Just upgraded to the Al129x alternator. Upgraded the alternator wiring to 4ga. and connected it directly to the battery. Its putting out 15.4 V at idle and stays that way at higher rpm. With the AC blower, condenser fan and lights on, it goes down to 14.4 V. Is this normal for the alternator?

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

That sounds too high. In my mind, 14.x is "charging voltage" at full RPM, and much more than that is battery-cooking territory.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

Yep, 15V is what you might see if the battery was nearly or completely dead and the charging system was putting out more volts to charge it. MB don't usually get over ~14V. Busted is right, 15V is a tad high.

Volts are dependant on the RPM of the rotor as well as the strength of the rotor field - field strength is a function of current through the windings of the rotor which is controlled by the regulator. Basically, output voltage is somewhat variable - load and engine speed are factors.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

I might check the meter. An analog meter may be out of cal a touch. Less chance of inaccuracy with a digital VOM. It would be easy to check against another vehicle with a known to good charging system.

14.5 is the Maximum Voltage you should see.

If it's not your meter ,look at the Voltage Regulator.

15.4 will cook your battery,over a period of time.

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'84 300SD sold
124.128

X2 compress,
Some times when using a fast charger the voltage can go high like that. With the alt having a voltage reg that shouldnt happen. Good idea to check the meter first.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

VOM is digital. Checked it again yesyerday, it was 13.8V with the radio on. I have a 100amp amplifier. Im getting different readings. When i pulled the alternator from the saab, it was grounded to the chassis with a 10awg wire. I didnt ground the alternator with the wire. Will try to get a new regulator and see if it helps.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

You meant to say a 100watt amp !!! if you get 13.8 v with the radio going, that sounds ok. Take the car for a 10 minute drive during the day. When you get back check it before you turn it off. I think it will be ok. That high reading may have been a bad reading. It happens some times. They some times jump around a little especially if it has just been installed. You can never have too many ground straps on a motor !!!

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Yes thats correct - a 100w amp.

__________________
'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles