I recently (2 months ago) changed the battery and the regulator on my 420 to fix a charging problem. Now, when I'm idling, I see the battery light go on. Once I pick up speed, it goes away (around 1500 rpm). Now, this is where I can't figure it out; When I put a multimeter on the battery posts with the engine running, I get 11.91 vdc no matter what engine speed I have the car at. at idle - 11.91v, at 1500 rpm - 11.91v, at 2500 rpm - 11.91v. Could I have another failed regulator or is there something I'm missing here???

Possibly the alternator itself, has the diode bridge been checked for rectification?

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

No, it hasn't. I don't have a manual, however I do know what you are talking about. Where is the bridge located??? Any help is greatly appreciated as this is my daily driver.

Many parts stores will check out-put for FREE.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Found it!!!!


After doing some more searching and looking for the bridge rectumfrier (lol), found that one of the carbon stacks in the regulator had slipped to one side and was only barely touching the stator. Smoothed down the stack a little and voila!!!! Instant 13.91 vdc charge at idle.

Thanks for the help and suggesstions.

If these cars do not have close to 14 volts- forget it. The diode usually will also drain your battery and no one will be able to figure it out!