt in this car.

Any ideas?

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1989 420 SEL

Depending on the temperature in the engine bay, 13.8V is completely normal for a charging voltage. Fully charged battery voltage is ~12.8V, anything higher means the alternator is charging the battery.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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